Grade 12 IEB Physical Science (Chemistry) Chemical Equilibrium (Section D) summary & notes

Chemical Equilibrium:


Closed system: Mass is conserved inside the system but energy can enter or leave the system freely

Open system: Both energy and matter can be exchanged between the system and its surroundings



Reversible reaction: Products can be converted back into reactants

(Reactions don't go to completion and occur in forward and reverse directions)


aA + bB « cC + dD
!
!


balancing compound
number




Dynamic equilibrium: When the forward and reverse reactions take place at the same rate.

• Concentrations of reactants and products remain constant

• (Need a reversible reaction and a closed system)



Example of reversible reaction:


CoCl42¡ + 6H2O ® Co(H2O)2+
6 + 4Cl
¡
¢H < 0
Blue Pink


• If energy is a product (on the right), reaction is exothermic (reverse reaction is endothermic)



• Add H2O then forward reaction favoured
¡
• Add Cl then reverse reaction favoured


exo
• Add heat: then reverse reaction favoured (because reverse reaction is endothermic)
endo

exo
• Remove heat: endo then forward reaction favoured (because forward reaction is exothermic)

, Le Chatelier's Principle: When an external stress (change in pressure, temperature or concentration)

is applied to a system in chemical equilibrium, the equilibrium point will change in such a way to

counteract the stress.



Factors that a ect chemical equilibrium:

• Temperature

• Pressure

• Concentration




Equilibrium constant:

Kc (no unit)
tconcentration


• Solids do not appear in Kc (phase solid)

• Solvents do not appear in Kc (phase liquid)

eg.
¡
N H3(g) + H2O(l) « N H4+(aq) + OH(aq)
!



!




must be closed Don't include in
container due to Kc (liquid)
NH3 (gas)