l. chapter 12 energy packet 2008

1

Energy

Many believe that man’s insatiable energy appetite is

destroying the planet. In this unit we will therefore look not

only at energy from a chemical perspective, but also from an

environmental one. We’d like to know: Which energy sources are practical, abundant, and

environmentally clean?

Which energy sources for vehicles are likely to replace gasoline?

Which is cheaper, gasoline or electricity?

How can energy transfer be measured?

How much energy is in a potato chip?

How much energy is involved during a phase change?

What units are used in energy and how do they relate?

What are enthalpy, free energy, and entropy, and why are they

important?

Tentative Schedule:

Day 1: Specific Heat

Lab: Specific Heat Capacity of a Metal

Homework: energyws2: Specific Heat

Day 2: Introduction to Energy

Lesson: Why Energy Matters; Definitions;

Units

Homework: energyws1: Energy Sources and

Conversions

Day 3: How much energy is in a potato chip:

Enthalpy

Lab: Potato Chip Calorimetry

Homework: Complete chip lab

Day 5: Enthalpy

Lesson: Enthalpy of vaporization and fusion

Homework: energyws3: Energy with phase

change

Day 6: Free Energy

Lesson: Free Energy; Energy in Connecticut

Homework: energyws4 and ws5: Phase

change II, Free Energy

Day 7: Review

Lesson: How to Ace the Energy Test

Homework: Review for Energy Test

Day 8: Energy Test

1. Intro

2. data

3. matter

4. the atom

5. electrons

6. periodic table

7. bonding

8. reactions

9. the mole

10. gases

11. solutions

12. Energy

13. Reaction rates

14. equilibrium

15. Acids and bases

What changes should we make?

2

Students: Please read this article and then answer the 2 questions at the end.

Electric Cars Offer Energy Independence

By Jeff Swicord

Washington

09 October 2008

With fuel prices still high, the electric car is becoming a more attractive form of

transportation. Electric cars were first introduced in the 1970s, but the technology has

dramatically improved in the last 10 years. By 2010 automakers like Mercedes and General

Motors plan to bring their models to showrooms. Jeff Swicord introduces us to one man who

uses electric cars built several years ago as his primary mode of transportation.

Like many people in the Washington D.C. area, Brian Murtha commutes five days a week, to

downtown and back. But, he does it in an electric car.

Brian owns two factory-made electric vehicles: a Toyota Rav 4 EV and a Ford Ranger pick-up truck

EV. These were produced in small numbers a few years ago. He charges them from electricity

produced by solar panels on the roof of his house.

"After I retired from the Air Force, I set a goal not to use energy from anyone else off my

property," Brian explained. "To make all my energy myself and be energy independent."

Major automakers are betting there will be more and more consumers like Brian in the future.

Toyota, General Motors, and Mercedes plan to have an electric vehicle in showrooms within two

years. The American made Chevy Volt prototype has received

widespread attention at auto shows.

For now, fans of electric vehicles like Brian buy their electric cars on

eBay. He paid $40,000 for the Toyota.

"It gives you 900 pounds on the lowest point of the car, which makes

the center of gravity better than the gasoline version," Brian said.

"Which makes roll-over less likely and gives you better handling."

The inside, with a few exceptions, looks like a gasoline-powered car.

"The electric motor is actually air cooled and produces so little heat it

is not really of use in the winter time for heating the passenger cabin. So, Toyota put a heat pump

in there," Brian explained.

He charges his cars in the garage by plugging into receptors. The electricity runs along wires that

are connected to the solar panels on his roof.

"It charges about 25 percent an hour," he said. "So if you ran it down a quarter on the fuel gauge,

Murtha's sun powered car

3

if you were three quarters and you wanted to fill it all the way up, it would take you an hour."

Brian says driving to work with a gasoline powered vehicle would cost him about $8 a day in fuel.

"With the electric car it's about 20 to 25 kilowatt hours to go in and back," he said. "And say about

.10 a kilowatt hour, that's about $3.00. And there really is not any maintenance."

His quest for energy independence includes his house. He has replaced appliances that run on

natural gas with electric ones, including the furnace. But his solar array is not big enough to power

the entire house and two cars.

"Well If I didn't have the electric vehicles to refuel, the 2200 watt array on the roof of my house,

right now, almost completely powers the house," Brian said.

