CHEM C7: Equilibria (Introduction)

In this (long-delayed) post, I’ll introduce Chemical Equilibria. We’ll cover:

  • Reversible reactions
  • Equilibrium
    • Dynamic equilibrium

Let’s start!


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 reactionA + B → C + D
The reverse reactionC + D → A + B

The whole equation is shown as:
A + B C + D

There are 2 kinds of reversible reactions:

Non-equilibrium reactionsForward & reverse reactions occur separately, not at the same timeConsider 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 reactionsForward & reverse reactions occur at the same timeConsider 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.

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