5.6 - Reversible Reactions & Equilibrium

Reversible reactions

• In a reversible reaction, the products can react together to re-form the original reactants, so the reaction can run in either direction.
• A reversible reaction is written with the ⇌ symbol in place of the usual arrow:
  • A + B ⇌ C + D

The direction a reversible reaction runs in, and how far it goes, depends on the conditions (such as temperature, pressure, and concentration).

Example - Heating ammonium chloride

• Heating white solid ammonium chloride breaks it down into ammonia and hydrogen chloride (both colourless gases).
• Cooling the gases lets them recombine into solid ammonium chloride again.
• Ammonium chloride ⇌ ammonia + hydrogen chloride

Energy changes in reversible reactions

• If a reversible reaction is exothermic in one direction, it is endothermic in the other.
• The same amount of energy is transferred in each direction: the energy taken in by the endothermic reaction equals the energy released by the exothermic reaction.

Hydrated and anhydrous compounds

• Hydrated - A compound with water chemically combined within its crystals
• Anhydrous - A compound with that water removed

Example - Copper(II) sulfate

• Heating blue hydrated copper(II) sulfate crystals drives off water to leave white anhydrous copper(II) sulfate powder. This is endothermic.
• Adding water back to the white powder turns it blue again and releases heat. This is exothermic.
• Hydrated copper(II) sulfate (blue) ⇌ anhydrous copper(II) sulfate (white) + water