Quiz 1

Quiz 1

Due: 11:59pm on Monday, September 3, 2012

Note: You will receive no credit for late submissions. To learn more, read your instructor's Grading Policy

Question 1

Part A

Calculate the change in internal energy of the following system: a balloon is cooled by removing 0.656 of

heat. It shrinks on cooling, and the atmosphere does 383 of work on the balloon.

ANSWER:

Correct

Part B

Determine whether the process above is endothermic or exothermic.

ANSWER:

Correct

Part C

Calculate the change in internal energy of the following system: a 100.0- bar of gold is heated from 25 to

50 during which it absorbs 322 of heat. Assume the volume of the gold bar remains constant.

ANSWER:

= -0.273

The process is endothermic.

The process is exothermic.

The process is neither endothermic nor exothermic.

= 322

Correct

Part D


ANSWER:

Correct

Part E

Calculate the change in internal energy of the following system: the surroundings do 1.41 of work

compressing gas in a perfectly insulated cylinder.

ANSWER:

Correct

Part F


ANSWER:

Correct




= 1.41




Question 2

Consider the following reaction:

Part A

Is this reaction exothermic or endothermic?

ANSWER:

Correct

Part B

Calculate the amount of heat transferred when 3.51 of reacts at constant pressure.

ANSWER:

Correct

Part C

How many grams of are produced during an enthalpy change of -232 ?

ANSWER:

Correct

exothermic

endothermic

= -86.9

= 15.5

Part D

How many kilojoules of heat are absorbed when 40.9 of is decomposed into and at

constant pressure?

ANSWER:

Correct

Question 3

The joule ( ) is a unit of energy. Recall that energy may be converted between many different forms such as

mechanical energy, thermal energy (heat), chemical energy, electrical energy, and light.

The mechanical energy produced by a system is called work. When work is accomplished through the changingvolume of a gas, it is called PV work and is given by the formula , where is the work, is the

external pressure, and is the change in volume. If the volume change and pressure are in liters and

atmospheres respecitvely, then the work will have units of liter-atmospheres, which can be converted to joules usingthe conversion factor .

Work can also be expressed as force multiplied by distance: , where is the work in joules, is the

force in newtons, and is the distance in meters. Note that .

Part A

A piston has an external pressure of 9.00 . How much work has been done if the cylinder goes from a

volume of 0.150 liters to 0.650 liters.

Express your answer with the appropriate units.

Hint 1. Calculate the change in volume

What is the value of the change in volume ?


ANSWER:

Hint 2. Calculate the work done

= 611

= 0.500

Calculate the amount of work done in units of . Notice that the units are already entered for you.

Express your answer numerically in liter-atmospheres.

ANSWER:

Hint 3. Conversion factor

Use the relation .

One way to remember this is to divide the two common values of the gas constant :

ANSWER:

Correct

Question 4

Learning Goal:

To understand the concepts of heat capacity, specific heat, and molar heat capacity.

Heat capacity, , is the amount of energy required to raise the temperature of a substance by exactly 1 degree

Celsius. The energy needed to warm an object increases as the mass of that object increases. We see this in oureveryday life. For example, we know that it takes much more energy to heat a large tank of water than a small cup.Because of this dependence on mass, experimentally determined heat capacities are always reported in terms ofthe amount of the substance that is heated. One method is to report how much energy it takes to raise thetemperature of one mole of a substance by exactly 1 degree Celsuis. This value is the molar heat capacity, whichhas the symbol .The molar heat capacity is given in the units . A second method is to report how

much energy it takes to raise the temperature of one gram of a substance by exactly 1 degree Celsius. This value isthe specific heat, which has been given the symbol . The units for specific heat are .

The heat capacity of a substance is therefore related to the energy needed to raise its temperature by an amount . That is, , where denotes the number of moles of the substance, or , where

denotes the number of grams of the substance.

Part A

-4.50

= -456

It takes 49.0 to raise the temperature of an 10.6 piece of unknown metal from 13.0 to 24.0 . What is the

specific heat for the metal?


Hint 1. How to approach the problem

You are given the values of and . You can calculate by subtracting the two temperatures. Then

you can plug all of these values into the formula to solve for .

Hint 2. Find the correct formula

In this problem, you have been given the mass, temperature change, and energy added to the system.Which formula can be used to find the specific heat?

ANSWER:

Hint 3. Calculate the change in temperature

The change in temperature, , is given by . What is the value of for the piece of

metal described in this part?


