Reaction with metals
4 marks · NormalEthanoic acid is a commonly used carboxylic acid.
State the term used to describe the strength of ethanoic acid as an acid.
Describe how ethanoic acid affects blue litmus paper.
Suggest a typical pH value for a solution of ethanoic acid.
Compare the reaction rate of magnesium with ethanoic acid to the reaction rate with hydrochloric acid. State which is faster and explain why.
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(a) weak acid
(b) turns blue litmus paper red
(c) pH 3–5
(d) The reaction with hydrochloric acid is faster because hydrochloric acid is a strong acid (fully ionised), whereas ethanoic acid is a weak acid (partially ionised).
Common mistakes: Saying 'strong' instead of 'weak' acid; Incorrect pH range; Not providing reason for reaction rate
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Marking points
B1 (a) · 1 mark · Identifies ethanoic acid as a 'weak acid'.
B2 (b) · 1 mark · States blue litmus paper turns red.
A1 (c) · 1 mark · States a typical pH value in range 3–5.
M1 (d) · 1 mark · States reaction with hydrochloric acid is faster, with correct explanation relating to strength/ionisation of acid.
Worked solution
a) Ethanoic acid is a weak acid.
b) It turns blue litmus paper red.
c) Typical pH value: 3–5.
d) The reaction of magnesium with hydrochloric acid is faster because hydrochloric acid is a strong acid (fully ionised), while ethanoic acid is a weak acid (partially ionised), so it produces fewer H+ ions in solution, leading to a slower reaction.