5. Chemical Energetics

Question 1

Define Standard Enthalpy change of reaction

Answer 1

The enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions.

Question 2

Define Standard Enthalpy change of formation

Answer 2

The enthalpy change when one mole of compounds are made from its elements under standard conditions.

Question 3

Define Standard Enthalpy change of combustion

Answer 3

The enthalpy change when one mole of substance burns in excess oxygen under standard conditions.

Question 4

Define Standard Enthalpy change of neutralization

Answer 4

The enthalpy change when one mole of water is formed from reacting an acid and alkali under standard conditions.

Question 5

Define activation energy

Answer 5

The minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction.

Question 6

Explain why exothermic reactions have lower activation energy than endothermic reactions.

Answer 6

In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state

Question 7

What are the standard conditions that all reactions should be taken under.

Answer 7

A pressure of 101 kPa
A temperature of 298 K (25 C)

Question 8

What is meant by a transition state?

Answer 8

The transition state is a stage during the reaction at which chemical bonds are partially broken and formed

Question 9

Define the term average bond energy

Answer 9

The energy needed to break one mole of bonds
In a gaseous molecule averaged over similar compounds