Higher and foundation tiers

Neutralisation using an acid and an alkali

Recall that acids:

Have a pH below 7.

They are usually formed by dissolving substances; usually non-metal oxides in water.

Are aqueous solutions that contain an excess of hydrogen ions (H⁺).

Alkalis on the other hand:

Have a pH above 7.

Are formed when substances; usually metal oxides and metal hydroxides dissolve in water

Are aqueous solutions that contain an excess of hydroxide ions (OH^-.

If a solution has a pH of 7 we say it is neither acidic nor alkaline but it is neutral.

Common acids and alkalis

The table below lists some common acids and alkalis you will probably have used in the lab. You should see from the table that all the acids contain hydrogen in their molecular formula and that all acids are solutions that contain an excess of hydrogen ions (H⁺) while all the alkalis contain the hydroxide ion (OH^-) in their molecular formula, they are also solutions which contain an excess of hydroxide ions (OH^-).

acid	molecular formula	alkali	molecular formula
hydrochloric acid	HCl	sodium hydroxide	NaOH
sulfuric acid	H₂SO₄	potassium hydroxide	KOH
nitric acid	HNO₃	calcium hydroxide	Ca(OH)₂
ethanoic acid	CH₃COOH	ammonium hydroxide	NH₄ OH

Neutralisation reactions

The reaction between an acid and an alkali or base is called neutralisation and the products of this neutralisation reaction are a salt and water, we can represent this reaction using the equations below:

equations for neutralisation

Neutralisation reactions and eqautions

Example 1- Hydrochloric acid and sodium hydroxide

The neutralisation equations shown below involve adding hydrochloric acid to the alkali sodium hydroxide. The products of this neutralisation reaction are the salt sodium chloride and water:

hydrochloric acid_(aq) + sodium hydroxide_(aq) → sodium chloride_(aq) + water_(l)

HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

If you study the equations above you will see that the hydrogen ion (H⁺) in the acid and the hydroxide ion (OH^-) in the alkali simply react together to form water, we can show this as:

Hydrogen ion + hydroxide ion → water

H⁺_(aq) + OH^-_(aq) → H₂O_(l)

What is left over; in this case the chloride ion (Cl^-) from the hydrochloric acid and the sodium ion (Na⁺) from the sodium hydroxide now simply forms the salt:

Cl^- + Na⁺ → NaCl

You can think of the salt as the acid where the hydrogen in the acid is replaced by the metal present in the alkali e.g. if we use hydrochloric acid and neutralise it with 3 different alkalis then the salt produced is always a chloride; as shown below (if you need help in working out the formula for the acids, alkalis and salts produced in these reactions then click here):

Neutralisation showing that a salt and water are formed.

Example 2: Hydrochloric acid and lithium hydroxide

hydrochloric acid_(aq) + lithium hydroxide_(aq) → lithium chloride_(aq) + water_(l)

HCl_(aq) + LiOH_(aq) → LiCl_(aq) + H₂O_(l)

Using hydrochloric acid in a neutralisation reaction produces salts called chlorides

Example 3: Hydrochloric acid and potassium hydroxide

hydrochloric acid_(aq) + potassium hydroxide_(aq) → potassium chloride_(aq) + water_(l)

HCl_(aq) + KOH_(aq) → KCl_(aq) + H₂O_(l)

Example 4: Hydrochloric acid and calcium hydroxide

hydrochloric acid_(aq) + calcium hydroxide_(aq) → calcium chloride_(aq) + water_(l)

2HCl_(aq) + Ca(OH)₂_(aq) → CaCl_2(aq) + 2H₂O_(l)

In each of the examples above all that really happens is the hydrogen ion (H⁺) in the hydrochloric acid is replaced by the metal present in the alkali, that is the metals lithium, potassium and finally calcium in the last example.

In each case the salt formed is a metal chloride. The metal comes from the alkali and the chloride from the acid. This means every time you neutralise hydrochloric acid you will make a salt called a chloride .

Neutralisation using sulfuric acid

Sulfuric acid (H₂SO₄) contains two acidic hydrogen ions (2H⁺_(aq)), it is called a diprotic acid. Whereas hydrochloric acid which can only donate one H⁺_(aq) is called a monoprotic acid. Sulfuric acid contains two hydrogen ions and a sulfate ion (S0₄^2-). When sulfuric acid is neutralised it will need two hydroxide ions (OH^-) to neutralise the two hydrogen ions in the acid an equation for this reaction is shown below:

2H⁺_(aq) + 2OH^-_(aq) → 2H₂O_(l)

What is left will be the metal ion from the alkali and the sulfate ion from the acid. This will form the salt as shown in the equations below:

Sulfuric acid and lithium hydroxide

sulfuric acid_(aq) + lithium hydroxide_(aq) → lithium sulfate_(aq) + water_(l)

Sulfuric acid during neutralisation reactions produces salts called sulfates.

