UNIT 14 - Energy Changes and Applications 1

GENII Engineering & Technology Training

UNIT 14 – Energy Changes, Sources and Applications

Accreditation with

A credit towards

BTEC

National Certificate in Applied Science (Level 3)

GENII Engineering & Technology Training National Certificate in Applied Science Level 3Scheme of Work 1


National Certificate in Applied Science

Each student will be provided with:

1. Unit Aims & Learning Outcomes Page 32. Delivery Method Page 33. Unit Contents Index Page 54. Unit Assessment & Accreditation Page 65. . Grade Descriptors Page 86. . Course Learning Materials Page 9



Aim

This unit gives learners an understanding of the fundamental concepts of energy and how energy is measured, with consideration of ‘useful’ energy and ‘wasted’ energy. Energy changes can cause a rise or fall in temperature or changes of state: learners will study more about temperature and how it relates to energy changes, as well as the relationship with volume and pressure. This unit allows learners to develop an understanding of the need for portable energy sources in a laboratory environment, and recognise the importance of tailoring the energy source to the application. They will also learn about energy transfer mechanisms and how they are used in industrial applications.

Learning Outcomes

On completion of this course a learner should:

1. Know the fundamental concepts associated with energy and its measurement

2. Be able to demonstrate and relate changes of temperature or physical state to changes in internal energy

3. Understand the differences between energy transfer mechanisms and the relationships between them

4. Understand the properties of electrical energy sources.

Delivery Methods

This unit covers much of the foundation work relating to energy, energy changes and transfer mechanisms to complement advanced scientific studies. The learners will have been introduced to the concept of energy as part of Unit 1: Fundamentals of Science. This unit aims to develop the quantitative aspects and qualitative understanding.

Practical investigation will form the backbone of the delivery strategy. Learners should carry out measurements and see effects for themselves, rather than passive note-taking and bookwork. Learners are not expected to carry out a series of standard practicals and should develop their own investigations. For example, investigating the effectiveness of double glazing on reducing the rate at which thermal energy is lost. It is important that all the work for this unit is related to industrial applications of energy, highlighting differences in lab-based investigations compared with industry. Consideration should be given to implications for industry, eg high pressure requires thickened pipes.

During delivery of this unit, tutors should adopt the sequence in the content section. This sequence starts by linking the concept of energy to physical work. Learners should recognise that energy is the capacity to do work. All the common energy labels, such as electrical energy, chemical energy and solar energy, are referring to kinetic or potential energies. These are energies due to the motion



or state of physical objects. Two concrete examples of potential energy are indicated in the unit content but tutors should make the learners aware of other forms of potential energy. It would be valuable for learners to carry out a practical investigation of PE to KE conversion (or KE to PE, or PE to PE, etc). This could be done on a simple level, by finding the height reached by a projectile fired by a spring, for example.

It is important for learners to see that temperature is related to the internal energy of a substance. It is not necessary for learners to do a detailed or quantitative study of distributions of quanta in a hot solid. They should recognise that the kinetic energy of the atoms or molecules is related to the temperature. Tutors should focus on how industrial processes make use of this concept. Learners should carry out simple experiments to measure the specific heat or latent heat of a substance. The aim is for learners to experience techniques used to measure physical quantities, rather than to learn a standard experimental technique. Learners could, for example, use a data logger to record the temperature, at regular intervals, of a container of crushed ice heated by an electric immersion heater. This experiment allows the determination of values for specific heat capacity and latent heat fusion of water. There is a very simple experiment for determining the latent heat of vaporisation of water. Water is boiled with the kettle on a balance so that change in mass can be noted. The power of the kettle is known, so the latent heat can be calculated from the electrical energy transferred during the time it takes for a measured mass loss. The industrial applications and implications must again be the focus here.

The treatment of thermal conductivity could be linked to insulation of buildings or the effectiveness of double glazing. It is useful for the learners to see the heat flow equation as ‘push = flow x resistance’. In this case the ‘push’ is indicated by the temperature difference, the flow is the energy flow and the resistance is the inverse of the conductance (note: conductance = conductivity area/length). Learners should comment on the effects of surface layers and the industrial applications. This is a good time to draw out the parallels in different flow systems, ie flow of fluid, flow of charge and flow of heat. This point need not be laboured, but it is useful for learners to draw parallels across different systems, as it will help their understanding of science. Forced convection produces a faster cooling rate than natural convection. Learners’ experience should tell them this. At this level, learners should be aware of the five-fourths power law for cooling by natural convection and the linear law for cooling in a steady draught. Quantitative questions requiring the recall of those laws is not necessary. When dealing with thermal radiation, learners should understand what is meant by a black body radiator, be able to complete calculations using Stefan’s law and be able to explain the industrial applications and implications.

The treatment of energy sources is restricted to those used to power portable equipment. Learners need to understand the basic principles, so they should study the simple cell, the leclanche dry cell and the lead-acid cell. They should consult catalogues of cell suppliers and get to know the range of cell types currently available. They should know how they differ from those studied in terms of energy capacity, convenience, load performance, suitability for particular applications, etc. Fuel cells are being developed as energy sources for vehicles and other devices. Learners should know how fuel cells differ from conventional cells.

They should also investigate the energy per square metre delivered by sunlight, so they understand the potential benefits and limitations of using solar panels to power remote instrumentation.



Unit Contents Index

Section 1.

Know the fundamental concepts associated with energy and its measurement

Definitions: work as force x distance moved in direction of force (W=Fd); energy in terms of work; kinetic energy (KE = ½ mv2); gravitational potential energy (PEg = mgh); elastic potential energy (PEe = ½ kx2); power as the rate of transfer of energy

Concepts: principle of the conservation of energy; recognition of energy types as forms of potential or kinetic energies; useful energy, wasted energy and efficiency

Quantities and units: energy (joule); power (watt, kilowatt)

Section 2

Be able to demonstrate and relate changes of temperature or physical state to changes in internal energy

Temperature: degree of hotness; temperature scales (Kelvin, Celsius) and fixed points (absolute zero); thermal expansion

Energy changes: transfer of energy can cause a rise or fall in temperature or changes of state; calculations eg specific heat capacity (Q = mcΔt), specific latent heat (Q = mL); fusion; vaporisation; condensation; applications

Gases: effect of changing temperature, pressure and volume of a gas; experimental evidence for a gas law eg Charles’ law, Boyle’s law, pressure law; the ideal gas law; kinetic theory dealt with qualitatively; applications

Section 3

Understand the differences between thermal energy transfer mechanisms and the relationships between them

Energy transfer mechanisms: conduction (transfer of kinetic energy between atoms, electrons or molecules); thermal conductivity of solids, liquids and gases; convection (bulk motion of liquids); radiation (absorption, emission and relation to surface properties); Stefan’s law of radiation (W = eσAT4 ); temperature gradient; applications

Relationships: eg differences between forced and natural convection



Section 4

Understand the properties of electrical energy sources

Structure and operating principles of common primary and secondary cells: characteristics, merits and limitations for particular applications; capacity and behaviour under load; ampere-hours, milliampere-hours; disposal hazards; applications

Fuel cells: eg simple cell, the leclanche dry cell and the lead-acid cell, zinc-air fuel cells (ZAFC), proton exchange membrane or solid polymer, direct methanol fuel cells, recent developments, their prospects and limitations

Solar cells: recent developments; their prospects and limitations

Assessment & Accreditation

Successful completion of this course will provide the learner with a unit credit leading towards the award of a BTEC National Certificate in Applied Science.

A BTEC certificate of achievement can be awarded for the completion of this single unit.

To gain the National Certificate additional units will be required:

Fundamentals of Science Mathematics for Science Technicians Application of Numbers for Science Technicians Working in the Science Industry Industrial Applications of Chemical Reactions Electrical Circuits and their Industrial Applications Scientific Practical Techniques Applied Engineering Science (Electrical/Mechanical)

This unit will be assessed with written tests, along with practical observation and practical write ups. These tests and observations will assess the unit outcomes.

All the pass grade criteria must be met in order for a learner to achieve this unit. This unit requires learners to build up a portfolio of laboratory investigations, reports and calculations. The majority of the pass criteria can be achieved through practical activity.

For P1, learners must describe the fundamental concepts of energy, in the context of industrial applications. They must be able to define each term, and know the associated unit of measure, as listed in the unit content.

For P2, learners must investigate a gas law. Experiments are available interactively on the web, but learners should perform at least one in a real laboratory. The Charles’ law tube is a fairly inexpensive piece of equipment containing a small amount of mercury. A risk assessment must be carried out.



For P3, learners must investigate conduction, convection and radiation. They could do this through cooling experiments, and this would be a useful opportunity for learners to use data loggers. Learners need to highlight and explain differences in their investigations compared with those used in industry.

