The Ions in Acids & Alkalis (Cambridge International IGCSE Chemistry)

Neutralisation reactions

Spec point — 7.1.2 · State that aqueous solutions of acids contain H+ ions and aqueous solutions of alkalis contain OH– ions Describe the neutralisation reaction between an acid and an alkali to produce water, H+ (aq) + OH– (aq) → H2O(l)

Neutralisation reactions

What is a neutralisation reaction?

• Acids are a source of hydrogen ions,H⁺
• Bases (or alkalis) are a sources of hydroxide ions, OH^–
• When they react together in a neutralisation reaction, the H⁺ ions react with the OH^– ions to produce water
• This is the net ionic equation of all acid-base neutralisations and is what leads to a neutral solution, since water has a pH of 7:

H⁺ (aq) + OH^– (aq)⟶ H₂O (l)

• Not all reactions of acids are neutralisations:

  • For example when a metal reacts with an acid, although a salt is produced there is no water formed so it does not fit the definition of neutralisation

Exam Hint Not all reactions of acids are neutralisations. For example, when a metal reacts with an acid, although a salt is produced there is no water formed so it does not fit the definition of neutralisation.

Hydrogen ion concentration & pH

Spec point — 7.1.2 · Describe how to compare hydrogen ion concentration, neutrality, relative acidity and relative alkalinity in terms of colour and pH using universal indicator paper

What is the pH scale?

• The pH scale goes from 1 – 14

• All acids have pH values of below 7, all alkalis have pH values of above 7

• The lower the pH then the more acidic the solution is:

  • pH 0-2 = typically strong acids (e.g. hydrochloric acid, sulfuric acid)

    • Extremely acidic substances can have values of below 1

  • pH 3-6 = typically weak acids (e.g. ethanoic acid, citric acid)

• The higher the pH then the more alkaline the solution is:

  • pH 8-11 = typically weak alkalis (e.g. ammonia solution, sodium hydrogencarbonate)
  • pH 12-14 = typically strong alkalis (e.g. sodium hydroxide, potassium hydroxide)

• A solution of pH 7 is described as being neutral

The pH scale

The pH scale showing acidity, neutrality and alkalinity

Exam Hint The terms strong and weak refer to the degree of dissociation of the acid or alkali in solution, not to the pH alone.

A dilute strong acid can have the same pH as a concentrated weak acid.

The pH scale and hydrogen ions

• We have already seen that acids are substances that contain hydrogen ions in solution
• The more hydrogen ions the stronger the acid, but the lower the pH
• The higher the concentration of hydroxide ions in a solution the higher the pH
• So pH is a measure of the concentration of H⁺ ions in solution, but they have an inverse relationship
• The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10

Concentration of hydrogen ions and pH

An acid with a pH of 3 has ten times the concentration of H⁺ions than an acid of pH 4. An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H⁺ions than an acid with a pH of 4

Exam Hint Acid strength is reflected in how many hydrogen ions are in solution. The more hydrogen ions the lower the pH and vice-versa.

How is universal indicator used?

• Universal indicator is a mixture of different indicators which is used to measure the pH
• A drop is added to the solution and the colour is matched with a colour chart which indicates the pH which matches specific colours

The pH scale with the Universal Indicator colours which can be used to determine the pH of a solution