H₂SO_4(aq) + 2LiOH_(aq) → Li₂SO_4(aq) + 2H₂O_(l)

Sulfuric acid and potassium hydroxide

sulfuric acid_(aq) + potassium hydroxide_(aq) → potassium sulfate_(aq) + water_(l)

H₂SO_4(aq) + 2KOH_(aq) → K₂SO_4(aq) + 2H₂O_(l)

Sulfuric acid and calcium hydroxide

sulfuric acid_(aq) + calcium hydroxide_(aq) → calcium sulfate_(aq) + water_(l)

H₂SO_4(aq)+ Ca(OH)_2(aq) → Li₂SO_4(aq) + 2H₂O_(l)

In these three examples the salts lithium sulfate, potassium sulfate and calcium sulfate are formed. There is an obvious pattern here, it's the same pattern for the examples with hydrochloric acid. The hydrogen in the acid is replaced by the metal present in the alkali; this means that with sulfuric acid the salt formed will always be a metal sulfate.

Neutralisation using nitric acid

Finally if we use nitric acid (HNO₃), then exactly the same pattern as above occurs e.g. nitric acid has the formula HNO₃ and it contains one hydrogen ion and nitrate ions (NO^-₃). So the hydrogen ions will be neutralised by the hydroxide ions and the salt formed will be a metal nitrate as shown in the equations below:

Nitric acid and sodium hydroxide

During neutralisation reactions nitric acid forms salts called nitrates.

nitric acid_(aq) + sodium hydroxide_(aq) → sodium nitrate_(aq) + water_(l)

HNO_{3 (aq)} + NaOH_(aq) → NaNO_{3 (aq)} + H₂O_(l)

Nitric acid and lithium hydroxide

nitric acid_(aq) + lithium hydroxide_(aq) → lithium nitrate_(aq) +water_(l)

HNO_{3 (aq)} + LiOH_(aq) → LiNO_{3 (aq)} + H₂O_(l)

Nitric acid and calcium hydroxide

nitric acid_(aq) + calcium hydroxide_(aq) → calcium nitrate_(aq) + water_(l)

2HNO_{3 (aq)} + Ca(OH)₂_(aq) → Ca(NO₃)_{2 (aq)} + 2H₂O_(l)

Key Points

Alkalis and bases will neutralise acids.

The products of a neutralisation reaction are salt and water.

All acids contain hydrogen ions (H⁺_(aq)) and all alkalis contain hydroxide ions (OH^-_(aq)).

Salts can be thought of as acids where the hydrogen in the acid is replaced by a metal.

Hydrochloric acid produces salts called chlorides, nitric acid produces salts called nitrates and sulfuric acid produces salts called sulfates and phosphoric acid produces salts called phosphates.

Neutralisation using an acid and an alkali

Common acids and alkalis

Neutralisation reactions

Neutralisation reactions and eqautions

Example 1- Hydrochloric acid and sodium hydroxide

hydrochloric acid(aq) + sodium hydroxide(aq) → sodium chloride(aq) + water(l)

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Hydrogen ion + hydroxide ion → water

H+(aq) + OH-(aq) → H2O(l)

Cl- + Na+ → NaCl

Example 2: Hydrochloric acid and lithium hydroxide

hydrochloric acid(aq) + lithium hydroxide(aq) → lithium chloride(aq) + water(l)

HCl(aq) + LiOH(aq) → LiCl(aq) + H2O(l)

Example 3: Hydrochloric acid and potassium hydroxide

hydrochloric acid(aq) + potassium hydroxide(aq) → potassium chloride(aq) + water(l)

HCl(aq) + KOH(aq) → KCl(aq) + H2O(l)

Example 4: Hydrochloric acid and calcium hydroxide

hydrochloric acid(aq) + calcium hydroxide(aq) → calcium chloride(aq) + water(l)

2HCl(aq) + Ca(OH)2(aq) → CaCl2(aq) + 2H2O(l)

Neutralisation using sulfuric acid

2H+(aq) + 2OH-(aq) → 2H2O(l)

Sulfuric acid and lithium hydroxide

sulfuric acid(aq) + lithium hydroxide(aq) → lithium sulfate(aq) + water(l)

H2SO4(aq) + 2LiOH(aq) → Li2SO4(aq) + 2H2O(l)

Sulfuric acid and potassium hydroxide

sulfuric acid(aq) + potassium hydroxide(aq) → potassium sulfate(aq) + water(l)

H2SO4(aq) + 2KOH(aq) → K2SO4(aq) + 2H2O(l)

Sulfuric acid and calcium hydroxide

sulfuric acid(aq) + calcium hydroxide(aq) → calcium sulfate(aq) + water(l)

H2SO4(aq)+ Ca(OH)2(aq) → Li2SO4(aq) + 2H2O(l)