Deck 6: Thermochemistry: Energy Flow and Chemical Change

Question 1

In a phase change of water between the liquid and the gas phases, 770.1 kJ of energy was released by the system. What was the product, and how much of it was formed in the phase change? (Data: H₂O(l) $\to$ H₂O(g) $\Delta$ H = 44.01 kJ/mol.)

A) 315 g water vapor was produced.
B) 17.5 g of water vapor was produced.
C) 17.5 mol of water vapor was produced.
D) 17.5 mol of liquid water was produced.
E) 17.5 g of liquid water was produced.

Answer 1

17.5 mol of liquid water was produced.

Question 2

Cold packs, whose temperatures are lowered when ammonium nitrate dissolves in water, are carried by athletic trainers when transporting ice is not possible. Which of the following is true of this reaction?

A) . $\Delta$ H < 0, process is exothermic
B) . $\Delta$ H > 0, process is exothermic
C) . $\Delta$ H < 0, process is endothermic
D) . $\Delta$ H > 0, process is endothermic
E) . $\Delta$ H = 0, since cold packs are sealed

Answer 2

. $\Delta$ H > 0, process is endothermic

Question 3

A system which undergoes an adiabatic change and does work on the surroundings has

A) w < 0, $\Delta$ E = 0.
B) w > 0, $\Delta$ E > 0.
C) w > 0, $\Delta$ E < 0.
D) w < 0, $\Delta$ E > 0.
E) w < 0, $\Delta$ E < 0.

Answer 3

w < 0, $\Delta$ E < 0.

Question 4

A system that does no work but which receives heat from the surroundings has

A) q < 0, $\Delta$ E > 0.
B) q > 0, $\Delta$ E < 0.
C) q = $\Delta$ E.
D) q = - $\Delta$ E.
E) w = $\Delta$ E.

Question 5

A system delivers 1275 J of heat while the surroundings perform 855 J of work on it. Calculate $\Delta$ E in J.

A) -2130 J
B) -420 J
C) 420 J
D) 2130 J
E) -1275 J

Question 6

A system which undergoes an adiabatic change and has work done on it by the surroundings has

A) w = $\Delta$ E.
B) w = - $\Delta$ E.
C) w > 0, $\Delta$ E < 0.
D) w < 0, $\Delta$ E > 0.
E) w > $\Delta$ E.

Question 7

In which of the following processes is $\Delta$ H = $\Delta$ E ?

A) Two moles of ammonia gas are cooled from 325°C to 300°C at 1.2 atm.
B) One gram of water is vaporized at 100°C and 1 atm.
C) Two moles of hydrogen iodide gas react to form hydrogen gas and iodine gas in a 40-L container.
D) Calcium carbonate is heated to form calcium oxide and carbon dioxide in a container with variable volume.
E) One mole of solid carbon dioxide sublimes to the gas phase.

Question 8

A system initially has an internal energy E of 501 J. It undergoes a process during which it releases 111 J of heat energy to the surroundings, and does work of 222 J. What is the final energy of the system, in J?

A) 168 J
B) 390 J
C) 612 J
D) 834 J
E) cannot be calculated without more information

Question 9

A system delivers 225 J of heat to the surroundings while delivering 645 J of work. Calculate the change in the internal energy, $\Delta$ E, of the system.

A) -420 J
B) 420 J
C) -870 J
D) 870 J
E) -225 J

Question 10

Two solutions (the system), each of 25.0 mL volume and at 25.0°C, are mixed in a beaker. A reaction occurs between them, and the temperature rises to 35.0°C. After the products have equilibrated with the surroundings, the temperature is again 25.0°C and the total volume is 50.0 mL. No gases are involved in the reaction. Which one of the following relationships concerning the change from initial to final states (both at 25.0°C) is correct?

A) . $\Delta$ E = 0
B) . $\Delta$ H = 0
C) . $\Delta$ E > 0
D) q = 0
E) w = 0

Question 11

An ideal gas (the system) is contained in a flexible balloon at a pressure of 1 atm and is initially at a temperature of 20.°C. The surrounding air is at the same pressure, but its temperature is 25°C. When the system has equilibrated with its surroundings, both systems and surroundings are at 25°C and 1 atm. In changing from the initial to the final state, which one of the following relationships regarding the system is correct?