For P4, learners must describe the characteristics of primary cells and secondary cells and then highlight the differences between these two types of cells including their uses.

For a merit grade, all the pass grade criteria and all the merit grade criteria must be met. All calculations must be carried out correctly and the correct units must be used.

For M1, learners must do accurate calculations involving changes of state of substances used in industrial process.

For M2, learners must perform calculations to determine the changes in pressure, volume and temperature for gases used in industrial processes, as given by the tutor.

For M3, learners must calculate energy flow for given thermal conductivities and temperature gradients, in an industrial application. They could involve the more practical applications of thermal conductivity in the insulation of buildings, eg heat energy lost through single and double glazed windows. The data for industrial calculations that use Stephan’s law will need to be providedby the tutor.

For M4, learners must describe the characteristics of primary and secondary cells used for industrial applications. They must also comment on the merits and limitations of the primary and secondary cells, and the implications of these for their industrial applications.

For a distinction grade, all the pass, merit and distinction grade criteria must be met.

For D1 and D2, learners must apply the principles, in the unit content, to at least one industrial system.

For D3, learners must explain the heat transfer mechanisms in solids, liquids, gases and combinations of substances. In doing so, they must describe molecular motion, bulk motion and surface properties, and highlight the differences of each in solids, liquids and gases. Learners must use examples contextualised to industrial processes.

For D4, learners must evaluate the use of primary and secondary cells for mobile electronic units. They could compare two portable devices which use primary and secondary cells, eg MP3 players, torches.



Grade descriptors

Grading Criteria

To achieve a pass grade the evidence must show that the learner is able to:

To achieve a merit grade the evidence must show that, in addition to the pass criteria, the learner is able to:

To achieve a distinction grade the evidence must show that, in addition to the pass and merit criteria, the learner is able to:

P1 describe the fundamental conceptsassociated with energy using industrialexamples

M1 perform calculations involving changes of state for industrial processes

D1 explain the behaviour and response of industrial systems in terms of latent heat, specific heat capacity, temperature changes and the gas laws

P2 report on the outcomes of an investigation of one gas law and relate them to industrial processes

M2 calculate pressure, volume and temperature changes for gases in given industrial processes

D2 explain gas pressure and how it affects industrial processes

P3 investigate and describe the processes of conduction, convection and radiation and their industrial applications

M3 calculate energy flow using industrial examples for given thermal conductivities and temperature gradients, and also for given emissivities

D3 explain the differences between heat transfer mechanisms in solids, liquids, gases and combinations of substances, in terms of molecular motion, bulk motion and surface properties in industrial processes

P4 describe the difference between primary and secondary cells.

M4 give examples of primary and secondary cells and describe their characteristics, merits and limitations, in industrial applications.

D4 evaluate the use of primary and secondary cells for portable applications.



Energy Changes, Sources and Applications

Accreditation with

BTEC


Chapter 1

Internal Energy

Unit 14 – Energy Changes, Sources and Application – Internal Energy 9


Properties of gases

Particles in solids, liquids and gases:

Gases Liquids Solids

Diagram

Shape and volum

e

They do not have a definite shape.

They have a definite volume.

Distance

between

particles

The particles are far apart

Movem

ent of particles

The particles can only move by vibrating next to

each other

Forces

between

particles

There are tiny forces of attraction between the

particles.

It is now accepted that matter consists of particles. The only difference between the ‘states’ is the arrangement of these particles. The particles (which may be atoms, ions or molecules) are very small (in the order of 0.07 nm 0.20 nm).

Solid: Particles are closely packed in a rigid structure. It allows no gross movement of particles – they are locked in position. It only allows vibration. Solids are hard to compress and cannot flow due to the rigid structure.

Liquid:Going from a solid to a liquid results in a small increase in volume (5 30 %) due to the looser arrangement of the particles. Liquids allow a little more compression and free movement of particles (they can flow and no longer have a fixed shape). The volume is fixed due to the attractive forces (either hydrogen bonding as with HF, NH3, H2O, or dipole-dipole as with H—Cl , or Van der Waals interactions with O2 or CS2) between the particles.



Gas: Going from a liquid to a gas there is a considerable increase in volume (over a 1000 times at atmospheric pressure). Gases are readily compressed as the majority of a gas is empty space between the particles. The forces between gas particles are negligible allowing free movement of the gas with no fixed volume.

Questions: 1. A solid usually expands on melting because ______________________________________

__________________________________________________________________________

__________________________________________________________________________

2. A liquid has a fixed volume at a specified temperature because _______________________

__________________________________________________________________________

__________________________________________________________________________

3. A liquid does not have a fixed shape at a specified temperature because ________________

__________________________________________________________________________

__________________________________________________________________________

4. The volume of a gas gets a lot smaller when it changes to a liquid because ______________

__________________________________________________________________________

__________________________________________________________________________

5. Gases are easily compressed, but solids are not because _____________________________

__________________________________________________________________________

__________________________________________________________________________

6. What effect has increased pressure on the volume of a liquid and why? _________________

__________________________________________________________________________

__________________________________________________________________________

__________________________________________________________________________

Terms used when dealing with gases

Temperature:

This is a measure of how hot a substance is.

The common scale most of us use is the Celsius scale (also called the centigrade scale). This is based on the changing states of water. The freezing point of water was taken to be 0 oC and the boiling point of water was taken to be 100 oC. These two temperatures are known as fixed points.

The scale used in chemistry is known as the Kelvin or Absolute scale. Absolute zero is thought to be the coldest temperature that can be obtained. It is the temperature at which a gas would have zero volume. It is given the symbol 0 K. Note that there is no degree sign.



Zero Kelvin is –273 oC (actually it is –273.15 oC but usually it is taken to be -273 oC). A Kelvin is equal to a degree centigrade in size. So 0 K is –273 oC, 273 K is 0 oC, 373 K is 100 oC and 473 K is 200 oC.

Temperature in centigrade Temperature in Kelvin

0 oC

100 oC

100 K

298 K

30 oC

-15 oC

The Kelvin is the S.I. unit of temperature.

A temperature of 0 K has not yet been achieved. However, temperatures to within 0.001 K have been. At this very low temperature the random movement of the particles almost vanishes. This causes substances to have strange properties such as some metals losing their electrical resistance and becoming superconductors.

Note that heat energy and temperature are two different things:

(i) A Bunsen burner was used to heat up a beaker of water and a bath of water. (a) Which has been supplied with the most heat__________________________(b) and which reaches the highest temperature? __________________________

(ii) A spark from a sparkler contains very little ____________ , but will have a very high ____________.

(iii) Bath water has a low ______________, but contains a lot of _______________ .

Pressure:

This is the force that a gas exerts, on the walls of the container which holds it, per unit area.The S.I. unit for pressure is Newtons per metre squared, Nm-2. This is also called the Pascal, Pa.Normal atmospheric pressure is 1 x 105 Nm-2, or 1 x 105 Pa.

(The old units of pressure were one atmosphere or 760 mm of mercury – the height the atmosphere would hold a column of mercury. You may see this in some old textbooks).

Volume:

The volume of a gas is the same as the container that holds it (remember that gases fill up any container). The S.I. unit of volume is the cubic metre, m3. This is too large a volume to use in most cases so we generally use a volume of one litre (L); more commonly called a decimetre cubed, dm3.

Note: 1 cm3 = 0.001 dm3 = 0.000,001 m3.1 dm3 = 1,000 cm3 = 0.001 m3.1 m3 = 1,000 dm3 = 1,000,000 cm3.



Standard temperature and pressure – s.t.p.:

When comparing volumes occupied by gases we always use a set temperature and pressure (so we can compare like with like). This is known as s.t.p. or standard temperature and pressure. The values used are

Standard temperature = 273 K (0 oC)Standard pressure = 1 x 105 Pa (1 atm or 760 mmHg)

Gas laws and the Kinetic Theory of Gases

All gases are observed to have a range of similarities between their physical properties.



Boyle’s Law:

Boyle, an Irish chemist, measured how the pressure exerted on a gas affected its volume. He discovered that at a constant temperature there was a link between the volume and the pressure. He proposed therefore that pressure was inversely proportional to volume.

Pressure 1 / Volume

i.e. Pressure = 1 x constantvolume

Written as P = (1/V) x constant if the temperature is constant.

This means that as you increase the pressure by a set amount, there is a corresponding reduction in the volume. i.e. You double the pressure and half the volume.

Boyles Law

At a constant temperature, the volume of a given mass of any gas is inverselyproportional to the pressure of the gas.

P=(1/V) x constant PV = constant

Demo a syringe

We can plot the data as a graph:



We can see that the data fit into a pattern called a hyperbola. If, however we plot pressure against 1/volume we get a linear (straight line) graph showing that the pressure is inversely (as it’s a graph of 1/V) proportional (as it is a straight line) to the volume.



