SPM Chemistry Form 5 Notes – Thermochemisty (Part 3)

by BerryBerryTeacher

in Berry Reference (Notes)

This is Part 3 of Berry Berry Easy‘s SPM Form 5 Chemistry chapter of Thermochemistry. Berry Readers who have read the previous two posts in this series would understand all about the basic definitions of thermochemistry, energy changes involving bonds and the various categories of ‘heats’. In this part, you’ll learn all about the Heat of Neutralisation.

While this topic might seemed difficult at first, it is essentially very easy once you understand that the heat of neutralisation is ALWAYS based on the pairs of acid-alkali involved. Try memorising all the four combinations of strong acid-strong alkali, weak acid-weak alkali, strong acid-weak alkali and weak acid-strong alkali. Yes, only four and that’s all that is, nothing more. Simple? Yes, so move on and read the following post.

SPM Chemistry Form 5 – Terminology and Concepts: Thermochemistry (Part 3)

Heat of Neutralisation (Form 4, Chapter 7 Acids and Bases)

  1. Heat of neutralisation – the heat change when one mole of water is formed from the reaction between an acid and an alkali.
  2. Neutralisation – a reaction between an acid reacts with a base (alkali) to form a salt and water.
  3. Hydrogen ion from acid reacts with hydroxide ions from alkali to form water. H+(aq) + OH-(aq) –> H2O(l)
  4. Neutralisation reaction gives out heat and always an exothermic reaction.

Example 1: (Strong acids – monoprotic acid and strong alkalis)
Chemical equation: HCl(aq) + NaOH(aq) –> NaCl(aq) + H2O(l)
Ionic equation: H+(aq) + OH-(aq) –> H2O(l)
Heat of neutralisation of strong acids and strong alkalis are the same (ΔH = -57.3 kJ mol-1)

Example 2: (Strong acids – diprotic acid and strong alkalis)
Chemical equation: H2SO4(aq) + NaOH(aq) –> Na2SO4(aq) + 2H2O(l)
Ionic equation: 2H+(aq) + 2OH-(aq) –> 2H2O(l)
Heat of neutralisation of strong acids and strong alkalis are the same (ΔH = -57.3 kJ mol-1)

Example 3: (Weak acids and strong alkalis)
Chemical equation: CH3COOH(aq) + NaOH(aq) –> CH3COONa(aq) + H2O(l)
Ionic equation: H+(aq) + OH-(aq) –> H2O(l)
Heat of neutralisation of weak acids and strong alkalis are lower (ΔH = -55.0 kJ mol-1) than heat of neutralisation of strong acids and strong alkalis (ΔH = -57.3 kJ mol-1).

Example 4: (Strong acids and weak alkalis)
Chemical equation: HCl(aq) + NH4OH(aq) –> NH4Cl(aq) + H2O(l)
Ionic equation: H+(aq) + OH-(aq) –> H2O(l)
Heat of neutralisation of strong acids and weak alkalis are lower (ΔH = -51.5 kJ mol-1) than heat of neutralisation of strong acids and strong alkalis (ΔH = -57.3 kJ mol-1).

Example 5: (Weak acids and weak alkalis)
Chemical equation: CH3COOH(aq) + NH4OH(aq) –> NH4Cl(aq) + H2O(l)
Ionic equation: H+(aq) + OH-(aq) –> H2O(l)
Heat of neutralisation of strong acids and weak alkalis are lower (ΔH = -50.4 kJ mol-1) than heat of neutralisation of strong acids and strong alkalis (ΔH = -57.3 kJ mol-1).

The next post in this series would be the final post in the series of Thermochemistry SPM Form 5 from Berry Berry Easy. Heat of Combustion will be discussed in detailed in the next part where list of energy contents for different fuel substances, types of chemical equations and their corresponding heat of combustions will be given.

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