Science with Tara

Acids are substances that produce hydrogen ions (H⁺) in aqueous solution. In this lesson, we will learn the typical chemical reactions of acids.

Common acids include Hydrochloric acid (HCl), Sulfuric acid (H₂SO₄) and Nitric acid (HNO₃).

Acids have pH values less than 7. The lower the pH value, the more acidic the solution. Strongest acid has pH of 0.

Reaction with Metals

Acids react with metals to form salt and hydrogen gas.

Example:
Reaction between hydrochloric acid and magnesium metal:
Chemical equation:
2HCl (aq) + Mg (s) → MgCl₂ (aq) + H₂ (g)

Note:
Not all metals react with acids. For example, copper and silver do not react with
acids. Lead does not react with sulfuric acid and hydrochloric acid due to the
insoluble layer of salt formed around the metal, protecting the metal from
further reaction with acid.

Test for Hydrogen gas:
Hydrogen gas can be tested with a burning splint. Hydrogen gas extinguishes burning splint with a ‘pop’ sound.

Reaction with Bases (Neutralisation)

Acids react with bases to form salt and water. This is called a neutralisation reaction.

Bases are either metal oxides or metal hydroxides.

Soluble metal hydroxides are called alkalis.

Example:
Reaction between hydrochloric acid and sodium hydroxide
Chemical equation:
HCl (aq)+ NaOH (aq)→ NaCl (aq) +H₂O(l)

Let’s work out the ionic equation of this reaction.

Substances that ionises in aqueous solution are represented by (aq)
state symbol.

Hydrochloric acid, HCl, will ionise into hydrogen ions and chlorides ions.
Sodium hydroxide, NaOH, will ionise into sodium ions and hydroxides ions.
Sodium chloride, NaCl, will ionise into sodium and chloride ions.
Water will not ionise, hence it is represented with (l) state symbol.

Ionic equation:
H⁺(aq) + Cl^–(aq) + Na⁺(aq) + OH^–(aq) → Na⁺(aq) + Cl^–(aq) + H₂O(l)

Any ions that appear at the left and right of the equation are considered as
spectator ions. They do not take part in the reaction and can be cancelled in the
equation. We will cancel Cl^–  (aq) and Na⁺ (aq), leaving us with H⁺ (aq) and OH^– (aq).

Ionic equation of neutralisation reaction:
H⁺ (aq)+ OH^– (aq)→ H₂O(l)

Reaction with Carbonates

Acids react with Carbonates to form salt, water and carbon dioxide gas.

Example:
Reaction between hydrochloric acid and calcium carbonate:
Chemical equation:
2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂ (g)

Test for Carbon Dioxide:
Carbon dioxide gas, an acidic gas, can be tested by bubbling the gas through limewater, Ca(OH)₂ (aq).
Carbon dioxide gas forms white precipitate with limewater.
The white precipitate is calcium carbonate, CaCO_3.
Chemical Equation:
CO₂ (g) + Ca(OH)₂ (aq) → CaCO₃ (s) + H₂O (l)