Acids, bases and salts

Key vocabulary:

• Aqueous: dissolved in water

• Acid: source of hydrogen ions (H⁺) in solution

• Hydrogen ion: hydrogen atom that has lost an electron (H⁺)

• Proton: another name for a hydrogen ion (H⁺)

• Alkali: source of hydroxide ions (OH⁻) in solution

• Solubility: how well a substance dissolves

• Soluble: does dissolve

• Insoluble: does not dissolve

• Dissociation: breaking apart into ions

• Ionisation: forming ions by gaining or losing electrons

• Neutralisation: reaction of acid with base/alkali to form a salt

Acids:
Acids release hydrogen ions (H⁺) in solution and act as proton donors.

They can be strong or weak depending on how much they ionise in water.

• Strong acids: Almost fully ionised in water, producing many H⁺ ions.
  Examples: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃).

Equation to show how strong acids ionise completely in water to form many H⁺ ions:

Weak acids are those that are partially ionised in water, forming few H⁺ ions 

e.g ethanoic acid,CH₃COOH_(aq) and carbonic acid, H₂CO_3(aq), 

Equation to show how ethanoic acid ionises partially in water to form few H⁺ ions:

Bases:

• Proton acceptors and opposites of acids.

• Include metal oxides, hydroxides, and ammonia.

• Neutralise acids to form a salt and water.

• Examples:

  • Sodium hydroxide (NaOH)

  • Ammonia solution (NH₃)

  • Magnesium oxide (MgO)

  • Copper(II) oxide (CuO)

  • Iron(II) hydroxide (Fe(OH)₂)

• Alkalis are bases that dissolve in water.

• Alkaline solutions contain hydroxide ions (OH⁻).

Alkalis:

• Strong alkalis: Fully dissociate in water, forming many OH⁻ ions.

  • Examples: sodium hydroxide (NaOH), potassium hydroxide (KOH)

  • Equation (for NaOH):
    NaOH(aq) → Na⁺(aq) + OH⁻(aq)

• Weak alkalis: Partially ionise in water, forming fewer OH⁻ ions.

  • Example: ammonia solution (NH₃)

  • Equation:
    NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

• The more H⁺(aq) ions, the stronger the acidity and the lower the pH.

• The more OH⁻(aq) ions, the stronger the alkalinity and the higher the pH.

• Indicators show if a solution is acidic, alkaline, or neutral by changing colour.

pH Scale:

• The pH scale ranges from 0 to 14.

• It measures how acidic or alkaline a solution is.

• pH < 7: Acidic

• pH = 7: Neutral

• pH > 7: Alkaline

pH values as shown by the different colours with Universal Indicator solution

pH Values:

• pH values are not always whole numbers (e.g., 2.4 or 5.8).

• pH > 7: Alkaline

• pH = 7: Neutral

• pH < 7: Acidic

Universal Indicator is used to find the approximate pH of a solution – it has a range of colours corresponding to different pH values, indicating whether a solution is acidic, alkaline, or neutral.

Reactions of Acids:

• Safety:

  • Wear goggles and a lab coat.

  • Ensure stools are tucked under the desks.

• Procedure:

  • In this practical, investigate the reactions of acids.

  • Make a note of your observations and consider the inferences based on these reactions.

Properties of Acids:

• pH:
  Acids have a pH < 7, indicating they are acidic.

• Reaction with Metals:

  • Reactive metal + acid → salt + hydrogen

  • Example:
    Zinc (Zn) + Hydrochloric acid (HCl) → Zinc chloride (ZnCl₂) + Hydrogen (H₂)

• Salt Formation:
  A salt is formed when an acid reacts with a metal or a base, replacing the hydrogen ions of the acid with metal or ammonium ions.

  • Neutralisation with different acids gives different salts:

    • Hydrochloric acid → Chlorides

    • Sulphuric acid → Sulfates

    • Nitric acid → Nitrates

    • Ethanoic acid → Ethanoates

• Solubility Rules:

  • State symbols can be added to chemical equations.

  • Acids are aqueous (aq) in water.

  • Water is a liquid (l).

  • Metals are solids (s).

  • Bases are solids (s) unless soluble in water, in which case they are alkalis (aq).

  • The state symbol for the salt depends on the metal or compound involved.

Solubility Rules 

Salts can be made using different methods according to their solubility. Learn the following table:

Examples of Metals Reacting with Acids:

1. Magnesium + Sulphuric Acid

   • Word equation: Magnesium + Sulphuric acid → Magnesium sulfate + Hydrogen

   • Chemical equation: Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

   • Ionic equation: Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)

2. Zinc + Hydrochloric Acid

   • Word equation: Zinc + Hydrochloric acid → Zinc chloride + Hydrogen

   • Chemical equation: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

   • Ionic equation: Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)

Observations:

• Bubbles of hydrogen gas are released (effervescence).

• The metal dissolves, and a salt solution forms.

Note:

• Reactions with highly reactive metals (K, Na, Li) are too violent and dangerous.

Acid Reaction Equations: IMPORTANT!!!

• Acid + Metal → Salt + Hydrogen

• Acid + Base → Salt + Water

• Acid + Metal Carbonate → Salt + Water + Carbon Dioxide

• Acid + Metal Hydrogen Carbonate → Salt + Water + Carbon Dioxide

• Acid + Ammonia → Ammonium Salt

Common Acids:

• Hydrochloric acid: HCl

• Sulphuric acid: H₂SO₄

• Nitric acid: HNO₃

• Ethanoic acid: CH₃COOH

3) Reaction with Base or Alkali – Neutralisation:

• Base or Alkali + Acid → Salt + Water

Examples:

• Word equation: Copper(II) oxide + Hydrochloric acid → Copper(II) chloride + Water

• Chemical equation: CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)

• Ionic equation: CuO(s) + 2H⁺(aq) → Cu²⁺(aq) + H₂O(l)

• Word equation: Potassium hydroxide + Sulphuric acid → Potassium sulfate + Water

• Chemical equation: 2KOH(aq) + H₂SO₄(aq) → K₂SO₄(aq) + 2H₂O(l)

• Ionic equation: 2OH⁻(aq) + 2H⁺(aq) → 2H₂O(l)

4) Reaction with Metal Carbonates and Metal Hydrogen Carbonates:

• Metal carbonate + Acid → Salt + Water + Carbon dioxide

• Metal hydrogen carbonate + Acid → Salt + Water + Carbon dioxide

Examples:

• Word equation: Magnesium carbonate + Sulphuric acid → Magnesium sulfate + Water + Carbon dioxide

• Chemical equation: MgCO₃(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂O(l) + CO₂(g)

• Ionic equation: MgCO₃(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂O(l) + CO₂(g)

• Word equation: Sodium hydrogen carbonate + Hydrochloric acid → Sodium chloride + Water + Carbon dioxide

• Chemical equation: NaHCO₃(aq) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)

• Ionic equation: NaHCO₃(aq) + H⁺(aq) → Na⁺(aq) + H₂O(l) + CO₂(g)

Observations:

• Bubbles of carbon dioxide gas are produced(effervescence).

• The metal carbonate or hydrogen carbonate dissolves.

5) Reaction with Ammonia Solution to Produce Ammonium Salts:

• Ammonia + Hydrochloric acid → Ammonium chloride

  • Equation: NH₃(aq) + HCl(aq) → NH₄Cl(aq)

• Ammonia + Sulphuric acid → Ammonium sulfate

  • Equation: 2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)

• Ammonia + Nitric acid → Ammonium nitrate

  • Equation: NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq)