Question

41. When 0.500 g of cyclohexane, C6H12, is combusted in a bomb calorimeter that has a water sheath containing 750.0 g of water, the temperature of the water increased by 5.5 °C. Assuming that the specific heat of water is 4.18 J/(g °C), and that the heat absorption by the calorimeter is negligible, calculate the enthalpy of combustion per mole of cyclohexane.

   (A). 2.90 x 106 J/mol

   (B). 4.20 x 106 J/mol

   (C). 1.19 x 104 J/mol

   (D). 2.97 x 105 J/mol

   (E). 8.21 x 106 J/mol

42. Choose the reaction that illustrates ΔH°f for Ca(NO3)2.

   (A). Ca(s) + N2(g) + 3O2(g)  Ca(NO3)2 (s)

   (B). Ca+(aq) + NO3-(aq)  Ca(NO3)2 (s)

   (C). Ca+(s) + NO3(aq)  Ca(NO3)2 (s)

   (D). Ca(s) + N2(g) + O2(g)  Ca(NO3)2 (s)

   (E). 2Ca(s) + 2N2(g) + 3O2(g)  Ca(NO3)2 (s)

45. Give the equation with the elements in Na2SO4 in their standard state as the reactants and Na2SO4 as the product.

   (A). 2 Na(s) + 1/8 S8 (s, rhombic) + 2 O2(g)  Na2SO4

   (B). Na(s) + S8 (s, rhombic) + 2 O2(g)  Na2SO4

   (C). Na(s) + S (s, rhombic) + ½ O2(g)  NaSO

   (D). Na(s) + 1/8 S8 (s, rhombic) + 2O2(g)  NaSO4

46. A gas cylinder contains 10.0 kg of C3H8 (propane) gas. Calculate the heat associated with the complete combustion of all of the propane in the cylinder. MW of C3H8 is 44.10 g/mol.

C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) Δrxn = -2044 kJ

   (A). -5.68 x 105 kJ

   (B). -4.63 x 105 kJ

   (C). +5.68 x 105 kJ

   (D). +4.63 x 105 kJ

   (E). 2.31 x 105 kJ

48. Use the Δof information provided to calculate Δorxn for the following reaction:

SO2Cl2(g) + 2H2O(l)  2HCl(g) + H2SO4(l) Δorxn = ?

Δof (kJ/mol)

SO2Cl2(g) -364

H2O(l) -286

HCl(g) -92

H2SO4(l) -814

49. Calculate the change in internal energy (Δ) for a system that is giving off 26.0 kJ of heat and is changing from 12.00 L to 6.00 L in volume at 1.50 atm pressure. (101.3 J = 1 L∙tm)

50. Use bond energies to calculate the Δrxn for the following reaction:

N2(g) + 3H2(g)  2NH3(g) (balanced?)

51. Which of the following (with specific heat capacity provided) would show the smallest temperature change upon gaining 155 J of heat?

   (A) 25.0 g Au, CAu = 0.128 J/g.°C

   (B) 25.0 g Ag, CAg = 0.235 J/g.°C

   (C) 50.0 g Al, CAl = 0.903 J/g.°C

   (D) 50.0 g Cu, CCu = 0.385 J/g.°C

   (E) 25.0 g granite, Cgranite = 0.790 J/g°C

52. Use the standard reaction enthalpies given below to determine ΔHorxn for the following reaction:

2 NO(g) + O2(g)  2 NO2(g) Δorxn = ?

Given:

N2(g) + O2(g)  2 NO(g) Δorxn = +183 kJ

½ N2(g) + O2(g)  NO2(g) Δorxn = +33 kJ

   (A) -211 kJ

   (B) -117 kJ

   (C) -153 kJ

   (D) +165 kJ

   (E) +795 kJ

Answer 1

41. The heat absorbed by the calorimeter is negligible. The heat absorbed by the water in the calorimeter is (750.0 gm)(4.18 J/gm.°C)(5.5°C) = 1.724*10⁴ J (the temperature of the water in the calorimeter rose by 5.5°C).

The same heat is given out by the combustion of cyclohexane. The moles of cyclohexane combusted is (0.500 gm)/(84 gm/mol) = 5.952*10^-3 mol (molar mass of cyclohexane is 84 gm/mol).

Therefore, the heat of combustion of cyclohexane is (1.724*10⁴ J)/(5.952*10^-3 mol) = 2.90*10⁶ J/mol

Ans: (A) 2.90*10⁶ J/mol

42. The heat of formation is defined as the enthalpy change for formation of 1 mole of a compound from its constituent elements in their standard states under 1 atm pressure. Of all the options given, only (A) gives the balanced equation for formation of 1 mole of Ca(NO₃)₂ from its constituent elements in their standard state at 1 atm; hence that’s our answer.

Ans: (A)

45. Option (A) gives the balanced equation for formation of 1 mole of Na₂SO₄from its constituent elements. All the elements are in their standard states.

Ans: (A)

46. The given equation is

C₃H₈ (g) + 5 O₂ (g) ---------> 3 CO₂ (g) + 4 H₂O (g )

ΔH⁰_rxn = -2044 kJ/mol

The given heat of reaction is for combustion of 1 mole of propane. We need to calculate the amount of propane in the 10.0 kg cylinder to find out the total heat of combustion. Molar mass of propane is 44.10 gm/mol.

Therefore, amount of propane in the cylinder = (10.0*1000 gm)/44.10 gm/mol = 226.76 mol.

Hence, total heat of combustion = (226.76 mol)(-2044 kJ/mol) = -4.63*10⁵ kJ.

Ans: (B) -4.63*10⁵ kJ

48. The equation is

SO₂Cl₂ (g) + 2 H₂O (l) -------> 2 HCl (g) + H₂SO₄ (l)

The enthalpy of the reaction is

ΔH⁰_rxn = [ΣΔH⁰_f]_pdts – [ΣΔH⁰_f]_rctnts where the enthalpy terms denote standard enthalpy of formation for the products and the reactants.

Therefore, ΔH⁰_{rxn =} {2(-92 kJ/mol) + (-814 kJ/mol)} – {(-364 kJ/mol) + 2(-286 kJ/mol)}

= (-998 kJ/mol) – (-936 kJ/mol) = -62 kJ/mol (ans)

Ans: The enthalpy of the given reaction is -62 kJ/mol .