Brian plans to add another 7,000 watt array on top of his garage. It will give him more than 9,000,

and that will be more power than he needs.

Questions:

1. One passage in the text states

“Brian says driving to work with a gasoline powered vehicle would cost him about $8 a day in

fuel. “

With gas currently at 2 dollars per gallon ($2/gallon), and assuming a gasoline powered

vehicle gets 30 miles per gallon (30 miles/gallon), how long is Brian’s round trip commute?

2. Another passage states

“With the electric car it's about 20 to 25 kilowatt hours (kwh) to go in and back," he said.

"And say about .10 a kilowatt hour, that's about $3.00.”

Based on Brian’s commute from #1, what is the fuel economy of his electric car in kwh/mile?

3. Soon people will have to try to compare gas mileage to electric car mileage. Perhaps the best way

would be to calculate the dollars per mile it costs to drive each car. Using the information above,

determine what is cheaper to drive by calculating the

A. dollars/mile cost of driving a 30 mile per gallon gasoline car at $2/gallon, and

B. the dollar/mile cost to drive a 25 kwh/mile electric car car at $.10/kwh

The cheaper car to drive is______________

4

Energy

What Changes Should We Make?Energy:

sour

ces

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type

s

units diets Specific heat enthalpy

Wate

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nergy

Hess’s L

aw

Welcome to planet earth. Please choose your primary energy source

Nuclear fissionNuclear fusion

wind solar geothermalwave

Natural gasoilcoal biomass wood

hydro

235U + 1n 236U 140Xe + 94Sr + 21n3H + 1H 4He

Cost(to thePlanet):

Chemical reaction:

#1 #2 #3 #4 #5Deluxe Meals:

All based on C,H CO2 + H2O

$$high

Specials:

Chemical reaction:

Medium?

All items$1

ValueMenu

Place your order:

Chemical reaction:

none

Energy


sour

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Hess’s L

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Welcome to planet earth

Please choose your eco-

friendly carTesla roadster Toyota Prius Chevy Volt Honda Civic GX

BMW hydrogen 7 Honda FCX ClarityVenturi astrolab

or eclecticBYD F3DM

Overall (1-10)

30k30k30K$100Kcost

now2010now100 nowavailability

CH4Electric or gasgaselectricfuel

Appearance (1-10)

Overall (1-10)

?30k40kalotcost

?Now (china)Limited nowunknownavailability

Electric or solarElectric or gasH2H2fuel

Appearance (1-10)

My eco-friendly car choice:________

Please rate from 1-10:

E-85 fuel biodiesel

Also:

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Energy


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Why the energy unit matters:

smog pollution

global warming deforestation

urban sprawlOzonedepletion

overpopulation

Primary sources of energy are usually chemicalcoal nuclear propane Natural gas gasoline

Energy


sour

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Eco

-car

sis

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type

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Water, ice

, and ste

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Hess’s Law

Energy and Weight Loss

One pound of body fat contains _____ Nutritional Calories

An average diet is about _____ Nutritional Caloriesper day

People burn roughly _____ Nutritional Calories per day without exercise

One hour of intense cardio exercise burns ______ Nutritional Calories

And I can expect to lose ____ pound(s) per week.

2,000

1,500

If I eat normal, and run hard one hour per day I should lose _____Nutritional Calories per day

3,500

1,000

500

1

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Energy


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Q = mcT

heat Mass (g)

c = Specific heat constant = 4.184 j/g oC for water

Temp. change oC

Specific heat: The amount of heat necessary to heat one g of water by one OC.

How many joules of heat are needed to heat one cup (237 mL) of water from room temp. (25 oC) to boiling (100 oC)?

Q = mcT = (237)(4.184)(75)

= 74,371 joules

7

Energy


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Enthalpy

• = heat change at constant pressure

• =H or Hrxn

H vs. q?

H = q when pressure is constant

• Exothermic Reactions

H is negative

• Endothermic Reactions

H is positive

H

Q

+-

Energy


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5. Standard Enthalpies of Vaporization and Fusion. For water:

• H2O(l)H2O(g)

Hovap = 2260J/g

• H2O(s)H2O(l)

Hofus = 334 J/g

Hvap

Hfus

Hcond

Hsolid

2260 J/g

334 J/g

-2260 J/g

-2260 J/g

gas

liquid

solid

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Energy


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G =H-TS

Gibb’s Free Energy (j)

Enthalpy (j)

Entropy (j/K)

What is Entropy??

