8.1    Dynamic Equilibria

Reversible reactions

• A reversible reaction is one which can proceed in both directions, as indicated by the “⇌” sign. For example:

H2 + I2 ⇌ 2HI                2SO2 + O2 ⇌ 2SO3

• In a reversible reaction, the reactants react to form the products. The products in turn also react to re-form the reactants. A mixture of reactants and products is present, an equilibrium is established between them
• Whether a reaction is reversible or not depends on its activation If the activation energy of the backward reaction is very high, the reaction is not reversible.

Characteristics of equilibrium

• An equilibrium have four particular features under constant conditions:

i.   It is dynamic

• Dynamic equilibrium is a situation where the rate of forward reaction becomes equal to the rate of reverse reaction and the amount of substance present to do not vary with time as long as the conditions remain the same
• In the microscopic scale, the reactants and products are continuously reacting but in the macroscopic scale, there seem to be no change at all

ii.     The concentrations of  the reactants and products remain  constant

• This is because at equilibrium, the rate of forward reaction equals to the rate of backward reaction
• For example, H2 + I2 ⇌ 2HI

At equilibrium, for every one mole of the reactants get converted to two moles of products, two moles of the products are re-forming the  reactants.

iii.        It requires a closed system

• A closed system is one which none of the substances can be added or removed, but energy can move in and out freely
• In an open system, the products formed will escape, this will not allow them to react and re-form the reactants, an equilibrium will never be reached.