Thermochemistry

Which of the following processes is endothermic?

 (A) O₂(g) + 2H₂(g) → 2H₂O(g) 

(B) H₂O(g) → H₂O(l) 

(C) 3O₂(g) + 2CH₃OH(g) → 2CO₂(g) + 2H₂O(g) 

(D) H₂O(s) → H₂O(l)

Copper metal has a specific heat of 0.385 J/g·°C. Calculate the amount of heat required to raise the temperature of 22.8 g of Cu from 20.0°C to 875°C. 

(A) 1.97 x 10^–5 J 

(B) 1.0 x 10^–2 J 

(C) 329 J 

(D) 7.51 kJ 

(E) 10.5 kJ

10.1 g CaO is dropped into a styrofoam coffee cup containing 157 g H₂O at 18.0°C. If the following reaction occurs, then what temperature will the water reach, assuming that the cup is a perfect insulator and that the cup absorbs only a negligible amount of heat? (the specific heat of water = 4.18 J/g·°C)

CaO(s) + H₂O(l) → Ca(OH)₂(s) ΔH°_rxn = –64.8 kJ/mol

(A) 18.02°C 

(B) 35.8°C 

(C) 311°C 

(D) 42.2°C 

(E) 117°C

At 25°C, the standard enthalpy of formation of KCl(s) is –435.87 kJ/mol. When one mole of KCl(s) is formed by reacting potassium vapor and chlorine gas at 25°C, the standard enthalpy of reaction is –525.86 kJ/mol. Find ΔH° for the sublimation of potassium, K(s) → K(g), at 25°C

(A) –345.88 kJ/mol

(B) 45.00 kJ/mol

(C) 345.88 kJ/mol

(D) 89.99 kJ/mol

(E) –525.86 kJ/mol 

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)

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

How many degrees of temperature rise will occur when a 25.0 g block of aluminum absorbs 10.0 kJ of heat? The specific heat of Al is 0.900 J/g·°C. 

(A) 0.44°C 

(B) 22.5°C 

(C) 225°C 

(D) 360°C 

(E) 444°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

For the reaction

C(graphite) + O₂(g) → CO₂(g) ΔH° = –393 kJ/mol

How many grams of C(graphite) must be burned to release 275 kJ of heat?

(A) 22.3 g 

(B) 0.70 g 

(C) 12.0 g 

(D) 17.1 g 

(E) 8.40 g

Pentaborane B₅H₉(s) burns vigorously in O₂ to give B₂O₃(s) and H₂O(l). Calculate ΔH°_rxn for the combustion of 1 mol of B₅H₉. 

ΔH°_f[B₂O₃(s)] = –1,273.5 kJ/mol 

ΔH°_f[B₅H₉(s)] = 73.2 kJ/mol 

ΔH°_f[H₂O(l)] = –285.8 kJ/mol

(A) –1,2735 kJ/mol

(B) –4,543 kJ/mol

(C) –18,170 kJ/mol

(D) –9,086 kJ/mol

(E) –8,448 kJ/mol