Summary Grade 12 IEB Physical Sciences - Energy Changes in Reactions Notes (Chemistry Section C 1,2)

Energy changes in reactions
Enthalpy: A measure of the internal energy (Ep) of the reactants or products
Enthalpy: Symbol = H

Enthalpy of reaction: Heat of reaction
Enthalpy of reaction: Symbol = ΔH (kg/mol)

Heat of reaction
= The net change in chemical potential energy of the system
= (The system is all chemicals involved in the reaction)

Temperature of a substance: The average kinetic energy of the composing particles

Exothermic
Temperature increased
Therefore, average Ek increased
Thus, Ep decreased
= ΔH < 0 (negative)

Endothermic
Temperature decreased
Therefore, average Ek decreased
Thus, Ep increased
= ΔH > 0 (positive)

Exothermic: Chemical potential energy is transformed into thermal energy
ΔH < 0
Echemp Ek

Endothermic: Thermal energy is transformed into chemical potential energy
ΔH > 0
Ek Echemp


Activation energy: Ea
The minimum energy required to start a chemical reaction.
Colliding molecules must have the kinetic energy equal to or larger than the Ea in order to react.

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

Energy is absorbed when bonds or forces are broken.
Energy is released when bonds or forces are formed.