1. Equilibrium Constant
Chapter 12.1

2. Video: chemistry crash course #28
Review Equilibrium
For a summary of equilibrium up until this point (qualitative equilibrium)
Video: chemistry crash course #28

3. Equilibrium
Chemical equilibrium is a dynamic reaction that results from two opposing chemical reactions that are occurring at the same rate.
The state of equilibrium can be expressed quantitatively by a constant ratio that is called the equilibrium constant (Kc). At a given temperature, there is a constant ratio between the product and reactant concentrations (the concentration of each substance is raised to the power of its stoichiometric coefficient) when a simple chemical reaction is at equilibrium.

4. Remember: r=k[reactants] OR r=k[products]
Equilibrium Constant
When we know the rate of the forward and reverse reactions we can deduce the expression for the equilibrium constant.
Remember: r=k[reactants] OR r=k[products]
When the rate law is applied to both the forward and the reverse reactions, the equilibrium constant can be isolated.

5. No Solids or Liquids!
It is important to know the phase of the reactants and products in the chemical reaction.
When calculating the equilibrium constant, only the gaseous and aqueous substances are taken into account because the quantity of a liquid or a solid cannot be measured as a concentration.

6. Examples C(s) + 2H2 (g)  CH4 (g)
SnO2 (s) + 2CO (g)  Sn (s) + 2CO2 (g)

7. Reverse?
Notice that the substances that are in the solid phase are not used to determine the equilibrium constant.
The expressions found above correspond to the direct (or forward) reaction.
If you want to find the equilibrium constant for the reverse reaction, take the reciprocal.

8. What is Favoured?
The equilibrium constant allows us to better understand or evaluate the progress of a reaction.
When the equilibrium constant is very high, Kc >>1, the system favours the direct reaction.
When the equilibrium constant is very low, Kc << 1, the system favours the reverse reaction.
When the equilibrium constant is close to one, Kc ≈ 1, the system does not favour the direct or the reverse reaction.
If Kc  1010, the reaction is considered to be complete.
If Kc  10-10, the reaction cannot occur.

9. Temperature
How does a change in temperature affect the value of the equilibrium constant?
The equilibrium constant will increase or decrease depending on whether the change in equilibrium favours the forward or reverse reaction.
Remember: an increase in temperature will always favour the endothermic reaction and a decrease in temperature will always favour the exothermic reaction.

10. Temperature In the case of an exothermic reaction: A  B + energy
The lower the temperature, the greater the equilibrium constant. This means that the forward reaction is favoured.
An increase in temperature will decrease the value of the equilibrium constant. This means that the reverse reaction will be favoured.

11. Temperature In the case of an endothermic reaction: A + energy  B
The higher the temperature, the greater the equilibrium constant. This means that the forward reaction is favoured.
A decrease in temperature will decrease the equilibrium constant. This means that the reverse reaction is favoured.

12. Concentration
How can we determine the concentrations of the reactants and products of a chemical reaction at equilibrium?
Examples done in class…
There will be some posted here in the near future…