If G < 0 we have a spontaneous process

FreeEnergy

Josiah GibbsNew Haven, CT

9

Energy


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Hess’s L

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S = positive = more random

• Liquid gas

• + more random

• Liquid solid

• - less random

• 1 particle 2 particles

• + more random

The big bang

Energy


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For a reaction:H=145,000 JS = 322 j/K

T= 382KIs it spontaneous? (Apply G =H-TS)

G =H-TS

• =145,000 – 382(322)

• = 22,000

• Positive: nonspontaneous.

10

Energy


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spontaneity

Usually

Depends on T--

Rarely. Depends on T++

never-+

always+-

Spontaneous?SH

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lab16.1

Name__________________________________Period________________Date______

Specific Heat Capacity of a Metal

10 Points

Please read and complete this pre-lab prior to performing this experiment.

Purpose: To use a calorimeter to find the specific heat capacity of a metal.

Theory: Have you observed how some metals stay hot longer than others? Specific heat

measures this property: it is the amount of heat required to raise the temperature of one gram of

a substance by one degree Celsius. For example, as a 1 g slug of hot lead (Pb) cools in water, it

releases 0.129 joules (j) of heat to the water for each degree that it cools. We can therefore

identify metals by measuring the specific heat of an unknown metal, and comparing it to known

values.

We will do this by heating a metal to 100 OC, then placing it in room temperature water to see how

much hotter the water gets. Metals with a high specific heat will heat up the water a lot. We will

then calculate the specific heat (called “c”) of the metal to identify it.

Questions:

1. Give a definition of specific heat in your own words- don’t use the definition above.

2. Why might it be important to use a large mass of metal for these measurements?

Procedure: We need 5 pieces of data.

Put water in a calorimeter (a Styrofoam cup) to just cover your metal; mass the water.

Mass of water in calorimeter:

Take the temperature of the water.

Initial calorimeter water temperature:

Take the temperature of the boiling water (it should be 100 OC)

Temperature of metal in boiling water:

Carefully put your metal in boiling water. Wait 3 minutes. Transfer to calorimeter (cup).

See how hot the water gets

Temperature of water in cup (and metal) after heated by metal:

Dry your metal and mass it. Mass of dry metal:

12

Calculations

q = m x c x T where q = heat change, c = specific heat, m = mass in grams, and T = change in

temperature in oC. .

The key assumption we will use is that heat absorbed by water = heat released by metal (qwater = qmetal).

q for water = 4.184 j/goC.

Solve for c and use this to identify your unknown metal. The sample calculation below should be very

helpful.

Question:

1. What is the identity of your unknown metal? Show your specific heat calculations below. If more

than one metal was identified, use common sense to rule out dangerous or expensive elements.

Since this calculation is trickier than most, a guide is presented below based on the following logic.

We’d like to plug directly into q = mcT, but there are 2 unknowns- q and T. However, since the

specific heat of liquid water is known we can calculate the heat gained by the water…which, if you think

about it, is exactly equal to the heat lost by the metal, since that’s what heated the water. So…

qwater = mwater x cwater x Twater = ____g x 4.184 J/g oC x (___ oC – ____ oC)

qwater = qmetal =_____ J

ometalmetal o

metal metal

q (___ j)rearranging, c ____ j g C

m T (___ g)(___ C)

By comparing this specific heat to known values, and looking at my metal, I conclude the metal is

_________.

Sample Calculation: 125 g water, initial temperature of water = 25.6 oC, initial temperature of

metal = 115.0 oC, Final temp of water = 29.3 0C. Mass of metal = 50.0 g. What is the metal?

qwater = mwater x cwater x Twater = 125 g x 4.184 J/g oC x (29.3 oC – 25.6 oC)

qwater = 1900J

qwater = qmetal

qmetal = 1900J = mmetal x cmetal x Tmetal

cmetal = 1900J/m x T = 1900J/50.0 g x (115 oC – 29.3 oC) = 0.44 J/g oC.

Comparing to known values, the metal may be Iron (Fe, c = 0.449 J/g oC).