A) . $\Delta$ E < 0
B) . $\Delta$ E = 0
C) . $\Delta$ H = 0
D) w > 0
E) q > 0

Question 12

A system absorbs 21.6 kJ of heat while performing 6.9 kJ of work on the surroundings. If the initial internal energy, E, is 61.2 kJ, what is the final value of E?

A) 32.7 kJ
B) 46.5 kJ
C) 75.9 kJ
D) 82.8 kJ
E) 89.7 kJ

Question 13

The dissolution of barium hydroxide in water is an exothermic process. Which of the following statements is correct?

A) The enthalpy of solid barium hydroxide plus pure water is less than that of the solution, at the same temperature.
B) The enthalpy of solid barium hydroxide plus pure water is greater than that of the solution, at the same temperature.
C) The enthalpy of solid barium hydroxide plus pure water is the same as that of the solution, at the same temperature.
D) The temperature of the solution is lower than of the barium hydroxide and water before mixing.
E) When barium hydroxide dissolves in water, the system does work on the surroundings.

Question 14

Which one of the following relationships is always correct?

A) potential energy + kinetic energy = constant
B) E = q + w
C) . $\Delta$ E = $\Delta$ H - P $\Delta$ V
D) H = E + PV
E) . $\Delta$ H = q_v

Question 15

A system expands from a volume of 1.00 L to 2.00 L against a constant external pressure of 1.00 atm. The work (w) done by the system, in J, is

A) 1.00 J.
B) 2.00 J.
C) 1.01 × 10² J.
D) 1.01 × 10⁵ J.
E) none of the above.

Question 16

A system that does no work but which transfers heat to the surroundings has

A) q < 0, $\Delta$ E > 0.
B) q < 0, $\Delta$ E < 0.
C) q > 0, $\Delta$ E > 0.
D) q > 0, $\Delta$ E < 0.
E) q < 0, $\Delta$ E = 0.

Question 17

For which one of the following reactions will $\Delta$ H be approximately (or exactly) equal to $\Delta$ E?

A) H₂(g) + Br₂(g) $\to$ 2HBr(g)
B) H₂O(l) $\to$ H₂O(g)
C) CaCO₃(s) $\to$ CaO(s) + CO₂(g)
D) 2H(g) + O(g) $\to$ H₂O(l)
E) CH₄(g) + 2O₂(g) $\to$ CO₂(g) + 2H₂O(l)

Question 18

A system contracts from an initial volume of 15.0 L to a final volume of 10.0 L under a constant external pressure of 0.800 atm. The value of w, in J, is

A) -4.0 J.
B) 4.0 J.
C) -405 J.
D) 405 J.
E) 4.05 × 10³ J.

Question 19

A system receives 575 J of heat and delivers 425 J of work. Calculate the change in the internal energy, $\Delta$ E, of the system.

A) -150 J
B) 150 J
C) -1000 J
D) 1000 J
E) 575 J

Question 20

In which one of the following reactions would you expect $\Delta$ H to be substantially greater than $\Delta$ E (i.e., $\Delta$ H > $\Delta$ E)?

A) H₂(g) + Br₂(g) $\to$ 2HBr(g)
B) CO₂(s) $\to$ CO₂(g)
C) C₂H₂(g) + H₂(g) $\to$ C₂H₄(g)
D) H₂O(s) $\to$ H₂O(l)
E) HCl(aq) + NaOH(aq) $\to$ NaCl(aq) + H₂O(l)

Question 21

The specific heat capacity c of a metal is approximately related to its molar mass as follows: c × = 3R, where R is the universal gas constant, 8.314 J/mol.K. Use this relationship to identify the metal which has a specific heat capacity of 0.900 J/g.K.

A) Li
B) Sn
C) Ca
D) Al
E) U

Question 22

A Snickers^® candy bar contains 280 Calories, of which the fat content accounts for 120 Calories. What is the energy of the fat content, in kJ?

A) 5.0 × 10^-1 kJ
B) 29 kJ
C) 5.0 × 10² kJ
D) 1.2 × 10³ kJ
E) 5.0 × 10⁵ kJ

Question 23

Galena is the ore from which elemental lead is extracted. In the first step of the extraction process, galena is heated in air to form lead(II) oxide.
2PbS(s) + 3O₂(g) $\to$ 2SO₂(g) $\Delta$ H = -827.4 kJ
What mass of galena is converted to lead oxide if 975 kJ of heat are liberated?

