The Ions in Acids & Alkalis (Cambridge (CIE) O Level Chemistry): Revision Note

Neutralisation Reactions

• When acids are added to water, they form positively charged hydrogen ions (H⁺)

• The presence of H⁺ ions is what makes a solution acidic

• When alkalis are added to water, they form negative hydroxide ions (OH^–)

• The presence of the OH^– ions is what makes the aqueous solution an alkali

• The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is, in other words it is a measure of the concentration of hydrogen ions in the solution

• A neutralisation reaction occurs when an acid reacts with an alkali

• When these substances react together in a neutralisation reaction, the H⁺ ions react with the OH^– ions to produce water

• For example, when hydrochloric acid is neutralised, sodium chloride and water are produced:

HCl + NaOH → H₂O + NaCl

• The net ionic equation of acid-alkali neutralisations, and what leads to a neutral solution, since water has a pH of 7, is:

H⁺  +  OH^– ⟶ H₂O

Hydrogen Ion Concentration & pH

The pH scale

• The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is

• It goes from 0 - 14 (very concentrated acids can have values below 0)

• 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

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

• A solution with a pH of 7, such as water, is described as being neutral 

The pH scale showing acidity, neutrality and alkalinity

• We have already seen that acids are substances that contain hydrogen ions in solution

• The higher the concentration of hydrogen ions, 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

  • Therefore 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