ANSWER:

ANSWER:

Correct

Parts B and C

The next two questions pertain to silver. They have nothing to do with unknown metal described in Part A.

Part B

= 11.0

= 0.420

The molar heat capacity of silver is 25.35 . How much energy would it take to raise the temperature

of 10.6 of silver by 13.6 ?



Be sure to choose the appropriate formula to answer the question. Since you are given the molar heatcapacity of silver, you will need to calculate how many moles of silver are being heated. Next, plug in theknown variables and solve for the unknown value, in this case the energy needed, .

Hint 2. Choosing the correct formula

You have been given the molar heat capacity of silver, but the amount of silver has been given in grams,not moles. You could convert the molar heat capacity to a specific heat, but it is easier to convert themass of silver to moles. Since it is easiest to use the given molar heat capacity, what is the best formulato use in this problem?

ANSWER:

Correct

Hint 3. Calculate the number of moles of silver

To use the formula for molar heat capacity, you need to know the number of moles of silver beingheated. How many moles of silver are being heated in this problem?


Hint 1. Molecular weight of silver

The molecular weight of silver is 108 .

ANSWER:

9.81!10"2

Answer Requested

ANSWER:

Answer Requested

Part C

What is the specific heat of silver?



You have been given the molar heat capacity of silver in Part B. The only difference between the units ofspecific heat and the units of molar heat capacity is that one is in terms of grams and the other is interms of moles. Therefore, use a conversion factor between grams and moles for silver.

Hint 2. What is the molar mass of silver?

What is the molar mass of silver?


ANSWER:

Hint 3. Find the correct unit factor setup

Which unit factor setup is correct?

ANSWER:

= 33.8

108

ANSWER:

Correct

Question 5

A gas is confined to a cylinder under constant atmospheric pressure, as illustrated in the following figure. When0.450 of heat is added to the gas, it expands and does 212 of work on the surroundings.

Part A

What is the value of for this process?

Express your answer using two significant figures.

ANSWER:

0.235

= 0.45

Correct

Part B

What is the value of for this process?

Express your answer using two decimal places.

ANSWER:

Correct

Question 6

Calorimetry is a method used to measure enthalpy, or heat, changes that occur during chemical processes. Twocommon calorimeters are constant-pressure calorimeters and constant-volume (or "bomb") calorimeters. Bombcalorimeters are used to measure combustion and other gas-producing reactions, where the reaction is observed ina strong, sealed vessel. A simple constant-pressure calorimeter can be made from a foam coffee cup and athermometer; energy changes in a reaction are observed via a temperature change of the solution in the cup. Theidea behind calorimeters is that if they are sufficiently insulated from the outside environment, any energy gained orlost in the chemical reaction will be directly observable as a temperature and/or pressure change in the calorimeter.

Part A

A total of 2.00 of a compound is allowed to react with water in a foam coffee cup and the reaction produces

125 of solution. The reaction caused the temperature of the solution to rise from 21.0 to 24.7 . What is the

enthalpy of this reaction? Assume that no heat is lost to the surroundings or to the coffee cup itself and that thespecific heat of the solution is the same as that of pure water.

Enter your answer in kilojoules per mole of compound to three significant figures.

Hint 1. How to approach this problem.

Enthalpy is the amount of heat absorbed or produced by a reaction:

Endothermic reactions have a positive enthalpy value. Exothermic reactions have a negative enthalpyvalue.

Hint 2. Specific heat of water

= 0.24

The specific heat of water is .

Hint 3. Calculate the temperature change

What is the temperature change for water in this problem?

Express your answer numerically in degrees Celsius.

ANSWER:

Hint 4. Calculate the heat absorbed by the water

How many joules of heat did the water gain?

Enter your answer numerically in joules to three significant figures.

ANSWER:

ANSWER:

Correct

Question 7

A 2.250 sample of quinone is burned in a bomb calorimeter whose total heat capacity is 7.854

. The temperature of the calorimeter increases from 24.20 to 31.49 .

Part A

What is the heat of combustion per gram of quinone?

ANSWER:

= 3.7

1940

= -0.968

= -25.5

Correct

Part B

What is the heat of combustion per mole of quinone?

ANSWER:

Correct

Question 8

The heat of combustion of fructose, , is -2812 .

Part A

If a fresh golden delicious apple weighing 4.31 (120 ) contains 18.0 of fructose, what caloric content doesthe fructose contribute to the apple?

