Chemical Equilibria

A reversible reaction can proceed in both directions under appropriate conditions. In a closed system initially containing reactants, the forward reaction forms products. As products accumulate, the reverse reaction becomes possible and its rate increases. Dynamic equilibrium is reached when the forward and reverse rates are equal, so macroscopic properties such as concentration and colour remain constant.

A closed system prevents material loss; it need not be thermally insulated. Constant temperature is needed when discussing one fixed equilibrium constant. The same equilibrium composition can be approached from either side if the overall amounts, temperature and volume or pressure constraints are the same.

Le Chatelier's Principle: when a system at equilibrium is disturbed, its position changes in the direction that tends to oppose that disturbance. The response usually offsets only part of the imposed change. Adding a reactant does not mean that all of the added material disappears.

Compression affects a gaseous equilibrium through the changed partial pressures. If both sides have the same total gaseous coefficient, as in H₂(g) + I₂(g) ⇌ 2HI(g), compression does not shift the ideal-gas equilibrium position. Pure solids and liquids are not counted as gaseous particles.

Adding an inert gas at constant volume and temperature raises total pressure but leaves each reacting gas partial pressure unchanged, so it causes no shift in the ideal-gas model. At constant total pressure, adding inert gas expands the mixture and lowers reacting partial pressures; the side with more gas moles is then favoured. State the constraint rather than using total pressure alone.

On a concentration-time graph, adding a species produces an immediate jump in its concentration, followed by a gradual reaction-driven adjustment. A volume decrease immediately increases all gaseous concentrations before any shift occurs. Distinguish that instantaneous physical change from the subsequent change in composition.

A catalyst changes kinetics through an alternative mechanism. It does not alter the thermodynamic free-energy difference between the reactants and products and therefore does not change K. An industrial catalyst may allow a different operating temperature; it is that temperature change, not the catalyst itself, that can alter the equilibrium yield.

For aA + bB ⇌ cC + dD, the concentration expression is K_c = [C]^c[D]^d / ([A]^a[B]^b), using equilibrium concentrations. K_p uses equilibrium partial pressures of gaseous species in the same pattern. Pure solids and pure liquids are omitted because their activities are constant; a dissolved species is not omitted merely because it is in a liquid solution.

In conventional examination expressions, derive any units from the concentration or pressure powers and use one consistent pressure unit. For N₂O₄(g) ⇌ 2NO₂(g), the concentration expression has units mol dm^-3 and a pressure expression in kPa has units kPa. The numerical pressure-based value changes if the pressure unit changes, so report the unit used.

Reversing a reaction takes the reciprocal of K; multiplying all coefficients by a factor raises K to that power. A large K favours products relative to reactants in its expression, but gives no direct information about the speed of reaching equilibrium. The relationship converting K_p into K_c is not required in this syllabus.

A useful table is not a substitute for checking the chemistry. Include any initial product, account for unequal coefficients, and use the actual equilibrium volume. When gaseous mole number changes, total pressure cannot be split using initial mole fractions. The required syllabus calculations do not need quadratic-equation solutions.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is exothermic, with ΔH about -92 kJ for the equation as written. Lower temperature favours the equilibrium amount of ammonia; higher pressure favours the side with fewer gaseous particles. Neither statement alone specifies the best operating plant.

The quoted conditions are representative, not a universal specification for every plant. In a data question, compare the actual equilibrium-yield curve, reaction-rate information, energy use and equipment constraints supplied. Keep the explanations separate: temperature affects both K and rate; a catalyst affects rate; recycling improves overall conversion.

Dynamic equilibrium means equal forward and reverse rates. A concentration or pressure disturbance can change composition while K stays fixed. Only temperature changes K for the same written reaction; a catalyst speeds approach without changing the endpoint.

Write K from the balanced equation, use equilibrium values and respect phases. Build an amount-change table before converting to concentrations or partial pressures. For Haber, explain yield, rate, compression cost and recycling as distinct considerations.