Test Yourself : Ch34 Standard enthalpy changes of reactions

Consider the following reaction:

2CH₃CH₂OH(l) + 6O₂(g) → 4CO₂(g) + 6H₂O(l)　　ΔH = −2733.6 kJ mol⁻¹

What is the ΔH value for the reaction as shown in the diagram?

• A. +683.4 kJ mol⁻¹
• B. −683.4 kJ mol⁻¹

• C. +10934.4 kJ mol⁻¹
• D. −10934.4 kJ mol⁻¹

Given that 2219.2 kJ of heat is released when 44.0 g of propane burns completely in oxygen to give carbon dioxide and water at 298 K and 1 atm. Which of the following statements concerning the standard enthalpy change of combustion of propane are correct?

(1) It is the enthalpy change when one mole of propane is completely burnt in oxygen under standard conditions.

(2) It is a negative value.

(3) The thermochemical equation for the reaction is C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g).

• A. (1) and (2) only
• B. (1) and (3) only
• C. (2) and (3) only
• D. (1), (2) and (3)

Direction: Questions 3 and 4 refer to the experimental determination of the enthalpy change of combustion of methanol by using the set-up shown in the diagram.

What is the enthalpy change of combustion of methanol under the experimental conditions?

(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0; specific heat capacity of water = 4.2 J g⁻¹ K⁻¹ )

• A. +537 600 kJ mol⁻¹
• B. −537 600 kJ mol⁻¹

• C. +537.6 kJ mol⁻¹
• D. −537.6 kJ mol⁻¹

Direction: Questions 3 and 4 refer to the experimental determination of the enthalpy change of combustion of methanol by using the set-up shown in the diagram.

What are the sources of error for this experiment?

(1) There was heat loss to the surroundings.

(2) Incomplete combustion of methanol occurred.

(3) The metal absorbed heat.

• A. (1) and (2) only
• B. (1) and (3) only
• C. (2) and (3) only
• D. (1), (2) and (3)

• A. (1) only
• B. (2) only
• C. (1) and (3) only
• D. (2) and (3) only

When 20 cm³ of 1.0 M HCl(aq) reacts with 20 cm³ of 1.0 M NaOH(aq), the temperature change is ΔT. If the reaction is repeated with 5 cm³ of each solution, the temperature change will be

• A. (1/4)ΔT.

• B. 4ΔT.

• C. ΔT.

• D. (ΔT)⁴.

Which of the following equations correctly represent(s) the formation of one mole of a substance from its constituent elements under standard conditions?

(1) 6C(diamond) + 6H₂(g) + 3O₂(g) → C₆H₁₂O₆(s)

(2) 2Na(s) + S(s) + 2O₂(g) → Na₂SO₄(s)

(3) H₂(g) + Cl₂(g) → 2HCl(g)

• A. (1) only
• B. (2) only
• C. (1) and (3) only
• D. (2) and (3) only

Which of the following statements concerning standard enthalpy changes of formation are correct?

(1) The standard enthalpy changes of formation of all compounds are negative values.

(2) The standard enthalpy changes of formation of all elements in their standard states are zero.

(3) The standard enthalpy changes of formation of some compounds cannot be determined by simple calorimetric methods.

• A. (1) and (2) only
• B. (1) and (3) only
• C. (2) and (3) only
• D. (1), (2) and (3)