A) 203 g
B) 282 g
C) 406 g
D) 478 g
E) 564 g

Question 24

A common laboratory reaction is the neutralization of an acid with a base. When 50.0 mL of 0.500 M HCl at 25.0°C is added to 50.0 mL of 0.500 M NaOH at 25.0°C in a coffee cup calorimeter, the temperature of the mixture rises to 28.2°C. What is the heat of reaction per mole of acid? Assume the mixture has a specific heat capacity of 4.18 J/(g·K) and that the densities of the reactant solutions are both 1.00 g/mL.

A) 670 J
B) 1300 J
C) 27 kJ
D) 54 kJ
E) > 100 kJ

Question 25

If, as a pioneer, you wished to warm your room by taking an object heated on top of a pot-bellied stove to it, which of the following 15-pound objects, each heated to 100°C, would be the best choice? The specific heat capacity (in J/(g·K)) for each substance is given in parentheses. Iron (0.450), copper (0.387), granite (0.79), gold (0.129), water (4.18).

A) iron
B) copper
C) granite
D) gold
E) water

Question 26

A 275-g sample of nickel at 100.0°C is placed in 100.0 mL of water at 22.0°C. What is the final temperature of the water? Assume that no heat is lost to or gained from the surroundings. Specific heat capacity of nickel = 0.444 J/(g·K)

A) 39.6°C
B) 40.8°C
C) 61.0°C
D) 79.2°C
E) 82.4°C

Question 27

When Karl Kaveman adds chilled grog to his new granite mug, he removes 10.9 kJ of energy from the mug. If it has a mass of 625 g and was at 25°C, what is its new temperature? Specific heat capacity of granite = 0.79 J/(g·K)

A) 3°C
B) 14°C
C) 22°C
D) 47°C
E) none of the above

Question 28

Your favorite candy bar, Gummy Beakers, contains 1.2 × 10⁶ J of energy while your favorite soft drink, Bolt, contains 6.7 × 10⁵ J. If you eat two packs of Gummy Beakers a day and drink 3 cans of Bolt, what percent of your 2000 Calorie daily food intake is left for broccoli, beans, beef, etc.?

A) 53%
B) 47%
C) 27%
D) 11%
E) 0%

Question 29

A backpacker collects snow at 0°C, and places it in a cooking pot on a camp stove. It takes 643 kJ of heat energy to melt the snow and bring the water to boiling. Assuming no heat loss, and neglecting the specific heat capacity of the pot, calculate the mass of snow that the backpacker collected. (Data: specific heat capacity of liquid water, c = 4.18 J/g.K; and: H₂O(s) $\to$ H₂O(l) $\Delta$ H = $\Delta$ H_fusion = 6.02 kJ/mol)

A) 1.92 kg
B) 1.90 kg
C) 1.52 kg
D) 855 g
E) < 800 g

Question 30

The Starship Enterprise is caught in a time warp and Spock is forced to use the primitive techniques of the 20th century to determine the specific heat capacity of an unknown mineral. The 307-g sample was heated to 98.7°C and placed into a calorimeter containing 72.4 g of water at 23.6°C. The heat capacity of the calorimeter was 15.7 J/K. The final temperature in the calorimeter was 32.4°C. What is the specific heat capacity of the mineral?

A) 0.124 J/(g·K)
B) 0.131 J/(g·K)
C) 0.138 J/(g·K)
D) 0.145 J/(g·K)
E) none of the above

Question 31

Calcium hydroxide, which reacts with carbon dioxide to form calcium carbonate, was used by the ancient Romans as mortar in stone structures. The reaction for this process is Ca(OH)₂(s) + CO₂(g) $\to$ CaCO₃(s) + H₂O(g) $\Delta$ H = -69.1 kJ
What is the enthalpy change if 3.8 mol of calcium carbonate is formed?

A) -18 kJ
B) -69 kJ
C) -73 kJ
D) -260 kJ
E) none of the above

Question 32

Benzene is a starting material in the synthesis of nylon fibers and polystyrene (styrofoam). Its specific heat capacity is 1.74 J/(g·K). If 16.7 kJ of energy is absorbed by a 225-g sample of benzene at 20.0°C, what is its final temperature?