ANSWER:

Correct

Question 9

Calorimetry is a method used to measure changes in enthalpy, or heat, that occur during chemical processes. Twocommon calorimeters are constant-pressure calorimeters and constant-volume (or "bomb") calorimeters. Bombcalorimeters are used to measure combustion and other gas-producing reactions, in which the reaction is observedin a strong, sealed vessel. A simple constant-pressure calorimeter can be made from a foam coffee cup and athermometer, in which energy changes in a reaction are observed via the change in temperature of the solution inthe cup. The idea behind calorimeters is that if they are sufficiently insulated from the outside environment, anyenergy gained or lost in the chemical reaction will be directly observable as a temperature and/or pressure changein the calorimeter.

= -2750

67.1

Part A

In the following experiment, a coffee-cup calorimeter containing 100 of is used. The initial

temperature of the calorimeter is 23.0 . If 4.90 of is added to the calorimeter, what will be the final

temperature of the solution in the calorimeter? The heat of solution of is .



To solve this problem, first consider how much energy would be liberated by the mass of that was

added to the calorimeter. Next, calculate what temperature change this amount of heat energy wouldproduce in the calorimeter. Finally, calculate the final temperature of the solution in the calorimeter, sincethe starting temperature is known.

Hint 2. Calculate the heat released by calcium chloride

How much heat is released from the dissolution of 4.90 of ?


Hint 1. How to find the heat released

The number tells us that 82.8 is released for every mole of calcium chloride.

But you don't have exactly one mole of calcium chloride. Therefore, you should find how manymoles of calcium chloride you actually have. Then multiply that by the expressed in joules

per mole. The units cancel to give you joules, a unit of heat: (the amount of

heat released).

Hint 2. Calculate the number of moles of calcium chloride

4.90 of has been added to the calorimeter. How many moles of does this

represent?


Hint 1. Find the molecular weight of calcium chloride

The molecular weight of is 40 . The molecular weight of is 35.5 . What

is the molecular weight of ?


ANSWER:

ANSWER:

Hint 3. Express the heat of solution in joules per mole

Convert to units of . Recall that 1 = 1000 .


ANSWER:

ANSWER:

Hint 3. Find the temperature change of the solution.

What is the change in temperature in the calorimeter?

where, is the total mass of solution in the calorimeter. Assume that the specific heat of this solution isthe same as that for water: .


Hint 1. Find the mass of the solution in the calorimeter

The variable in the calorimeter equation refers to the mass of the solution in the calorimeter.The calorimeter in this problem contains 100 of , which has a mass of 100 . To get the

total mass of the solution in the calorimeter, be sure to add to this the mass 4.90 of

added to the . What is the total mass of the solution in the calorimeter after the has

been added?


ANSWER:

111

4.41!10"2

= "8.28!104

3660

Hint 2. Identify the calorimetric equation in terms of

Which of the following equation is correct calorimetric equation in terms of ?

ANSWER:

ANSWER:

ANSWER:

Correct

Question 10

A 1.800- sample of solid phenol ( ) was burned in a bomb calorimeter whose total heat capacity is

11.66 . The temperature of the calorimeter plus contents increased from 21.36 to 26.37 .

Part A

Write a balanced chemical equation for the bomb calorimeter reaction.

Express your answer as a chemical equation. Identify all of the phases in your answer.

ANSWER:

105

8.33

31.3

Correct

Part B

What is the heat of combustion per gram of phenol?

ANSWER:

Correct

Part C

Per mole of phenol?

ANSWER:

Correct

Question 11

A coffee-cup calorimeter contains 150.0 of water at 25.3 . A 124.0 block of copper metal is heated to 100.4

by putting it in a beaker of boiling water. The specific heat of is 0.385 . The is added to the

calorimeter, and after a time the contents of the cup reach a constant temperature of 30.1 .

Part A

Determine the amount of heat, in , lost by the copper block.

ANSWER:

= -32.5

= -3060

= 3360

Correct

Part B

Determine the amount of heat gained by the water. The specific heat of water is 4.18 .

ANSWER:

Correct

Part C

The difference between your answers for (a) and (b) is due to heat loss through the Styrofoam cups and the

heat necessary to raise the temperature of the inner wall of the apparatus. The heat capacity of the calorimeteris the amount of heat necessary to raise the temperature of the apparatus (the cups and the stopper) by 1 .

Calculate the heat capacity of the calorimeter in .

Express your answer using two significant figures.

ANSWER:

Correct

Part D

What would be the final temperature of the system if all the heat lost by the copper block were absorbed by thewater in the calorimeter?