13

Name _______________________________ Period ________ energy lab 2

Potato Chip Calorimetry

In this experiment we will burn a snack chip and see how many calories of heat it produces. We will

then compare it to the nutritional information on the container.

We can find how many calories are contained in any combustible material by simply measuring the

ability of the burning material to heat water. For every degree it can heat up a milliliter of water

that is equal to exactly 1 calorie of heat energy. It is also equal to 4.184 joules, or 0.001

Nutritional Calories.

To do this we put a burning chip under a beaker of water and measure how much warmer the water

gets. This is known as calorimetry.

The problem: Every year we perform this experiment we get lousy results.

The reason: The transfer of heat from a burning chip is inefficient. Too much heat is lost to the

air, and to the beaker. We’d like all of the energy from that flame to heat up the water, with no

loss. It’s a similar problem to that faced at home with your furnace, or to the design of a

woodstove.

Let’s improve it. Draw and describe below a calorimetry apparatus that will be as efficient as

possible, while still being safe. Don’t forget that your chip needs oxygen to burn, and that your

design must be safe. Draw it, and describe it fully below; your homework assignment for tonight

is to build your design and bring it to class tomorrow. Note that no construction will be allowed

in class.

Most common errors Possible solutions

-nice design but chip won’t burn- not enough air pull air through or open up design

-too much water- very little temperature change use less than 5 mL of water

-heat loss. insulate

Calorimeter Design Blueprint

14

Safety Warning: This lab includes flames, burning materials, and potentially hot water. Wear

goggles, gloves, aprons, and covered shoes. It can also get smoky. Working in the hood or near a

window is a good idea. Be cautious working with burning materials. Please let me know if you need

fresh air.

Enter your data here:

Observations of the Burning of a Potato Chip

______Mass of water in beaker (For water mass = volume)

______Initial temperature of water

______Highest temperature of water

______Change in temperature

4.184__ Specific heat of liquid water in J/gOC

4,184__ Number of joules in a nutritional calorie

Calculate the number of nutritional calories in your chip below; show your work including

cancelled units for credit

Read the nutritional information on the bag and calculate the actual number of nutritional

calories in your chip below. Show your work including cancelled units, and explain any

estimations you had to make.

Why is the measured chip calories different from the actual chip calories? Provide a thoughtful

explanation paragraph based on your experimental design and performance.

Measured Chip Calories Calculation (5 points):

Actual Chip Calories Calculation (5 points):

Explanation of results (5 points):

15

Write up this lab the in the form of a poster that follows the format shown below:

Title

Name, Date

(For example:

Potato Chip Calorimetry

Or Energy Analysis of a common Snack Food)

Schematic drawing

With labels

Of your

calorimeter

caption

Conclusions

Include the Nutritional

Calories calculated for

your chip, the estimated

real nutritional calories for

your chip, and an

explanation for the

difference.

Data

Q = mcT

Q=

M=

C=

T =

= ( ) ( ) ( )

= ___ J

= ___Nutritional

Calories

Pick a topic:

1. What is calorimetry?

2. Sources of Error

in our calorimeter design

3. A better design for

the next experiment

10 points:

1. Effort: 5 points

-does this represent

45 minutes of effort?

2. Calculations: 3 Points

-are they accurate?

3. Analysis: 2 points:

Why are the results so

bad (or so good).

16

Name________________________Period________ energy ws1

Energy: Sources, definitions, and conversions

Many people, including scientists, believe the Earth, and the people on it, are in trouble. Problems

include global warming, air, land, and water pollution, ozone depletion, overpopulation, and urban

sprawl, among others. People may disagree as to how serious these issues are, but most would agree

that many of the problems relate to energy use. Here we will consider what energy is, how we use

it, and how we measure it.

Energy is defined as the ability to do work or produce heat. Our primary sources of energy are

the combustion of fossil fuels, nuclear fission, and more passive sources including solar, wind, and

geothermal energy. Currently, fossil fuels and the nuclear fission of uranium-235 provide most of

our energy needs. In Connecticut, about 50% of our electrical energy comes from nuclear fission,

40% from burning oil, gas, biomass, and coal, and the rest is from solar, wind, geothermal, and

other sources.

Energy is measured using a variety of units. A Nutritional Calorie is one most of us are familiar

with. Not to be confused with the scientific calorie, (note the small c), a Nutritional Calorie is the

amount of energy required to raise the temperature of a liter of water by one degree Celsius.

