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

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