5.2 Calculation of Enthalpy Changes

Which of the following statements best describes Hess's Law?

A. The enthalpy change of a reaction is independent of the path taken.

B. The enthalpy change of a reaction is directly proportional to the amount of reactants used.

C. The enthalpy change of a reaction is always positive.

D. The enthalpy change of a reaction is equal to the sum of the enthalpies of the products.

Given the following reactions:

• A + B -> C with ΔH = -10 kJ/mol
• C -> D + E with ΔH = 20 kJ/mol

What is the enthalpy change for the reaction A + B -> D + E?

A. 10 kJ/mol

B. 30 kJ/mol

C. -10 kJ/mol

D. -30 kJ/mol

Which of the following is the correct definition of standard enthalpy change of formation?

A. The enthalpy change when one mole of a compound is formed from its elements in their standard states.

B. The enthalpy change when one mole of a compound is burned in excess oxygen.

C. The enthalpy change when one mole of a compound reacts with an acid.

D. The enthalpy change when one mole of a compound is dissolved in water.

Given the standard enthalpy changes of formation for A and B are -100 kJ/mol and -200 kJ/mol respectively, what is the enthalpy change for the reaction A + B -> 2C if the standard enthalpy change of formation for C is -150 kJ/mol?

A. -100 kJ/mol

B. 50 kJ/mol

C. -50 kJ/mol

D. 100 kJ/mol

Which of the following reactions represents the standard enthalpy change of combustion for methane?

A. CH4 + O2 -> CO2 + 2H2O

B. CH4 + 2O2 -> CO2 + 2H2O

C. CH4 + 3O2 -> CO2 + 2H2O

D. CH4 -> CO2 + 2H2O

a) Define Hess's Law and explain its significance in determining enthalpy changes. [3]

b) Given the following reactions:

i. A + B -> C with ΔH = +50 kJ/mol ii. C + D -> E with ΔH = -30 kJ/mol

Calculate the enthalpy change for the reaction A + B + D -> E. [2]

a) Describe the difference between standard enthalpy changes of formation and combustion. [3]

b) Given that the standard enthalpy change of formation for water is -286 kJ/mol, calculate the enthalpy change when 2 moles of water are formed. [2]

a) What is meant by the term "energy cycle" in the context of enthalpy changes? [3]

b) Using the concept of energy cycles, explain how the enthalpy change of a reaction can be determined indirectly. [2]

a) Explain the role of a catalyst in terms of activation energy and its effect on the rate of a reaction. [3]

b) If a reaction has an activation energy of 100 kJ/mol without a catalyst and 50 kJ/mol with a catalyst, by what factor does the activation energy decrease? [2]

c) Does the presence of a catalyst alter the overall enthalpy change of a reaction? Justify your answer. [2]

a) Describe what is meant by the standard enthalpy change of neutralisation. [3]

b) If the enthalpy change of neutralisation for a strong acid and strong base is -57 kJ/mol, what is the energy change when 0.5 moles of the acid reacts with an equivalent amount of the base? [2]

c) Why might the enthalpy change of neutralisation for a weak acid and strong base differ from that of a strong acid and strong base? [3]