ANSWER:

Correct

= 3010

72

= 30.7

Question 12

The heat capacity of an object indicates how much energy that object can absorb for a given increase in thatobject's temperature. In a system in which two objects of different temperatures come into contact with one another,the warmer object will cool and the cooler object will warm up until the system is at a single equilibrium temperature.Note the difference between the terms molar heat capacity, which has units of , and specific heat,

which has units of .

Part A

A volume of 80.0 of is initially at room temperature (22.0 ). A chilled steel rod at 2.00 is placed

in the water. If the final temperature of the system is 21.5 , what is the mass of the steel bar?

Use the following values:

specific heat of water = 4.18

specific heat of steel = 0.452

Express your answer to three significant figures and include the appropriate units.


According to the law of conservation of energy, the heat lost by the water is equal to the amount of heatgained by the steel, that is, :

where and are the masses of water and steel, respectively, and

are the specific heats of water and steel respectively, and and are the

temperature changes for water and steel, respectively. Also recall that the density of water is 1.00

.

Hint 2. Find the temperature change for the water

What is the change in temperature, , of the water? Recall that is given by .


ANSWER:

= -0.500

Answer RequestedThus the temperature change is

Hint 3. Find the temperature change for the steel

What is the temperature change of the steel? Recall that the change in temperature is given by

.


ANSWER:

CorrectThus the temperature change is

Hint 4. Determine the mass of the water

What is the mass of 80.0 of water if the density of water is 1.00 ?


ANSWER:

Correct

Hint 5. Rearrange the equation to solve for the mass of steel

Rearrange the equation

to solve for the mass of steel ( ).

ANSWER:

= 19.5

mass of the water ( ) = 80.0

Correct

ANSWER:

Correct

Part B

The specific heat of water is 4.18 . Calculate the molar heat capacity of water.



The only difference between the units of specific heat and the units of molar heat capacity is that onehas joules per gram and the other has joules per mole. Therefore, use a conversion factor betweengrams and moles for water.

Hint 2. Find the molar mass of water

What is the molar mass (MM) of water, ?

Express your answer as an integer and include the appropriate units.

ANSWER:

mass of the steel = 19.0

Answer Requested

Hint 3. Identify the unit factor setup

Which unit factor setup is correct for determining the molar heat capacity of water?

ANSWER:

Correct

ANSWER:

Correct

Question 13

Chemical energy is released or absorbed from reactions in various forms. The most easily measurable form ofenergy comes in the form of heat, or enthalpy. The enthalpy of a reaction can be calculated from the heats offormation of the substances involved in the reaction:

where represents the stoichiometric coefficients.

Part A

Calculate the standard enthalpy change for the reaction

MM = 18

molar heat capacity for water = 75.2

Use the following data:

Substance

(

-231

-419

177

-499



The enthalpy of a reaction can be calculated from the sum of the standard enthalpies of formation of theproducts minus those of the reactants, keeping in mind the stoichiometric coefficient of each reactant orproduct involved in the reaction. Thus

Hint 2. Calculate the total heat of formation for the reactants

Taking the reaction coefficients into account, what is the sum of the values of the reactants,

?


ANSWER:

Answer Requested

Hint 3. Calculate the total heat of formation for the products

Taking the reaction coefficients into account, what is the sum of the values of the products?


ANSWER:

= -881

= -644

Answer Requested

ANSWER:

All attempts used; correct answer displayed

Part B

For the reaction given in Part A, how much heat is absorbed when 3.70 of reacts?



In the previous part, you determined that was 237 for the reaction as written. This means that

237 of heat is absorbed when exactly 2 of and 1 of react to produce 2 of and 2

of according to the balanced chemical equation

When less reacts, less heat is absorbed. When more reacts, more heat is absorbed. So use

just as you would a conversion factor; that is, use the factor

Hint 2. Determine the amount of heat absorbed per mole of A

Given the enthalpy of the reaction as written

how much heat is absorbed for each mole of that reacts?

Express your answer to four significant figures and include the appropriate units.

ANSWER:

= 237

118.5

Answer RequestedThus the conversion factor becomes .

ANSWER:

Answer Requested

Question 14

Learning Goal:

To understand how to use Hess's law to find the enthalpy of an overall reaction.