Demo Boyles’ Law apparatus

The picture below shows a data-logging experiment for Boyle's Law.


Tube with volume of air trapped by oil

Volumescale

Bicycle pump

Reservoir of oil

Pressure gauge

Valve


Explanation of Boyle’s Law:

Gases are in continuous motion. They exert a pressure when they collide with the sides of the container. If the volume of the container is decreased, then the molecules are moving around in a smaller space. This will result in more of the molecules crashing into the sides of the container per second. The increased number of collisions will therefore cause an increase in the pressure.

See NASA web page

Charles’ Law:

The volume of a gas varies when the temperature is changed. Demo heating a test tube of air, bubbling into water.

Charles, a French scientist, discovered that equal volumes of different gases (at constant pressure) all expanded by the same amount for a given rise in temperature. This can be written as:

V = T x constant

Where V = volume (in m3) and T = temperature (measured in Kelvins).

Charles’ Law

The volume of a fixed mass of any gas at constant pressure is directlyproportional to the temperature in Kelvins.

V = T x constant V/T = constant

Charles investigated how the volume varied with temperature (under a constant pressure) using a capillary tube blocked at one end by a bead of concentrated sulphuric acid. As he cooled the apparatus, the air trapped in it reduced in volume. As he heated it, the gas increased in volume (moving the bead of sulphuric acid). The expansion and contraction was found to be the same for all gases. He plotted a graph of the volume for a range of temperatures. He was unable to obtain very low temperatures, so he extrapolated (continued the line with the same gradient backwards) until the volume was zero. This gave a temperature of –273 oC, or 0 K. From this he stated the above law.



Whatever the gas we use, we find that the line always passes through the temperature axis at -273.15 oC. This led to the concept of absolute zero, discovered by Lord Kelvin. If we put the absolute zero point, 0 Kelvin, we get:



Explanation of Charles’ Law:

If the temperature of the container of gas is raised, the particles of gas will have more energy and therefore move faster. This means that they will make more collisions with the walls of the container, resulting in a rise in pressure. Now, to keep the pressure constant the volume of the container must be increased. (Think about what happens when a can of coke heats up and is opened – the gas rushes out, as it wants to occupy a larger volume).

See NASA web page

The Pressure Law:

This law tells us that pressure is proportional to the Kelvin temperature, if the volume is fixed. The traditional way to demonstrate this is with a large glass sphere immersed in water, connected to a manometer, a way of detecting small differences in gas pressure.



We can show this on a graph like this:



So P µ T

Therefore P = kT and rearranging, P/T = constant

The combined gas law:

Boyle’s Law: V 1/P at constant T. PV = constant P1V1 = P2V2

Charles’ Law:V T at constant P. V/T = constant V1/T1 = V2/T2

Pressure Law:P T at constant V P/T = constant P1/T1 = P2/T2

Combined Laws: PV T under all conditions PV = constant P1V1 = P2V2 T T1 T2

Where P = pressure (Nm-2)V = volume (m3)T = temperature in Kelvin (K)

When using the Combined gas law you must use the temperature in Kelvin.

You may use whatever units you wish for volume and pressure, as long as you are consistent (use the same on both sides of the equation).

Questions:1. A sample of nitrogen has a volume of 75 cm3 at a temperature of 27 oC and a pressure of 95

kPa. Find the volume that the gas would occupy at s.t.p.

2. A sample of hydrogen of volume 100 cm3 at a pressure of 1 x 105 Pa is compressed to 55 cm3 at a constant temperature. What is the new pressure of this gas?

3. A chemical plant produced 2500 litres of ammonia at a pressure of 500 kPa and a temperature of 100 oC. A chemical engineer wishes to store the ammonia at a pressure of 100 kPa and a temperature of 20oC. What volume would the gas occupy at this new temperature and pressure?



Gay-Lussac’s Law

Gay-Lussac, a French chemist, discovered that if you react 2 volumes of hydrogen with one of oxygen then water is produced. All the reactants were used up in the reaction. He found that there was always a simple whole number ratio between the volume of gases reacting and produced. He came up with the following law:

Law of combining volumes

In reactions involving gases, the ratio of volumes reacting bear a simple ratio to each other and to the product gas (assuming the temperature and pressure remain

constant).

Other examples are:

(one) methane + (two) oxygen (one) carbon dioxide + (two) water

(two) carbon monoxide + (one) oxygen (two) carbon dioxide

But Gay-Lussac could not explain this observation.

Avogadro’s Law

Avogadro was the one to explain Gay-Lussac’s Law of combining volumes.

Avogadro’s Law

Equal volumes of gases, under the same conditions of temperature and pressure, contain equal numbers of molecules.

This means that 2 dm3 of oxygen gas at 300K and 100,000 Nm-2 has the same number of molecules as 2 dm3 of nitrogen gas under the same conditions.

This helps to explain Gay-Lussac’s observations. He stated that 2 volumes of hydrogen react with one volume of oxygen to make 2 volumes of water vapour.

Hydrogen + Oxygen water vapour2 volumes 1 volume 2 volumes

Assume there are N molecules of oxygen present, then, applying Avogadro’s Law which relates the volume to the number of molecules present:

Hydrogen + Oxygen water vapour2N molecules 1N molecules 2N molecules

Then diving by N:

Hydrogen + Oxygen water vapour2 molecules 1 molecule 2 molecules



Therefore the ratio of the volumes of gas is equal to the ratio of number of molecules involved in a reaction. This is one way of deducing the equation for the reaction.

Hydrogen + Oxygen water vapour 2H2 + O2 2H2O

Questions:1. Sulphur dioxide reacts with hydrogen sulphide according to the equation:

2H2S (g) + SO2 (g) 3S (S) + 2H2O (g)

What volume of sulphur dioxide would be required to react completely with 140 cm3 of hydrogen sulphide (all volumes being measured under the same conditions of temperature and pressure)?

Answer: ____________

2. 140 cm3 of methane is found to react completely with oxygen according to the equation:

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)

Assume that all volumes are measured under the same conditions of temperature and pressure.

a) What volume of oxygen is consumed in the reaction? ________________

b) What volume of carbon dioxide is formed in the reaction? ________________



The Mole

It is impossible to weigh out one molecule of an element or compound because it is far too light. But we can measure out volumes of gas accurately. We know that at the same temperature and pressure, a set volume of all gases contain the same number of particles (Avogadro’s Law). So if we want to produce hydrogen chloride gas from hydrogen and chlorine: ( H2 (g) + Cl2 (g) 2HCl (g) ) we need to react one volume of hydrogen and one volume of chlorine to form two volumes of hydrogen chloride. This will leave us with no unreacted hydrogen or chlorine.

We can only do this for reactants which are gases. If they are solids or liquids we cannot measure out the amount of molecules by volume (Gay-Lussac’s and Avogadro’s Laws do not apply). An example of this is the reaction between sodium and hydrogen: H2 (g) + 2Na (s) 2NaH (s).

As we can’t always use volumes to ensure the right number of reacting particles, there was a need to create a unit for the quantity of atoms, molecules and ions which is easily measurable. This unit is known as the MOLE. As we cannot measure one molecule, it was decided to measure a much larger number of particles and call this quantity the mole. In 12 grams of 12C there are 6.02 x 1023 particles. This number of particles makes up one mole of 12C, and is known as Avogadro’s constant (L).

The definition of a mole is as follows:

A mole is the amount of substance which contains as many particles as there are atoms of 12C in 12 grams of 12C. This number of particles (6.02 x 1023) is known as Avogadro’s Constant.

Now linking back to the reaction to produce water:

Hydrogen + Oxygen water vapour2 molecules 1 molecule 2 molecules

Multiplying by Avogadro’s number:

Hydrogen + Oxygen water vapour2 x 6.02 x 1023 1 x 6.02 x 1023 2 x 6.02 x 1023

molecules molecules molecules(two moles) (one mole) (two moles)

Molar volume of gases:



According to Avogadro’s Law, equal volumes of all gases (at s.t.p.) contain the same number of particles.

One mole of gas at s.t.p. occupies 22.4 dm3 (or 22,400 cm3).

e.g. one mole of CO2 occupies 22.4 dm3 (or 22,400 cm3) at s.t.p.and one mole of H2 occupies 22.4 dm3 (or 22,400 cm3) at s.t.p.

22.4 dm3 is known as the molar volume (volume occupied by one mole of any gas).

Standard temperature and pressure – s.t.p.:

The volumes of gases are compared at a standard temperature and pressure as follows:

Pressure: 1 atmosphere = 760 mm of mercury = 1.0 x 105 Nm-2 Temperature: 273 K = 0oC.

