UNIT 3 Review from yesterday - enthalpy Chapter 5: Energy Changes - enthalpy, H, is a way to express the change in thermal energy and it accounts for changes

UNIT 3

Review from yesterday - enthalpy

Chapter 5: Energy Changes

- enthalpy, H, is a way to express the change in thermal energy and it accounts for changes in P and V

ΔH = ΔE + Δ(PV)

For reactions of solids and liquids in solutions, Δ(PV) = 0

If heat enters a system• ΔH is positive• the process is endothermic

If heat leaves a system• ΔH is negative• the process is exothermic

Therefore: ΔH = ΔE and since ΔE=Q

Q and ΔH are often used interchangeable for solids & liquids

UNIT 3 Section 5.1

The Second Law of Thermodynamics


When in thermal contact, energy from hot particles will transfer to cold particles until the energy is equally distributed and thermal equilibrium is reached.

The second law of thermodynamics states that:

when two objects are in thermal contact, heat is transferred from the object at a higher temperature to the object at the lower temperature until both objects are the same temperature (in thermal equilibrium)

UNIT 3 Section 5.1

Types of enthalpy changes


The ΔH for one phase change is the negative of the ΔH for the opposite phase change.

1. Enthalpy changes associated with physical changes

a)ΔH solution – the total change in energy associated with breaking and forming intermolecular bonds of solutes and solvents (e.g. NaCl dissolving in water)

b) ΔH with phase changes-Heat must be added to or removed from a substance in order for the phase of the substance to change.

- ΔH for each phase change has a particular symbol (e.g. ΔHmelt - enthalpy

of melting)

b) Enthalpy changes associated with phase changes

ΔHmelting and ΔHvaporization are always positive (endothermic processes – the substance is absorbing energy from the surroundings

Therefore:

ΔHfreezing and ΔHcondensation are always negative (exothermic processes – energy is released by the substance)

Heating and cooling curves

Heating curves – show how T changes of a substance as heat is added

e.g. heating curve for 100.0 g of ice at -10.0oC warmed to 120.0oC

A: Ice warmed from -10 °C to 0°C: ↑T; ↑Ek; ↑q

B: The ice is melting: no ΔT; ↑Ep; ↑q

C: Water is warming from 0 °C to 100 °C: ↑T; ↑Ek; ↑q

D: Water is boiling (vaporizing): no ΔT; ↑Ep; ↑q

E: Water vapour is warming: ↑T; ↑Ek; ↑q

Types of enthalpy changes

Those associated with:

1. Physical changes

2. Chemical reactions (enthalpies of reaction)E.g. The combustion of methane is

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

For every mole of methane combusted ΔHr = –890 kJ

3. Nuclear changes (more on this soon)

e.g. Alpha and Beta Decay: Atoms of some elements emit alpha (α) or beta (β) particles, transforming them into other elements with smaller masses.

UNIT 3

Comparing Enthalpy Changes

• ΔH for physical changes, such as phase changes and dissolving of a solute in solvent, are the smallest

• ΔHr are much larger than ΔH for physical changes, but smaller than ΔH for nuclear changes.

Nuclear > Chemical > Physical (phase)

UNIT 3 Section 5.2

2. Enthalpy changes associated with chemical reactions

Chemical reactions involve

• initial breaking of chemical bonds (endothermic)

• then formation of new bonds (exothermic)

ΔHr is the difference between the total energy required to break bonds and the total energy released when bonds form.

UNIT 3

Thermochemical Equations

Thermochemical equations include the enthalpy change.

The enthalpy term can also be written beside the equation.

Exothermic reaction:CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + 890.8 kJ

Endothermic reaction:N2(g) + 2O2(g) + 66.4 kJ → 2NO2(g)

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔHr = –890.8 kJ

N2(g) + 2O2(g) → 2NO2(g) ΔHr = +66.4 kJ

UNIT 3

Enthalpy Diagrams

Enthalpy diagrams clearly show the relative enthalpies of reactants and products.

• For exothermic reactions, reactants have a larger enthalpy than products and are drawn above the products and ΔH is negative.

• For endothermic reactions, the products have a larger enthalpy and are drawn above the reactants ΔH is positive.

UNIT 3

Molar Enthalpy of Combustion

Combustion reactions have their own symbol: ΔHcomb

For standard molar enthalpy of combustion data in reference tables (back of your textbook):• reactants and products are at standard conditions

• refer only to the compound undergoing combustion

• are for combustion of 1 mol of the compound being combusted, not for equations “as written.” For some compounds to be written with a coefficient of 1, equations are written with fractions as coefficients

2C4H10 + 13O2 → 8CO2 + 10H2O vs.

