Reactions of metals with dilute acids
5 marks · HardA student investigates how sodium, magnesium, zinc, and silver react with dilute hydrochloric acid. She records her results in the table below:
| Metal | Observation |
|---|---|
| Sodium | Rapid fizzing, heat produced, gas collected, test tube very hot. |
| Magnesium | Vigorous fizzing, solution gets warm, gas collected. |
| Zinc | Steady fizzing, slight warmth, some gas collected. |
| Silver | No visible change. |
Rank the four metals in order of reactivity with dilute hydrochloric acid, starting with the most reactive.
Explain how the student’s observations support the order of reactivity you have given. Refer to at least two different metals in your explanation.
State one reason why it is important to use the same number of moles of each metal when comparing their reactivity in this experiment.
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) Sodium > Magnesium > Zinc > Silver
(b) Sodium produced the most heat and gas the fastest, showing it is the most reactive. Silver did not react at all, indicating it is the least reactive. Magnesium and zinc produced heat and gas, but less than sodium, so they are less reactive than sodium but more reactive than silver.
(c) Using the same number of moles ensures a fair comparison because more moles of metal would produce more reaction/products, affecting observations like temperature change or amount of gas.
Common mistakes: Reversing most/least reactive; Giving reasons unrelated to experiment variables
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correctly ranks metals (Sodium > Magnesium > Zinc > Silver). Accept Na > Mg > Zn > Ag.
M1 (b) · 1 mark · Explains why a highly reactive metal (e.g. sodium) is most reactive based on observation (e.g. fastest fizzing, most heat/gas).
M2 (b) · 1 mark · Explains why a less reactive metal (e.g. silver) is least reactive based on observation ('no change/no reaction').
M3 (c) · 1 mark · States that using same number of moles enables fair comparison (fair test).
A2 (c) · 1 mark · Correctly links amount of metal/moles to amount of product/reaction observed (e.g. more moles = more gas/heat).
Worked solution
a) Sodium > Magnesium > Zinc > Silver
b) Sodium produced rapid fizzing, the most heat and gas, so is most reactive. Silver showed no visible change and is least reactive. Magnesium and zinc produced some fizzing and heat, making them less reactive than sodium but more reactive than silver.
c) Using the same number of moles allows for a fair test because more moles would produce more gas and heat. Comparing the same amount of substance means differences are due to reactivity, not to the amount of substance used.