What "reversible" means
- A reversible reaction can proceed in both directions: the products can themselves combine, or break apart, to give the original reactants back again
- The forward reaction turns reactants into products
- The reverse (backward) reaction turns products back into reactants
- A reversible equation uses a special two-headed arrow, ⇌, instead of the single arrow → used for one-way reactions
- The top half-arrow points right (forward); the bottom half-arrow points left (reverse)
- Whether the reaction settles toward the products side or toward the reactants side depends on the conditions — temperature, pressure, concentration
Example — thermal decomposition of ammonium chloride
- Heat solid ammonium chloride and it breaks down into two colourless gases:
NH4Cl(s) → NH3(g) + HCl(g)
- Lift the test tube away from the heat and the gases meet cooler glass higher up; the two recombine and a white solid of ammonium chloride condenses on the wall:
NH3(g) + HCl(g) → NH4Cl(s)
- The same chemistry written as a reversible equation:
NH4Cl(s) ⇌ NH3(g) + HCl(g)
- The forward step is endothermic (heating drives it); the reverse step is exothermic (cooling drives it)
Example — hydration of copper(II) sulfate
- Many ionic salts crystallise with water locked into their crystal lattice — this water is called water of crystallisation, and the salt is described as hydrated
- Blue copper(II) sulfate crystals are the pentahydrate, CuSO4·5H2O
- Heat the blue crystals strongly and the water is driven off, leaving the white solid called anhydrous copper(II) sulfate, CuSO4
- Add water back to the white anhydrous powder and the blue hydrate forms again, with a noticeable release of heat
- Written as a reversible reaction:
CuSO4·5H2O(s) ⇌ CuSO4(s) + 5 H2O(l)
- The forward step (dehydration) is endothermic; the reverse step (rehydration) is strongly exothermic — which is why anhydrous copper(II) sulfate is also the chemical test for the presence of water (see topic 17 section 5)