Chemistry - ENERGY CHANGES (C5)

Exothermic & Endothermic reactions -

What is an exothermic reaction?

A reaction that transfers energy to the surroundings, usually as heat, so the temperature increases.

Exothermic & Endothermic reactions -

Give two examples of exothermic reactions.

Combustion, neutralisation, many oxidation reactions.

Exothermic & Endothermic reactions -

What is an endothermic reaction?

A reaction that takes in energy from the surroundings, so the temperature decreases.

Exothermic & Endothermic reactions -

Give two examples of endothermic reactions.

Thermal decomposition, reaction of citric acid + sodium hydrogen carbonate.

Exothermic & Endothermic reactions -

What are some uses of exothermic reactions?

Hand warmers + self-heating cans.

Exothermic & Endothermic reactions -

What are some uses of endothermic reactions?

Instant cold packs for injuries.

Reaction Profiles (Energy Level Diagrams) -

What does a reaction profile show?

The relative energies of reactants + products + the activation energy.

Reaction Profiles (Energy Level Diagrams) -

In an exothermic reaction profile, are products higher / lower in energy than reactants?

Lower - because energy’s been released.

Reaction Profiles (Energy Level Diagrams) -

In an endothermic reaction profile, are products higher / lower in energy than reactants?

Higher - because energy’s been absorbed.

Reaction Profiles (Energy Level Diagrams) -

What is activation energy?

The minimum amount of energy needed for a reaction to start.

Bond Energy Calculations -

What happens to bonds in a chemical reaction?

Bonds are broken in reactants (requires energy), + new bonds - formed in products (releases energy).

Bond Energy Calculations -

Is bond breaking exothermic / endothermic?

Endothermic - it requires energy.

Bond Energy Calculations -

Is bond forming exothermic / endothermic?

Exothermic - it releases energy.

Bond Energy Calculations -

How do you calculate the overall energy change of a reaction?

Energy change = Energy in (bonds broken) - Energy out (bonds made)

Bond Energy Calculations -

What does a negative energy change mean?

The reaction is exothermic.

Bond Energy Calculations -

What does a positive energy change mean?

The reaction is endothermic.

Practical Work: Temperature Changes in Reactions -

What simple experiment can measure energy change in reactions?

Measuring the temperature change of a solution using a thermometer when chemicals react.

Practical Work: Temperature Changes in Reactions -

What are the variables in a temperature change experiment?

Independent: Reactants ( / concentration)

Dependent: Temperature change

Control: Volume, starting temperature

Practical Work: Temperature Changes in Reactions -

How do you reduce heat loss in this experiment?

Use a polystyrene cup, lid, + insulate the container.