The change in enthalpy, , is the heat absorbed or produced during any reaction at constant pressure. Hess's

law states that for an overall reaction is the sum of the values for the individual reactions. For example, if

we wanted to know the enthalpy change for the reaction

we could calculate it using the enthalpy values for the following individual steps:

Step 1:

Step 2:

Overall:

If the enthalpy change is for step 1 and for step 2, then the enthalpy change for the

overall reaction is calculated as follows:

It is also important to note that the change in enthalpy is a state function, meaning it is independent of path. In otherwords, the sum of the values for any set of reactions that produce the desired product from the starting

materials gives the same overall .

Now consider the following set of reactions:

438

1.

2.

Part A

The equations given in the problem introduction can be added together to give the following reaction:

overall:

However, one of them must be reversed. Which one?

Hint 1. How to add equations

In adding two equations together, substances that appear both on the left and right sides of the arrowcan be canceled. For example, imagine adding the equations and . The C's

can be canceled, as well as one A from each side:

After the cancellations are made, we are left with the following overall equation:

ANSWER:

Correct

Part B

What is the enthalpy for reaction 1 reversed?

reaction 1 reversed:

Express your answer numerically in kilojoules per mole.

ANSWER:

Correct

Part C

What is the enthalpy for the following reaction?overall:

Express your answer numerically in kilojoules per mole.

Hint 1. Hess's law

According to Hess's law, the values for the individual reaction steps may be added together to give

for the overall reaction.

ANSWER:

CorrectBy adding these two reaction, you get the overall reaction. Therefore by adding the enthalpy values, youget the overall enthalpy. Thus

+

gives

Question 15

Learning Goal:

To understand how standard enthalpy of reaction is related to the standard heats of formation of the reactants andproducts.

= 283

= -110

The standard enthalpy of reaction is the enthalpy change that occurs in a reaction when all the reactants andproducts are in their standard states. The symbol for the standard enthalpy of reaction is , where the

subscript "rxn" stands for "reaction." The standard enthalpy of a reaction is calculated from the standard heats offormation ( ) (subscript "f" for formation) of its reactants and products. Therefore, the standard enthalpy

of any reaction can be mathematically determined, as long as the standard heats of formation ( ) of its

reactants and products are known.

In a generic chemical reaction involving reactants A and B and products C and D, , the

standard enthalpy of the reaction is given by

Notice that the stoichiometric coefficients, , , , , are an important part of this equation. This formula is often

generalized as follows, where the first sum on the right-hand side of the equation is a sum over the products and thesecond sum is over the reactants:

where and represent the appropriate stoichiometric coefficients for each substance.

Part A

What is for the following chemical reaction?

You can use the following table of standard heats of formation to calculate the enthalpy of the given

reaction.

Element/ Compound Standard Heat ofFormation (kJ/mol) Element/ Compound Standard Heat of

Formation (kJ/mol)

Express the standard enthalpy of reaction to three significant figures and include the appropriate units.


First, go to a table of standard heats of formation (such as the one given in the problem introduction) and

find the standard heats of formation for each of the reactants and products in the chemical reaction.Then, add together the values for all the products, taking care to multiply each value by the

stoichiometric coefficient if necessary. Next, add together the values for all the reactants, again

taking care to multiply each value by the stoichiometric coefficient if necessary. Finally, subtract the sumof the values for the reactants from the sum of the values for the products, according to the formula

where and denote the appropriate stoichiometric coefficients for each substance.

Hint 2. Calculate the heats of formation of the reactants

The sum of the heats of formation of the reactants is given by

where and represent the appropriate stoichiometric coefficients for each reactant in the given

reaction.What is the sum of the heats of formation of the reactants?

Express the heat of formation to five significant figures and include the appropriate units.

Hint 1. Find the heat of formation of

What is the standard heat of formation of given in the standard heats of formation table?

Express the standard heat of formation to four significant figures and include theappropriate units.

ANSWER:




ANSWER:

ANSWER:

-139.5

-285.8

Hint 3. Calculate the heats of formation of the products

The sum of the heats of formation of the products is given by

where and represent the appropriate stoichiometric coefficients for each product in the given

reaction.What is the sum of the heats of formation of the products?

Express the heat of formation to five significant figures and include the appropriate units.


What is the standard heat of formation of given in the standard heats of formation

table?


ANSWER:




ANSWER:

ANSWER:

ANSWER:

-425.30

-218.8

-92.3

-403.40

CorrectAlthough we generally write the units for simply as , the units for are in kilojoules per mole of

reaction. For a reaction that involves 1 of reactant A, 2 of reactant B, and of enthalpy

change per mol reaction, the is of A, but of B.

Score Summary:

Your score on this assignment is 88.1%.You received 13.21 out of a possible total of 15 points.

21.9

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