Carboxylic Acids
5 marks · EasyPropanoic acid is a member of the carboxylic acid family.
Draw the displayed formula of propanoic acid.
State why propanoic acid is described as a weak acid.
A piece of magnesium ribbon is added to propanoic acid. What do you expect to observe? Explain your answer.
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(a) Displayed formula: CH3CH2COOH, all bonds drawn, including COOH group showing O=C–O–H and hydrocarbon chain. · must show all bonds, -COOH group fully shown, three carbon atoms
(b) Propanoic acid is a weak acid because it only partially ionises in water; most molecules stay unionised. · must show partial ionisation, most molecules remain un-ionised
(c) Effervescence/fizzing occurs as hydrogen gas is released, because propanoic acid reacts with magnesium. The reaction is slow because it is a weak acid producing fewer hydrogen ions. · must show observation: fizzing, hydrogen gas, reaction is slower due to weak acid
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Marking points
B1 (a) · 1 mark · Correct displayed formula for propanoic acid: all bonds shown, including COOH group
B2 (b) · 1 mark · States propanoic acid is only partially ionised in water/is a weak acid
B3 (b) · 1 mark · Mentions most molecules remain unionised/undissociated
B4 (c) · 1 mark · States observation: effervescence/fizzing or hydrogen bubbles
B5 (c) · 1 mark · Explains reaction is slow due to the acid being weak/few hydrogen ions
Worked solution
a) The displayed formula of propanoic acid is drawn as:
b) Propanoic acid is a weak acid because it only partially ionises in water. Most molecules remain unionised.
c) Fizzing/effervescence occurs as hydrogen gas forms when magnesium reacts with propanoic acid. The reaction is slow because few hydrogen ions are available from the weak acid.