A) -22.7°C
B) 36.7°C
C) 42.7°C
D) 62.7°C
E) none of the above

Question 33

Calculate q when 28.6 g of water is heated from 22.0°C to 78.3°C.

A) 0.385 kJ
B) 1.61 kJ
C) 6.74 kJ
D) 9.37 kJ
E) 1.61 × 10³ kJ

Question 34

The combustion of glucose (C₆H₁₂O₆) with oxygen gas produces carbon dioxide and water. This process releases 2803 kJ per mole of glucose. When 3.00 mol of oxygen react in this way with glucose, what is the energy release in kcal? (Hint: Write a balanced equation for the combustion process.)

A) 223.5 kcal
B) 335.3 kcal
C) 1402 kcal
D) 2012 kcal
E) 5858 kcal

Question 35

Natural gas, or methane, is an important fuel. Combustion of one mole of methane releases 802.3 kilojoules of energy. How much energy does that represent in kilocalories?

A) 1.92 × 10^-1 kcal
B) 1.92 × 10² kcal
C) 3.36 × 10³ kcal
D) 1.92 × 10⁵ kcal
E) 3.36 × 10⁶ kcal

Question 36

Ethylene glycol, used as a coolant in automotive engines, has a specific heat capacity of 2.42 J/(g·K). Calculate q when 3.65 kg of ethylene glycol is cooled from 132°C to 85°C.

A) -1900 kJ
B) -420 kJ
C) -99 kJ
D) -0.42 kJ
E) -4.2 × 10^-6 kJ

Question 37

A piece of copper metal is initially at 100.0°C. It is dropped into a coffee cup calorimeter containing 50.0 g of water at a temperature of 20.0°C. After stirring, the final temperature of both copper and water is 25.0°C. Assuming no heat losses, and that the specific heat (capacity) of water is 4.18 J/(g·K), what is the heat capacity of the copper in J/K?

A) 2.79 J/K
B) 3.33 J/K
C) 13.9 J/K
D) 209 J/K
E) none of the above

Question 38

What is the final temperature when 20.0 g of water at 25°C is mixed with 30.0 g of water at 80°C?

A) 35°C
B) 42°C
C) 53°C
D) 58°C
E) 70°C

Question 39

Which of the following is not a state function?

A) internal energy
B) volume
C) work
D) pressure
E) enthalpy

Question 40

Sand is converted to pure silicon in a three step process. The third step is
SiCl₄(g) + 2Mg(s) $\to$ 2MgCl₂(s) + Si(s) $\Delta$ H = -625.6 kJ
What is the enthalpy change when 25.0 mol of silicon tetrachloride is converted to elemental silicon?

A) -25.0 kJ
B) -7820 kJ
C) -1.56 × 10⁴ kJ
D) -3.13 × 10⁴ kJ
E) none of the above

Question 41

The compound carbon suboxide, C₃O₂, is a gas at room temperature. Use the data supplied to calculate the heat of formation of carbon suboxide.
(Data: 2CO(g) + C(s) $\to$ C₃O₂(g) $\Delta$ H° = 127.3 kJ/mol
And: $\Delta$ H_f^° of CO(g) = -110.5 kJ/mol)

A) 116.8
B) -93.7
C) 227.8
D) -348.3
E) 93.7

Question 42

Consider the equation $\Delta$ E = q + w
Explain fully the meaning of all three terms in the equation, and also the implied sign convention for q and w.

Question 43

Starting from equations relating pressure to force and force to work, derive the relationship w = - P $\Delta$ V, explaining the steps in your argument.

Question 44

Calculate the $\Delta$ H°_rxn for the following reaction. ( $\Delta$ H°_f [AsH₃(g)] = 66.4 kJ/mol; $\Delta$ H°_f [H₃AsO₄(aq)] = -904.6 kJ/mol; $\Delta$ H°_f [H₂O(l)] = -285.8 kJ/mol)
H₃AsO₄(aq) + 4H₂(g) $\to$ AsH₃(g) + 4H₂O(l)

A) -1981.4 kJ
B) -685.2 kJ
C) -172.2 kJ
D) 172.2 kJ
E) 685.2 kJ

Question 45

Which one of the following statements about standard states is incorrect?