Key equations:

Volume of gas at s.t.p. = No of moles x 22.4 dm3

No of moles of gas = volume of gas 22.4

In all calculations, always convert masses and volumes into moles before you manipulate them in equations. It makes life easier and always works.

1. What is the volume in (a) dm3 at s.t.p. of each of the following?

(i) 0.5 moles of oxygen (O2) gas ________ dm3

(ii) 0.025 moles of nitrogen (N2) gas ________ dm3

(iii) 3 moles of hydrogen sulfide (H2S) gas ________ dm3

(iv) 0.125 moles of methane (CH4) gas ________ dm3

(v) 7 moles of ammonia (NH3) gas ________ dm3

2. How many moles are there in each of the following (all volumes are measured at s.t.p.)

(i) 560 cm3 of oxygen gas _________ mol

(ii) 1120 cm3 of hydrogen gas _________ mol

(iii) 56 cm3 of methane gas _________ mol

(iv) 44.8 dm3 of carbon dioxide gas _________ mol

(v) 1.12 dm3 of ammonia gas _________ mol

3. How many molecules are there at s.t.p. in

(i) 280 cm3 of nitrogen (N2) gas _________ molecules



(ii) 2.24 dm3 of oxygen (O2) gas _________ molecules

4. How many atoms are there at s.t.p. in

(i) 280 cm3 of nitrogen (N2) gas _________ atom

(ii) 2.24 dm3 of oxygen (O2) gas _________ atom




Where T = temperature in Kelvins (K)

1. Using the combined gas law, convert the following to volumes at s.t.p.:

(i) 37.5 cm3 of carbon dioxide at 300 K and 100,000 Pa

(ii) 82 cm3 of chlorine at 18 oC and 100,000 Pa

(iii) 224 dm3 of ammonia at 19 oC 1,000,000 Pa

(iv) 57 cm3 of nitrogen at 373 K and 90,000 Pa

2. Answer the following

(i) State Avogadro’s Law

(ii) How would you demonstrate experimentally that gases diffuse faster than

liquids?

(iii) What is Brownian motion? Explain how it arises.



3. Answer the following

(i) How do the motion of particles differ in solids, liquids and gases?

(ii) How does the spacing between the particles differ in solids, liquids and gases?

(iii) What effect do you think a decrease in temperature would have on the rate of diffusion of a gas? Why do you think this?

(iv) Explain why a gas exerts pressure on the walls of a closed vessel in which it is contained. How is this pressure affected by increasing the temperature, and why?

Extension question:

4. In a suitable apparatus at 423 K, 100 cm3 of hydrogen sulphide, measured at s.t.p., reacted with 100 cm3 of sulphur dioxide, measured at s.t.p. The volume (when corrected to s.t.p.) after the reaction was found to be150 cm3. When the water vapour was removed, a volume (when corrected to s.t.p.) of 50 cm3 of gas remained. This gas was found to be sulphur dioxide.

(i) What is the combining volume ratio of hydrogen sulphide and sulphur dioxide? Explain how this data leads to this conclusion.

(ii) If the formulae for hydrogen sulphide gas and sulphur dioxide gas are H2S and SO2 respectively, write a correct chemical equation for the reaction, given that solid sulphur is the only other product formed.



The Kinetic Theory

The above laws were explained by the Kinetic Theory of Gases. This theory came up with some assumptions to account for these laws: They are:

1. Gases are made up of particles. The particles are in constant random motion.2. There are no forces of attraction or repulsion between these particles.3. The diameters of the particles are negligible in comparison to the distance between particles.4. All collisions are perfectly elastic. 5. The average kinetic energy of particles is proportional to the Kelvin temperature.

The following Laws apply to an IDEAL GAS

An ideal gas is one which follows all the ideas behind the Kinetic Theory stated above.

Boyle’s Law: V 1/P at constant T. PV = constant P1V1 = P2V2

Charles’ Law:V T at constant V. V/T = constant V1/T1 = V2/T2


Avogadro’s Law: V n at constant T and P

Where P = pressure (Nm-2)V = volume (m3)T = temperature in Kelvin (K)n = number of moles (mol)

Combining Boyle’s, Charles’ and Avogadro’s Laws, V is proportional to 1/P x T x nThis can be re-written as V = R x 1/P x T x n

Where R is a constant, known as the gas constant.

This equation is known as the equation of state for an ideal gas, or the Ideal Gas Law. It is usually written as:

PV = nRT The value for R = 8.31 JK-1 mol-1. Care with units

This equation is only obeyed by ideal gases, ones which follow the ideas behind the Kinetic Theory. This is also true of all the other gas laws stated above.

An ideal gas can be described as one which follows the ideal gas law, PV = nRT, for all temperatures and pressures.



Properties of gases

No moles = mass / Ar

No moles = mass / Mr

No moles = volume / 22.4 dm3

No moles = No of particles / 6.02 x 1023

PV = nRT (where R = 8.4 N m mol-1 K-1)

(P1V1) / T1 = (P2V2) / T2

a) State Boyles’s Law.

b) Define relative molecular mass (Mr).

c) What do you understand by an ideal gas?

d) What does n represent in the Ideal Gas Equation (PV = nRT)

e) 0.3 g of a gas occupies 169 cm3 at 300 K and a pressure of 1.0 x 105 Nm-2. Calculate the relative molecular mass (Mr) of the gas. (R = 8.4 N m mol-1K-1).

f) A mass of 5.6 grams of a gaseous diatomic element (X2) occupies a volume of 4.98 x 10-3 m3

at 27oC and a pressure of 1.0 x 105 Nm-2. Find the relative molecular mass of the element and give its name.

g) What is the volume at s.t.p. of one gram of nitrogen gas?

h) A gas occupies 500 cm3 at 273 K and 101 x 105 Nm-2 pressure. What volume will it occupy at 819 K and 2.02 x 105 Nm-2 pressure?



Effect of heat

1. Introduction

If we heat a solid material, it gets hotter and at the melting point it melts and changes to a liquid. More heating will make the liquid hotter and when it reaches the boiling point it evaporates to form a gas. Note that melting and evaporating does not occur instantly. Energy is added to change the state of the substance without raising the temperature.

2. Solids, liquids and gases

Bonding

In a solid material, the atoms or molecules are held together very strongly and cannot wander about freely. As the temperature is raised the atoms vibrate and so obtain kinetic energy. If the temperature is raised enough, this vibration becomes so severe that the bonds break and the molecules are no longer able to maintain a fixed structure so it melts and becomes a liquid.

A liquid has bonds strong enough to hold the particles together but also weak enough that the substance will flow and can be poured into a vessel of any shape. If the temperature of a liquid is raised the weak bonds get even weaker and the particles move around with increased kinetic energy. The liquid normally becomes more fluid – think of cold and hot custard.

At some point the particles will break free all together and unless constrained in a vessel, they will fly off. This is evaporation. The substance has become a gas.

3. Internal energy

The molecules of a substance possess kinetic energy. There may be elements of other energy involved but basically it is the kinetic energy of the molecules inside the substance that gives us the internal energy.



If a substance changes temperature by ΔT and does not change state (melting or evaporating), thechange in internal energy ΔU is directly proportional to the change in ΔT and the mass m.

ΔU = m c ΔT

Note that a change in absolute temperature is the same as a change in degrees Celsius. The units of temperature change may be stated in oC or K. The constant of proportionality is c and this is called the specific heat capacity. This is different for different materials. The units of c are J/kg K or kJ/kg K. The symbol for internal energy is U kJ or u kJ/kg.

4. Heat transfer

Heat transfer occurs because one place is hotter than another. Heat will only flow from a hot body to a cold body due to the temperature difference. There are 3 mechanisms for this; conduction, convection and radiation.

A quantity of energy transferred as heat is given the symbol Q and its basic unit is the Joule. The quantity transferred in one second is the heat transfer rate and the unit is the Watt.

An example of this is when heat passes from the furnace of a steam boiler through the walls separating the combustion chamber from the water and steam. In this case, conduction, convection and radiation all occur together. In many problems, the heat transfer is the same as the change in internal energy so we often use the formula

Q = ΔU = m c ΔT

Calculate the energy needed to heat up 50 kg of aluminium from 20oC to 300oC. The specific heat capacity is 913 J/kg K.

