Energetics / thermochemistry (2)

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. Which is a correct statement about an endothermic reaction?

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

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

3. 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)

4. 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.

5. 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

6. 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.

7. 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

8. 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

9. 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

10. 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

11. The standard enthalpy of combustion of propanone is – 1786 kJ mol^-1

CH₃COCH₃(l) + 4O₂₍g) → 3CO₂(g) + 3H₂O(l) ∆H_c= – 1786 kJ mol^-1

The standard enthalpies of formation of CO₂(g) and H₂O(l) are – 394 kJ mol^-1 and
– 286 kJ mol^-1 respectively. Which is the correct calculation to give the standard
enthalpy of formation of propanone?

A. 3(– 394) + 3(– 286) – (– 1786)

B. 3(– 394) + 3(– 286) + (– 1786)

C. 3(– 394) - 3(– 286) – (– 1786)

D. – 1786 – 3(– 394) – 3(– 286)

12. Which is an exothermic reaction?

A. Br₂(l) → Br₂(g)

B. Br₂(g) → 2Br(g)

C. Br(g) → Br⁺(g) + e^-

D. Br(g) + e^- → Br^-(g)

13. Which is the correct equation for the lattice enthalpy of calcium sulfide?

A. Ca²⁺(s) + S^2-(s) → CaS(s)

B. Ca²⁺(g) + S^2-(g) → CaS(s)

C. Ca²⁺(g) + S^2-(g) → CaS(g)

D. Ca(s) + S(s) → CaS(s)

14. Which are characteristics of compounds with a large lattice enthalpy value?

I. The positive ions have a small ionic radius

II. The negative ions have a large ionic radius

III. The positive ions have a high ionic charge

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

15. Which reaction is the most exothermic?

A. Na(g) → Na⁺(g) + e^-

B. Na(s) + ½ Cl₂(g) → NaCl(s)

C. Cl(g) + e^- → Cl^-(g)

D. Na⁺(g) + Cl^-(g) → NaCl(s)

16. Which reaction has the largest negative value for the change in entropy?

A. H₂(g) + Br₂(g) → 2HBr(g)

B. 2C(s) + O₂(g) → 2CO(g)

C. 2SO₂(g) + O₂(g) → 2SO₃(g)

D. N₂O₄(g) → 2NO₂(g)

17. But-2-ene and steam react to form butan-2-ol.

C₄H₈ (g) + H₂O(g) → C₄H₉OH(l)

The standard entropy values, S, for but-2-ene, water vapour and butan-2-ol are 220,
189 and 215 J K^-1 mol^-1 respectively.
What is the standard entropy change (in J K^-1 mol^-1) for this reaction?

A. – 194

B. + 194

C. – 184

D. + 184

18. The values of ∆H and ∆S for a reaction are both positive. What will happen to the spontaneity of the reaction as the temperature is altered?

A. The reaction will become more spontaneous as the temperature is decreased

B. The reaction will remain spontaneous at all temperatures

C. The reaction will remain non spontaneous at all temperatures

D. The reaction will become more spontaneous as the temperature is increased.

19. The table gives the standard free energy changes for some compounds.

Compound	Na2O(S)	H2O(l)	NaOH(s)
ΔGf / kJ mol-1	– 376	– 237	– 380

What is the value of ∆G for the reaction?

Na₂O(s) + H₂O(l) → 2NaOH(s)

A. + 147 kJ mol^-1

B. – 147 kJ mol^-1

C. + 233 kJ mol^-1

D. – 233 kJ mol^-1

20. What are the signs for ∆H, ∆S and ∆G when gasoline (petrol) is used as a fuel in an internal
combustion engine?

C₈H₁₈(g) + 12½O₂(g) → 8CO₂(g) + 9H₂O(g)

Answers

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