A) The standard state of a solid compound is the pure solid.
B) The standard state of a liquid compound is the pure liquid.
C) The standard state of a gaseous compound is the gas at a pressure of 1 atmosphere.
D) The standard state of an aqueous solute is a saturated solution in water.
E) The standard state of an element is the form in which it is stable at 1 atm and a specified temperature, usually 25°C.

Question 46

Which one of the following equations represents the formation reaction of CH₃OH(l)?

A) C(g) + 2H₂(g) + O₂ (g) $\to$ CH₃OH(l)
B) C(g) + 4H(g) + O(g) $\to$ CH₃OH(l)
C) C(graphite) + 4H(g) + O(g) $\to$ CH₃OH(l)
D) C(diamond) + 4H(g) + O(g) $\to$ CH₃OH(l)
E) C(graphite) + 2H₂(g) + O₂(g) $\to$ CH₃OH(l)

Question 47

Ethanol, C₂H₅OH, is being promoted as a clean fuel and is used as an additive in many gasoline mixtures. Calculate the $\Delta$ H°_rxn for the combustion of ethanol.
( $\Delta$ H°_f [C₂H₅OH(l)] = -277.7 kJ/mol; $\Delta$ H°_f [CO₂(g)] = -393.5 kJ/mol; $\Delta$ H°_f [H₂O(g)] = -241.8 kJ/mol)

A) -1234.7 kJ
B) -751.1 kJ
C) -357.6 kJ
D) 357.6 kJ
E) 1234.7 kJ

Question 48

a. A gas sample absorbs 53 kJ of heat and does 18 kJ of work. Calculate the change in its internal energy.
b. A system expands against a constant pressure of 1.50 atm, from an initial volume of 1.00 L to a final volume of 10.0 L. Calculate the work (w) involved in this process, in kJ.

Question 49

Use the following data to calculate the standard heat (enthalpy) of formation, $\Delta$ H°_f , of manganese(IV) oxide, MnO₂ (s).
2MnO₂(s) $\to$ 2MnO(s) + O₂(g) $\Delta$ H = 264 kJ
MnO₂(s) + Mn(s) $\to$ 2MnO(s) $\Delta$ H = -240 kJ

A) -504 kJ
B) -372 kJ
C) -24 kJ
D) 24 kJ
E) 504 kJ

Question 50

An important step in the synthesis of nitric acid is the conversion of ammonia to nitric oxide. 4NH₃(g) + 5O₂(g) $\to$ 4NO(g) + 6H₂O(g)
Calculate $\Delta$ H°_rxn for this reaction.
$\Delta$ H°_f [NH₃(g)] = -45.9 kJ/mol; $\Delta$ H°_f [NO(g)] = 90.3 kJ/mol; $\Delta$ H°_f [H₂O(g)] = -241.8 kJ/mol

A) -906.0 kJ
B) -197.4 kJ
C) -105.6 kJ
D) 197.4 kJ
E) 906.0 kJ

Question 51

Calculate the $\Delta$ H°_rxn for the following reaction. ( $\Delta$ H°_f [SiO₂(s)] = -910.9 kJ/mol; $\Delta$ H°_f [SiCl₄(g)] = -657.0 kJ/mol; $\Delta$ H°_f [HCl(g)] = -92.3 kJ/mol; $\Delta$ H°_f [H₂O (g)] = -241.8 kJ/mol) SiO₂(s) + 4HCl(g) $\to$ SiCl₄(g) + 2H₂O(g)

A) -139.5 kJ
B) -137.4 kJ
C) -104.4 kJ
D) 104.4 kJ
E) 139.5 kJ

Question 52

Use Hess's Law to calculate the enthalpy change for the reaction
WO₃(s) + 3H₂(g) $\to$ W(s) + 3H₂O(g)
From the following data:
2W(s) + 3O₂(g) $\to$ 2WO₃(s) $\Delta$ H = -1685.4 kJ
2H₂(g) + O₂(g) $\to$ 2H₂O(g) $\Delta$ H = -477.84 kJ