Unfortunately, lots of different energy units are used, so we need to be familiar with them and

know how to convert them. Here are the most common ones:

Use this information above to answer the following questions

1. What is the primary source of energy that powers your rechargeable ipod?

a. The battery b. The electrical outlet c. Sunlight d. Power Plants e. Power lines

2. What is the primary source of energy that heats this school?

a. Power lines b. Oil c. Natural Gas d. Nuclear energy d. Solar energy e. Radiators

3. What is the primary source of energy that heat our Bunsen burners in class?

a. gasoline b. propane c. natural gas d. power plants e. The gas jets

4. Electrical energy in Connecticut comes from

a. Electrical outlets b. Power lines c. Nuclear power d. Nuclear power and combustion e. solar

energy

5. What is energy?

6. What do you believe is the primary cause of global warming?

1 Nutritional Calorie = 4 BTU (British Thermal Units) = 1000 calories = 4184 joules = 0.0016 KWH

17

7. How much energy is required to heat a liter of water from 25 OC to 27 OC? Please show your

calculation including cancelled units.

8. You just ate a chocolate bar, which will provide your body with 300 Nutritional Calories of sugar-

rush energy. Please show your calculation including cancelled units.

Convert this 300 Nutritional Calories to

a. Joules (Hint: there are 4184 joules in a Nutritional Calorie)

b. British Thermal Units (btu)

c. kilowatt hours (KWH)

9. Here is the calculation that tells me I should buy a plug-in hybrid car as soon as they come out:

My gas-powered Mazda gets 30 miles per gallon (that’s 30 miles/gallon), and gas currently

costs $3.20 per gallon.

How much does it cost to drive my Mazda one mile?

Each mile driven by a Toyota Prius uses 0.25 kilowatt hours of electrical energy when the

combustion engine is off (that’s 1 mile/0.253 kwh) and the electricity costs 10 cents per

kwh.

How much does it cost to drive the Prius one mile?

Which is cheaper to drive- the Mazda, or the Toyota?

18

Name________________________Period________ energy ws2

Specific Heat Worksheet

Have you ever noticed how some substances feel colder than others in a room? Or have you noticed

that some metals like iron retain their heat for a long time, while others like aluminum cool very

quickly? Clearly, substances vary in their responses to heating and cooling.

The amount of energy needed to heat a substance is known as specific heat, and is unique for each

pure substance. The units are j/gOC, which literally means the amount of energy in joules needed to

raise the temperature of one gram of the substance one degree Celsius. Liquid water requires over

4.184 joules, while gold only requires about 1/10 of a joule for each gram to get one degree Celsius

hotter.

Use the specific heat formula below to learn about the thermal properties of various substances.

The first problem is solved for you. The answers are given; you must show the work to get there.

1. When 15.0 g of steam drops in temperature from 275.0 °C to 250.0 °C, how much heat energy is

released?

(754 J)

2. How much energy is required to heat 120.00 g of water from 2.0000 °C to 24.000 °C?

(11,046 J)

3. If 720.0 g of steam at 400.0 °C absorbs 800.0 joules of heat energy, what will be its increase in

temperature?

(400.6 OC)

All of these problems may be solved using the specific heat equation:

q = mcT where

q = heat in joules

m= the mass of the substance in g

c = the specific heat of the substance in j/goC (2.03 for ice, 4.184 for liquid

water, and 2.01 for steam)

And T is the absolute temperature change in oC.

Example: How much energy must be absorbed by 20.0 g of liquid water (Cwater = 4.184 j/g0C) to

increase its temperature from 283.0 °C to 303.0 °C?

Solution:

q = mcT

= (20.0 g)(4.184 J/g0C)(20 oC) = 1,673.6 J

19

4. How much heat (in J) is given out when 85.0 g of lead cools from 200.0 °C to 10.0 °C? (Clead =

0.129 J/g °C)

(2080 J)

5. If it takes 41.72 joules to heat a piece of gold weighing 18.69 g from 10.0 °C to 27.0 °C, what is

the specific heat of the gold?

(0.131 J/gOC)

6. If 35 g of a substance absorbs 4000 J of heat when the temperature rises from 25 to 5100C,

what is the specific heat of that substance?

