Form 6 STPM students by now would have been familiar with the notion of chemical reactions. However, little thoughts have been given to the “direction” and “rate” of the reactions. If reactant A and B can be converted to product C (forward reaction), will product C become a ‘reactant’ and be converted back to ‘products’ A and B (backward reaction)? If so, how ‘fast’. Those are simple terms used by layman. And like many physical phenomena in this universe, chemical reactions moves towards equilibrium too.
So Berry Berry Easy would like to share with our Berry Readers about Chemical Equilibrium. This will be the first part of the Chemical Equilibrium chapter. Before we start, the key phrase to note is “no net change”. So chemical equilibrium is simply the state of a chemical reaction where activities and concentration of both the reactants and products involved do not have net change over time, due to the forward and backward reaction being the same rate. So try to learn the nuances involved.
STPM Form 6 – Terminology and Concepts: Chemical Equilibrium – Part 1
1) Reversible Reaction
- Chemical reaction is in both directions.
- Forward reaction: from left to right.
- Reverse reaction: from right to left.
- React in a closed container.
- Reversible reaction is never complete (a mixture of reactants and products).
- An intermediate position (equilibrium) is reached.
2. Dynamic Equilibrium
- Reversible reaction is at a position of dynamic equilibrium.
- the rate of the forward reaction = rate of the reverse reaction.
- no change is observed (concentrations of the reactants and the products remain unchanged.)
- all the external conditions remain unchanged (closed container).
3. The Equilibrium Law and the Equilibrium Constant
- Equilibrium expression / equilibrium equation,
- Kc = [product]x / [reactant]y
- Forward reaction:
- aA + bB <—-> cC + dD
Equilibrium expression / equilibrium equation,
Kc = [C]c[D]d / [A]a[B]b
[A] = concentration of A
[B] = concentration of B
[C] = concentration of C
[D] = concentration of D
Kc = equilibrium constant for the reaction (in terms of concentration of the reactant) and the units depend on the chemical equation.
- Reverse reaction:
cC + dD <—-> aA + bB
Equilibrium expression / equilibrium equation
The equilibrium constant, Kc‘ = [A]a[B]b / [C]c[D]d
- This relationship is equilibrium law / equilibrium expression where:
Equilibrium constant of the reverse reaction is the reciprocal of the equilibrium constant of the forward reaction, Kc‘ = 1 / Kc
- Equilibrium constant is constant at a particular temperature.
- Exothermic reaction (forward reaction), the value of the equilibrium constant decreases as temperature increases.
- Endothermic reaction (forward reaction), the value of the equilibrium constant increases as temperature increases.
- Reaction quotients, Q (predicting the direction of reaction)
Q = Kc, no net reaction will take place (the system is at equilibrium).
Q < Kc, a net forward reaction to the right occurs.
(If the value Kc (equilibrium constant) is very large, the equilibrium position lies very much to the right and the yield of the products are higher than the reactants.
Q > Kc, a net reverse reaction to the left occurs.
(If the value Kc (equilibrium constant) is very small, the equilibrium position lies more to the left and the the yield of the products are lower than the reactants.
So stay log on to Berry Berry Easy for part 2 of this Chemical Equilibrium chapter. It’ll cover the basic of what you need to know to master this chapter.