A) 125.9 kJ
B) 252.9 kJ
C) 364.9 kJ
D) 1207.6 kJ
E) none of the above

Question 53

Calculate, in J, the work done by 10.0 g of CO₂ when it sublimes against a pressure of 1.00 atm to form gaseous CO₂ at 0.0°C. The volume of CO₂(s) can be neglected; CO₂(g) can be assumed to behave ideally. The process occurring is
CO₂(s) $\to$ CO₂(g)

Question 54

Calculate the enthalpy change for the reaction NO(g) + O(g) $\to$ NO₂(g)
From the following data:
NO(g) + O₃(g) $\to$ NO₂(g) + O₂(g) $\Delta$ H = -198.9 kJ
O₃(g) $\to$ 1.5O₂(g) $\Delta$ H = -142.3 kJ
O₂(g) $\to$ 2O(g) $\Delta$ H = 495.0 kJ

A) -551.6 kJ
B) -304.1 kJ
C) 190.9 kJ
D) 153.8 kJ
E) 438.4 kJ

Question 55

Which one of the following is a correct formation reaction?

A) C(diamond) $\to$ C(graphite)
B) H₂(g) + O(g) $\to$ H₂O(l)
C) C(graphite) + 4H(g) $\to$ CH₄(g)
D) 6C(graphite) + 6H₂O(s) $\to$ C₆H₁₂O₆(s)
E) 2C(graphite) + 3H₂(g) + O₂(g) $\to$ C₂H₅OH(l)

Question 56

Stoichiometric amounts of nitrogen gas and hydrogen gas react in a calorimeter to produce 5.00 g of ammonia gas. The calorimeter temperature rises 0.42°C. The calorimeter and water have a combined heat capacity of 32.16 kJ/K. Calculate the heat of formation of ammonia, $\Delta$ H_f^°, in kJ/mol. The formation reaction for ammonia is: 0.5N₂(g) + 1.5H₂(g) $\to$ NH₃(g).

A) -46 kJ/mol
B) -13.5 kJ/mol
C) -3.97 kJ/mol
D) 3.97 kJ/mol
E) 13.5 kJ/mol

Question 57

The highly exothermic thermite reaction, in which aluminum reduces iron(III) oxide to elemental iron, has been used by railroad repair crews to weld rails together.
2Al(s) + Fe₂O₃(s) $\to$ 2Fe(s) + Al₂O₃(s) $\Delta$ H = -850 kJ
What mass of iron is formed when 725 kJ of heat are released?

A) 47 g
B) 65 g
C) 95 g
D) 112 g
E) 130 g

Question 58

Nitric acid, which is among the top 15 chemicals produced in the United States, was first prepared over 1200 years ago by heating naturally occurring sodium nitrate (called saltpeter) with sulfuric acid and collecting the vapors produced. Calculate $\Delta$ H°_rxn for this reaction. $\Delta$ H°_f [NaNO₃(s)] = -467.8 kJ/mol; $\Delta$ H°_f [NaHSO₄(s)] = -1125.5 kJ/mol; $\Delta$ H°_f [H₂SO₄(l) = -814.0 kJ/mol; $\Delta$ H°_f [HNO₃(g)] = -135.1 kJ/mol
NaNO₃(s) + H₂SO₄(l) $\to$ NaHSO₄(s) + HNO₃(g)

A) -644.2 kJ
B) -291.4 kJ
C) -21.2 kJ
D) 21.2 kJ
E) 644.2 kJ

Question 59

Calculate the $\Delta$ H°_rxn for the decomposition of calcium carbonate to calcium oxide and carbon dioxide. $\Delta$ H°_f [CaCO₃(s)] = -1206.9 kJ/mol; $\Delta$ H°_f [CaO(s)] = -635.1 kJ/mol; $\Delta$ H°_f [CO₂(g)] = -393.5 kJ/mol
CaCO₃(s) $\to$ CaO(s) + CO₂(g)

A) -2235.5 kJ
B) -1448.5 kJ
C) -178.3 kJ
D) 178.3 kJ
E) 2235.5 kJ

Question 60

Which one of the following is not a correct formation reaction? (products are correct)

A) H₂(g) + O(g) $\to$ H₂O(l)
B) H₂(g) + Cl₂(g) $\to$ HCl(g)
C) 6C(graphite) + 3H₂(g) $\to$ C₆H₆(l)
D) C(graphite) $\to$ C(diamond)
E) 6C(graphite) + 6H₂(g) + 3O₂(g) $\to$ C₆H₁₂O₆(s)

Question 61

For a reaction in a sealed, rigid container, $\Delta$ H is always greater than $\Delta$ E.

