36. Neutralisation Reactions: Reactions of Acids

1. General Reaction Patterns

All reactions between an acid and a base are classified as neutralisation reactions. The salt produced depends on the acid used:

  • Hydrochloric Acid produces Chlorides.

  • Sulfuric Acid produces Sulfates.

  • Nitric Acid produces Nitrates.

Patterns for specific bases:

  • Acid + Metal Oxide → Salt + Water

  • Acid + Metal Hydroxide → Salt + Water

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

    • Note: Carbonates are the only base in this group that also produce bubbles of CO₂ gas.

2. Forming the Salt Formula

To determine the salt's formula, combine the positive ion from the base with the negative ion from the acid. You must ensure the charges balance:

  • Example: Sodium (1+) and Chloride (1-) form NaCl.

  • Example: Calcium (2+) and Nitrate (1-) form Ca(NO₃)₂.

3. Practical: Making a Soluble Salt (Required Practical)

This process uses an insoluble base (like copper oxide) and an acid (like sulfuric acid) to create pure crystals of a soluble salt.

Step 1: Reaction

  • Gently heat the dilute acid in a beaker.

  • Add the insoluble base a little at a time until it is in excess (it stops disappearing and sinks to the bottom). This ensures all the acid has been neutralised.

Step 2: Filtration

  • Filter the mixture using filter paper and a funnel to remove the excess insoluble base. The liquid left behind (filtrate) is the dissolved salt solution.

Step 3: Crystallisation

  • Gently heat the filtered solution in an evaporating dish (ideally using a water bath) to evaporate some of the water.

  • Stop heating once crystals start to form. Leave the remaining solution to cool slowly so more crystals grow.

Step 4: Isolation and Drying

  • Filter the crystals out of the remaining liquid.

  • Dry the pure crystals by dabbing them with filter paper or leaving them in a warm place.

4. Summary Checklist

  • Check for CO₂: If you see fizzing when adding the base, it is a carbonate.

  • Heating: Use a water bath for crystallisation to avoid "spitting" or damaging the salt crystals with high heat.