Higher chemistry: Enthalpy

What is enthalpy (H)?

The heat stored by a substance.

What does ΔH represent in a chemical reaction?

The overall enthalpy change for a reaction.

What type of reaction releases energy to the surroundings?

Exothermic reaction.

What happens to the temperature in an exothermic reaction?

The temperature rises.

Provide an example of an exothermic reaction.

Neutralisation reactions or burning magnesium.

What type of reaction absorbs energy from the surroundings?

Endothermic reaction.

What happens to the temperature in an endothermic reaction?

The temperature decreases.

Provide an example of an endothermic reaction.

The reaction of barium hydroxide and ammonium thiocyanate.

In exothermic reactions, what is the sign of ΔH?

Negative.

In endothermic reactions, what is the sign of ΔH?

Positive.

What is activation energy (EA)?

The energy required to start the reaction.

What equation is used to calculate enthalpy change?

Enthalpy change = enthalpy of products - enthalpy of reactants.

What is an activated complex?

A high energy intermediate state where bonds are breaking and forming.

What effect does a catalyst have on activation energy?

It lowers the activation energy.

What does a potential energy diagram show in relation to chemical reactions?

It can be used to calculate both the enthalpy change and the activation energy.