Here Are the Most Important Questions for Chemical Science Students - THERMOCHEMISTRY!

Thermochemistry - Most Important Questions! (Download PDF)

1. One mole of naphthalene was burnt in oxygen gas at constant volume and produced carbon dioxide gas and liquid water at 25°C. The heat evolved was found to be 5138.8 kJ. Calculate the enthalpy of the reaction. R = 8.314 J K^–1 mol^–1.

1. -5143.8 kJ
2. -403.21 kJ
3. -382.65 kJ
4. -636.24 kJ

2. For glucose C₆H₁₂O₆(s), the enthalpy of combustion is –2816 kJ mol^–1 at 25° C. Calculate ΔHf° (C₆H₁₂O₆). The ΔHf° value for CO₂(g) and H₂O(l) are –393.5 and –285.9 kJ mol^–1, respectively.

1. –1360.2 kJ mol^–1
2. –1132.4 kJ mol^–1
3. –1260.4 kJ mol^–1
4. D. –1462.3 kJ mol^–1

3. The heat evolved on dissolving CuSO₄(s) in water is 86.6 kJ mol^–1. If ΔH°f(Cu²⁺) is 64.4 kJ mol^–1, what is ΔH°f(SO₄^2–)? ΔH°f(CuSO₄(s)) = –770.0 kJ mol^–1

A. – 642.3 kJ

B. – 543.8 kJ

C. – 747.8 kJ

D. – 345.9 kJ

4. In a bomb calorimeter, cyanamide was subjected to combustion at constant volume and the heat evolved was found to be 742.7 kJ at 25°C. Calculate qp for the reaction.

A. – 643.2 kJ

B. – 452.8 kJ

C. – 741.5 kJ

D. – 327.8 kJ

5. Determine the enthalpy of formation of OH^– ions at 25°C from the following thermochemical data: H₂O(l) → H⁺(aq) + OH–(aq); ΔH°= 57.3 kJ

H₂(g) + O₂(g) → H₂O(l); ΔH°=-285.9 kJ

A. – 324.8 kJ

B. – 228.6 kJ

C. – 278.5 kJ

D. – 356.8 kJ

6. The enthalpy of reaction (ΔH) for the formation of ammonia according to the reaction:

 N₂ + 3H₂ = 2NH₃ at 27°C was found to be –91.94 kJ. 

What will be the enthalpy of the reaction (ΔH) at 50°C? The molar heat capacities at constant pressure and at 27°C for nitrogen, hydrogen, and ammonia are 28.45, 28.32, and 37.07 joules, respectively.

A. – 78.85 kJ

B. – 62.70 kJ

C. – 56.35 kJ

D. – 92.85 kJ

7. Calculate the enthalpy change at 125° C for the reaction:

    N₂(g) + 3H₂(g) → 2NH₃(g);

The molar heat capacities (in J K ^–1 mol^–1) for the various gases involved in the reaction vary with temperature as follows:

Cp(N₂) = 27.26 + 5.23 × 10^–3 T – 4.18 × 10^–9 T2

Cp(H₂) = 29.02 – 8.35 × 10–4 T + 20.80 × 10^–7 T2

Cp(NH₃) = 25.86 + 32.94 × 10–2 T – 30.42 × 10^–7 T2

A. – 83.20 kJ

B. – 96.50 kJ

C. – 45.60 kJ

D. – 76.83 kJ

8. H₂ gas is mixed with air at 25°C under the pressure of one atmosphere and explodes in a closed vessel. The enthalpy of reaction H₂(g) + 1/2 O₂(g) → H₂O(g) at constant volume, ΔU298 = – 240.6 kJ and CPs for H₂O vapor and N₂ in the temperature range 298 K and 3,200 K are 39.1 J K^–1 and 36.4 J K^–1, respectively. Calculate the explosion temperature under adiabatic conditions.

A. 3216 K

B. 4230 K

C. 2916 k

D. 5200 K

9. 0.50 g of benzoic acid was subjected to combustion in a bomb calorimeter when the temperature of the calorimeter system (including water) was found to rise by 0.55°C. Determine the enthalpy of combustion of benzoic acid. The ΔT calorimeter constant was found to be 23.85 kJ K^–1.

A. – 3234.7 kJ mol^–1

B. – 2301.9 kJ mol^–1

C. – 4032.6 kJ mol^–1

D. – 3201.9 kJ mol^–1

10. For the hypothetical reaction: 2B(g) → B₂(g) , ΔCP (in joules) = 6.0 + 2.0 × 10–3 T and ΔH°298 = – 20.0 kJ mol^–1. Estimate the temperature at which ΔH° = 0 for this reaction.

A. 2527 K

B. 3245 K

C. 1234 K

D. 1200 K

Solutions

 Solution 1:

Solution 2:

Solution 3:

Solution 4:

Solution 5:

Solution 6:

Solution 7:

Solution 8:

Solution 9:

Solution 10:

ANSWER KEYS

1. A
2. C
3. C
4. C
5. B
6. D
7. B
8. C
9. D
10. A

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