(0.235 J/gOC)

7. A certain mass of water was heated with 41,840 Joules, raising its temperature from 22.0 °C to

28.5 °C. Find the mass of water.

(1540 g)

8. Calculate the number of joules given off when 32.0 grams of steam cools from 90 degrees

celsius to 31 degrees celsius.

(3800 J)

9. How many joules of heat are lost by 3580 kilograms of granite as it cools from 41.2 OC to -12.9 OC? The specific heat of granite is 0.803 J/gOC

(1.56 x 108 J)

10. In reality we are making a lot of assumptions for these calculations to be accurate. For

example, in many cases we are assuming a perfectly insulated container. List two other assumptions

we are making in these calculations.

1.

2.

20

Name____________________________________________ Period__________ energy ws3

Energy of Heating and Cooling Water

In the previous worksheet we learned to use the specific heat equation:

q = mcT (where q = joules of heat, m = grams of mass, c = specific heat in j/gOC, and

T = degrees of temperature change)

To heat or cool water we have to consider the phase it is in, as well as the energy

associated with phase changes

Symbol and value Symbol What it means

cwater(l) = 4.18 J/goC The specific heat of liquid water It takes 4.18 joules of heat

energy to raise the

temperature of 1 gram of

liquid water by one degree

Celsius

cwater(s) = 2.03 J/goC The specific heat of ice It takes 2.03 joules of heat

energy to raise the


solid water (ice) by one

degree Celsius

cwater(g) = 2.01 J/goC The specific heat of steam It takes 2.01 joules of heat

energy to raise the


gaseous water (steam) by

one degree Celsius

Hvap = 2,259 J/g The enthalpy of vaporization of

water

It takes 2,259 joules of

energy to boil one gram of

liquid water (convert it

from a liquid to a gas).

Hfus =334 J/g The enthalpy of fusion of water It takes 334 joules of

energy to melt one gram of

liquid water (convert it

from a solid to a liquid).

To find out how much energy it takes to heat water, we may have to include the energy of phase

changes. For example, there are five separate energy steps involved when ice is heated into steam:

1. The ice warms to 0OC 2. The ice melts at 0OC 3. The water heats to 100OC 4. The water boils at

100OC. 5. The steam raises to it’s final temperature. Note for the last step we assume a closed

system that would pressurize.

Use these ideas to answer the guided questions below.

21

1) A 12 oz. can of soda contains 450 g of water. How many joules are released when a can of

soda is cooled from 25 degrees Celsius (room temperature) to 4 degrees Celsius (the temperature

of a refrigerator). The specific heat of liquid water is 4.18 J / gram x oC.

Hint: q=mcT

(39.5kJ, or 39,500 J)

2) How many joules are required to heat 250 grams of liquid water from 00 to 1000 C ?

Hint: q=mcT

(104.5 kJ)

3) How many joules are required to melt 100 grams of water? The heat of fusion of water is

6010 J / mole.

Hint: Each mole we melt consumes 6010 J. How many moles of water do we have?

(33.4 kJ)

4) How many joules are required to boil 150 grams of water? The heat of vaporization of

water is 40,670 J / mole.

Hint: Each mole we boil consumes 40,670 J. How many moles of water do we have?

(338.8 kJ)

5) How many joules are required to heat 200 grams of water from 25 0C to 125 0C? The

specific heat of steam is 2.01 J / g . 0C (Hint: there are 3 parts to this)

Hint: Here are the parts

Part 1: 25-1000C: q = mcT = ( )( )( )=_______

Part 2: boiling at 100oC: 40,670J/mole . ______moles = ______

Part 3: 100-1250C: q = mcT = ( )( )( ) =_______

Total = 1 + 2 + 3 =________ + _________ + _______ +_______

(524.7 kJ)

7. How much heat is required to warm 225 g of ice from -46.8°C to 0.0°C, melt the ice, warm the

water from 0.0°C to 100.0°C, boil the water, and heat the steam to 173.0°C?

(732 kJ)

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Name_____________________________________ Period________ energy ws4

Energy with Phase change WS II

1) How many joules are required to heat 75 grams of water from -85 0C to 1850C?

2) How many joules are required to heat a frozen can of ice (360 grams) from -5 0 C (the

temperature of an overcooled refrigerator) to 110 0C (the highest practical temperature within a

microwave oven)?

