Carboxylic Acids
7 marks · HardThis question is about carboxylic acids and their properties.
Draw the displayed formula of propanoic acid.
State how many carbon atoms are present in a molecule of propanoic acid.
Describe what is meant by a weak acid. Use propanoic acid as an example in your answer.
Propanoic acid reacts with sodium metal. Write a word equation for this reaction.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show Displayed formula with three C atoms and full bonds to H/COOH
(b) 3
(c) A weak acid only partially ionises in solution. Propanoic acid does not release all its hydrogen ions in water.
(d) propanoic acid + sodium → sodium propanoate + hydrogen
Must show: all bonds shown for -COOH group
Also accept: For the displayed formula, accept correct bond diagrams with or without lone pairs, as long as standard conventions are met.
Common mistakes: Leaving out one carbon atom; Not drawing all the bonds in the -COOH group; Saying 'fully ionised' for weak acid
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correct carbon skeleton and all atoms present for propanoic acid.
A1 (a) · 1 mark · All bonds in -COOH group correctly shown.
B1 (b) · 1 mark · States there are 3 carbon atoms.
M1 (c) · 1 mark · States weak acids only partially ionise in water.
A1 (c) · 1 mark · Example linked to propanoic acid or idea of not all H+ released.
M1 (d) · 1 mark · Correct reactants and products in the word equation.
A1 (d) · 1 mark · Products are sodium propanoate and hydrogen (must not use formula).
Worked solution
b) There are 3 carbon atoms in propanoic acid.
c) A weak acid only partially ionises in water. Propanoic acid does not release all its hydrogen ions when dissolved in water.
d) Word equation: propanoic acid + sodium → sodium propanoate + hydrogen