Keys for CHEMICAL EQUILIBRIUM in AP Chemistry exams

Chemical Equilibrium plays an very important role in AP chemistry exams. It appears very often in both multiple choice and free response sections. Following are the keys to chemical equilibrium:

1. All chemical equilibrium are dynamic equilibrium. At macro level, all concentration of reactants and products are constant.  At molecular level, the reaction is still going on in both forward and backward direction. The same amount of reactants consumed in the forward reaction are generated in the backward reaction, making their concentrations constant. It is the same case with the products.

2. For a particular reaction:

the equilibrium constant K is expressed as:

Where [A], [B], [C] and [D] are concentration at equilibrium in molarity, a, b, c and d are the corresponding coefficients in the balanced chemical equation. 

Pure solid, liquid and water are not included in the equilibrium expression.

3. If K>1, the forward reaction is favoured and the forward reaction is spontaneous. If K<1, the backward reaction is favoured and the backward reaction is spontaneous.

4. If the reaction is reversed, the equilibrium constant is the reciprocal of the original one.

5. If the balanced reaction is multiplied by a factor, the equilibrium constant is raised to that power.

6. If several equations are used to obtain a net equation, the equilibrium constant of the net equation is the product of the equilibrium constants of each individual equation.

7. For a particular type of  reaction, the equilibrium constant is given a particular name.

• for a weak acid, the equilibrium constant is called the acid dissociation constant Ka.  

• for a weak base, the equilibrium constant is called the base dissociation constant Kb. 

• for a compound with very low solubility, the equilibrium constant is called the solubility product Ksp. 

• for a complexation ion, the equilibrium constant is called the formation constant Kf. and the reciprocal is called the dissociation constant for the complexation ion.

8. For a particular reaction:

the reaction quotient is expressed as:

Where [A], [B], [C] and [D] are concentration at a particular moment (NOT AT EQUILIBRIUM) in molarity, a, b, c and d are the corresponding coefficients in the balanced chemical equation. 

• If Q<K, the reaction is not at equilibrium and the formation of the products is favoured.
• If Q=K, the reaction is at equilibrium.
• If Q>K, the reaction is not at equilibrium and the formation of the reactants is favoured.

9. Le Chatelier's Principle is used to predict the direction of the shift of the equilibrium when changes occur to the system. Le Chatelier's Principle states: " A change in the factors that govern the chemical equilibrium of a system will cause the system to respond in a manner to counteract the change."

• Increased temperature will favour the endothermic direction, while decreased temperature will shift the equilibrium towards the exothermic direction. A reaction is endothermic if it absorbs heat and a reaction is exothermic if it gives out heat. If a reaction is endothermic in the forward reaction, it is exothermic in the backward direction and vice versa.
• Increase in reactants concentration will shift the equilibrium towards the products and decrease in reactants concentration will shift the equilibrium towards the backward direction. Increase in products concentration will shift the equilibrium towards the reactants and decrease in products concentration will shift the equilibrium towards the product direction. 
• Volume and pressure affect the equilibrium when gases are involved. Increase in volume (or decrease in pressure) will shift towards the side with more gases (the side with the bigger sum of coefficients of gases) and vice versa. If a gas that is not participating the reaction was introduced to the equilibrium, even there is volume or pressure change, it will not shift the equilibrium in any direction. The reaction. will stay in equilibrium.

10. At the same temperature, the equilibrium constant is a constant. It only changes when temperature changes.

Hopefully the above summary will clear your vision regarding Chemical Equilibrium in AP Chemistry exams.  Another blog focussing on calculations involving chemical equilibrium is to follow.

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