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Strength of Acids & Bases (CIE AS Chemistry)

Revision Note

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Strong & Weak Acids & Bases

  • Strong and weak acids can be distinguished from each other by their:
    • pH value (using a pH meter or universal indicator)
    • Electrical conductivity
    • Reactivity

pH

  • An acid dissociates into H+ in solution according to:

HA → H+ + A-

  • The stronger the acid, the greater the concentration of H+ and therefore the lower the pH

pH value of a strong & weak acid table

Acid pH of 0.1 mol dm-3 solution
HCl (strong) 1
CH3COOH (weak) 2.9

 

  • The most accurate way to determine the pH is by reading it off a pH meter
  • The pH meter is connected to the pH electrode which shows the pH value of the solution

Using a digital pH meter

Equilibria pH-Meter, downloadable AS & A Level Chemistry revision notes

The diagram shows a digital pH meter measures the pH of a solution using a pH electrode

  • A less accurate method is to measure the pH using universal indicator paper
  • The universal indicator paper is dipped into a solution of acid upon which the paper changes colour
  • The colour is then compared to those on a chart which shows the colours corresponding to different pH values

How to use universal indicator paper

Equilibria Universal Indicator Paper, downloadable AS & A Level Chemistry revision notes

The diagram shows the change in colour of the universal indicator paper when dipped in a strong (HCl) and weak (CH3COOH) acid. The colour chart is used to read off the corresponding pH values which are between 1-2 for HCl and 3-4 for CH3COOH

Electrical conductivity

  • Since a stronger acid has a higher concentration of H+ it conducts electricity better
  • Stronger acids therefore have a greater electrical conductivity
  • The electrical conductivity can be determined by using a conductivity meter
  • Like the pH meter, the conductivity meter is connected to an electrode
  • The conductivity of the solution can be read off the meter

Using a digital conductivity meter

Equilibria Conductivity Meter, downloadable AS & A Level Chemistry revision notes

The diagram shows a digital conductivity meter that measures the electrical conductivity of a solution using an electrode

Reactivity

  • Strong and weak acids of the same concentrations react differently with reactive metals
  • This is because the concentration of H+ is greater in strong acids compared to weak acids
  • The greater H+ concentration means that more H2 gas is produced

The reaction of 0.1 mol dm-3 of a strong acid, HCl, with Mg

Equilibria Strong Acid with Reactive Metal, downloadable AS & A Level Chemistry revision notes

The reaction produces a lot of bubbles and hydrogen gas due to the high concentration of H+ present in the solution

The reaction of 0.1 mol dm-3 of a weak acid, CH3COOH, with Mg

Equilibria Weak Acid with Reactive Metal, downloadable AS & A Level Chemistry revision notesThe reaction produces fewer bubbles and hydrogen gas due to the lower concentration of H+ present in the solution

Examiner Tip

  • The above-mentioned properties of strong and weak acids depend on their ability to dissociate and form H+ ions.
  • Stronger acids dissociate more, producing a greater concentration of H+ ions and therefore showing lower pH values, greater electrical conductivity and more vigorous reactions with reactive metals.

Neutralisation Reactions

  • A neutralisation reaction is one in which an acid (pH <7) and a base/alkali (pH >7) react together to form water (pH = 7) and a salt:

Acid + base (alkali) → salt + water

  • The proton of the acid reacts with the hydroxide of the base to form water:

H+ (aq) + OH (aq) → H2O (l)

  • The spectator ions which are not involved in the formation of water are Na+ (aq) + Cl (aq)
    • These react to form the salt:

Na+ (aq) + Cl (aq) → NaCl (aq)

  • The name of the salt produced can be predicted from the acid that has reacted

Table of salts produced from certain acids

Acid reacted Salt produced
Hydrochloric Chloride
Sulfuric Sulfate
Nitric Nitrate
Ethanoic Ethanoate

Examiner Tip

Note that the reaction of an acid and metal carbonate also forms carbon dioxide:

acid + metal carbonate → salt + water + carbon dioxide

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Caroline

Author: Caroline

Expertise: Physics Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.