Question 62

For all processes, both q and w will have the same sign.

Question 63

The only way in which a system can do work on the surroundings is by expansion against the external pressure.

Question 64

Clearly state the thermodynamic standard state of
a. an element or compound.
b. a solute.

Question 65

Although internal energy (E) is more fundamental and conceptually easier than enthalpy (H), in most chemical applications $\Delta$ H is more relevant and useful than $\Delta$ E. Why?

Question 66

Diborane (B₂H₆) has been considered as a possible rocket fuel. Calculate $\Delta$ H° for the reaction
B₂H₆(g) $\to$ 2B(s) + 3H₂(g)
using the following data: O₂(g) $\to$ H₂O(g)
-242

Question 67

The enthalpy (H) of liquid water is greater than that of the same quantity of ice at the same temperature.

Question 68

The reaction
2NaOH(aq) + H₂SO₄(aq) $\to$ Na₂SO₄(aq) + 2H₂O(l)
was studied in a coffee cup calorimeter. 100. mL portions of 1.00 M aqueous NaOH and H₂SO₄, each at 24.0°C, were mixed. The maximum temperature achieved was 30.6°C. Neglect the heat capacity of the cup and the thermometer, and assume that the solution of products has a density of exactly 1 g/mL and a specific heat capacity of 4.18 J/(g·K).
a. Calculate the heat of reaction, q, in J.
b. Calculate $\Delta$ H, the heat (enthalpy) of reaction, in kJ/mol of Na₂SO₄ produced.

Question 69

A mass of 1.250 g of benzoic acid (C₇H₆O₂) was completely combusted in a bomb calorimeter. If the heat capacity of the calorimeter was 10.134 kJ/K and the heat of combustion of benzoic acid is -3226 kJ/mol, calculate (to three decimal places) the temperature increase that should have occurred in the apparatus.

Question 70

a. Starting from the equation H = E + PV, show how the relationship $\Delta$ H = q_p is derived. Clearly indicate any necessary assumptions or conditions.
b. In one sentence, state in full what is meant by the equation: $\Delta$ H = q_p.

Question 71

a. State Hess's Law.
b. Use the $\Delta$ H° data given below to calculate $\Delta$ H° for the reaction:

Question 72

$\Delta$ H does not depend on the path of a reaction, but $\Delta$ E does.

Question 73

In a reaction with high energy reactants and low energy products, q is negative.

Question 74

Different chemical bonds have different potential energies.

Question 75

Given the following data: calculate $\Delta$ H° for the reaction
NO(g) + O(g) $\to$ NO₂(g)

Question 76

a. Explain fully what is meant by the term "state function."
b. (i) Give two examples of thermodynamic quantities which state functions.
(ii) Give two examples of thermodynamic quantities which are not state functions.

Question 77

For all processes, both q and $\Delta$ E will have the same sign.

Question 78

a. Write a balanced equation for the combustion of benzene, C₆H₆(l) in oxygen.
b. The standard heat of combustion of benzene is -3271 kJ/mol. Calculate its standard heat of formation, $\Delta$ H°_f , given the data:
$\Delta$ H°_f [CO₂(g)] = -394 kJ; $\Delta$ H°_f [H₂O(l)] = -286 kJ

Question 79

When 1.00 g of solid NH₄Cl is dissolved in 25.00 g of water contained in a coffee cup calorimeter, both reagents initially being at 25.0°C, the temperature falls to 22.4°C. Assuming that the heat capacity of the ammonium chloride solution is 4.18 J/(g·K), calculate the heat (enthalpy) of solution of NH₄Cl, (a) in J/g and (b) in kJ/mol.

Question 80

a. Define, or explain fully what is meant by the standard enthalpy of formation of a substance, $\Delta$ H°_f..
b. What is the standard state of the element oxygen?
c. Write down in full the formation reaction for liquid ethanol, C₂H₅OH(l). The equation should be balanced and should indicate the physical state of each substance.