Chapter Six
Thermochemistry
Question
Label the following process as exothermic
or endothermic:
The freezing of water.
a) Exothermic
b) Endothermic
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Answer
a) Exothermic
For water to freeze, heat must be
released from the water to the
surroundings.
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Question
Label the following process as exothermic
or endothermic:
Your hand gets cold when you touch ice.
a) Exothermic
b) Endothermic
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Answer
a) Exothermic
For your hand to get cold, heat must be
transferred from your hand to the ice.
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Question
Label the following process as exothermic
or endothermic:
The ice increases in temperature when
you touch it.
a) Exothermic
b) Endothermic
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Answer
b) Endothermic
Heat is transferred from the hand
(surroundings) to the ice (system).
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Question
Label the following process as exothermic
or endothermic:
Water vapor condenses on a cold pipe.
a) Exothermic
b) Endothermic
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Answer
a) Exothermic
For water vapor to condense, heat must
be transferred from the water vapor to the
cold pipe.
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Question
Which of the following statements is true?
a) Change in enthalpy is a state function.
b) In exothermic reactions, the reactants are lower
in potential energy than the products.
c) A chemist takes the point of view of the
surroundings when determining the sign for
heat, q.
d) The heat of reaction and change in enthalpy
can always be used interchangeably.
e) More than one of the above statements is true.
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Answer
a) Change in enthalpy is a state function.
Change in enthalpy does not depend on
the pathway, making it a state function.
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Question
Given the equation:
S (s) + O2 (g)  SO2 (g),
H = –296 kJ,
which of the following statement(s) is (are) true?
I.
II.
III.
a)
b)
c)
d)
e)
The reaction is exothermic.
When 0.500 mol sulfur is reacted,
148 kJ of energy is released.
When 32.0 g of sulfur is burned,
2.96  105 J of energy is released.
All are true.
None is true.
I and II are true.
I and III are true.
Only II is true.
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Answer
a) All are true.
A reaction with a negative change in enthalpy is
exothermic.
Based on the stoichiometry, if 0.500 mol of
sulfur was consumed, H = (½)(–296 kJ). If
1.00 mol (32.0 g) of sulfur is burned, 296 kJ of
energy is released.
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Question
Two different metals of equal mass with
different specific heat capacities absorb the
same amount of heat. Which undergoes the
smallest change in temperature?
a) The metal with the higher specific heat capacity
b) The metal with the lower specific heat capacity
c) Because they have equal mass, both metals
undergo the same change in temperature.
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Answer
a) The metal with the higher specific heat
capacity
Since each metal absorbs the same amount of heat:
qmetal1 = qmetal2. Since the mass of each metal is the
same,
(specific heat capacitymetal1)(tmetal1) =
= (specific heat capacitymetal2)(tmetal2),
and since the heat capacity and the change in
temperature are inversely related, for a smaller change
in temperature, the heat capacity would have to be
higher.
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Question
If 5.0 kJ of energy is added to a 15.5-g
sample of water at 10.°C, the water is
a)
b)
c)
d)
boiling.
completely vaporized.
frozen solid.
still a liquid.
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Answer
d) still a liquid.
In order to decide on an answer, use the
equation q = s × m × Δt (where Δt = tf − ti) so

J 
5.0  10 J =  4.18
 (15.5 g )(tf  10.°C).


