pH Scale (CIE A Level Chemistry)

• The pH scale is a numerical scale that shows how acidic or alkaline a solution is
• The values on the pH scale go from 1-14 (extremely acidic substances have values of below 1)
• All acids have pH values of below 7, all alkalis have pH values above 7
• The lower the pH then the more acidic the solution is
• The higher the pH then the more alkaline the solution is

The pH scale showing acidity, neutrality and alkalinity

pH of water

• An equilibrium exists in water where few water molecules dissociate into proton and hydroxide ions

H₂O(l) ⇌ H⁺(aq) + OH^-(aq)

• The equilibrium constant for this reaction is:

K_c x [H₂O] = [H⁺] [OH^-]

• Since the concentration the H⁺ and OH^- ions is very small, the concentration of water is considered to be a constant, such that the expression can be rewritten as:

K_w = [H⁺] [OH^-]

Where K_w (ionic product of water)      =         K_c x [H₂O]

 =         10^-14 mol² dm^-3 at 298K

• Water at 298K has equal amounts of OH^- and H⁺ ions with concentrations of 10^-7 mol dm^-3
• To calculate the pH of water, the following formula should be used:

pH       = -log (10^-7)

               = 7

• Thus, water has a pH of 7

pH of acids

• Acidic solutions (strong or weak) always have more H⁺ than OH^- ions
• Since the concentration of H⁺ is always greater than the concentration of OH^- ions, [H⁺] is always greater than 10^-7 mol dm^-3
• Using the pH formula, this means that the pH of acidic solutions is always below 7
• The higher the [H⁺] of the acid, the lower the pH

pH of bases

• Basic solutions (strong or weak) always have more OH^- than H⁺ ions
• Since the concentration of OH^- is always greater than the concentration of H⁺ ions, [H⁺] is always smaller than 10^-7 mol dm^-3
• Using the pH formula, this means that the pH of basic solutions is always above 7
• The higher the [OH^-] of the base, the higher the pH