C4H10 + 13/2O2 → 4CO2 + 5H2O ΔHcomb = -2260 kJ/mol

UNIT 3 Section 5.2

Reactant Amounts and Enthalpy of Reaction


• The enthalpy of a reaction is directly proportional to the amount of substance that reacts.

• The combustion of 2 mol of propane releases twice as much energy as the combustion of 1 mol of propane.

• Knowing the thermochemical equation, the enthalpy change associated with a certain amount of reactants or products can be calculated.

Determine ΔHcomb for 15.0 g of propane.

UNIT 3 Section 5.2

TRY THIS

The molar mass of propane: 44.1 g/molTherefore, 15.0 g is 0.340 mol

ΔHcomb = ΔH°comb

ΔHcomb = (0.340 mol) (–2219.2 kJ/mol)ΔHcomb = 755 kJ/mol

UNIT 3

UNIT 3

Using Calorimetry To Study Energy Changes

For an endothermic process, heat will be transferred from the water to the process system. The temperature of the water will decrease.

Calorimeters are used to measure heat released or absorbed by a chemical or physical process.

The water of a calorimeter is considered one system and the container in which the process occurs another system. These two systems are in thermal contact.

For an exothermic process, heat will be transferred from the process to the water. The temperature of the water will increase.

UNIT 3 Section 5.2

Using a Simple Calorimeter

TO PREVIOUS SLIDE


A simple calorimeter.

• nested polystyrene cups can be used (good insulators)

• a known mass of water is in the inner cup, where the process occurs

• the process often involves compounds dissolved in water

• the change in temperature of the water is measured; the solution absorbs or releases energy

• the solution is dilute enough so that the specific heat capacity of water is used

UNIT 3 Section 5.2

Using a Simple Calorimeter

TO PREVIOUS SLIDE


The change in thermal energy caused by the process can be calculated:

Q = mcΔT

m is the mass of the water, c is the specific heat capacity of water, and ΔT is the change in temperature of the water

If the “system” is the process being studied and the “surroundings” is the water in the calorimeter:

UNIT 3 Section 5.2

How to Use a Simple Calorimeter

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UNIT 3 Section 5.2

Using Calorimetry Data To Determine the Enthalpy of Reaction

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Q = mwcwΔTw is the thermal energy absorbed or released by the water.

The opposite sign of that value is the thermal energy released or absorbed by the system (compounds in solution).

Since pressure is constant, Q exchanged by the system and the water is the same as the enthalpy change of the system ΔH. This is used to determine molar enthalpy change, ΔHr.

UNIT 3 Section 5.2

Using Flame Calorimetry To Determine the Enthalpy of Combustion

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Flame calorimeters are flame-resistant and often made of metals cansA flame calorimeter

• is used for determining ΔHcomb

• absorbs a great deal of energy, which must be included in energy calculations

• is used for burning impure materials like food; ΔHcomb is reported in kJ/g

UNIT 3 Section 5.2

Using Bomb Calorimetry To Measure Enthalpy Changes during Combustion

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Bomb calorimeters are much more sophisticated than flame calorimeters or simple calorimeters.

A bomb calorimeter

• is used for more accurately determining ΔHcomb

• determines ΔHcomb at constant volume

• has a particular heat capacity, C

Q = CΔT is used for bomb calorimetry calculations

A chemical reaction is carried out in a dilute aqueous solution using a simple calorimeter. Calculate the enthalpy change of the reaction using the data below.

UNIT 3 Section 5.2

Answer on the next slide

TO PREVIOUS SLIDE


mass of solution in calorimeter: 2.00 х 102 gspecific heat capacity of water: 4.19 J/g°CInitial temperature: 15.0°CFinal temperature: 19.0°C

LEARNING CHECK

Q = msolutioncsolution∆Tsolution

Q = (2.00 х 102 g)(4.19 J/g°C)(4.0°C)

Q = 3.35 х 103 JThis is the thermal change of the “surroundings.”

Section 5.2UNIT 3

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∆Hsystem = –Q∆Hsystem = –3.35 kJ

LEARNING CHECK

Section 5.2 Review

UNIT 3 Section 5.2

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UNIT 3 Section 5.3

5.3 Hess’s Law

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The enthalpy change of nearly any reaction can be determined using collected data and Hess’s law.

