C2aL15 Reversible Reactions

Key Words

Backwards - products turn into reactants.

Chemical reaction - chemical interaction in which new substances are formed.

Forward - reaction makes products

Products - new substances made in a reaction

Reactants - substances that react in the reaction.

Reversible reaction - reaction that can go forwards or backwards.

Water of crystallisation - water trapped in the crystal lattice.

Grade E

Some reactions are reversible.  That means that as well as going forwards, the reaction can go backwards:

reactant 1 + reactant 2  ® product 1 + product 2  (forwards)

product 1 + product 2  ® reactant 1 + reactant 2 (backwards)

For example:

ammonium chloride D ammonia + hydrogen chloride

NH₄Cl  D NH₃ + HCl

You can heat ammonium chloride.  It will decompose into ammonia and hydrogen chloride.  When cool enough, the two gases will react to form ammonium chloride again.

Grade C

Copper sulphate crystals are blue.  This is because there is water in the crystal structure, water of crystallisation.  If we heat copper sulphate, it turns white, forming anhydrous copper sulphate.  If we later add water, the crystals become blue, and heat is given off.

hydrated copper sulphate D anhydrous copper sulphate + water

CuSO₄.5H₂O D CuSO₄ + 5H₂O

A similar thing is seen with cobalt chloride paper.  It turns from blue to pink in the presence of water.  Cobalt chloride paper can end up going pink as it picks up moisture from the air.  It can be "refreshed" by heating in a warm oven.

hydrated cobalt chloride D anhydrous cobalt chloride + water

Grade A

Reversible reactions don't always go to completion.  Chemists optimise the conditions so that the yield of wanted product is as high as possible.