CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.7.11 Strength of Acids & Bases

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 acid & base table

Equilibria Table 1_Strong & Weak Acids & Bases, downloadable AS & A Level Chemistry revision notes

 

  • 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

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

The diagram shows a digital pH meter that 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

 

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

 

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

 

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

The diagram shows the reaction of 0.1 mol dm-3 of a strong acid (HCl) with Mg. The reaction produces a lot of bubbles and hydrogen gas due to the high concentration of H+ present in solution

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

The diagram shows the reaction of 0.1 mol dm-3 of a weak acid (CH3COOH) with Mg. The reaction produces less bubbles and hydrogen gas due to the lower concentration of H+ present in solution

Exam 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

Equilibria Neutralisation Reaction, downloadable AS & A Level Chemistry revision notes

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

Equilibria Neutralisation Reaction Ions, downloadable AS & A Level Chemistry revision notes

  • The spectator ions which are not involved in the formation of water, form the salt

 

Equilibria Neutralisation Reaction of HCl and NaOH, downloadable AS & A Level Chemistry revision notes

The diagram shows a neutralisation reaction of HCl and NaOH and the two individual reactions that take place to form the water and salt

  • The name of the salt produced can be predicted depending as it depends on the acid that has reacted

Acid reacted & salt table

Equilibria Table 1_Neutralisation Reactions, downloadable AS & A Level Chemistry revision notes

Exam Tip

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

acid + metal carbonate → salt + water + carbon dioxide

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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