metal acid reactions

Higher and foundation tier

Metal acid reactions

Metals react with dilute acids according to the equation below:

metal_(s) + acid_(aq) → salt_(aq) + hydrogen_(g)

The image below shows the reactions of metals magnesium, zinc, iron, tin, lead and copper with dilute acid. It would be foolish to add any of the alkali metals from group 1 of the periodic table to acids as the reactions with water are violent enough, so adding them to acid would be considered too dangerous in a classroom, as the reactions are likely to be violent or explosive. For the reactions shown in the image you can measure the rate or speed of the reaction by measuring how quickly the hydrogen gas is produced or even a simple thermometer would give an indication of the amount of energy released and the speed of the reaction.

You can see that the most bubbles are produced by magnesium and that the metals become less reactive as we move from magnesium to zinc all the way down to the unreactive copper metal.

Equations for reactions

You should already know that:

all acids contain hydrogen ions, H⁺_(aq).
salts are solids formed when the hydrogen in the acid is replaced by a metal. (sometimes it maybe ammonium ions (see section on ammonia)
The name of the salt depends on the acid used, as shown in the table below:

Acid	Formula	Salt formed
hydrochloric	HCl	chloride
sulfuric	H₂SO₄	sulfate
nitric	HNO₃	nitrate

Using hydrochloric acid

1. magnesium

magnesium_(s) + hydrochloric acid_(aq) → magnesium chloride_(aq) + hydrogen_(g)

Mg_(s) + 2HCl_(aq) → MgCl₂_(aq) + H_{2 (g)}

2. calcium

calcium_(s) + hydrochloric acid_(aq) → calcium chloride_(aq) + hydrogen_(g)

Ca_(s) + 2HCl_(aq) → CaCl₂_(aq) + H₂(g)

You can see that hydrochloric acid always produces salts called chlorides

Using sulfuric acid

1. magnesium

magnesium_(s) + sulfuric acid_(aq) → magnesium sulfate_(aq) + hydrogen_(g)

Mg_(s) + H₂SO_4(aq) → MgSO₄_(aq) + H_2(g)

2. calcium

calcium_(s) + sulfuric acid_(aq) → calcium sulfate_(aq) + hydrogen_(g)

Ca_(s) + H₂SO_4(aq) → CaSO₄_(aq) + H_2(g)

You can see that sulfuric acid always produces salts called sulfate

Using nitric acid

1. magnesium

magnesium_(s) + nitric acid_(aq) → magnesium nitrate_(aq) + hydrogen_(g)

Mg_(s) + 2HNO_3(aq) → Mg(NO₃)_2(aq) + H_2(g)

You can see that nitric acid always produces salts called nitrate

In each of the reactions of the metal with acid the metal is oxidised, that is they lose electrons e.g. Consider the metal magnesium, it is an alkaline earth metal in group 2 of the periodic table, this means it has 2 electrons in its last shell, so it will lose these in its reactions to form a metal ion with a 2+ charge.

Mg_(s) → Mg²⁺_(aq) + 2e
this is an oxidation reaction.

the acid contains H⁺(aq) ions and these gain electrons and form hydrogen gas:

2H⁺_(aq) + 2e → H_2(g)
this is a reduction reaction.

The metal is oxidised and the hydrogen ions are reduced, so this is an example of a redox reaction. A redox reaction is one where one substance is reduced (gains electrons) and another substance is oxidised (loses electrons)

Practice questions

Check your understanding - Questions on metal acid reactions

Key Points

Some metals will react with acids to form a salt and release hydrogen gas.
The salt formed depends on which acid is used. Hydrochloric acid forms salts called chlorides, sulfuric acid forms sulfates and nitric acid forms nitrates.
Metal atoms are oxidised and lose electrons when they react with acids. The acids are reduced and gain the electrons from the metal
Metal acid reactions involve oxidation and reduction reactions and so are redox reactions.