Rates of Reaction
6 marks · NormalA student compares two reactions:
• Reaction 1: 5.0 g of solid calcium carbonate powder is added to 25.0 cm³ of 2.00 mol/dm³ hydrochloric acid. • Reaction 2: 5.0 g of solid calcium carbonate lumps is used with 25.0 cm³ of 1.00 mol/dm³ hydrochloric acid.
For both experiments, the mass of CO₂ gas given off in the first 30 seconds is recorded.
In Reaction 1, the student finds the mass of CO₂ lost after 30 seconds is 0.36 g. Calculate the average rate of reaction during this time. Give your answer in g/s.
Explain why Reaction 1 is expected to be faster than Reaction 2, using both surface area and concentration in your answer.
In Reaction 2, the student records a mass loss of 0.13 g in the first 30 seconds. Calculate the average rate of reaction for Reaction 2.
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) 0.012 g/s · must show rate = mass loss / time
(b) Reaction 1 uses powder so has higher surface area (more particles exposed), and a higher acid concentration. Both give more frequent collisions per second, so Reaction 1 is faster. · must show mentions greater surface area, mentions higher concentration, links both to more collisions per second/faster rate
(c) 0.0043 g/s · must show rate = mass loss / time
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses average rate = mass loss / time.
A1 (a) · 1 mark · 0.36 / 30 = 0.012 g/s (correct answer to 2 or 3 significant figures, unit required).
B1 (b) · 1 mark · Mentions powder has greater surface area than lumps (more particles exposed).
B2 (b) · 1 mark · Mentions higher concentration gives more particles per volume; both increase frequency of collisions or successful collisions per second.
M1 (c) · 1 mark · Uses average rate = mass loss / time.
A1 (c) · 1 mark · 0.13 / 30 = 0.0043 g/s (accept 0.004 g/s if rounded, unit required).
Worked solution
a) Average rate = mass lost / time = 0.36 g / 30 s = 0.012 g/s
b) Reaction 1 uses powder (higher surface area), so more CaCO₃ particles are available to collide with acid. The acid is also at higher concentration, so more particles per volume. Both lead to more collisions per second and a faster reaction.
c) Average rate = 0.13 g / 30 s = 0.0043 g/s