Useful information (see previous worksheet and powerpoint presentation for a

full explanation of terms):

q = mcT

cwater(l) = the specific heat of liquid water = 4.184 J/goC

cwater(s) = the specific heat of ice = 2.03 J/goC

cwater(g) = the specific heat of steam = 2.01 J/goC

Hvap the energy required to convert liquid water to steam = 2259 J/g

Hfus = the energy required to convert ice to liquid water = 334 J/g

Example: How many joules of energy are required to heat 25 grams of water from -28 to +130oC?

Solution: Heating up that 25 grams of ice involves 5 separate steps:

1. The ice heats from -28 OC to 0OC: q = mcT = (25 g)(2.03 J/gOC)(28OC) = 1,421 J

2. At 0OC the ice melts: 334 j/g x 25 g = 8350 J

3. The water then heats from 0 to 100 OC: q = mcT = (25 g)(4.18 J/gOC)(100OC) = 10,450 J

4. The water boils at 100 OC: 2259 J/g x 25 g = 56,475 J

5. The steam heats from 100 to 185 (in reality this could only happen as pressure increases);

q= mcT = (25)(2.01)(85) = 4271 J

Total: 80,967 J

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3) (Level 1 only) How many joules are released when 450 grams of water are cooled from 4 x 107

0C (the hottest temperature ever achieved by man) to 1 x 10-9 0C (the coldest temperature achieved

by man).

4) (Level 1 only) How many joules are required to raise the temperature of 100 grams of water

from -269 0C (the current temperature of space) to 1.6 x 1015 0C (the estimated temperature of space

immediately after the big bang)?

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Energy ws5

Name________________________Date_________________Period____________

Free Energy Worksheet

We have just learned that if the free energy (G) of a reaction is

negative, that reaction will occur spontaneously. This can be

calculated using the Gibbs Free Energy Equation shown on the left,

named after New Haven’s own J. Willard Gibbs.

Now, usually a reaction is spontaneous if it is exothermic. Bang.

Sometimes however, an endothermic reaction can be spontaneous. The

melting of ice is a good example. H is slightly positive, but it has

something else going for it: the process produces disorder. This

provides enough boost to make the reaction spontaneous at room

temperature. This randomness factor is called entropy (S). Examples

of increased randomness include a liquid becoming a gas, something

splitting in two, or things spreading out, like the expansion of the universe. As Entropy increases it

becomes more positive. If we know the temperature, enthalpy, and entropy of a reaction or system we

can use the Gibbs Free energy equation to predict if the reaction will be spontaneous.

Calculate G and indicate if the process is spontaneous or nonspontaneous

1. H = 145 kJ, T = 293K, S = 195 J/K

G = H –TS

Where G = Gibbs Free Energy

H = Enthalpy in Joules

T = Temperature (K)

And S = Entropy in Joules/K

Example: Calculate DG and indicate if the reaction is spontaneous when H = 2.3 kJ, T = 25OC, and

S = 195 J/K

Solution: Note that we need to convert kilojoules (kJ) to joules (1000 J = 1 kJ), and degrees

Celsius to Kelvin (K = OC + 273).

G = H-TS = 2300 J – (298 K . 195 J/K) = -55,810 J = spontaneous

Most common errors: Did not change kJ to J; did not change OC to K; subtracting before

multiplying.

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2. H = -232 kJ, T = 273K, S = 138 J/K

3. H = -15.9 kJ, T = 100 oC, S = -268 J/K

4. Calculate the temperature at which G = 34.7 kJ if H = 40.2 kJ and S = 22.2 J/K.

Iron ore can be converted to iron by the following reaction:

Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(g) H = 149.8 kJ

For this reaction S = 610 J/K.

5. Is this reaction spontaneous at 298K? What is the value of G?

6. Why is the entropy for this reaction positive?

26

How to ace the Energy exam

In this our 12th unit we investigated Energy, both from a scientific and environmental perspective. We

began by considering the primary sources of energy we use, and their environmental consequences,

as well as their abundance. This is perhaps the most important information to answer our essential

question for this unit:

We looked at some efforts to answer this question in the form of transportation when we took a look

at some eco-friendly cars. We then focused on the hard science that relates to energy: what it is,

types of energy, Enthalpy, and exothermic and endothermic chemical reactions. During this time we

performed two experiments where we measured the specific heat of an unknown metal, and the

energy contained in a potato chip, using a calorimeter of our own design.

