Physical Chemistry
8 marks · NormalThe reaction between magnesium and dilute hydrochloric acid is used to investigate both the energy changes and the rate of the reaction.
Write a balanced chemical equation for the reaction between magnesium and dilute hydrochloric acid.
State whether this reaction is exothermic or endothermic. Explain your answer in terms of temperature change observed.
Describe a simple experiment you could carry out to compare the rate of this reaction using magnesium powder and magnesium ribbon. Include the measurement you would take.
Based on your knowledge of energetics and rates, explain why the reaction may occur faster with magnesium powder than with magnesium ribbon, but the temperature rise observed may be similar in both cases if the same mass is used.
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.
Show answer
(a) Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g) · must show correct formulae, balanced equation
(b) Exothermic. The temperature of the reaction mixture increases as heat is released to the surroundings during the reaction. · must show identified as exothermic, reference to temperature rise
(c) Add magnesium powder to dilute hydrochloric acid in a conical flask fitted with a gas syringe or an upside-down measuring cylinder in water to collect hydrogen gas. Repeat with the same mass of magnesium ribbon. Measure and compare the time taken to produce a set volume of hydrogen in both cases. · must show comparison of powder vs ribbon, measure gas produced or time taken
(d) Magnesium powder reacts faster because its greater surface area allows more frequent collisions with acid particles, increasing the rate. However, if the same mass reacts completely in both cases, the total energy given out (and therefore the temperature rise) will be similar, since the same amount of reaction occurs. · must show surface area effect on rate, same total energy/temperature change
Common mistakes: confusing exothermic/endothermic terms; thinking temperature rise is greater for faster rate (if total reactant is the same)
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correct, fully balanced chemical equation.
B1 (b) · 1 mark · States reaction is exothermic.
E1 (b) · 1 mark · Explains that temperature rises due to heat released.
C1 (c) · 1 mark · Describes correct use of both forms of magnesium and acid.
C2 (c) · 1 mark · Includes measurement of rate (e.g., gas volume over time).
C3 (c) · 1 mark · Specifies fair comparison (same mass/conditions).
D1 (d) · 1 mark · Explains powder reacts faster due to greater surface area.
D2 (d) · 1 mark · Explains why the temperature rise is similar if same mass is used.
Worked solution
a) Balanced equation:
b) The reaction is exothermic. This is shown by the temperature rise of the reaction mixture when magnesium reacts with hydrochloric acid; heat is released to the surroundings.
c) To compare the rates, add the same mass of magnesium powder to dilute hydrochloric acid in a conical flask connected to a gas syringe to collect hydrogen gas, and measure the time to produce a fixed volume, e.g., 50 cm³. Repeat the experiment with magnesium ribbon, using the same mass. Compare the times taken or volumes of gas produced over a specific period.
d) Magnesium powder has a much greater surface area than ribbon, so acid particles collide with it more frequently, increasing the reaction rate. However, if the same mass of magnesium reacts completely, the total energy released (and thus the final temperature increase) will be about the same, because it depends on the total amount of reaction, not the speed.