5. Change of state

When a solid is heated, a temperature is eventually reached when it cannot absorb any more energy and it melts. Whilst it is melting, its temperature stays constant. The energy absorbed by melting is called the latent energy of fusion. The values of latent energy may be looked up in tables. The figures are always given for 1 kg so it is called the specific latent energy of fusion. The change in



specific internal energy is denoted as ui. If this is entirely due to a heat transfer Q then :

Q = ΔU = m ui

Once a solid has melted, the addition of more energy will make the temperature rise again. The specific heat capacity of the liquid is different to that of the solid but the change in internal energy is again given by ΔU = m c ΔT

There is also a maximum amount of energy that the liquid can absorb. When this point is reached, it starts to evaporate and turns into vapour. The temperature is more commonly called the boiling point. We all should know that water boils at 100oC but we should also know that this is only true if the pressure is standard atmospheric pressure. Liquids boil at higher temperatures if it is at a higher pressure. (pressure cooker, gas lighters)

Q = ΔU = m c ΔT + m ui

80 kg of copper at 20oC is heated up to its melting point of 932oC and then just exactly melted. Calculate the energy needed. The latent heat of fusion is 210 kJ/kg and the specific heat capacity is 385 J/kg K

Whilst liquid is evaporating, the temperature stays constant and energy is absorbed. This energy is called the latent energy of evaporation. Because the liquid changes volume as it turns into vapour, the energy transfer may be more than the change in internal energy. The values of these latent energies must be found in tables at the pressure and temperature required.

After the liquid has evaporated it becomes vapour. If this is heated, its temperature rises again. A vapour does not accurately behave as a gas because of the tendency to turn back into liquid if cooled. When it is heated well above the boiling it becomes a gas and obeys gas laws. Steam and other vapours are also called superheated vapours when heated above the boiling point. The internal energy of vapours is found from charts and tables.

1. A boiler heats up 2.5 kg of water per second from 20oC to 100oC and then evaporates it without raising the temperature. The specific heat capacity is 4.186 kJ/kg K and the latent heat ufg is2090 kJ/kg. Calculate the change in internal energy. .



2. Calculate the energy needed to heat up 2 kg of copper from 100oC to 250oC. The specific heat capacity is 386 J/kg K.

3. Calculate the change in internal energy when 30 kg of water is heated from 20oC to 80oC. Take c = 4186 J/kg K.

4. 5 kg of lead at 20oC is heated up to its melting point of 600oC and then just exactly melted. Calculate the energy needed. The specific heat capacity of solid lead is 126 J/kg K and the latent energy of fusion ui is 26 kJ/kg.

5. A boiler heats high pressure water from 80oC to 152oC and then evaporates it without raising the temperature. The specific heat capacity is 4.86 kJ/kg K and the latent heat ufg is 1923 kJ/kg. Calculate the change in internal energy of 1 kg.

5. Thermal expansion

When solids and liquids are heated, the molecules vibrate more and take up more space so the material expands. Consider first the expansion in one direction. If a bar of material of length Lo has its temperature increased by T degrees, the increase in length is ΔL.



This is directly proportional to the original length L and to the temperature change ΔT. It followsthat :-

ΔL = constant x Lo ΔT

The constant of proportionality is called the coefficient of linear expansion (α).

ΔL = α Lo ΔT

A thin steel band 850 mm diameter must be expanded to fit around a disc 851 mm diameter. Calculate the temperature change needed. The coefficient of linear expansion is 15 x 10-6 per oC.



Superficial expansion

This is about the change in area of a flat shape. Consider a flat plate of metal with area Ao. The change in area is ΔA and this is directly proportional to the temperature change so:-

ΔA = constant x Ao ΔT

The constant is the coefficient of superficial expansion β

ΔA = β Ao ΔT

A steel sheet has an area of 500 cm2 at 20oC. Calculate the area when it is heated to 300 oC. The coefficient of superficial expansion is 30 x 10-6 per oC.

Cubical expansion

Since a material expands in all directions, the volume changes. The change in volume is . ΔV. This is directly proportional to the original volume Vo and to the temperature change ΔT. It follows that

ΔV = constant x Vo ΔT

The constant of proportionality is called the coefficient of cubical expansion (γ).

ΔV = γ L ΔT

1. Calculate the change in volume of 1 m3 of water when it is heated from 10 oC to 80 oC. The coefficient of cubical expansion is 210 x 10-6 per oC



2. A brass bar is 600 mm long and 100 mm diameter. It is heated from 20 oC to 95 oC. Calculate the change in length. α is 18 x 10-6 per oC.

3. A steel ring is 50 mm diameter and 2 mm thick. It must be fitted onto a shaft 50.1 mm diameter. Calculate the temperature to which it must be heated in order to fit on the shaft. The initial temperature is 20 oC and the coefficient of linear expansion is 15 x 10-6 per oC.

4. A stub shaft 85.2 mm diameter must be shrunk to 85 mm diameter in order to insert it into a housing. By how much must the temperature be reduced? Take the coefficient of linear expansion is 12 x 10-6 per oC.

5. A tank contains 40 m3 of oil at 10oC. Calculate the volume at 40oC given γ = 700 x 10-6 per oC.

6. Copper sheet covers a wall and has an area of 20 m2 at 15oC. What is the change in area when it is heated to 80oC? β = 34 x 10-6 per oC.



Energy Changes, Sources and Applications

Accreditation with

BTEC


Chapter 2

Electrical Energy Sources

Unit 14 – Energy Changes, Sources and Application – Electrical Energy Sources 38


Electric Batteries and Cells

A cell is a device that produces electricity from a chemical reaction. It consists of a negative electrode, an electrolyte which conducts ions and a positive electrode. So, basically, there are two electrodes separated by an electrolyte. The name of the cell usually includes reference to the materials used in the construction and the voltage of each cell depends on the chemicals used, eg an Alkaline Manganese AA 1.5 V, or a Lead-acid cell of about 2 V.

A battery is a collection of two or more cells, but the term is commonly used for a single cell. Examples include the PP3 9V (6 cells in series) or the Lead-acid car battery (typically 12 V, also 6 cells). Although the voltages are quoted usually as if they were accurate, mostly they are not. Voltages vary according to many factors including the temperature and state of charge.

Symbol for a cell

Symbol for a battery

Battery History

1748 - Benjamin Franklin first coined the term "battery" to describe an array of charged glass plates.

1780 to 1786 - Luigi Galvani demonstrated what we now understand to be the electrical basis of nerve impulses and provided the cornerstone of research for later inventors like Volta. He got frogs legs to kick when passing electric current through them.

1800 - Alessandro Volta invented the voltaic pile and discovered the first practical method of generating electricity. Constructed of alternating discs of zinc and copper with pieces of cardboard soaked in brine between the metals, the voltic pile produced electrical current. Alessandro Volta's voltaic pile was the first "wet cell battery" that produced a reliable, steady current of electricity.

1836 - John F. Daniel invented the Daniel Cell that used two electrolytes: copper sulphate and zinc sulphate. The Daniel Cell was somewhat safer and less corrosive then the Volta cell.

1839 - William Robert Grove developed the first fuel cell, which produced electricity by combining hydrogen and oxygen.

1859 - Gaston Plante developed the first practical storage lead-acid battery that could be recharged (secondary battery). This type of battery is mainly used in cars.



1866 - Georges Leclanche patented the carbon-zinc wet cell battery called the Leclanche cell. George Leclanche's original cell was assembled in a porous pot. The positive electrode consisted of crushed manganese dioxide with a little carbon mixed in. The negative pole was a zinc rod. The cathode was packed into the pot, and a carbon rod was inserted to act as a currency collector. The anode or zinc rod and the pot were then immersed in an ammonium chloride solution. The liquid acted as the electrolyte, readily seeping through the porous cup and making contact with the cathode material.

1881 - J.A. Thiebaut patented the first battery with both the negative electrode and porous pot placed in a zinc cup.

1881 - Carl Gassner invented the first commercially successful dry cell battery (zinc-carbon cell).

1899 - Waldmar Jungner invented the first nickel-cadmium rechargeable battery. 1901 - Thomas Alva Edison invented the alkaline storage battery. 1949 - Lew Urry invented the small alkaline battery. 1954 - Gerald Pearson, Calvin Fuller and Daryl Chapin invented the first solar battery.

These Baghdad batteries from 2000 years ago are thought to have been used to electroplate artifacts and also as a medicine to relieve pain. They consist of a vase made of clay. Inside this vase was a cylinder made of copper.



The above is a replica. The assembly can generate a weak electric current. Close to the vases pieces of thinner iron and copper rods were found which might have been used as conductive wires.



Cell types:

There are two fundamental types of cells, Primary and Secondary.

Primary cells are meant to be used once only, through to the discharged state.

Secondary cells are designed to be recharged over and over again, typically several hundreds of times.

Primary Cells:

A primary cell is any kind of electrochemical cell in which the electrochemical reaction is irreversible. So these batteries can only be used once and are disposed of when they run down.