g
·°C


3
Since tf is 87°C, the water is still a liquid.
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Question
A 50.0-g sample of water at 80°C is
added to a 50.0-g sample of water at
20°C. The final temperature of the water
should be
a) between 20°C and 50°C.
b) 50°C.
c) between 50°C and 80°C.
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Answer
b) 50°C.
In both samples, mass and specific heat
capacity are the same. So, the final
temperature should be the average of the
two temperatures.
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Question
A 50.0-g sample of water at 80°C is
added to a 100.0-g sample of water at
20°C. The final temperature of the water
should be
a) between 20°C and 50°C.
b) 50°C.
c) between 50°C and 80°C.
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Answer
a) between 20°C and 50°C.
The mass of the cooler water is greater,
so the final temperature should be lower
than the average of the two temperatures.
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Question
You have a 50.0-g sample of water at
20°C and add a 50.0-g sample of iron at
80°C. The final temperature of the water
should be
a) between 20°C and 50°C.
b) 50°C.
c) between 50°C and 80°C.
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Answer
a) between 20°C and 50°C.
Iron has a lower specific heat capacity than
water (and the masses are the same).
Therefore, the change in temperature of the
water will be less than that of the iron. So, the
final temperature will be less than the average
temperature of the iron and the water.
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Question
Hypothetical elements A2 and B2 react according to the
following reaction, forming the compound AB:
A2 (aq) + B2 (aq)  AB (aq) H° = −265 kJ/mol
If solutions A2 (aq) and B2 (aq), starting at the same
temperature, are mixed in a coffee-cup calorimeter, the
reaction that occurs is
a) exothermic, and the temperature of the
resulting solution rises.
b) endothermic, and the temperature of the
resulting solution rises.
c) exothermic, and the temperature of the
resulting solution falls.
d) endothermic, and the temperature of the
resulting solution falls.
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Answer
a) exothermic, and the temperature of the
resulting solution rises.
Since the H° is negative, the reaction is
exothermic. Exothermic reactions release heat
to their surroundings (the water), and so the
temperature of the solution rises.
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Question
Given the following reaction:
H° = −566.0 kJ
Calculate the enthalpy change for the reaction:
2CO (g) + O2 (g)  2CO2 (g)
CO (g) + ½ O2 (g)  CO2 (g).
a) −566.8 kJ
b) −283.0 kJ
c) 283.0 kJ
d) 566.0 kJ
e) −1132 kJ
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Answer
b) −283.0 kJ
Since each of the stoichiometric coefficients in the
reaction is cut in half, the enthalphy change is also cut
in half.
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Question
Given the following reaction:
H° = −566.0 kJ
Calculate the enthalpy change for the reaction:
2CO (g) + O2 (g)  2CO2 (g)
2CO2 (g)  2CO (g) + O2 (g).
a) −566.8 kJ
b) −283.0 kJ
c) 283.0 kJ
d) 566.0 kJ
e) −1132 kJ
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Answer
d) 566.0 kJ
Since the reaction is reversed, the sign of the
enthalphy change is (−1) times the original reaction.
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Question
Use the following equations:
C3H8 (g) + 5O2 (g)  3CO2 (g) + 4H2O (l)
Hrxn 1 = −2219.9 kJ
Hrxn 2 = −283.0 kJ
to calculate the enthalpy change for the reaction:
CO (g) + ½ O2 (g)  CO2 (g)
C3H8 (g) +
7
2
O2 (g)  3CO (g) + 4H2O (l)
Hrxn 3= ?
a) −1936.9 kJ
b) −1370.9 kJ
c) 1370.9 kJ
d) −3068.9 kJ
e) 3068.9 kJ
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Answer
b) −1370.9 kJ
7
2
C3H8 (g) + 5O2 (g)  3CO2 (g) + 4H2O (l) H = −2219.9 kJ
3CO2 (g)  3CO (g) + 3/2O2 (g)
H = −3(−283.0 kJ)
C3H8 (g) + 7 2O2 (g)  3CO (g) + 4H2O (l) H = −1370.9 kJ
Since C3H8 is a reactant in the first reaction and the desired
reaction, the reaction can be used as is. Since 3 moles of CO
are required as a product in the desired reaction and the
second reaction has only 1 mole of CO as a reactant, the
reaction must be reversed and multiplied by 3. Thus, the H =
− 3 Hrxn 2. Adding the reactions cancels CO2 and excess O2.
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Question
The standard enthalpy of formation of sulfuric
acid, H2SO4 (l), refers to the H° of which of the
following reactions?
a)
b)
c)
d)
e)
H2SO4 (l)  H2 (g) + S (s) + 2O2 (g)
H2 (g) + S (g) + 2O2 (g)  H2SO4 (l)
H2SO4(l)  2H (g) + S (s) + 4O (g)
H2 (g) + S (s) + 2O2 (g)  H2SO4 (l)
2H (g) + S (g) + 4O (g)  H2SO4 (l)
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Answer
d)
H2 (g) + S (s) + 2O2 (g)  H2SO4 (l)
The standard enthalpy of formation of a
substance is the H° of the reaction where 1
mol of the compound is formed from its
elements in their reference form and in their
standard states. For sulfuric acid, H2SO4 (l), the
elements are H2 (g), S (s), and O2 (g).
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Question
Calculate Hrxn for the combustion reaction of ethanol,
C2H5OH, shown below given the following data:
Hf C2H5OH (l) = −276.98 kJ/mol
Hf H2O (l) = −285.5 kJ/mol
Hf H2O (g) = −241.8 kJ/mol
Hf CO2 (g) = −393.5 kJ/mol
C2H5OH (l) + 3O2 (g)  2CO2 (g) + 3H2O (l).
a) 1356.5 kJ
b) 993.6 kJ
c) −1356.5 kJ
d) −993.6 kJ
e) −1197.8 kJ
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Answer
c) −1356.5 kJ
Use Hess’s Law to calculate the enthalpy change:
Hrxn   n H f (products)   m H f (reactants)
 [2( H f CO2 ( g ))  3( Hf H2O (l ))] 
 [1( H f C2H5OH (l ))  3( H f O2 ( g ))]
 [2( 393.5 kJ)  3( 285.5 kJ)]  [1( 276.98 kJ)  3(0.0 kJ)]
 [ 1643.5 kJ]  [ 276.98 kJ]
 1356.5 kJ
Be sure to choose the proper phase for each species
and the proper stoichiometric coefficient. Combustion
reactions are exothermic.
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