Topic 5: Energetics & Thermochemistry (SL & HL)

Using the standard enthalpies of combustion given below, what is enthalpy change for the following reaction?

C₇H₁₆(l) → C₂H₄(g) + C₅H₁₂(l)

• +1411 + 3509 - 4817

• -1411 - 3509 + 4817

• +1411 - 3509 - 4817

• +1411 + 3509 + 4817

Answer explanation

Which equation represents the P-H bond enthalpy in PH₃?

• PH₃(g) → P(g) + 1.5H₂(g)

• PH₃(g) → P(g) + 3H(g)

• PH₃(g) → PH₂(g) + H(g)

• 1/3PH₃(g) → 1/3P(g) + H(g)

Answer explanation

When equal masses of X and Z absorb the same amount of energy their temperatures rise by 10°C and 20°C respectively. Which is correct?

• The specific heat capacity of X is half that of Z

• The specific heat capacity of X is ten times that of Z

• The specific heat capacity of X is twice that of Z

• The specific heat capacity of X is the same as Z

Answer explanation

Which is correct for the reaction?

2NO₂ → N₂O₄ ΔH = -57.6 kJ mol^-1

• The products are less stable than the reactants and the reaction is endothermic.

• The products are more stable than the reactants and the reaction is exothermic.

• The products are less stable than the reactants and the reaction is exothermic.

• The products are more stable than the reactants and the reaction is endothermic.

Answer explanation

What is the enthalpy change for the incomplete combustion of ethene using average bond enthalpies given below?

C₂H₄(g) + 2O₂(g) → 2CO(g) + 2H₂O(g)

Bond enthalpies (in kJ mol^-1) that you may require: C-H 414 / C=C 614 / O=O 498 / C=O 1077 / O-H 463

• [2(1077) + 2(463)] - [614 + 4(414) + 2(498)]

• [614 + 4(414) + 2(498)] - [2(1077) + 2(463)]

• [614 + 4(414) + 2(498)] - [2(1077) + 4(463)]

• [2(1077) + 4(463)] - [614 + 4(414) + 2(498)]

Answer explanation

Using the standard enthalpies given below, what is the standard enthalpy of combustion of ethane in kJ mol^-1?

2C₂H₆(g) + 7O₂(g) → 4CO₂(g) + 6H₂O(l)

• 2X + 3Y + Z

• 4X + 6Y + 2Z

• 4X + 6Y - 2Z

• 2X + 3Y - Z

Answer explanation

Two 50.0 cm³ aqueous solutions containing 0.020 mol hydrochloric acid and 0.020 mol sodium hydroxide respectively are both at 298 K.

When the two solutions are mixed, the temperature rises by y°C.

Assume that the density of the final solution is 1.00 g cm^-3 and the specific heat capacity is 4.18 J K^-1 g^-1.

What is the enthalpy change of neutralization in kJ mol^-1?

Answer explanation

Which expression gives the mass, in grams, of propan-1-ol required to produce 790 kJ of heat energy upon complete combustion?

(M_r for propan-1-ol = 60.1; ΔH°_c = -2021 kJ mol^-1)

Answer explanation

Why is the enthalpy change for this reaction calculated from bond enthalpy data less accurate than that calculated using standard enthalpies of formation?

C₃H₆(g) + 4.5O₂(g) → 3CO₂(g) + 3H₂O(g)

• Elements do not have standard enthalpies of formation

• All of the reactants and products are gases

• Bond enthalpies are average values of many compounds

• The equation is incorrectly balanced

Answer explanation

What can be deduced from the knowledge that ozone absorbs ultraviolet radiation of a greater wavelength than oxygen?

• The bonds between atoms in ozone are stronger than those in oxygen.

• Only ozone can form free radicals.

• Ozone absorbs radiation of a higher energy than oxygen.

• The bonds between atoms in oxygen require more energy to break than those in ozone.

Answer explanation