We finished by considering the energy required to heat water, including the phase changes that may

be involved, and we familiarized ourselves with Free energy, which includes the esoteric concept of

entropy.

To ace this unit be familiar with the terminology associated with energy, know how to measure it, and

think about the sources of energy we use, and what changes we must make for the sake of our planet.

As usual, review your notes, worksheets, and lab experiments, and answer all of the questions below.

In our next unit we will ask why some reactions such as explosions are rapid, while others such as rust

are slow- this is our Rates of Reaction unit, coming up.

Useful information to be provided on test:

q = mcT

where m = mass (g), c = specific heat (J/g0C; see examples for H2O below), and T =

change in temperature (0C)

cwater(l) = 4.18 J/goC

cwater(s) = 2.03 J/goC

cwater(g) = 2.01 J/goC

Hvap = 2260 J/g

Hfus = 334 J/g

At 1 atm:

Water boils/condenses at 100oC

Water melts/freezes at 0oC

1 Nutritional Calorie = 4 BTU (British Thermal Units) = 1000 calories = 4184 joules = 0.0016 KWH

G = H-TS

Where G = change in free energy (J), H = change in enthalpy (J), T = temperature (K),

and S = change in entropy (J/K)

What Changes Should We Make?

27

1. Know the meaning and usage of the following terms

Energy

Enthalpy (H)

Potential Energy

Kinetic Energy

Exothermic

Endothermic

Specific Heat

Heat of vaporization (Hvap)

Heat of fusion (Hfus)

Heat of condensation (Hcond)

Heat of solidification (Hsolid)

Hess’s Law

calorie

Nutritional Calorie

Joule

Standard enthalpy of formation (Ho)

Free Energy

Entropy

2. Understand concepts, with sample questions:

1. Energy

Which of the following are exothermic processes: melting, freezing, boiling,

condensing, subliming, depositing?

2. Thermochemical properties of fuels

28

Based on the table below, how much heat would be generated if 92 grams of ethanol

were burned?

3. Energy unit conversions

Convert 1000 joules to kJ, calories, and Nutritional Calories

4. Specific heat

2. When 15.0 g of steam drops in temperature from 275.0 °C to 250.0 °C, how much

heat energy is released?

5. Typical specific heat numbers and their meaning

6. If it takes 41.72 joules to heat a piece of gold weighing 18.69 g from 10.0 °C to 27.0 °C, what is the specific heat of the gold?

6. Enthalpy for temperature changes compared to phase changes.

Calculate how much energy is required to convert 1 kg of ice at -10 oC to steam at 101 oC.

1.

2.

3.

4.

5.

Answer:

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7. Free Energy

1. Calculate G and indicate if the process is spontaneous or nonspontaneous

a. H = 145 kJ, T = 100 0C, S = 195 J/K

b. 2. Calculate the temperature at which G = 34.7 kJ if H = 40.2 kJ and S = 22.2 J/K.

8. Energy and the Environment

a. Describe details concerning the energy usage of two of your

favorite eco-friendly cars

b. How can the energy costs of an electric car be compared to a

gasoline car?

c. Use this real data to make a comparion of energy costs between a prius and a Matrix

Prius: Requires 0.253 kwh/mile where each kwh costs ten cents

Matrix: Requires 1 gallon of gas for every thirty miles, where each gallon costs two

dollars

30

d. Rate the following energy sources in terms of cost, abundance and damage to the

environment”

Energy

Source

Chemical (if

any)

involved

Cost (cheap,

medium,

expensive)

Abundance

(running out,

medium, plenty,

renewable)

Damage to the

environment

(benign,

medium, bad)

My opinion

(excellent, okay,

yuck) plus

comments

Coal

oil

Natural gas

Nuclear

fission

Nuclear

Fusion

tidal

wave

geothermal

E-85

biodiesel

solar

wind

8. Depending on your class, you may have learned some additional topics such as protein Kinase Mzeta

and the ZIP molecule, the used and abused molecule tetrahydrocannabinol (THC), or details

concerning nuclear fusion. Review your notes and have a good general knowledge of each topic.

8. Review the rest of your notes, worksheets, and lab experiments. Good Luck on the test.

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Documents

l. chapter 12 energy packet 2008