The enthalpy change of any reaction can be determined if:

• the enthalpy changes of a set of reactions “add up to” the overall reaction of interest

• standard enthalpy change, ΔH°, values are used

UNIT 3 Section 5.3

Combining Sets of Chemical Equations

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To find the enthalpy change for formation of SO3 from O2 and S8, you can use

UNIT 3 Section 5.3

Techniques for Manipulating Equations

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1.Reverse an equation

• the products become the reactants, and reactants become the products

• the sign of the ΔH value must be changed

2.Multiply each coefficient

• all coefficients in an equation are multiplied by the same integer or fraction

• the value of ΔH must also be multiplied by the same number

UNIT 3 Section 5.3

Standard Molar Enthalpies of Formation

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Data that are especially useful for calculating standard enthalpy changes:

standard molar enthalpy of formation, ΔH˚fthe change in enthalpy when 1 mol of a compound is synthesized from its elements in their most stable form at SATP conditions

• enthalpies of formation for elements in their most stable state under SATP conditions are set at zero

• since formation equations are for 1 mol of compound, many equations include fractions

• standard molar enthalpies of formation are in Appendix B

UNIT 3 Section 5.3

Formation Reactions and Thermal Stability

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The thermal stability of a substance is the ability of the substance to resist decomposition when heated.

• decomposition is the reverse of formation• the opposite sign of an enthalpy change of formation

for a compound is the enthalpy change for its decomposition

• the greater the enthalpy change for the decomposition of a substance, the greater the thermal stability of the substance

UNIT 3 Section 5.3

Using Enthalpies of Formation and Hess’s Law

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For example:CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

Determine ∆H˚r for the following reaction using the enthalpies of formation that are provided.

UNIT 3 Section 5.3

Answer on the next slide

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C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)

∆H˚f of C2H5OH(l): –277.6 kJ/mol∆H˚f of CO2(g): –393.5 kJ/mol∆H˚f of H2O(l): –285.8 kJ/mol

LEARNING CHECK

∆H˚r = [(2 mol)(∆H˚f CO2(g)) + (3 mol)(∆H˚f H2O(l))] – [(1 mol)(∆H˚f C2H5OH(l)) + (3 mol)(∆H˚fO2(g)]

Section 5.3UNIT 3

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∆H˚r = [(2 mol)(–393.5 kJ/mol) + (3 mol)(–285.8 kJ/mol)] – [(1 mol)(–277.6 kJ/mol) + (3 mol)(0 kJ/mol)]

∆H˚r = (–1644.4 kJ) – (–277.6 kJ) ∆H˚r = –1366.8 kJ

LEARNING CHECK

Section 5.3 Review

UNIT 3 Section 5.3

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UNIT 3 Section 5.4

5.4 Energy Efficiency and Energy Resources

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Energy efficiency can be calculated using the equation:

UNIT 3 Section 5.4

Using Energy Efficiently

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Energy use distribution in Canadian homes

A challenge in the development of energy efficient technology is to find ways to best convert energy input into useful forms.

For example, efficiency of appliances:• conversion of input of electrical energy versus output of

energy usually all that is considered• but should also consider efficiency

of the source of the electricity

UNIT 3 Section 5.4

Conventional Energy Sources in Ontario

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The distribution of energy sources in Ontario

The three main sources of electrical energy in Ontario:

• nuclear power plants

• power plants that burn fossil fuels (natural gas and coal)

• hydroelectric generating stations

UNIT 3 Section 5.4

Alternative Renewable Energy Sources in Ontario

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Renewable energy sources in Ontario:

• account for about 25% of energy production

• are projected to increase to as high as 40% by 2025

• include hydroelectric power (major source), wind energy (currently ~ 1% and projected to 15% in 2025), and solar energy (currently low but may be as high as 5% in 2025). Much lower contributors are biomass, wave power, and geothermal energy.

UNIT 3 Section 5.4

What Is a “Clean” Fuel?

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Different fuels have differing impacts on the environment. One way this impact is measured is through emissions.

For example, CO2(g) emissions per kJ of energy produced.

Fuel kg CO2/ kJ energy

Anthracite coal 108.83

Oil 78.48

Natural gas 56.03

Nuclear 0.00

Renewables 0.00

Section 5.4 Review

UNIT 3 Section 5.4

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Documents

UNIT 3 Review from yesterday - enthalpy Chapter 5: Energy Changes - enthalpy, H, is a way to express the change in thermal energy and it accounts for changes