Energetics / thermochemistry

For each question choose the answer you consider to be the best.

1. Which is an exothermic process?

A. K(s) → K(g)

B. H₂O(l) → H⁺(aq) + OH^–(aq)

C. Cl(g) + e^– → Cl^–(g)

D. Mg(g) → Mg⁺(g) + e^–

2. When some solid ammonium nitrate was dissolved in water the temperature decreased
from 22 ^oC to 3 ^oC. What can be deduced from this observation?

A. The dissolving is endothermic and ∆H is positive.

B. The dissolving is endothermic and ∆H is negative.

C. The dissolving is exothermic and ∆H is positive.

D. The dissolving is exothermic and ∆H is negative.

3. Which is a correct statement about an endothermic reaction?

A. The bonds in the reactants are stronger than in the products and ∆H is positive.

B. The bonds in the products are stronger than in the reactants and ∆H is positive.

C. The bonds in the reactants are stronger than in the products and ∆H is negative

D. The bonds in the products are stronger than in the reactants and ∆H is negative.

4. Which of the following conditions commonly apply to the standard enthalpy change for a reaction, ∆H?

I. A pressure of 100 kPa

II. A temperature of 298 K

III. One mol of all reactants and all products

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

5. Which is a correct statement about the following enthalpy level diagram for a reaction?

A. The reaction is exothermic and ∆H is positive.

B. The reaction is exothermic and ∆H is negative.

C. The reaction is endothermic and ∆H is positive.

D. The reaction is endothermic and ∆H is negative.

6. Which statements are correct for all exothermic reactions?

I. The products are more stable than the reactants

II. The bonds in the products are stronger than the bonds in the reactants

III. The enthalpy of the products is less than the enthalpy of the reactants

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

7. For which of the following is the sign of the enthalpy change different to the other three?

A. Br₂(g) → 2Br(g)

B. Br(g) → Br⁺(g) + e^–

C. Br(g) + e^– → Br^–(g)

D. Br(l) → Br(g)

8. Which is a correct statement concerning the reaction shown?

N₂(g) + 3H₂(g) → 2NH₃(g) ∆H = – 92 kJ

A. 92 kJ of energy are absorbed for every mol of ammonia formed.

B. 92 kJ of energy are released for every mol of ammonia formed.

C. 46 kJ of energy are absorbed for every mol of ammonia formed.

D. 46 kJ of energy are released for every mol of ammonia formed.

9. The heat capacity of manganese is 0.480 kJ kg^-1 K^-1. How much energy is required to raise
the temperature of 3.00 kg of manganese by 20.0 K?

A. 28.8 kJ

B. 3.20 kJ

C. 0.0720 kJ

D. 288 kJ

10. The specific heat capacities (in kJ kg^-1 K^-1) of four elements are:

carbon (graphite) 0.709

copper 0.385

gold 0.129

iron 0.449

Which element will show the greatest increase in temperature if 2 kJ of heat is supplied to 0.100 kg samples of each element at the same initial temperature?

A. carbon (graphite)

B. copper

C. gold

D. iron

11. When 25 cm³ of 1.0 mol dm^-3 HCl was added to 25 cm³ of 1.0 mol dm^-3 KOH the temperature
increased by 7 ^oC.
Which combination of HCl and KOH will also give a temperature rise of 7 ^oC?

A. 50 cm³ of 0.25 mol dm^-3 HCl and 50 cm³ of 0.25 mol dm^-3 KOH.

B. 50 cm³ of 0. 50 mol dm^-3 HCl and 50 cm³ of 0.50 mol dm^-3 KOH.

C. 50 cm³ of 1.0 mol dm^-3 HCl and 50 cm³ of 1.0 mol dm^-3 KOH.

D. 50 cm³ of 2.0 mol dm^-3 HCl and 50 cm³ of 2.0 mol dm^-3 KOH.

12. The heat produced when 0.010 mol of propanone was combusted raised the temperature
of 250 g of water by 14 ^oC.
The specific heat capacity of water is 4.2 J g^-1 K^-1.
Which is the correct expression for value of the enthalpy of combustion of propanone (in J mol^-1)?

