Page 28 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

Law of mass action

▪ The rate of a chemical reaction is directly proportional to the product of molar

concentrations of the reacting substances, each raised to a power equal to the number

of ions or molecules appearing in the balanced equation for the reaction.

Where;
aA + bB ↔ C + D
➢ K is the rate constant Reaction Rate α [A][B]
➢ a and b are the stoichiometric coefficients with a b
Rate = K [A] [B]
respect to compounds A and B, respectively.
➢ Assuming the following chemical reaction: A + B ↔ C + D

Formation direction (forward reaction)

Decomposition direction (backward or reverse reaction)
➢ As the reaction proceeds, the concentration of the reactants decreases, and so also the

rate at which they react decreases, the concentration of the products increases, and so

does the rate at which they form.

➢ A state of equilibrium is reached when the rate of the forward reaction is equal to the

rate of the backward reaction (Dynamic equilibrium).

Equilibrium constant (Keq)

aA+bB ↔ cC+dD

➢ The rate of the forward reaction is proportional to a b
r = K .[A] . [B]
the molar concentration of A and B (r  [A]. [B]). 1 1
1

➢ The rate of the reverse (backward) reaction r c d
2 r = K .[C] . [D]
(r  [C]. [D]). 2 2
2

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