Some battery uses require long dormancy periods so charge retention is important. In these circumstances, certain rechargeable battery technologies may not be appropriate as many have a high self-discharge rate compared to equivalent non-rechargeable batteries. For example, a flashlight used for emergency purposes must work when needed, even if it has sat on a shelf for an extended period of time.

The following are some examples of Primary Cells in use today.

Zinc Carbon: nominal 1.5 V, an old type, cheap and voltage varies considerably during use. These have mainly been replaced by Alkaline Manganese cells, commonly known as alkaline batteries.

Alkaline Manganese: nominal 1.5V, the main replacement for zinc-carbon, higher energy more expensive.

Mercury: phased out in many applications because mercury is very poisonous, was/is used in small applications, eg cameras, hearing aids.

Lithium: a wide ranging group with voltages varying from 1.5 V to 3.6 V, high energy density, voltage almost constant and very long shelf life. Lithium is a very reactive element which can lead to explosions (there were some laptops which were withdrawn due to the batteries being unstable with a possibility of explosions). They are also very expensive.

Zinc Air: nominal 1.4V, as its name suggests must be exposed to the air (a seal is usually removed when first used), high self-discharge rate.

Silver Oxide: nominal 1.55V. Often sold as button cells, for use in calculators, cameras, watches etc where its stable discharge characteristics are valuable.



Electrochemical Cells

A primary cell is an electrochemical cell.

A simple electrochemical cell can be made from copper and zinc metals with solutions of their sulphates. In the process of the reaction, electrons are transferred from the zinc to the copper through an electrically conducting path as an electric current.

An electrochemical cell can be created by placing metallic electrodes into an electrolyte where a chemical reaction generates an electric current. Electrochemical cells which generate an electric current are called voltaic cells or galvanic cells.

In an electrochemical cell hydrogen ions are converted into hydrogen atoms and bubbles of hydrogen gas appear at the positive plate. These bubbles interfere with the chemical action of the cell, causing the voltage and thus current to be reduced, and becoming severely polarized as the bubbles cover the entire plate. A depolarizer prevents this from happening.

Voltaic Cells

An electrochemical cell which causes external electric current flow can be created using any two different metals since metals differ in their tendency to lose electrons. Zinc more readily loses electrons than copper, so placing zinc and copper metal in solutions of their salts can cause electrons to flow through an external wire which leads from the zinc to the copper.

English chemist John Frederick Daniell developed a voltaic cell in 1836 which used zinc and copper and solutions of their ions.



As a zinc atom provides the electrons, it becomes a positive ion and goes into aqueous solution, decreasing the mass of the zinc electrode. On the copper side, the two electrons received allow it to convert a copper ion from solution into an uncharged copper atom which deposits on the copper electrode, increasing its mass. The two reactions are typically written

Zn(s) Zn2+(aq) + 2e-

Cu2+(aq) + 2e- Cu(s)

The zinc "half-reaction" is classified as oxidation since it loses electrons. The terminal at which oxidation occurs is called the "anode". For a battery, this is the negative terminal.

The copper "half-reaction" is classified as reduction since it gains electrons. The terminal at which reduction occurs is called the "cathode". For a battery, this is the positive terminal.

In order for the voltaic cell to continue to produce an external electric current, there must be a movement of the sulfate ions in solution from the right to the left to balance the electron flow in the external circuit. The metal ions themselves must be prevented from moving between the electrodes, so some kind of porous membrane or other mechanism must provide for the selective movement of the negative ions in the electrolyte from the right to the left.

Energy is required to force the electrons to move from the zinc to the copper electrode, and the amount of energy per unit charge available from the voltaic cell is called the electromotive force (emf) of the cell. Energy per unit charge is expressed in volts (1 volt = 1 joule/coulomb).

To get energy from the cell, you must get more energy released from the oxidation of the zinc than it takes to reduce the copper.

The cell can yield a finite amount of energy from this process, the process being limited by the amount of material available either in the electrolyte or in the metal electrodes. This is why cells and batteries run down. The supply of chemicals has been used up. Larger cells of the same type last longer as they have a greater supply of chemicals.

If there was one mole of copper sulphate on the copper side, then the process is limited to transferring two moles of electrons through the external circuit. Once this has occurred all the copper ions have been used up.

The amount of electric charge contained in a mole of electrons is called the Faraday constant, and is equal to Avogadro's number times the charge on an electron:

Faraday constant = F = 6.022 x 1023 x 1.602 x 10-19 = 96,485 Coulombs/mole

The energy yield from a voltaic cell is given by the cell voltage Ecell times the number of moles of electrons transferred, n, times the Faraday constant.

Electrical energy output = nFEcell

The cell emf Ecell may be predicted from the standard electrode potentials for the two metals. For the zinc/copper cell under the standard conditions, the calculated cell potential is 1.1 volts.



Leclanché Cell

Georges Leclanché invented and patented in 1866 his battery, the Leclanché cell. It contained a conducting solution (electrolyte) of ammonium chloride, a cathode (negative terminal) of carbon, a depolarizer of manganese dioxide, and an anode (positive terminal) of zinc.

The original form of the cell used a porous pot. The depolarizer, which consisted of crushed manganese dioxide, was packed into a porous pot, and a carbon rod was inserted to act as the cathode. The anode, which was a zinc rod, was then immersed along with the pot in a solution of ammonium chloride. The liquid solution acted as the electrolyte, permeating through the porous pot to make contact with the cathode.

Zinc atoms on the surface of the anode oxidize, ie they give up both their electrons to become positively-charged ions. As the zinc ions move away from the anode, leaving their electrons on its surface, the anode becomes more negatively charged than the cathode. When the cell is connected in an external electrical circuit, the excess electrons on the zinc anode flow through the circuit to the carbon rod, the movement of electrons forming an electrical current.



When the electrons enter the rod, they combine with molecules of manganese dioxide and molecules of water, which react with each other to produce manganese oxide and negatively charged hydroxide ions. This is accompanied by a secondary reaction in which the negative hydroxide ions react with positive ammonium ions in the ammonium chloride electrolyte to produce molecules of ammonia and water.

The Leclanché battery (or wet cell as it was referred to) was the forerunner of the modern dry cell zinc-carbon battery.

Zinc Carbon Cells

A Zinc-carbon dry cell or battery is packaged in a zinc can that serves as both a container and anode. It was developed from the wet Leclanché cell – doesn’t spill so more convenient to use in appliances. The cathode is a mixture of manganese dioxide and carbon powder. The electrolyte is a paste of zinc chloride and ammonium chloride dissolved in water. Carbon-zinc batteries are the least expensive primary batteries and thus a popular choice by manufacturers when devices are sold with batteries included. They can be used in remote controls, flashlights, toys, or transistor radios, where the power drain is not too heavy.

The container of the zinc-carbon dry cell is a zinc can. This contains a layer of NH4Cl paste separated by a paper layer from a mixture of powdered carbon & manganese (IV) oxide (MnO2) which is packed around a carbon rod.

In a dry cell, the outer zinc container is the anode (-). The zinc is oxidised according to the following half-equation.

Zn(s) → Zn2+(aq) + 2 e-

A graphite rod surrounded by a powder containing manganese(IV) oxide is the cathode(+). The manganese dioxide is mixed with carbon powder to increase the conductivity of the cathode mixture. The cathode reaction is as follows:

2MnO2(s) + 2 H+(aq) + 2 e- → Mn2O3(s) + H2O(l)


http://en.wikipedia.org/wiki/Image:Zincbattery.png


The H+ comes from the NH4+

(aq):

NH4+

(aq) + H2O(l) → H+(aq) + NH3(aq)

and the NH3 combines with the Zn2+.

In this half-reaction, the manganese is reduced from an oxidation state of (+4) to (+3).

There are other possible side-reactions, but the overall reaction in a zinc-carbon cell can be represented as:

Zn(s) + 2 MnO2(s) + 2 NH4+

(aq) → Mn2O3(s) + Zn(NH3)22+

(aq)

The battery has an e.m.f. of about 1.5 V.

When the dry cell has been used for a certain time, the zinc container becomes thinner because zinc metal is oxidised to zinc ions. Therefore Zinc Chloride Solution leaks out the battery. The old dry cell is not leak proof. It becomes very sticky as the paste leaks through the holes in the zinc case. The service life of the battery is short, with a shelf life of around 1.5 years. Also, the zinc casing in the dry cell gets thinner slowly, even when the cell is not being used. It is because the ammonium chloride inside the battery is acidic, reacting with the zinc.

Cell capacity

The number of different battery types, properties and applications is enormous, quite apart from the fact that each type comes in a variety of sizes, usually dictated by voltage and capacity. Capacity is measured in ampere-hours (Ah) or thousandths of ampere hours (mAh).

