A reversible reaction is one in which can proceed in both directions i.e. forward and reverse reactions.
The reactants are not completely converted to products. Instead, an intermediate position or EQUILIBRIUM is reached whereby both reactants and products are present.
This also means that reversible reactions are never complete, a mixture of reactants and products is obtained.
At equilibrium in a reversible reaction:
1. Rate of forward reaction = Rate of reverse reaction
2. Concentration of the reactants and the products are unchanged
It is important to know what is the EQUILIBRIUM CONSTANT, which is defined as the measure of the extent to which the reaction are coverted into products before equilibrium is reached.
Consider the equilibrium, aA + bB <=> cC + dD
Equilibrium constant, Kc = [C]c[D]d / [A]a[B]b and is a constant at a given temperature
For reactions involving gases, the equilibrium constant can be represented in terms of the partial pressure, P, of the gases and is give by the symbol, Kp.
Consider the equilibrium, aA (g) + bB (g) <=> cC (g) + dD (g)
Equilibrium constant, Kp = (PC)c (PD)d / (PA)a (PB)b and is a constant at a given temperature
Something to note for Partial Pressure:
Partial pressure, P, of a gas is equal to the product of its mole fraction and the total pressure, PT.
Example, in a mixture of two gases A and B (where nx = no. of moles of gas X)
Partial pressure of Gas A, PA = [ nA / (nA + nB) ] x PT
1. Concentration of a SOLID is constant, and is omitted in expressions of Kc and Kp.
2. State symbols are not essential in writing the expressions for equilibrium constant.
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