Neutralisation
Application of Neutralisation Reactions in Daily Life
Neutralisation Between “Strong Acid and Strong Alkali” vs “Weak Acid and Strong Alkali”
1. Comparison of the heat of neutralisation of the reaction between “strong acid and strong alkali” and weak acid and strong alkali”.
Strong Acid and Strong Alkali Weak Acid and Strong Alkali
HCl + NaOH → NaCl + H2O CH3COOH + NaOH → CH3COONa +H2O
∆H = -57kJ mol-1 ∆H = -55kJ mol-1
H2SO4 + 2KOH → K2SO4 + 2H2O HCN + KOH → KCN + H2O
∆H = -57kJ mol-1 ∆H = -12kJ mol-1
2HNO3 + Ba(OH)2 → Ba(NO3)2 + 2H2O
∆H = -57kJ mol-1
2. The heat of neutralisation between strong acid and strong alkali is almost the same (-57 kJmol-1) for all acid and all alkali.
3. Nevertheless, the heat of neutralisation between weak acid and strong alkali is always less than -57 kJmol-1
4. This is because weak acids are partially decomposed.
5. During the reaction of neutralisation, the acid molecules will be decomposed to form hydrogen ions and react with the hydroxide ions from alkali.
6. Part of the heat released during the formation of water molecule is used to decompose the acid to form hydrogen ion.
Acid-base Titration
Indicator Alkali Neutral Acid
Litmus Blue Orange Red
Methyl orange Yellow Orange Red
Phenolphthalein Pink Colourless Colourless
- Neutralisation = a reaction between an acid and a base/alkali to produce salt and water only.
- Acid + alkali –> salt + water
- Ionic equation of neutralisation: H+(aq) + OH-(aq) –> H2O(l)
Application of Neutralisation Reactions in Daily Life
- Digestive juices in stomach: Break up food (only in acidic condition) and maintained at pH of between 1 and 2,
- Insect stings: Bees and ants inject an acidic liquid into the skin but wasps inject an alkaline liquid.
- Toothpaste: An alkaline compound (magnesium hydroxide) in toothpastes neutralises the organic acids produced by the food.
- pH of the swimming pool: Calcium hypochlorite, Ca(OCl)2 is added to the water.
- Latex industries: Ammonia is used to neutralise the organic acid produces by bacteria to prevent coagulation.
- Neutralisation: Manufacture fertilisers, soaps and detergents
- Effluent from factories: It can be treated with lime.
- Sulphuric acid: Manufacture of dyes, explosives, steel, fertilisers, paints and plastics.
- Soil treatment: Plants grow best when the pH of the soil is about 7. The soil is too acidic, quick lime (calcium oxide) or slaked lime (calcium hydroxide) is added to the soil to neutralise the excess acid.
Neutralisation Between “Strong Acid and Strong Alkali” vs “Weak Acid and Strong Alkali”
1. Comparison of the heat of neutralisation of the reaction between “strong acid and strong alkali” and weak acid and strong alkali”.
Strong Acid and Strong Alkali Weak Acid and Strong Alkali
HCl + NaOH → NaCl + H2O CH3COOH + NaOH → CH3COONa +H2O
∆H = -57kJ mol-1 ∆H = -55kJ mol-1
H2SO4 + 2KOH → K2SO4 + 2H2O HCN + KOH → KCN + H2O
∆H = -57kJ mol-1 ∆H = -12kJ mol-1
2HNO3 + Ba(OH)2 → Ba(NO3)2 + 2H2O
∆H = -57kJ mol-1
2. The heat of neutralisation between strong acid and strong alkali is almost the same (-57 kJmol-1) for all acid and all alkali.
3. Nevertheless, the heat of neutralisation between weak acid and strong alkali is always less than -57 kJmol-1
4. This is because weak acids are partially decomposed.
5. During the reaction of neutralisation, the acid molecules will be decomposed to form hydrogen ions and react with the hydroxide ions from alkali.
6. Part of the heat released during the formation of water molecule is used to decompose the acid to form hydrogen ion.
Acid-base Titration
- Titration – a quantitative analysis method to determine the volume of an acid that is required to neutralise a fixed volume of an alkali / a quantitative analysis method to determine the volume of an alkali that is required to neutralise a fixed volume of an acid.
- End point of a titration – a point which neutralisation occurs when the acid has completely neutralised the alkali / the alkali has completely neutralised the acid.
- Reactants: Acid and alkali
- Products: Salt and water
- End point can be determined by (i) the use of acid-base indicators during titration, (ii) measuring the pH values of the solution during titration and (iii) measuring the electrical conductivity of solution during titration.
Indicator Alkali Neutral Acid
Litmus Blue Orange Red
Methyl orange Yellow Orange Red
Phenolphthalein Pink Colourless Colourless
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