Thermochemistry

Methanol (CH₃OH) burns according to the equation

2CH₃OH(l) + 3O₂(g) → 2CO₂(g) + 4H₂O(l), ΔH°_rxn = –1454 kJ/mol.

How much heat, in kilojoules, is given off when 75.0 g of methanol is burned?

(A) 727 kJ

(B) 3.22 x 10³ kJ

(C) 1.45 x 10³ kJ

(D) 1.70 x 10^–3 kJ

(E) 3.41 x 10³ kJ

Suppose a 50.0 g block of silver (specific heat = 0.2350 J/g·°C) at 100°C is placed in contact with a 50.0 g block of iron (specific heat = 0.4494 J/g·°C) at 0°C, and the two blocks are insulated from the rest of the universe. The final temperature of the two blocks 

(A) will be higher than 50°C. 

(B) will be lower than 50°C. 

(C) will be exactly 50°C. 

(D) is unrelated to the composition of the blocks. 

(E) cannot be predicted.

For which of these reactions will the difference between ΔH° and ΔE° be the smallest? 

(A) N₂(g) + 3H₂(g) → 2NH₃(g) 

(B) 4PH₃(g) → P₄(g) + 6H₂(g) 

(C) H₂(g) + Cl₂(g) → 2HCl(g) 

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

(E) P₄(s) + 10Cl2(g) → 4PCl₅(s)

The heat of solution of ammonium nitrate is 26.2 kJ/mol. If a 5.368 g sample of NH₄NO₃ is added to 40.0 mL of water in a calorimeter at 23.5°C, what is the minimum temperature reached by the solution? (The specific heat of water = 4.18 J/g·°C; the heat capacity of the calorimeter = 650. J/°C.) 

(A) 14.3°C 

(B) 20.8°C 

(C) –7.7°C 

(D) 25.6°C 

(E) 21.4°C

At 25°C, the following heats of reaction are known: 

2ClF(g) + O₂(g) → Cl₂O(g) + F₂O(g ΔH°_rxn = 167.4 kJ/mol

2ClF₃(g) + 2O₂(g) → Cl₂O(g) + 3F₂O(g) ΔH°_rxn = 341.4 kJ/mol

2F₂(g) + O₂(g) → 2F2O(g) ΔH°_rxn = –43.4 kJ/mol

At the same temperature, use Hess’s law to calculate ΔH°_rxn for the reaction:

ClF(g) + F₂(g) → ClF₃(g)

(A) –217.5 kJ/mol

(B) –130.2 kJ/mol

(C) 217.5 kJ/mol

(D) –108.7 kJ/mol

Glycine, C₂H₅O₂N, is important for biological energy. The combustion reaction of glycine is given by the equation

4C₂H₅O₂N(s) + 9O₂(g) →8CO₂(g) + 10H₂O(l) + 2N₂(g) ΔH°_rxn = –3857 kJ/mol

Given that DH°_f[CO₂(g)] = –393.5 kJ/mol and ΔH°_f[H₂O(l)] = –285.8 kJ/mol, calculate the enthalpy of formation of glycine.

(A) –537.2 kJ/mol

(B) –268.2 kJ/mol

(C) 2,149 kJ/mol

(D) –3,178 kJ/mol

(E) –964 kJ/mol 

Given that CaO(s) + H₂O(l) →Ca(OH)₂(s), ΔH°_rxn = –64.8 kJ/mol, how many grams of CaO must react in order to liberate 525 kJ of heat? 

(A) 6.92 g 

(B) 56.1 g 

(C) 455 g 

(D) 606 g 

(E) 3.40 x 10⁴ g

If 325 g of water at 4.2°C absorbs 12.28 kJ, what is the final temperature of the water? The specific heat of water is 4.184 J/g·°C. 

(A) 4.21°C 

(B) 4.8°C 

(C) 9.0°C 

(D) 13.2°C 

(E) 2,938°C

Find the standard enthalpy of formation of ethylene, C₂H₄(g), given the following data: heat of combustion of C₂H₄(g) = –1411 kJ/mol; ΔH°_f[CO₂(g)] = –393.5 kJ/mol; 

ΔH°f[H₂O(l)] = –285.8 kJ/mol.

(A) 52 kJ/mol

(B) 87 kJ/mol

(C) 731 kJ/mol

(D) 1.41 x 10³ kJ/mol

(E) 2.77 x 10³ kJ/mol