Chemical Energetics

Question 1

Exothermic & Endothermic

Answer 1

An exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings

An endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings

Question 2

Enthalpy Change & Activation Energy

Answer 2

The transfer of thermal energy during a reaction is called the enthalpy change, ∆H, of the
reaction.

∆H is negative for exothermic reactions and positive for endothermic reactions

Activation energy, Ea, is the minimum energy that colliding particles must have to react

Question 3

Graphs of Exothermic & Endothermic Reactions

Answer 3

Exothermic Reaction:

Begins with high energy. Energy is given off as the reaction proceeds. Ends with lower energy. Net release of energy. ΔH (Change in Enthalpy): Negative

Endothermic Reaction:

Starts with low energy. Energy is absorbed as the reaction proceeds. Ends with higher energy. Net absorption of
energy. ΔH (Change in Enthalpy): Positive

Question 4

Bond Breaking & Bond Making
- Enthalpy Change (ΔH)

Answer 4

Bond Breaking:

ΔH: Positive (Endothermic)
Energy is absorbed to break bonds.

Bond Making:

ΔH: Negative (Exothermic)
Energy is released when new bonds form.