Thermochemistry

To which one of the following reactions occurring at 25°C does the symbol ΔH°_f[HNO₃(l)] refer?

(A) H(g) + N(g) + O₃(g) → HNO₃(l) 

(B) (1/2)H₂(g) + (1/2)N₂(g) + (3/2)O₂(g) → HNO₃(l) 

(C) HNO₃(l) → (1/2)H₂(g) + (1/2)N₂(g) + (3/2)O₂(g) 

(D) HNO₃(l) → H(g) + N(g) + 3O(g) 

(E) H₂(g) + N₂(g) + O₃(g) → HNO₃(l)

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

When 18.5 g of HgO(s) is decomposed to form Hg(l) and O₂(g), 7.75 kJ of heat is absorbed at standard-state conditions. What is the standard enthalpy of formation (ΔH°_f) of HgO(s)?

(A) –90.7 kJ/mol

(B) –7.75 kJ/mol

(C) 0.419 kJ/mol

(D) 27.9 kJ/mol

(E) 143 kJ/mol 

Define specific heat.

Aluminum oxide can be reduced to aluminum metal using carbon, the other reaction product being carbon monoxide. Determine the enthalpy change when 12.5 g of aluminum is produced by this method. [ΔH°_f(carbon monoxide) = –110.5 kJ/mol; ΔH°_f(aluminum oxide) = –1669.8 kJ/mol] 

(A) 725 kJ 

(B) 697 kJ 

(C) 310 kJ 

(D) 361 kJ 

(E) 1504 kJ

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

The combustion of butane produces heat according to the equation 

2C₄H₁₀(g) + 13O₂(g) →8CO₂(g) + 10H₂O(l) ΔH°_rxn= –5,314 kJ/mol 

How many grams of butane must be burned to release 1.00 x 10⁴ kJ of heat?

(A) 30.9 g 

(B) 61.8 g 

(C) 109 g 

(D) 153 g 

(E) 219 g

The specific heat of gold is 0.129 J/g·°C. What is the molar heat capacity of gold?

(A) 0.039 J/mol·°C

(B) 0.129 J/mol·°C

(C) 25.4 J/mol·°C

(D) 39.0 kJ/mol·°C

(E) 197 J/mol·°C

At 25°C, the standard enthalpy of formation of anhydrous sodium carbonate is –1130.9 kJ/mol, whereas the standard enthalpy of formation of sodium carbonate monohydrate is –1430.1 kJ/mol. Determine ΔH° at 25°C for the reaction

Na₂CO₃(s) + H₂O(l) → Na₂CO₃·H₂O(s).

(Given: ΔH°_f[H₂O(l)] = –285.8 kJ/mol)

(A) –13.4 kJ/mol

(B) –285.8 kJ/mol

(C) –585.0 kJ/mol

(D) –299.2 kJ/mol

(E) –156.3 kJ/mol

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