Question

Write the equilibrium constant expression for the decomposition of the baking soda. Deduce the unit from the above expression.

Answer 1

The equilibrium constant, Kc, is the ratio of the equilibrium concentrations of products over the equilibrium concentrations of reactants each raised to the power of their stoichiometric coefficients. The ratio of concentration of products and reactants at equilibrium will be the required equilibrium constant

Complete step-by-step answer: We know that The equilibrium constant of a chemical reaction (usually denoted by the symbol K) provides insight into the relationship between the products and reactants when a chemical reaction reaches equilibrium. For example, the equilibrium constant of concentration (denoted by Kc) of a chemical reaction at equilibrium can be defined as the ratio of the concentration of products to the concentration of the reactants, each raised to their respective stoichiometric coefficients
So the decomposition reaction of baking soda which is sodium bicarbonate is as follows
$2NaHCO_3(s) \rightleftharpoons Na_2CO_3(s)+Co_2(g)+H_2O(g)$
The equilibrium constant can be written as
$K_c = \dfrac{[Na_2CO_3][CO_2][H_2O]}{{[Na_2CO_3]^2}}$
Pure solid concentration is taken as unity
Then $K_c = [CO_2][H_2O]$
The units of $K_c$ are = $(mol\,\, dm^{-3}.\,\,mol\,\,\, dm^{-3})$
So the units of $K_c$ are = $mol^{2}\,\,dm^{-6}$
So the equilibrium constant $K_c$ = $[CO_2][H_2O]$ and the units are $mol^{2}\,\,dm^{-6}$

Note: For a chemical reaction, the equilibrium constant can be defined as the ratio between the amount of reactant and the amount of product which is used to determine chemical behaviour.
At equilibrium, Rate of the forward reaction = Rate of the backward reaction