A. −
B. −

C. −

D. − (250 x 4.2 x 14 x 0.010)

13. How much energy is released when 14.0 g of carbon monoxide are completely combusted?

2CO(g) + O₂(g) → 2CO₂(g) ∆H = – 560 kJ

A. 1120 kJ

B. 560 kJ

C. 280 kJ

D. 140 kJ

14. When one mol of magnesium and one mol of hydrogen are combusted the enthalpy changes are - 602 kJ
and - 242 kJ respectively.

Mg(s) + ½ O₂(g) → MgO(s) ∆H = – 602 kJ
H₂(g) + ½ O₂(g) → H₂O(g) ∆H = – 242 kJ

What is the enthalpy change for the reduction of one mol of magnesium oxide by hydrogen?

MgO(s) + H₂(g) → Mg(s) + H₂O(g)

A. + 844 kJ

B. + 360 kJ

C. – 360 kJ

D. – 844 kJ

15. The enthalpy change for the dimerisation of two mol of nitrogen dioxide to form one
mol of dinitrogen tetroxide is – 57 kJ.

2NO₂(g) → N₂O₄ ∆H = – 57 kJ

The enthalpy change for the formation of one mol of NO₂(g) from its elements is + 33 kJ

½ N₂(g) + O₂(g) → NO₂(g) ∆H = +33  kJ

What is the enthalpy change for the formation of one mol of dinitrogen tetroxide from its elements?

N₂(g) + 2O₂(g) → N₂O₄(g)

A. + 9 kJ

B. – 9 kJ

C. + 24 kJ

D. – 24 kJ

16. The enthalpy changes for the formation of one mol of each of the two oxides of phosphorus
from their elements are given below:

P₄(s) + 3O₂(g) → P₄O₆(s) ∆H = – 1600 kJ

P₄(s) + 5O₂(g) → P₄O₁₀(s) ∆H = – 3000 kJ

What is the enthalpy change for the oxidation of one mol of P₄O₆ to form P₄O₁₀?

P₄O₆(s) + 2O₂(g) → P₄O₁₀(s)

A. – 4600 kJ

B. + 4600 kJ

C. + 1400 kJ

D. – 1400 kJ

17. Consider the following three bond enthalpies (in kJ mol^-1)

H—H 436
Cl—Cl 243
H—Cl 432

What is the enthalpy change (in kJ) for the reaction below?

2HCl(g) → H₂(g) + Cl₂(g)

A. + 185

B. + 237

C. – 185

D. – 237

18. Which of the following enthalpy changes can be calculated using only bond enthalpies
and average bond enthalpies?

I. C₂H₄(g) + H₂(g) → C₂H₆(g)

II. C₈H₁₆(l) + H₂(g) → C₈H₁₈(l)

III. CH₄(g) + 4F₂(g) → CF₄(g)  +  4HF(g)

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

19. The bond enthalpies for N≡N and H—H are 945 kJ mol^-1 and 436 kJ mol^-1 respectively.

When nitrogen and hydrogen react to form ammonia the enthalpy change is – 92 kJ.

N₂(g) + 3H₂(g) → 2NH₃(g) ∆H = – 92 kJ

What is the average bond enthalpy (in kJ) for the N—H bond?

A. 212

B. 424

C. 391

D. 782

20. Ethane can react with chlorine in ultraviolet light to form chloroethane:

C₂H₆(g) + Cl₂(g) → C₂H₅Cl(g) + HCl(g)

The relevant bond enthalpies are

What is the enthalpy change, ∆H, for this reaction (in kJ)?

A. – 99

B. + 99

C. + 341

D. – 315

Answers

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