Grade 12 IEB Physical Science (Chemistry) Energy Change & Rates of Reactions (Section C) summary & notes

Energy Change & Rates of Reactions:


Enthalpy: (H) Measure of the internal energy (Ep) of reactants or products

Heat of reaction: Net change of chemical potential energy of the system ¢H



Temperature: Average kinetic energy of composing particles



Exothermic reaction: Reaction which transforms chemical potential energy into thermal energy

(Energy released: ¢H < 0 )
Endothermic reaction: Reaction which transforms thermal energy into chemical potential

energy (Energy ¢H > 0 ) absorbed:




• When heat is absorbed by a chemical reaction, Ep will increase and temperature will decrease (Ek

decrease)

• When heat is released by a chemical reaction, the temperature will increase (Ek increase) and Ep

will decrease



Activation energy (Ea): Minimum energy required to start a chemical reaction

Activated complex: High energy, unstable, temporary transition state between reactants and

products

, Collision theory:

In order for reaction to take place, colliding molecules must have:

• Correct orientation

• Kinetic energy equal to or bigger than the activation energy of reaction



• If Ek1 + Ek2 ¸ Ea then a successful collision occurs

• If Ek1 + Ek2 < Ea then no reaction is possible



Catalyst: Substance that increases the rate of reaction but remains unchanged at the end of the

reaction

(Catalyst works by providing an alternate path between reactants and products. This path has a

lower activation energy)




Rate of reaction: Change in concentration per unit time of either a reactant or product
¢c
• Rate =
¢t


• Factors that a ect rate:

- Nature of reacting substances (can't change)

- Temperature

- Catalyst

- Pressure (gas)

- Concentration (liquid)