Chapter 6 Thermochemistry. Copyright Houghton Mifflin Company. All rights Question, 62 QUESTION

Chapter 6

ThermochemistryThermochemistry

Copyright © Houghton Mifflin Company. All rights reserved. CRS Question, 6–2

QUESTION

A gas absorbs 0.0 J of heat and then performs 15.2 J of work. The change in internal energy of the gas is: 1) –24.8 J. 2) 14.8 J. 3) 55.2 J. 4) –15.2 J. 5) none of these.


ANSWER

–

q

4) 15.2 J.

Section 6.1 The Nature of Energy (p. 229)

When = 0, then the internal energy equals the work.


QUESTION

Of energy, work, enthalpy, and heat, how many are state functions? 1) 0 2) 1 3) 2 4) 3 5) 4


ANSWER

3) 2


The original experiment determining that heat was not a state function was conducted during the boring of cannons. It was found that if the drill was dull, heat could be produced indefinitely without boring deeper into the metal.


QUESTION

For a particular process q = 20 kJ and w = 15 kJ. Which of the following statements is true? 1) Heat flows from the system to the

surroundings. 2) The system does work on the surroundings. 3) E = 35 kJ. 4) All of these are true. 5) None of these are true.


ANSWER

E

q w

3) = 35 kJ.


Internal energy = + . The heat and work both have a positive sign indicating energy flowed from the system to the surroundings.


QUESTION

Which of the following statements is correct? 1) The internal energy of a system increases when more

work is done by the system than heat was flowing into the system.

2) The internal energy of a system decreases when work is done on the system and heat is flowing into the system.

3) The system does work on the surroundings when an ideal gas expands against a constant external pressure.

4) All statements are true. 5) All statements are false.


ANSWER

3) The system does work on the surroundings when an ideal gas expands against a constant external pressure.


The gas molecules of the atmosphere have an average distance from each other.


ANSWER (continued)

A gas from a reaction expanding into the atmosphere must force all the gas molecules nearby closer together. This is the work the gas does on the atmosphere.


QUESTION

Which one of the following statements is false? 1) The change in internal energy, E, for a process is

equal to the amount of heat absorbed at constant volume, qv.

2) The change in enthalpy, H, for a process is equal to the amount of heat absorbed at constant pressure, qp.

3) A bomb calorimeter measures H directly. 4) If qp for a process is negative, the process is

exothermic. 5) The freezing of water is an example of an exothermic

reaction.


ANSWER

)

3) A bomb calorimeter measures H directly.

Section 6.2 Enthalpy and Calorimetry (p. 235

Enthalpy is heat at constant pressure. The pressure can change drastically inside of a bomb calorimeter.


QUESTION

You take 200. g of a solid at 30.0C and let it melt in 400. g of water. The water temperature decreases from 85.1C to 30.0C. Calculate the heat of fusion of this solid. 1) 125 J/g 2) 285 J/g 3) 461 J/g 4) 518 J/g 5) cannot without the heat capacity of the solid


ANSWER

)

3) 461 J/g


The heat of fusion of the ice is found indirectly from the heat loss of the water.


QUESTION

For a particular process q = –17 kJ and w = 21 kJ. Which of the following statements is false? 1) Heat flows from the system to the

surroundings. 2) The system does work on the surroundings. 3) E = +4 kJ 4) The process is exothermic. 5) None of these is false.


ANSWER

E

. E

3) = +4 kJ


For a thermodynamic process we are only interested in the change of energy, E is the energy of a particular state.


QUESTION

In the lab, you mix two solutions (each originally at the same temperature) and the temperature of the resulting solution decreases. Which of the following is true? 1) The chemical reaction is releasing energy. 2) The energy released is equal to s m T. 3) The chemical reaction is absorbing energy. 4) The chemical reaction is exothermic. 5) More than one of these.


ANSWER

)

3) The chemical reaction is absorbing energy.


The temperature drops because the reactants are absorbing energy from the solution faster than the surroundings can replace it.


QUESTION

For the reaction H2O(l) H2O(g) at 298 K, 1.0 atm, H is more positive than E by 2.5 kJ/mol. This quantity of energy can be considered to be: 1) the heat flow required to maintain a constant

temperature. 2) the work done in pushing back the atmosphere. 3) the difference in the H—O bond energy in

H2O(l) compared to H2O(g). 4) the value of H itself. 5) none of these


ANSWER

)

2) the work done in pushing back the atmosphere.


The gas molecules need to use some energy to move atmospheric gas molecules out of their way.


QUESTION

If 5.0 kJ of energy is added to a 15.5-g sample of water at 10.C, the water is: 1) boiling. 2) completely vaporized. 3) frozen solid. 4) decomposed. 5) still a liquid.


ANSWER

q s T q s

T

5) still a liquid.

Section 6.2 Enthalpy and Calorimetry (p. 235)

= m . = 5.0 kJ, = 4.184 J/g C and m = 15.5 g. Solving for shows that the water was not raised to the 100 C necessary for boiling.


