AnswersTX-Chemistry B-CRUnit Test

Thermochemical Equations Answers

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The enthalpy of formation for C6 H6 (I) is 49.0 kJ/mol. Consider the following reaction.Is the reaction endothermic or exothermic, and what is the enthalpy of reaction? Use .

Question illustration
A
exothermic; Hrxn = 49.0 kJ
Option A
B
exothermic; Hrxn = –49.0 kJ
Option B
C
endothermic; Hrxn = 49.0 kJ
Option C
D
endothermic; Hrxn = –49.0 kJ
Option D
2
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Consider the reaction.Standard Enthalpies of FormationSubstanceDHf (kJ/mol)C2 H2 (g)226.73CaCO3 (s)–1206.92CaO (s)–635.09CO (g)–110.525CO2 (g)–393.509H 2 O (I)–285.8H 2 O (g)–241.818C (s), diamond1.895C (s), graphite0.0Based on the equation and the information in the table, what is the enthalpy of the reaction? Use .

Question illustration
A
–453.46 kJ
B
–226.73 kJ
C
226.73 kJ
D
453.46 kJ
3

Butane C4 H10 (g),(Hf = –125.6), combusts in the presence of oxygen to form CO2 (g) (Hf = –393.5 kJ/mol), and H2 O(g) (Hf = –241.82) in the reaction: What is the enthalpy of combustion, per mole, of butane? Use .

Question illustration
A
–5,315 kJ/mol
B
–2,657.5 kJ/mol
C
2,657.4 kJ/mol
D
5,314.8 kJ/mol
4

Which statement is true for most chemical reactions?

A
An energy change occurs during the breaking and forming of bonds.
B
The internal energy of the system increases during a reaction.
C
Energy is released during the formation of reactants.
D
The enthalpy of the products is higher than the enthalpy of the reactants.
5

Manganese dioxide (MnO2(s), Hf = –520.0 kJ) reacts with aluminum to form aluminum oxide (AI2O3(s), Hf = –1699.8 kJ/mol) and manganese according to the equation below.What is the enthalpy of the reaction? Use .

Question illustration
A
–1,839.6 kJ
B
–1,179.8 kJ
C
1,179.8 kJ
D
1,839.6 kJ
6

Hydrogen ( H2 ) reacts with bromine ( Br2 ) to produce hydrogen bromide (HBr) gas as shown in the equation below.Standard Enthalpies of FormationSubstanceDHf (kJ/mol)Br2 (l)0.0Br2 (g)30.907HBr (g)–36.4HCl (g)–92.307HI (g)26.48H2 (g)0.0I2 (g)62.438Based on the equation and the information in the table, what is the enthalpy of the reaction?

Question illustration
A
–134.6 kJ
B
–103.7 kJ
C
103.7 kJ
D
134.6 kJ
7

Which statement is true if the Hrxn of a reaction is positive?

Question illustration
A
The reaction is endothermic.
B
The reaction is exothermic.
C
There is more reactant than product.
D
There is more product than reactant.
8

Which statement about Hf is true?

Question illustration
A
It is positive when the bonds of the product store more energy than those of the reactants.
B
It is zero for any pure substance in its standard state.
C
It is negative when a compound forms from elements in their standard states.
D
It is zero for any compound that is in the liquid state.
9

The Hrxn of formation of carbon dioxide is negative. Which statement is true?

Question illustration
A
The reaction is endothermic.
B
The reaction is exothermic.
C
The amount of carbon dioxide is more than the amount of reactants.
D
The amount of carbon dioxide is less than the amount of reactants.
10

Which graph represents an exothermic reaction?

A
A graph has Reaction progression on the horizontal axis and energy on the vertical axis. A line starts about halfway up the vertical axis and travels a short distance parallel to the horizontal axis; this segment is A plus B. The line then curves up sharply, peaks, and falls sharply to run parallel to and near the horizontal axis; the final segment is C. A double-headed arrow extends from A plus B to C.
Option A
B
A graph has Reaction progression on the horizontal axis and energy on the vertical axis. A line segment labeled A plus B starts low and runs parallel to the horizontal axis. The line then curves up sharply, peaks, and falls a short distance to run parallel to the horizontal axis; the final segment is labeled C. A double-headed arrow extends from A plus B to C.
Option B
C
A graph has Reaction progression on the horizontal axis and energy on the vertical axis. A line segment labeled A plus B starts halfway up the vertical axis and runs parallel to the horizontal axis for a short distance. The line then peaks and returns to the same level as A plus B and continues parallel to the horizontal axis; this segment is labeled C.
Option C
D
A graph has Reaction progression on the horizontal axis and energy on the vertical axis. A line segment labeled A plus B starts low on the vertical axis and runs parallel to the horizontal axis for a short distance. The line then rises sharply and levels off; this level section is labeled C. A double-headed arrow extends from the level of A plus B to that of C.
Option D
11

Ammonia (NH3 (g), Hf = –45.9 kJ/mol ) reacts with oxygen to produce nitrogen and water (H2 O(g), Hf = –241.8 kJ/mol) according to the equation below.What is the enthalpy of the reaction? Use .

Question illustration
A
–3,572.1 kJ
B
–1,267.2 kJ
C
1,267.2 kJ
D
3,572.1 kJ
12

The enthalpies of formation of the compounds in the combustion of methane, ,are CH4 (g): Hf = –74.6 kJ/mol; CO2 (g): Hf = –393.5 kJ/mol; and H2 O(g): Hf = –241.82 kJ/mol.How much heat is released by the combustion of 2 mol of methane? Use .

Question illustration
A
80.3 kJ
Option A
B
802.5 kJ
Option B
C
1,605.1 kJ
Option C
D
6,420.3 kJ
Option D
13

Nitrogen reacts with hydrogen to form ammonia according to the equation below.Standard Enthalpies of FormationSubstanceDHf (kJ/mol)C2 H2 (g)–26.7NH3 (g)–46.19HBr (g)236.23HCl (g)–92.30HF (g)–268.6HI (g)25.9NaCl (s)–411.0Based on the equation and the information in the table, what is the enthalpy of the reaction?

Question illustration
A
Hrxn = 46.19 kJ
Option A
B
Hrxn = –46.19 kJ
Option B
C
Hrxn = 92.38 kJ
Option C
D
Hrxn = –92.38 kJ
Option D
14

Hydrogen reacts with chlorine to form hydrogen chloride (HCl (g), Hf = –92.3 kJ/mol) according to the reaction below.Which statement is correct? Use .

Question illustration
A
The enthalpy of the reaction is –184.6 kJ, and the reaction is exothermic.
B
The enthalpy of the reaction is –184.6 kJ, and the reaction is endothermic.
C
The enthalpy of the reaction is 184.6 kJ, and the reaction is endothermic.
D
The enthalpy of the reaction is 184.6 kJ, and the reaction is exothermic.

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