pH Scale (Cambridge International AS Chemistry)

The pH Scale

• The pH scale is a numerical scale that shows how acidic or alkaline a solution is

• The values on the pH scale usually go from 0-14 (very concentrated strong acids can have values below 0)

  • All acids have pH values below 7
  • All alkalis have pH values above 7

• The lower the pH, the more acidic the solution is

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

The pH scale

The pH scale shows the acidity, neutrality and alkalinity of chemicals

pH of water (A Level only)

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

H₂O (l) ⇌ H⁺ (aq) + OH^– (aq)

• The equilibrium constant expression for this reaction is:

K_c=$\frac{[ H^{+} ] [ OH^{-} ]}{[ H_{2} O ]}$

• The equilibrium constant expression can be rearranged to:

K_c x [H₂O] = [H⁺] [OH^–]

• Since the concentration of the H⁺ and OH^- ions is very small, the concentration of water is considered to be a constant

  • So, 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-6 at 298K

• Water at 298K has equal amounts of OH^- and H⁺ions with concentrations of 10^-7mol dm^-3

• To calculate the pH of water, the following formula should be used:

pH = –log [H⁺(aq)]

• Where [H⁺ (aq)] is the concentration of H⁺ / H₃O⁺ ions

• So, the calculation is:

  • 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