2.1.4 (c) Neutralisation Reactions

The simple definition of an acid is an H⁺ ion donor.

The simple definition of a base is an H⁺ ion acceptor.

Soluble bases form alkalis and these accept H⁺ because they produce OH^- ions.

So and Acid-Alkali neutralisation is always H⁺_(aq)+ OH^-_(aq) → H₂O_(l)

_{It doesn't really matter which acid is reacting with which base.}

HCl _(aq)+ NaOH_(aq) → H₂O_(l) + NaCl_(aq)

This is unrealistic because the HCl isn't associated when it reacts - it has ionised into H⁺ and Cl^- ions.

And the NaOH isn't associated when it reacts - it has ionised into Na⁺ and OH^- ions.

Even the NaCl produced isn't associated - it has ionised into Na⁺ and Cl^- ions.

So it would be more realistic to write:

H⁺_(aq) +Cl^-_(aq)+ Na⁺_(aq) + OH^-_(aq) → H₂O_(l) + Na⁺_(aq) + Cl^-_(aq)

And if we remove the ions that haven't actually changed - the spectator ions...

H⁺_(aq) +Cl^-_(aq)+Na⁺_(aq) + OH^-_(aq) → H₂O_(l) +Na⁺_(aq) +Cl^-_(aq)

So we're left with the same ionic equation for any pair of acids and alkalis.

H⁺_(aq)+ OH^-_(aq) → H₂O_(l)

In Year 9 you (hopefully) learned that:

Acid + Base → Salt + Water

Acid + Alkali → Salt + Water

Acid + Carbonate → Salt + Water + Carbon Dioxide

You may also remember that most Bases are Metal Oxides and Alkalis are Metal Hydroxides.

You should certainly still be able to name the salts involved.

The difference between GCSE and A level is that examiners will expect you to write a full chemical equation more frequently.

This requires you to be able to work out the formulae of the salts formed or the balancing will be incorrect.

eg. 1

Write a chemical equation for the reaction between Calcium Hydroxide and Hydrochloric acid.

First you should think that:

Hydrochloric acid + Calcium Hydroxide → Calcium Chloride + Water

Secondly you would need to recall (or look up) the position of Calcium in Group 2 - telling you it is Ca²⁺.

Then you'd need to know that Hydroxide ions are OH^-.

Telling you that Calcium Hydroxide is Ca(OH)₂.

Next you would need to recall (or look up) the position of Chlorine in Group 7 - telling you it is Cl^-.

Telling you that Calcium Chloride is CaCl₂.

Then you can write: HCl_(aq) + Ca(OH)₂(aq) → CaCl₂(aq)+ H₂O_(l)

And only then can you balance it: 2 HCl_(aq) + Ca(OH)₂(aq) → CaCl₂(aq)+ 2 H₂O_(l)

eg 2.

Write a chemical equation for the reaction between Sodium Oxide and Sulphuric acid.

First you should think that:

Sulphuric acid + Sodium Oxide → Sodium Sulphate+ Water

Secondly you would need to recall (or look up) the position of Sodium in Group 1 - telling you it is Na⁺.

Then you'd need to know that Oxide ions are O^2-.

Telling you that Sodium Oxide is Na₂O.

Next you would need to recall the formula of Sulphate ions - SO₄^2- .

Telling you that Sodium Sulphate is Na₂SO₄

Then you can write: H₂SO_{4 (aq)} + Na₂O(s) → Na₂SO_{4 (aq)}+ H₂O_(l)

No balancing required

eg 3.

Write a chemical equation for the reaction between Magnesium and Nitric acid.

First you should think that: Nitric acid + Magnesium → Magnesium Nitrate + Hydrogen

Secondly you would need to recall (or look up) the position of Magnesium in Group 2 - telling you it is Mg²⁺.

Then you'd need to know that Nitrate ions are NO₃^-.

Telling you that Magnesium Nitrate is Mg(NO₃)₂.

Then you can write: Mg _(s) + HNO₃(aq) → Mg(NO₃)_{2 (aq)}+ H_2(g)

And only then can you balance it: Mg _(s) + 2 HNO₃(aq) → Mg(NO₃)_{2 (aq)}+ H_2(g)

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