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]

What is the standard enthalpy change of neutralisation?

A. The enthalpy change when one mole of water is formed from the reaction of an acid and a base.

B. The enthalpy change when one mole of an acid reacts with a base.

C. The enthalpy change when one mole of a base reacts with water.

D. The enthalpy change when one mole of an acid reacts with water.

Given the following energy cycle:

• X + Y -> Z with ΔH = 50 kJ/mol
• Z -> A + B with ΔH = -20 kJ/mol

What is the enthalpy change for the reaction X + Y -> A + B?

A. 30 kJ/mol

B. 70 kJ/mol

C. -30 kJ/mol

D. -70 kJ/mol

Which of the following reactions represents the standard enthalpy change of formation for water?

A. H2 + O2 -> H2O

B. 2H2 + O2 -> 2H2O

C. H2 + 0.5O2 -> H2O

D. 2H2 + 2O2 -> 2H2O

In an energy cycle, which of the following is true regarding the enthalpy changes of the individual reactions?

A. They are always positive.

B. They are always negative.

C. They can be either positive or negative.

D. They are always zero.

What is the significance of the standard enthalpy change of combustion?

A. It indicates the stability of a compound.

B. It indicates the amount of energy released when one mole of a substance is burned in excess oxygen.

C. It indicates the amount of energy required to break the bonds in a compound.

D. It indicates the amount of energy required to form one mole of a compound.

a) Define and explain the significance of Hess's Law in relation to enthalpy changes. [3]

b) Using Hess's Law, describe how you would determine the enthalpy change for a reaction that is difficult to measure directly. [3]

c) Why is it important to ensure reactions are under standard conditions when using Hess's Law? [2]

a) What is the significance of the energy barrier in a reaction? [3]

b) How does the magnitude of the energy barrier relate to the rate of a reaction? [2]

c) If a reaction has a high energy barrier but is exothermic, what can be inferred about the stability of the reactants and products? [3]

a) Define the standard enthalpy change of combustion. [2]

b) Given that the enthalpy change of combustion for methane is -890 kJ/mol, calculate the energy released when 0.25 moles of methane are burned completely in oxygen. [2]

c) How does the enthalpy change of combustion for alkanes vary as the number of carbon atoms increases? [3]

d) Why might the experimental value for the enthalpy change of combustion differ from the theoretical value? [3]

a) Describe the concept of an energy cycle in relation to enthalpy changes. [3]

b) How can energy cycles be used in conjunction with Hess's Law to determine enthalpy changes? [3]

c) Given two known enthalpy changes in an energy cycle, how would you determine the third unknown enthalpy change? [2]

d) Why is it essential to ensure all reactions in an energy cycle are under the same conditions? [2]

a) What is the significance of the standard enthalpy change of formation in understanding the stability of compounds? [3]

b) Given that the standard enthalpy change of formation for water is -286 kJ/mol, what does this value indicate about the stability of water relative to its elements? [3]

c) How would you determine the enthalpy change of a reaction using the standard enthalpy changes of formation of the reactants and products? [3]

d) Why might the enthalpy change of formation for an element be zero? [2]