In this (long-delayed) post, I’ll introduce Chemical Equilibria. We’ll cover:
- Reversible reactions
- Dynamic equilibrium
This whole chapter relies on this fact:
Chemical reactions are NOT limited to one direction.
- Some reactions cannot be reversed:
they go into completion (products cannot be converted into reactants)
- Some reactions CAN be reversed:
these are called REVERSIBLE REACTIONS
What is a reversible reaction?
A chemical reaction in which the products can react to form their original reactants.
In this case, there are 2 reactions:
|The forward reaction||A + B → C + D|
|The reverse reaction||C + D → A + B|
The whole equation is shown as:
A + B ⇌ C + D
There are 2 kinds of reversible reactions:
|Non-equilibrium reactions||Forward & reverse reactions occur separately, not at the same time||Consider copper (II) sulphate. |
CuSO4 (s) + 5H2O (l) ⇌ CuSO4.5H2O (s)
Water can be added to form a hydrated salt. The hydrated salt can be heated to return to its anhydrous form.
In this case, the copper sulphate does not continuously shift between hydrated & anhydrous states. The reactions must happen separately.
|Equilibrium reactions||Forward & reverse reactions occur at the same time||Consider the Haber process. |
H2 (g) + N2 (g) ⇌ NH2 (g)
Hydrogen & nitrogen gas continuously react to form ammonia. At the SAME TIME, ammonia continuously breaks apart into hydrogen & nitrogen gas. Both reactions occur simultaneously.
We will focus on equilibrium reactions now.
What is equilibrium?
The state when a reaction has no overall change from reactants to products (or products to reactants).
Most equilibria are described as a DYNAMIC EQUILIBRIUM: where there are still forward and reverse reactions occurring, but they balance out.
How to describe a dynamic equilibrium?
- Rate of forward reaction is equal to the rate of reverse reaction
- The concentration of reactants & products remains constant
- Requires a closed system (no reactants or products enter or escape)
When a reversible reaction reaches equilibrium, the mixture is called an EQUILIBRIUM MIXTURE.
In the next post, we’ll look into equilibrium mixtures with more detail.