An ampere-hour (abbreviated as Ah or A-h) is a unit of electric charge. One ampere-hour is equal to 3600 coulombs, and is the amount of electric charge transferred by a steady current of one ampere for one hour.

The ampere-hour is a unit frequently used in measurements associated with electrochemical processes such as electroplating and electrical batteries. Although it is not a direct measure of the energy in a battery (like the joule (J) or watt-hour (Wh)), it is a common rating of how long a battery will last (or in the case of a rechargeable battery, how long it will last when fully charged).The commonly seen milliampere-hour (mAh) is equal to 3.6 coulombs



Secondary Cells:

A secondary cell is one which is rechargeable. These cells can be restored to full charge by the application of electrical energy. In other words, they are electrochemical cells in which the electrochemical reaction that releases energy is readily reversible.

Rechargeable batteries can offer an economic benefit when used instead of one-time-use disposable batteries. Most rechargeable battery technology has been adapted into the standard “AA,” “AAA,” “C,” “sub-C,” “D,” and “9-volt” configurations that consumers are familiar with. While the rechargeable versions of these types of cells have a higher up-front cost than disposable batteries, rechargeable batteries can be discharged and recharged many times. Some manufacturers of NiMH type rechargeable batteries claim a lifespan up to 3000 charge cycles for their batteries.

The following are some examples of Secondary Cells in use today.

Lead-acid: nominal 2 V per cell, as used in cars, well known to be rechargeable, large capacities but lead is very poisonous. Can be either wet or dry. Probably the easiest to recycle because the large amount of lead in each battery has a value in the scrap metal industry. Lead-acid and Sealed Lead-acid (SLA) are used where relatively large energy ratings are called for but weight is not a major problem.

Nickel-Cadmium (Ni-Cd): 1.2 V per cell. Used extensively in rechargeable situations and because it exhibits a memory effect is either continuously trickle-charged or recharged after complete discharge (ie not partially discharged and then charged). Ni-Cd can be recharged a large number of times (say, more than a thousand), which may be the reason why they are so popular. Cadmium is very poisonous.

Nickel Metal Hydride (NiMH): 1.2V per cell. They are a new generation which can replace Ni-Cd and have higher energy density and longer life cycle, and don't exhibit memory effect, are usually charged with constant-current source. They do not contain the the most dangerous heavy metals so are more environmentally friendly than Ni-Cd. The disadvantage is that they cannot be recharged so many times as Ni-Cd (maybe less than a thousand).

Nickel-zinc: 7 cells for 12V, is claimed to provide the lowest impact to the environment of any standard rechargeable battery technology mainly because of the absence of contamination from the dangerous heavy metals. They have lower cost than NiMH, are lighter and better performers than lead acid, have a high capacity per cycle and high cycle life and they also have low maintenance requirement. Size may be less than Lead-acid and about the same as Ni-Cd.

Lithium Ion (Li-Ion): used, as well as NiMH, in special applications eg laptop computers, are expensive to produce and cost a lot. They have the advantage that they have about twice the energy density of Ni-Cd and around 50% more than NiMH and therefore can be much lighter and smaller for the same capacity. They can be recharged for up to 1000 times approximately.

Lithium Polymer (LI-Polymer): apparently could become the battery of the future. They are reputed to have similar characteristics to Li-Ion but should be much cheaper to produce.



Usage and application

Unlike primary cells, secondary cells must be charged before use (attempting to recharge non-rechargeable batteries may cause an explosion).

Rechargeable batteries currently are used for lower power applications such as portable consumer devices, tools, and uninterruptible power supplies. Emerging applications in hybrid vehicles and electric vehicles are driving the technology to improve cost, reduce weight, and increase lifetime.

Charging

During charging, the positive active material is oxidized, producing electrons, and the negative material is reduced, consuming electrons. These electrons constitute the current flow in the external circuit. The electrolyte may serve as a simple buffer for ion flow between the electrodes, as in lithium-ion and nickel-cadmium cells, or it may be an active participant in the electrochemical reaction, as in lead-acid cells.

The energy used to charge rechargeable batteries mostly comes from mains electricity using an adapter unit. Most battery chargers can take several hours to charge a battery.

General Comments:

There have been significant advances in battery technology over the past few years. The battery market is a lucrative one. Some batteries contain metals which are very dangerous to humans and animals which is why proper disposal rather than disposing of in standard waste is important. This allows the expensive and dangerous metals to be recycled

Three heavy metals present in batteries, namely lead, cadmium and mercury, pose serious health hazards. The effects of these elements on animals (including humans) are terrible, leading to debilitating pain with fatal consequences. There are some minor recycling schemes, including experimental council schemes.

Lead-acid car batteries are relatively well catered for because they have an obvious scrap value (a substantial amount of lead) and you could possibly collect money by taking them to the scrap metal dealer. Local councils will usually take them for recycling.

Ni-Cd rechargeable batteries contain Cadmium and are increasingly popular. In any piece of equipment they should be easily removable. As long as they're being used they are not dangerous but if they are dumped into landfill they are. Some firms will take them back in part exchange.

Mercury is used in miniature primary cells (hearing aids, some cameras). Certain hospitals and jewellers may take these for safe disposal.



Fuel Cells

A fuel cell is an electrochemical energy conversion device. It produces electricity from external supplies of fuel (on the anode side) and oxidant (on the cathode side). These react in the presence of an electrolyte. Generally, the reactants flow in and reaction products flow out while the electrolyte remains in the cell. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.

Fuel cells are different from batteries in that they consume reactant which must be replenished, while batteries store electrical energy chemically in a closed system. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic.

Many combinations of fuel and oxidant are possible. A hydrogen cell uses hydrogen as fuel and oxygen as oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include air, chlorine and chlorine dioxide.

Fuel cell design

A fuel cell works by catalysis, separating the electrons and protons of the reactant fuel, and forcing the electrons to travel through a circuit, converting them to electrical power. Another catalytic process takes the electrons back in, combining them with the protons and the oxidant to form waste products (typically simple compounds like water and carbon dioxide).

Zinc-air battery

Zinc-air batteries, also called “zinc-air fuel cells,“ are non-rechargeable electro-chemical batteries powered by the oxidation of zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce. They are used in hearing aids and in experimental electric vehicles.

Particles of zinc are mixed with an electrolyte (usually potassium hydroxide solution); water and oxygen from the air react at the cathode and form hydroxidel ions which migrate into the zinc paste and form zincate (Zn(OH)4

2-), at which point electrons are released and travel to the cathode. The zincate decays into zinc oxide and water is released back into the system. The water and hydroxide ions from the anode are recycled at the cathode, so the water serves only as a catalyst. The reactions produce a maximum voltage level of 1.65 volts, but this is reduced to 1.4–1.35 V by reducing air flow into the cell; this is usually done for hearing aid batteries to reduce the rate of water drying out.

The term zinc-air fuel cell usually refers to a zinc-air battery in which zinc fuel is replenished and zinc oxide waste is removed continuously. This is accomplished by pushing zinc electrolyte paste or pellets into an anode chamber. Waste zinc oxide is pumped into a waste tank or bladder inside the fuel tank, and fresh zinc paste or pellets are taken from the fuel tank. The zinc oxide waste is pumped out at a refuelling station and sent to a recycling plant.

Zinc-air batteries have properties of fuel cells as well as batteries: the zinc is the fuel, the rate of the reaction can be controlled by controlling the air flow, and used zinc/electrolyte paste can be



removed from the cell and replaced with fresh paste. Research is being conducted in powering electric vehicles with zinc-air batteries.

Proton exchange membrane fuel cell

Proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells (PEMFC), are a type of fuel cell being developed for transport applications as well as for stationary and portable applications. Their distinguishing features include lower temperature/pressure ranges and a special polymer electrolyte membrane.

A proton exchange membrane fuel cell transforms the chemical energy liberated during the electrochemical reaction of hydrogen and oxygen to electrical energy, as opposed to the direct combustion of hydrogen and oxygen gases to produce thermal energy.

A stream of hydrogen is delivered to the anode side of the membrane-electrode assembly (MEA).

At the anode side it is catalytically split into protons and electrons. This oxidation half-cell reaction is represented by:

Eo = 0 V

The newly formed protons permeate through the polymer electrolyte membrane to the cathode side. The electrons travel along an external load circuit to the cathode side of the MEA, thus creating the current output of the fuel cell.

