Reversible reactions and equilibria

Some chemical changes refuse to be one-way journeys. Heat a blue crystal and it turns white; add water and the blue returns. Burn fuel and the products sit there forever; but split ammonium chloride by heating and the two products quietly recombine the moment they cool. A reversible-reaction is a reaction whose products can turn back into the original reactants — and once you start looking, you find them everywhere in chemistry.

This chapter has one driving question. If a reaction can go both ways, how do you arrange conditions so that the mixture ends up containing as much as possible of the substance you want? That question runs the whole chapter: first the idea of reversibility and its arrow, then what happens inside a sealed container where the two opposing changes fight to a stand-off called dynamic-equilibrium, and finally how shifting temperature, pressure or a catalyst moves that stand-off one way or the other.

The chapter runs on two tracks. The core track (assessed in both Paper 1C and Paper 2C) covers reversibility and the special double arrow ⇌ used in equations, plus two named reversible reactions in full: the dehydration of hydrated copper(II) sulfate and the effect of heat on ammonium chloride. The chemistry-only track (Paper 2C) builds on that foundation: dynamic equilibrium in a sealed container, the two characteristics that define it, why a catalyst leaves the position of equilibrium untouched, and how changing temperature or pressure moves the position of equilibrium in a predictable direction.

Two habits will carry you through. First, always ask "which direction?" — for any change you make to a reversible reaction, the mixture responds in a direction, and naming that direction precisely (towards reactants or towards products) is the mark-earning skill. Second, keep the two words in dynamic equilibrium separate in your mind: dynamic means both reactions never stop, equilibrium means nothing overall changes. Every exam answer in this chapter is built from those two halves.