Thermochemistry Diploma Questions -...

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Thermochemistry Diploma Questions

Sources of Energy

1. Reactions producing carbon dioxide cause concern among environmentalists because

CO2(g) is

a) a poisonous gas

b) a major greenhouse gas

c) a major contributor to acid rain

d) an important component of combustion

2. During photosynthesis, energy that originated from the i is converted to

ii energy and stored in chemical bonds.

The statement above is completed by the information in row

Row i ii

a) sun light

b) sun chemical

c) plant light

d) plant chemical

Use the following information to answer the next question.

Processes

1) Combustion 2) Photosynthesis 3) Bacterial decay 4) Energy from the sun

3. From the formation of methane to its eventual use as automobile fuel, the order of the

processes listed above is _____, _____, _____, and ______.

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Definition of Formation and Decomposition

4. Write the thermochemical equation for the formation of following compounds.

a) NaOH(s)

b) C2H6(g)

c) CaO(s)

5. Write the thermochemical equations for the decomposition of the compounds into their

standard state elements. Put them in order of thermal stability.

a) hydrogen iodide

b) glucose

c) ethanol

d) dinitrogen tetroxide

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Na(s) + ½ H2(g) + C(s) + 3

2 O2(g) → NaHCO3(s) + 947.7 kJ

6. In the reaction represented by the equation above, energy is i the

surroundings because the bonds in the products contain ii energy than the

reactants.


Row i ii

a) absorbed from more potential

b) absorbed from more kinetic

c) released to less potential

d) released to less kinetic

7. When one mole of sodium bicarbonate is formed from its elements, 947.7 kJ of heat

energy is released into the surroundings. This enthalpy change can be represented as

a) Na(s) + ½ H2(g) + C(s) + 3

2 O2(g) → NaHCO3(s) + 947.7 kJ

b) Na(s) + ½ H2(g) + C(s) + 3

2 O2(g) + 947.7 kJ → NaHCO3(s)

c) Na+(aq) + HCO3

- (aq) → NaHCO3(s) + 947.7 kJ

d) Na+(aq) + HCO3

-(aq) + 947.7 kJ → NaHCO3(s)

8. Which of the following equations represents the balanced chemical equation for the

decomposition of water and the energy transfer during the reaction?

a) H2(g) + ½ O2(g) → H2O(l) + 285.8 kJ

b) H2O(l) → H2(g) + 1

2 O2(g) + 285.8 kJ

c) 2 H2O(l) + 285.8 kJ → 2 H2(g) + O2(g)

d) H2O(l) + 285.8 kJ → H2(g) + 1

2 O2(g)

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The Concept of Enthalpy

9. Which statements about exothermic reactions are correct?

I. They have negative ∆H values.

II. The products have a lower enthalpy than the reactants.

III. The products are more energetically stable than the reactants.

a) I and II only

b) I and III only

c) II and III only

d) I, II and III

Reaction Equations

1. H2(g) + ½ O2(g) → H2O(l)

2. CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)

3. H12 + H1

2 → He24 (*This is a very high energy nuclear reaction, no longer in the curriculum)

4. CH3OH(l) + 3

2 O2(g) → CO2(g) + 2 H2O(l)

10. Numerical Response

For the reactions represented above, when their enthalpy changes are ranked from largest

to smallest according to magnitude, the order of the equations is _____, ______, ______

and .

(Record all four digits of your answer in the numerical-response section on the answer

sheet.)

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11. Dr. Richard Trotter has developed what could be the first cost-effective process for

limiting methane emissions from underground coal mines. In this process, methane and

oxygen are reacted at 800 °C in the presence of a catalyst. The products of this process

are carbon dioxide gas and liquid water.

Which of the following potential energy diagrams represents both the catalyzed (---) and

uncatalyzed reactions (—) for this process?

a)

b)

c)

d)

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Combustion of Methane Gas

Reaction progress

12. According to the diagram above, the enthalpy change for the combustion of 1.00 mol of

methane gas is

a) endothermic and represented by a negative ∆H value

b) endothermic and represented by a positive ∆H value

c) exothermic and represented by a negative ∆H value

d) exothermic and represented by a positive ∆H value

13. Toluene is reacted with nitric acid to produce TNT (trinitrotoluene), a powerful

explosive. The reaction requires H2SO4(aq) to act as a catalyst. Without H2SO4(aq) the

reaction would

a) react faster

b) react slower

c) produce more TNT

d) produce less TNT

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14. The energy barrier that must be overcome for the forward reaction to occur is called

the i , and in the diagram above, it is the difference between ii .


Row i ii

a) enthalpy change I and II

b) enthalpy change I and III

c) activation energy I and II

d) activation energy I and III

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15. The energy changes that occur when propane undergoes combustion are primarily due

to changes in i energy resulting from ii .


Row i ii

a) potential a rearrangement of bonds

b) potential an increase in molecular motion

c) kinetic a rearrangement of bonds

d) kinetic an increase in molecular motion

16. What energy changes occur when chemical bonds are formed and broken?

a) Energy is absorbed when bonds are formed and absorbed when they are broken.

b) Energy is released when bonds are formed and released when they are broken.

c) Energy is absorbed when bonds are formed and released when they are broken.

d) Energy is released when bonds are formed and absorbed when they are broken.


