Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 14.13 Experimental pH analysis-Study Notes - New Syllabus

14.13 Analysis of pH Experiments

These experiments develop understanding of how acid/base strength and dilution affect pH. The key skill is to interpret trends in pH data rather than just calculate values.

(i) Comparing pH of Equimolar Solutions

When solutions have the same concentration, differences in pH arise due to the degree of dissociation and nature of the species present.

Key Observations

Strong acids (e.g. \( \mathrm{HCl} \)):

• • Fully dissociate.
  • High \( \mathrm{[H^+]} \) → low pH.

Weak acids (e.g. \( \mathrm{CH_3COOH} \)):

• • Partially dissociate.
  • Lower \( \mathrm{[H^+]} \) → higher pH than strong acid.

Strong bases (e.g. \( \mathrm{NaOH} \)):

• • Fully dissociate.
  • High \( \mathrm{[OH^-]} \) → high pH.

Weak bases (e.g. \( \mathrm{NH_3} \)):

• • Partially react with water.
  • Lower \( \mathrm{[OH^-]} \) → lower pH than strong base.

Salts:

• • Neutral salts (e.g. \( \mathrm{NaCl} \)) → pH ≈ 7.
  • Acidic salts (e.g. \( \mathrm{NH_4Cl} \)) → pH < 7.
  • Basic salts (e.g. \( \mathrm{Na_2CO_3} \)) → pH > 7.

Explanation Principle

• pH depends on extent of ionisation, not just concentration.
• Strong electrolytes → greater ion concentration → more extreme pH.

(ii) Effect of Dilution on Strong vs Weak Acids

Dilution decreases concentration, but the effect on pH differs between strong and weak acids.

Strong Acids

• Fully dissociated at all concentrations.
• Dilution directly reduces \( \mathrm{[H^+]} \).
• pH increases significantly:
  • 10× dilution → pH increases by 1 unit.
  • 100× dilution → pH increases by 2 units.
  • 1000× dilution → pH increases by 3 units.

Weak Acids

• Only partially dissociated.
• Dilution shifts equilibrium to the right (more dissociation).
• Increase in pH is less than expected:
  • 10× dilution → pH increases by less than 1.
  • Because extra \( \mathrm{H^+} \) is produced as equilibrium shifts.

Key Comparison

• Strong acid: predictable pH change (logarithmic).
• Weak acid: smaller pH change due to increased dissociation.

Important Conclusions

• Strength affects initial pH and response to dilution.
• Weak acids resist pH change due to equilibrium shift.
• Experimental data should be explained using:
  • Degree of dissociation
  • Le Chatelier’s Principle

Key Features

• Strong vs weak → based on ionisation.
• Dilution effect differs due to equilibrium.
• Must explain trends, not just state values.