Reactions of Acids with Metals (GCSE Chemistry)

Reactions of Acids with Metals

Reactions of Acids

Acids with Water and Metals

• Acids can dissolve in water. An acid is a substance which dissolves in water to produce hydrogen ions (H⁺). Acids can also react with some metals.
• Acids can form a salt and hydrogen gas. During a metal and acid reaction, acids form a salt and hydrogen gas. The hydrogen gas can be observed in the form of bubbles. These bubbles can be used to measure the rate of reaction.
• We can write out equations. Since acids and metals react to form a salt and hydrogen, we can express this as an equation:

Acid + Metal → Salt + Hydrogen

E.g. H₂SO₄ + Mg → MgSO₄ + H₂

What is a Redox Reaction?

• A redox reaction example is when metals react with acids. When a metal reacts with an acid, the loss and gain of electrons occurs simultaneously. This is known as a redox reaction, where spectator ions are present.
• Spectator ions remain the same. During a redox reaction, the spectator ions don’t lose or gain any electrons. They simply stay as an ion during the whole reaction.
• Metals are oxidised. During a neutralisation reaction, metals are oxidised. This means that the metal will lose electrons, becoming oxidised into a positive ion.
• Hydrogen is reduced. During a neutralisation reaction, hydrogen is reduced. This means that the hydrogen will gain electrons, becoming reduced into a hydrogen molecule.

Example Reactions between Metals and Acids

For exams, you need to be familiar with certain reactions involving metals and acids. We will outline some key examples in this section.

H₂SO₄ + Mg → MgSO₄ + H₂

2HCl + Zn → ZnCl₂ + H₂

Fe + 2HCl → FeCl₂ + H₂

Practice Question 1: Identify which species are reduced and oxidised in the reaction between sulfuric acid and magnesium.

1.Write the symbol equation. Firstly, we need to write out the metal and acid reaction equation and balance it.

H₂SO₄ + Mg → MgSO₄ + H₂

2. Separate out the ions in the equation. Remember that not all compounds have ionic bonds: some might be covalent. Also watch out for compounds which have 2 of the same ions.

2H⁺ SO₄^2- + Mg → Mg²⁺ SO₄^2- + H₂

3. Identify the spectator ions. Now, look at the ions and cross out any which are the same on both sides of the equation.

2H⁺ SO₄^2- + Mg → Mg²⁺ SO₄^2- + H₂

2H⁺ + Mg → Mg²⁺ + H₂

4. Write the half equations. We need to write each set of half equations separately. In the half equation, we write the number of electrons gained or lost.

In this case, the 2 hydrogen ions have gained 2 electrons from the magnesium – 1 for each hydrogen ion. The magnesium has lost 2 electrons from its outer shell.

2H⁺ + 2e^– → H₂Mg – 2e^– → Mg²⁺

5. Identify the oxidised and reduced species. From these half equations, we can see that Hydrogen has gained electrons and been reduced. Magnesium has lost electrons and been oxidised.

Practice Question 2: Identify which species are reduced and oxidised in the reaction between hydrochloric acid and zinc.

1.Write the symbol equation. Firstly, we need to write out the equation for the reaction and balance it.

2HCl + Zn → ZnCl₂ + H₂   

2. Separate out the ions in the equation. Remember that not all compounds have ionic bonds: some might be covalent. Also watch out for compounds which have 2 of the same ions.

2H⁺ + 2Cl^– + Zn → Zn⁺² 2Cl^–  + H₂   

3. Identify the spectator ions. Now, look at the ions and cross out any which are the same on both sides of the equation.

2H⁺ + 2Cl^– + Zn → Zn⁺² 2Cl^–  + H₂   

2H⁺  + Zn → Zn⁺² + H₂   

4. Write the half equations. We need to write each set of half  equations separately. In the half equation, we write the number of electrons gained or lost.

In this case, the 2 hydrogen ions have gained 2 electrons from the Zinc – 1 for each hydrogen ion. The Zinc has lost 2 electrons from its outer shell.

2H⁺  + 2e^– → H₂   

Zn -2e^– → Zn⁺²

5. Identify the oxidised and reduced species. From these half equations, we can see that Hydrogen has gained electrons and been reduced. Zinc has lost electrons and been oxidised.