Energy change, rates of reactions and chemical equilibrium

Heat of a reaction

The net change of chemical potential energy of the system

Exothermic reactions

Reactions which transform chemical potential energy into thermal energy

Endothermic reactions

Reactions which transform thermal energy into chemical potential energy

Activation energy

The minimum energy required to start a chemical reaction.

Activated complex

A high energy, unstable, temporary transition state between the reactants and the products.

Reaction rate

The change in concentration per unit time of either a reactant or product

Catalyst

A substance that increases the rate of the reaction but remains unchanged at the end of the reaction.

Closed system (in chemistry)

One in which mass is conserved inside the system but energy can enter or leave the system freely

Open system (in chemistry)

One in which the both the energy and matter can be exchanged between the system and its surroundings

State Le Chatelier’s principle

When an external stress (change in pressure, temperature, or concentration) is applied to a system in dynamic chemical equilibrium, the equilibrium point will change in such a way as to counteract the stress.