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.
- -5143.8 kJ
- -403.21 kJ
- -382.65 kJ
- -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.
- –1360.2 kJ mol–1
- –1132.4 kJ mol–1
- –1260.4 kJ mol–1
- 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(SO42–)? 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
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ANSWER KEYS
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- C
- C
- C
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- B
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- A
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