QUESTION

Given the equation S(s) + O2(g) SO2(g), H = –296 kJ, which of the following statement(s) is (are) true?

a. The reaction is exothermic. b. When 0.500 mole sulfur is reacted,

148 kJ of energy is released. c. When 32.0 g of sulfur are burned,

2.96 105 J of energy is released.


QUESTION (continued)

1) All are true. 2) None is true. 3) a and b are true. 4) a and c are true. 5) Only b is true.


ANSWER

)

1) All are true.


The minus sign indicates that the reaction is exothermic.


QUESTION

What is the specific heat capacity of gold if it requires 48.8 J to raise the temperature of 15 grams of gold 25C? 1) 29 J/gC 2) 0.13 J/gC 3) 79 J/gC 4) 0.011 J/gC 5) none of these


ANSWER

2) 0.13 J/g C Section 6.2 Enthalpy and Calorimetry (p. 235 ) A common mistake is attempting to solve for the heat capacity instead of specific heat capacity. Using and keeping track of the units of measure

takes care of this type of problem.

2) 0.13 J/g C


A common mistake is attempting to solve for the heat capacity instead of specific heat capacity. Using and keeping track of the units of measure


QUESTION

The heat of formation of Fe2O3(s) is –826 kJ/mol. Calculate the heat of the reaction 4Fe(s) + 3O2(g) 2Fe2O3(s) when a 55.8-g sample of iron is reacted. 1) –206 kJ 2) –413 kJ 3) –826 kJ 4) –1650 kJ 5) –3.30 103 kJ


ANSWER

–

)

2) 413 kJ


Don’t forget to convert grams to moles in this problem. Enthalpy is always in units of kJ/mol.


QUESTION

When 0.157 mol NH3 is reacted with excess HCl, 6.91 kJ of energy is released as heat. What is H for this reaction per mole of NH3 consumed? 1) –22.7 J 2) –1.08 kJ 3) –44.0 kJ 4) +22.7 J 5) +44.0 kJ


ANSWER

3) –44.0 kJ Section 6.2 Enthalpy and Calorimetry (p. 235) A common mistake is use the wrong sign for the enthalpy. Careful reading of the problem will indicate the sign that must be used.

44.0 kJ3)

Section 6.2 Enthalpy and Calorimetry (p. 235)

A common mistake is use the wrong sign for the enthalpy. Careful reading of the problem will indicate the sign that must be used.

44.0 kJ


ANSWER (continued)

Phrases like “heat is released”, “heat is given off” or “heat is emitted” indicate that a minus sign is used. Phrases like “heat is absorbed” or “heat is added” indicate that a plus sign is used.


QUESTION

If a student performs an endothermic reaction in a calorimeter, how does the calculated value of H differ from the actual value if the heat exchanged with the calorimeter is not taken into account?


QUESTION (continued)

1) Hcalc would be more negative because the calorimeter always absorbs heat from the reaction.

2) Hcalc would be less negative because the calorimeter would absorb heat from the reaction.

3) Hcalc would be more positive because the reaction absorbs heat from the calorimeter.

4) Hcalc would be less positive because the reaction absorbs heat from the calorimeter.

5) Hcalc would equal the actual value because the calorimeter does not absorb heat.


ANSWER

Hcalc

)

4) would be less positive because the reaction absorbs heat from the calorimeter.


Most calorimeters are at least partly made of metal and metals have high thermal conductivity, so ignoring the heat loss to the calorimeter will lead to po or accuracy in your results.


QUESTION

Consider the following numbered processes:

a. A 2B b. B C + D c. E 2D

H for the process A 2C + E is


QUESTION

1) H1 + H2 + H3 2) H1 + H2 3) H1 + H2 – H3 4) H1 + 2H2 – H3 5) H1 + 2H2 + H3


ANSWER

1 – 3

)

–

4) H + 2 H2 H

Section 6.3 Hess’s Law (p. 242

The final equation is found by summing the reactions as follows: I + 2(II) III. The minus sign means the reaction is reversed.


QUESTION

Consider the following standard heats of formation:

P4O10(s) = –3110 kJ/mol H2O(l) = –286 kJ/mol H3PO4(s) = –1279 kJ/mol

Calculate the change in enthalpy for the following process:

P4O10(s) + 6H2O(l) 4H3PO4(s)


ANSWER

)

–290 kJ

Section 6.4 Standard Enthalpies of Formation(p. 246

Remember to multiply the heat of formation of each compound by its coefficient.


QUESTION

Which of the following is not being considered as an energy source for the future? 1) Ethanol 2) Methanol 3) Seed oil 4) Shale oil 5) Carbon dioxide


ANSWER

5) Carbon dioxide

Section 6.6 New Sources of Energy (p. 256)

Carbon dioxide is a very stable product of most combustion reactions. It will not react further to produce more useful heat.

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Chapter 6 Thermochemistry. Copyright Houghton Mifflin Company. All rights Question, 62 QUESTION