Chemistry

Chemical Equilibrium

4,762 words, 11 sections

Chemical equilibrium is where chemistry stops pretending that reactions run in one direction. The unit sits near the centre of physical chemistry on the paper: the bank holds 216 distinct questions on it, drawn from 85 official papers between 2000 and 2026 and set by all six boards, Balochistan, DUHS, Federal, KMU, NUMS and UHS. Chemistry carries 45 of the 180 marks, and within this chapter dynamic equilibrium with Le Chatelier's principle accounts for 84 questions, the equilibrium constants $K_c$ and $K_p$ for 63, solution equilibrium covering solubility and buffers for 49, and the Haber process for 20. Mean item difficulty is 1.79 on the bank scale, and 994 active questions sit in the practice bank. A student who has this chapter under control can predict which way a system shifts when it is pushed, calculate $K_c$ from a table of equilibrium concentrations, and turn a $K_a$ or a $K_{sp}$ into a pH or a solubility without hesitating over the algebra.

A reversible reaction is one in which the products are reactive enough to re-form the reactants, so the reaction never runs to completion. The forward reaction is the one written left to right in the equation, and the reverse reaction runs right to left; both happen in the same vessel at the same time. Write the synthesis of ammonia as $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ and the forward direction builds ammonia while the reverse direction breaks it back down. At the instant of mixing, the forward rate is at its largest and the reverse rate is zero, because there is no product yet to decompose; as products accumulate the reverse rate climbs and the forward rate falls, and equilibrium is the state the two rates reach when they are equal.

That state is dynamic, not static. The forward and reverse reactions have not stopped; they run at the same rate, so every molecule of ammonia that forms is matched by one that breaks down, and the concentrations of reactants and products stay constant. The rate relationship at equilibrium is $k_f[A]^a[B]^b = k_r[C]^c[D]^d$ , which rearranges to $\frac{k_f}{k_r}=\frac{[C]^c[D]^d}{[A]^a[B]^b}$ , the ratio of the two rate constants. Macroscopically the mixture looks finished, constant colour, constant pressure, constant concentration; microscopically it is as busy as ever.

A catalyst changes only how quickly equilibrium arrives. It lowers the activation energy of the forward and the reverse step by the same amount, so both rates rise together and the ratio that defines the equilibrium constant does not move. That is why a catalyst is used to reach the same equilibrium in less time, never to change where the equilibrium sits.

Worked example

A sealed flask is charged with $A$ only, and the reversible reaction $A(g) \rightleftharpoons B(g)$ begins. At the instant of mixing the forward rate is at its maximum and the reverse rate is zero. Describe how the two rates change and what the flask contains once equilibrium is established. As $B$ forms, the reverse reaction has something to work with, so its rate rises while the concentration of $A$ falls and the forward rate drops. Equilibrium is the point where the two curves cross, after which both rates are equal and the concentrations of $A$ and $B$ are constant. The step students get wrong is treating the crossing point as the point where the concentrations become equal; equal rates and equal concentrations are different conditions unless the stoichiometry and the constant happen to make them coincide.

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Introduction

1. Reversible reactions and dynamic equilibriumA reversible reaction is one in which the products are reactive enough to re-form the reactants, so the reaction never runs to completion. The forward reaction