The dilute hydrochloric acid supplied has a concentration of 1.0 mol/dm³. On your bench there are three measuring cylinders, a beaker of distilled water, a ...
Question 1 Report
The dilute hydrochloric acid supplied has a concentration of 1.0 mol/dm³. On your bench there are three measuring cylinders, a beaker of distilled water, a stop-clock, a conical flask with a bung and a gas syringe, and a tube of magnesium ribbon.
A student says that the reaction goes faster when the acid is more concentrated. You are going to test this by measuring the volume of gas collected in the first 60 s of the reaction.
(a) Plan an experiment to find out how the concentration of the hydrochloric acid affects the volume of gas collected in the first 60 s. In your plan, give the volumes of acid and of water you would measure out, state what you would keep the same, and state what you would record. [4]
(b) Give one reason why some gas may be lost before the bung is fitted. [1]
(c) Describe one change to the method that would reduce this loss of gas. [1]
Answer Details
This is a planning question on how acid concentration affects reaction rate, tested by measuring the gas collected in a fixed time. Magnesium reacts with hydrochloric acid, \( \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \), giving hydrogen; a more concentrated acid has more acid particles per unit volume, so collisions with the magnesium are more frequent and more gas forms in the first 60 s.
(a) The plan [4]. Any four of the following points score (maximum 4). A good plan changes only the concentration while keeping everything else fixed:
- Make up 50 cm3 portions of acid of different concentrations by mixing measured volumes of acid and water, for example 50 + 0, 40 + 10, 30 + 20 and 20 + 30 cm3 of acid and water [1].
- Use the same length or mass of magnesium ribbon each time [1].
- Keep the total volume of liquid, the temperature and the flask the same, so the comparison is fair [1].
- Start the stop-clock as the magnesium is added and fit the bung at once [1].
- Record the volume of gas in the syringe at 60 s for each concentration in a table [1].
- Repeat each concentration and take a mean to improve reliability [1].
Diluting a fixed total volume keeps the amount of liquid constant so that only the concentration varies - the whole point of a fair test.
(b) Why gas may be lost before the bung is fitted [1]. The reaction starts the instant the magnesium touches the acid, so some hydrogen escapes while the bung is still being pushed in [1].
(c) One change to reduce this loss [1]. Any one workable method scores: hold the magnesium above the acid in the flask and tip it in only after the bung is fitted; or use a tight-fitting bung already attached to the syringe; or add the magnesium through a small side arm [1]. Each avoids the gap between starting the reaction and sealing the flask.
Exam takeaway: in a rate plan, name the one variable you change, list what you keep the same, and state exactly what you measure - here the gas volume at a fixed 60 s.
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