Energetics - Model Answers

In terms of bonds broken and bonds formed, explain why the calculated values of enthalpies of combustion of these alcohols, when plotted against Mr, follow a straight line (2)

1. Each alcohol in the series (compared with the previous one) has one more C-C and two more C-H 2. Combustion of each alcohol in the series breaks one more C-C and two more C-H compared with the previous one AND forms one more mole CO2 and one more mole H2O

State Hess's Law (2)

1. Energy change is independent of route 2. ΔH = ΣΔHf prods - ΣΔHf reactants

Define the term standard enthalpy of formation (3)

1. Enthalpy change when 2. One mole of a substance 3. Is formed from its elements 4. All substances in their standard states

Define the term standard enthalpy of combustion (3)

1. Enthalpy change when 1 mole of a substance 2. Is completely burned in excess oxygen 3. At 298K and 100kPa

Explain the meaning of the term mean bond enthalpy (2)

1. Enthalpy to break a bond 2. Averaged over different molecules

What is the meaning of the term enthalpy change? (2)

1. Heat energy change 2. Measured at constant pressure

Give two reasons why the experimental values obtained by the student are lower than the calculated values using the enthalpy of formation data (2)

1. Heat loss or heat absorbed by the apparatus 2. Incomplete combustion 3. The water may end up in the gaseous state