Reaction with metals
4 marks · NormalThe table below shows the reaction of four different carboxylic acids with magnesium ribbon. In each case, 10 cm³ of acid at the same concentration is used. The time taken for the reaction to stop is measured:
| Carboxylic Acid | Time taken for reaction to stop / s |
|---|---|
| methanoic acid | 110 |
| ethanoic acid | 98 |
| propanoic acid | 99 |
| butanoic acid | 112 |
Which acid reacts fastest with magnesium ribbon? Support your answer using data.
Suggest why the reactions do not all take the same time, even though the acids are at the same concentration.
Write the formula for the salt formed when butanoic acid reacts with magnesium.
State one safety precaution when carrying out these reactions.
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) ethanoic acid; It has the shortest time (98 s), so reacts fastest.
(b) Small differences in structure or impurities, measurement errors, or slight differences in temperature or mixing, despite identical concentration.
(c) (CH3CH2CH2COO)2Mg
(d) Wear safety goggles (or gloves or work in a well-ventilated area, accept any suitable).
Common mistakes: Identifying fastest acid by longest time; Incorrect salt formula
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly identifies ethanoic acid as fastest (shortest time), with data cited.
M1 (b) · 1 mark · Suggests at least one valid reason for differences in reaction time (e.g. impurities, apparatus, measurement error, temperature).
A1 (c) · 1 mark · Correct formula: (CH3CH2CH2COO)2Mg.
B2 (d) · 1 mark · States a valid safety precaution (e.g. goggles, gloves, ventilation).
Worked solution
a) Ethanoic acid reacts fastest; shortest time is 98 s.
b) Differences in structure (chain length), impurities, minor variations in temperature or measurement could affect reaction speed.
c) The salt formed is (CH3CH2CH2COO)2Mg (magnesium butanoate).
d) Safety precaution: Wear safety goggles when mixing acids and metals.