Meanwhile, a stream of oxygen is delivered to the cathode side of the MEA. At the cathode side oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit to form water molecules. This reduction half-cell reaction is represented by:

Eo = 1.229V

To function, the membrane must conduct hydrogen ions (protons) but not electrons as this would in effect "short circuit" the fuel cell. The membrane must also not allow either gas to pass to the other


http://en.wikipedia.org/wiki/Redox

http://en.wikipedia.org/wiki/Current_(electricity)

http://en.wikipedia.org/wiki/Cathode

http://en.wikipedia.org/wiki/Electrical_network

http://en.wikipedia.org/wiki/Reaction

http://en.wikipedia.org/wiki/Oxidation

http://en.wikipedia.org/wiki/Electron

http://en.wikipedia.org/wiki/Proton

http://en.wikipedia.org/wiki/Catalysis

http://en.wikipedia.org/wiki/Electrode

http://en.wikipedia.org/wiki/Anode

http://en.wikipedia.org/wiki/Thermal_energy

http://en.wikipedia.org/wiki/Electrochemical

http://upload.wikimedia.org/wikipedia/en/5/5c/Fc_diagram_pem.gif


side of the cell, a problem known as gas crossover. Finally, the membrane must be resistant to the reducing environment at the cathode as well as the harsh oxidative environment at the anode.

Unfortunately, while the splitting of the hydrogen molecule is relatively easy by using a platinum catalyst, splitting the stronger oxygen molecule is more difficult, and this causes significant electric losses. An appropriate catalyst material for this process has not been discovered, and platinum is the best option. Another significant source of losses is the resistance of the membrane to proton flow, which is minimized by making it as thin as possible, on the order of 50 μm.

Despite their success in space programs, fuel cell systems were limited to space missions and other special applications, where high cost could be tolerated. It was not until the late 1980s and early 1990s that fuel cells became a real option for wider application base. Several pivotal innovations, e.g. low platinum catalyst loading and thin film electrodes drove the cost of fuel cells down, making development of PEMFC systems more or less realistic. However, there is significant debate as to whether hydrogen fuel cells will be a realistic technology for use in automobiles or other vehicles for many decades.

Direct methanol fuel cell

Direct-methanol fuel cells or DMFCs are a subcategory of proton-exchange fuel cells where, the fuel, methanol (CH3OH), is not reformed, but fed directly to the fuel cell. Because methanol is fed directly into the fuel cell, complicated catalytic reforming is unneeded. Storage of methanol is much easier than that of hydrogen because it does not need to be done at high pressures or low temperatures, as methanol is a liquid from -97.0 °C to 64.7 °C (-142.6 °F to 148.5 °F). The energy density of methanol, the amount of energy contained in a given volume of methanol, is an order of magnitude greater than even highly compressed hydrogen.

However, the efficiency of direct-methanol fuel cells is low due to the high permeation of methanol through the membrane, which is known as methanol crossover. Other problems include the management of carbon dioxide created at the anode. Current DMFCs are limited in the power they can produce, but can still store a high energy content in a small space. This means they can produce a small amount of power over a long period of time. This makes them ill-suited for powering vehicles, but ideal for consumer goods such as mobile phones, digital cameras or laptops.

The DMFC relies upon the oxidation of methanol on a catalyst layer to form carbon dioxide. Water is consumed at the anode and is produced at the cathode. Positive ions (H+) are transported across the proton exchange membrane (often Nafion) to the cathode where they react with oxygen to produce water. Electrons are transported via an external circuit from anode to cathode providing power to external devices.

The half-reactions are:

Anode: CH3OH + H2O → CO2 + 6H+ + 6e-

The methanol is adsorbed on a catalyst, usually Pt, and deprotonized, until a CO species is left. Another catalyst, usually Ru is used to oxidize water, producing an OH species which reacts with the CO to form carbon dioxide.

Cathode: 1.5 O2 + 6H+ + 6e- → 3H2O



Net reaction: CH3OH + 1.5 O2 → CO2 + 2H2O

Because water is consumed at the anode in the reaction, pure methanol cannot be used without provision of water via either passive transport such as back diffusion (osmosis), or active transport such as pumping. The need for water limits the energy density of the fuel.

Currently, platinum is used as a catalyst for both half-reactions. This contibutes to the loss of cell voltage potential, as any methanol that is present in the cathode chamber will oxidize. If another catalyst could be found for the reduction of oxygen, the problem of methanol crossover would likely be significantly lessened. Furthermore, platinum is very expensive and contributes to the high cost per kilowatt of the fuel cell.

In one of the steps of the methanol oxidation reaction, CO is produced, which adsorbs strongly on the platinum catalyst reducing the surface area for the catalyst reaction. The addition of other catalysts such as Ruthenium or Gold tends to reduce this problem because these catalysts oxidize water. The oxygen atom from the oxidized water molecule combines with the CO to produce CO2 which can then be released as a gas.



Solar Cells

A solar cell or photovoltaic cell is a device that converts light energy into electrical energy.

Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified.

The device needs to fulfil two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material separation of the charge carriers to a conductive contact that will transmit the electricity

(simply put, carrying electrons off through a metal contact into a wire or other circuit).

This conversion is called the photovoltaic effect.

Solar cells have many applications. They have long been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth-orbiting satellites and space probes, consumer systems, e.g. handheld calculators, remote radiotelephones and water pumping applications. More recently, solar cells are starting to be used in assemblies of solar modules (photovoltaic arrays) connected to the electricity grid through an inverter, often in combination with a net metering arrangement.

History of solar cells

The first generation photovoltaic, consists of a large-area, single layer p-n junction diode, which is capable of generating usable electrical energy from light sources with the wavelengths of sunlight. These cells are typically made using a silicon wafer. First generation photovoltaic cells (also known as silicon wafer-based solar cells) are the dominant technology in the commercial production of solar cells, accounting for more than 86% of the solar cell market.

The second generation of photovoltaic materials is based on the use of thin-film deposits of semiconductors. These devices were initially designed to be high-efficiency, multiple junction photovoltaic cells. Later, the advantage of using a thin-film of material was noted, reducing the mass of material required for cell design. This contributed to a prediction of greatly reduced costs for thin film solar cells. Typically, the efficiencies of thin-film solar cells are lower compared with silicon (wafer-based) solar cells, but manufacturing costs are also lower, so that a lower cost per watt can be achieved. Another advantage of the reduced mass is that less support is needed when placing panels on rooftops and it allows fitting panels on light or flexible materials, even textiles.

Third generation photovoltaics are very different from the previous semiconductor devices as they do not rely on a traditional p-n junction to separate photogenerated charge carriers. These new devices include photoelectrochemical cells, polymer solar cells, and nanocrystal solar cells. Dye-sensitized solar cells are now in production.

Fourth generation Composite photovoltaic technology with the use of polymers with nano particles can be mixed together to make a single multispectrum layer. Then the thin multi spectrum layers can be stacked to make multispectrum solar cells more efficient and cheaper based on polymer solar cell and multi junction technology used by NASA on Mars missions. The layer that converts different types of light is first, then another layer for the light that passes and last is an infra-red spectrum layer for the cell - thus converting some of the heat for an overall solar cell composite.



What happens:

1. Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon.

2. Electrons are knocked loose from their atoms, allowing them to flow through the material to produce electricity. The complementary positive charges that are also created are called holes and flow in the direction opposite of the electrons in a silicon solar panel.

3. An array of solar panels converts solar energy into a usable amount of direct current (DC) electricity.

4. The DC current enters an inverter. The inverter turns DC electricity into 120 or 240-volt AC (alternating current) electricity needed for home appliances. The AC power enters the utility panel in the house.

5. The electricity is then distributed to appliances or lights in the house. The electricity that is not used will be re-routed and used in other facilities.

Comparison of energy conversion efficiencies

Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 42.8% with multiple-junction research lab cells. Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14-16%. The highest efficiency cells have not always been the most economical.

To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected PV systems); in stand alone systems, batteries are used to store the energy that is not needed immediately.

A common method used to express economic costs of electricity-generating systems is to calculate a price per delivered kilowatt-hour (kWh). Using the commercially available solar cells (as of 2006) and system technology leads to system efficiencies between 5 and 19%. As of 2005, photovoltaic electricity generation costs ranged from ~0.60 US$/kWh down to ~0.30 US$/kWh in regions of high solar irradiation. This electricity is generally fed into the electrical grid on the customer's side of the meter. The cost can be compared to prevailing retail electric pricing (as of 2005), which varied from between 0.04 and 0.50 US$/kWh worldwide. (Note: in addition to solar irradiance profiles, these costs/kwh calculations will vary depending on assumptions for years of useful life of a system.

Solar cells and energy payback

In the 1990s, when silicon cells were twice as thick, efficiencies 30% lower than today and lifetimes shorter, it may well have cost more energy to make a cell than it could generate in a lifetime. The energy payback time of a modern photovoltaic module is anywhere from 1 to 20 years (usually under five) depending on the type and where it is used (see net energy gain). This means solar cells can be net energy producers, meaning they generate more energy over their lifetime than the energy expended in producing them.




Documents

UNIT 14 - Energy Changes and Applications 1