Rates of Reaction

6 marks · Normal

The graph below shows how the rate of reaction changes with the concentration of hydrochloric acid for two experiments. In both experiments, a different form of magnesium is reacted with hydrochloric acid at 20°C. Experiment A uses magnesium ribbon cut into strips; Experiment B uses powdered magnesium. All other conditions are the same.

Rate of reaction (y-axis) vs concentration of HCl (x-axis), two lines: A (strips), B (powder); B is always above A but both pass through the origin and are straight lines.

(a)

Describe the relationship between rate of reaction and concentration in Experiment A.

[1 mark]

(b)

Suggest why at every concentration, Experiment B has a greater rate than Experiment A.

[2 marks]

(c)

Explain how both concentration and surface area affect the frequency of successful collisions in these experiments.

[3 marks]

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) Rate is directly proportional to concentration (straight line through origin). · must show directly proportional, line passes through origin

(b) Powder has a much larger surface area than strips, so more magnesium particles are exposed for collision, producing a faster rate at each concentration. · must show mention of larger surface area, more exposed particles/collisions

(c) Increasing concentration means more acid particles per unit volume, so more frequent collisions with magnesium. Increasing surface area (using powder) exposes more magnesium particles to acid, also increasing frequency of successful collisions. Both factors together cause more successful collisions per second and raise the rate of reaction. · must show increasing concentration: more particles, more frequent collisions, increasing surface area: more exposed particles, more collisions, both increase successful collision frequency/per second

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · States rate is directly proportional to concentration or line passes through origin/straight line.

B1 (b) · 1 mark · States powder has greater surface area than strips.

B2 (b) · 1 mark · States more magnesium particles exposed or more collisions per second; links to higher rate.

B1 (c) · 1 mark · Explains higher concentration: more particles per volume, more frequent collisions.

B2 (c) · 1 mark · Explains higher surface area: more exposed particles, more collisions with acid.

B3 (c) · 1 mark · Links both factors together to increased frequency of successful collisions and greater rate.

Worked solution

a) The rate is directly proportional to concentration because the graph is a straight line through the origin.

b) Powdered magnesium has a much higher surface area than strips, so more magnesium particles are available to react with acid at the surface. This allows more successful collisions per second and increases the rate at every concentration.

c) Increasing concentration means more acid particles are present per volume, raising the chance of collision per second. Increasing surface area exposes more magnesium atoms for reaction, also increasing collision frequency. Both together mean more successful collisions each second, so the reaction rate is higher.