Reactions of acids with metals

AQA·Chemistry (Combined Science)·5.4.2.1·Trilogy — Higher

Acid-metal reactions

Some metals react with acids to produce a salt and hydrogen gas. In each reaction, the metal replaces hydrogen from the acid and becomes part of the salt. The general word equation is:

metal+acid→salt+hydrogen\text{metal} + \text{acid} \rightarrow \text{salt} + \text{hydrogen}

The metal gives the salt the first part of its name. The acid determines the second part: hydrochloric acid forms a chloride, while sulfuric acid forms a sulfate. For example, magnesium forms magnesium chloride with hydrochloric acid and magnesium sulfate with sulfuric acid.

Acid-metal word and symbol equations

A word equation names the reactants and products. A symbol equation uses their chemical formulae and is balanced so that there are equal numbers of each type of atom on both sides.

Magnesium and hydrochloric acid produce magnesium chloride and hydrogen:

$$\text{magnesium} + \text{hydrochloric acid} \rightarrow \text{magnesium chloride} + \text{hydrogen}$$

Mg+2 HCl→MgClX2+HX2\ce{Mg + 2HCl -> MgCl2 + H2}

Zinc and dilute sulfuric acid produce zinc sulfate and hydrogen:

$$\text{zinc} + \text{sulfuric acid} \rightarrow \text{zinc sulfate} + \text{hydrogen}$$

Zn+HX2SOX4→ZnSOX4+HX2\ce{Zn + H2SO4 -> ZnSO4 + H2}

Iron and hydrochloric acid produce iron(II) chloride and hydrogen:

$$\text{iron} + \text{hydrochloric acid} \rightarrow \text{iron(II) chloride} + \text{hydrogen}$$

Fe+2 HCl→FeClX2+HX2\ce{Fe + 2HCl -> FeCl2 + H2}

Redox in acid-metal reactions

When a metal reacts with an acid, electrons pass from metal atoms to hydrogen ions from the acid. The metal atoms become positive metal ions, while the hydrogen ions form hydrogen gas.

This is a redox reaction because oxidation and reduction happen together:

  • Oxidation: the metal loses electrons.
  • Reduction: hydrogen ions gain electrons.

Magnesium and hydrochloric acid

Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen gas:

Mg+2 HCl→MgClX2+HX2\ce{Mg + 2HCl -> MgCl2 + H2}

Each magnesium atom loses two electrons and becomes a magnesium ion, $$\ce{Mg^{2+}}$$. The oxidation half-equation shows this loss:

Mg→MgX2++2 eX−\ce{Mg -> Mg^{2+} + 2e^-}

Two hydrogen ions from the acid gain those two electrons and form one hydrogen molecule. The reduction half-equation shows this gain:

2 HX++2 eX−→HX2\ce{2H+ + 2e^- -> H2}

The number of electrons lost by magnesium equals the number gained by the hydrogen ions. Chloride ions are present in the solution and form magnesium chloride with the magnesium ions, but they do not take part in the electron transfer.

Identifying oxidation and reduction in other reactions

Zinc and iron follow the same pattern when they react with hydrochloric or sulfuric acid. For example, $$\ce{Zn + H2SO4 -> ZnSO4 + H2}$$ forms zinc ions, $$\ce{Zn^{2+}}$$, while $$\ce{Fe + 2HCl -> FeCl2 + H2}$$ forms iron(II) ions, $$\ce{Fe^{2+}}$$. In each case, the metal atoms lose electrons and hydrogen ions gain them.

To identify the species oxidised and reduced from an equation, compare the reactants with the products and track electron loss and gain. The metal is oxidised because its atoms lose electrons to form positive ions. The hydrogen ions are reduced because they gain electrons to form hydrogen gas.

Reactions of acids with metals | AQA GCSE Combined Science: Trilogy (8464) — Chemistry — Higher Revision Notes | KnowledgeSet