17. The decomposition of hot potassium chlorate to solid potassium chloride and oxygen

gas occurs at a faster rate in the presence of solid manganese (IV) oxide, as represented

by the following equation.

2 KClO3(s)MnO2(s) catalyst→ 2 KCl(s) + 3 O2(g)

When manganese (IV) oxide is used as a catalyst, the enthalpy change for the

decomposition of potassium chlorate

a) stays the same

b) becomes more exothermic

c) becomes more endothermic

d) changes from exothermic to endothermic

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18. A student performed a lab in which a sample of ice was warmed at a constant rate. She

produced the following graph from her results.

Explain what is happening in each region of the graph in terms of kinetic and potential energy.

A-B

B-C

C-D

D-E

E-F

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19. Which of the following potential energy diagrams represent the reaction that occurs at

the head of a match?

a) b)

c) d)

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Hess’s Law

20. Using the equations below:

C(s) + O2(g) → CO2(g) ∆H° = −390 kJ

Mn(s) + O2(g) → MnO2(s) ∆H° = −520 kJ

What is ∆H° (in kJ) for the following reaction?

MnO2(s) + C(s) → Mn(s) + CO2(g)

a) 910 kJ

b) 130 kJ

c) -130 kJ

d) -910 kJ

21. What is ∆H° for the following reaction in kJ?

CS2(g) + 3 O2(g) → CO2(g) + 2 SO2(g)

Compound ∆H°f / kJ mol-1

CS2(g) 110

CO2(g) -390

SO2(g) -290

a) -570

b) -790

c) -860

d) -1080

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22. Use the following information to answer the next question

C3H6(g) +9

2 O2(g) → 3 CO2(g) + 3 H2O(l) ∆H° = −1959.2 kJ

C(s) + O2(g) → CO2(g) ∆H° = −393.5 kJ

2 H2(g) + O2(g) → 2 H2O(l) ∆H° = −571.6 kJ

The molar enthalpy of formation of cyclopropane, 3 C(s) + 3 H2(g) → C3H6(g), is +/- _____ kJ.


The following diagram illustrates the formation enthalpies of V(s), Cl2(g) and a selection of their compounds. Enthalpies of Formation for Vanadium Chlorides

23. The energy absorbed when 0.350 mol of VCl4(l) decomposes to form VCl2(s) and Cl2(g) is

kJ.

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Calorimetry

24. If 44.7 kJ of energy is transferred when 1.65 g of ethanal, CH3CHO(l), is burned in a

calorimeter, then the molar enthalpy of combustion of ethanal is

a) -1.67 kJ/mol

b) -2.71 × 101 kJ/mol

c) -7.38 × 101 kJ/mol

d) -1.19 × 103 kJ/mol


2 CH3OH(l) + 3 O2(g) → 2 CO2(g) + 4 H2O(l) ∆H0 = -1275.8 kJ

25. The amount of methanol that must be burned to raise the temperature of 250.0 g of

water from 20.0 °C to 35.0 °C is

a) 6.16 × 10-3 mol

b) 1.23 × 10-2 mol

c) 2.46 × 10-2 mol

d) 2.46 × 101 mol

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2 C2H2(g) + 5 O2(g) → 4 CO2(g) + 2 H2O(g) ∆H = -2511.0 kJ


The amount of energy released by the combustion of 100 g of C2H2(g) is _______ MJ.

(Record your three-digit answer in the numerical-response section on the answer sheet.)

27. The temperature of a 2.0 g sample of aluminum increases from 25 °C to 30 °C. How

many joules of heat energy were added? (Specific heat of Al = 0.90 J g-1 K-1)

a) 0.36

b) 2.3

c) 9.0

d) 11

28. ©PS Suppose that you drop a 50.0 g chunk of metal, at 100.0 °C, into an insulated

vessel containing 100.0 mL (i.e. 100.0 g) of water at 25.0 °C.

What is the specific heat of the metal if the final temperature of the combined metal/water system is 30.0 °C? (The specific heat of water is 4.19 J/(g °C))

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29. In an experiment, 115.24 g of isopropyl alcohol at 20.1°C was mixed with 56.31 g of

water at 50.3 °C. After thermal equilibrium was reached, the temperature of the

mixture was 36.5 °C.

The energy lost by the water was

a) 3.25 kJ

b) 8.61 kJ

c) 11.9 kJ

d) 26.2 kJ

Challenge Questions


When 0.500 g of peanut oil was burned, the temperature of 0.950 kg of water in a

calorimeter increased by 4.60 °C. The enthalpy of combustion of the peanut oil was

If + record 1

If – record 2

kJ/g

first second third fourth column

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In a calorimetry experiment, a student found that the theoretical value for the molar

enthalpy of combustion for methanol was higher than the value that she obtained in the

laboratory.

31. Which of the following explanations could account for the discrepancy between the

experimental value and the theoretical value for the molar enthalpy of combustion of

methanol?

a) The mass of methanol was lower than measured.

b) The final temperature of the calorimeter contents was higher than measured.

c) The initial temperature of the calorimeter contents was lower than measured.

d) The incomplete combustion of methanol produced a mixture of CO(g) and CO2(g) in the

calorimeter.

Thermochemistry Diploma Questions -...

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