Rates of reaction

QUICK POINTS

A change in the amount of reactant or product with time describes the reaction rate. If the course of a reaction is followed over time, the concentrations of the reactants decrease while the concentrations of the products increase gradually. This depicts the rate of a reaction. By monitoring loss in mass or gain in volume of gases, the rate of a reaction can be determined.

Several factors such as concentration, pressure, temperature, particle size and catalysts affect reaction rates.

The collision theory states that particles must collide in order to react, and they must do so with minimum energy to break the bonds. Most factors work by increasing the number of collisions in the reaction.

Reaction rate increases when concentration, temperature and pressure are increased. This causes the slope of the graph to get steeper. The graph of a slower reaction is not as steep. Smaller pieces of reactants can also lead to an increase in reaction rate, as more surface area becomes available for the reaction to take place. Larger pieces lead to a slower reaction.

QUESTION 1

Magnesium is reacted with dilute sulphuric acid and the volume of hydrogen produced is measured over time. The reaction is done using two different conditions. (a) Which of the reactions show a faster reaction rate?

(b) Suggest what conditions may be different in reactions R and S.

ANSWERS

(a) Reaction R has a steeper slope, which suggests a faster reaction rate.

(b) Several conditions could be responsible for the faster reaction rate in R. These could be as follows:

Reaction R – higher concentration or temperature; use of powdered solid or catalyst.

Reaction S – lower concentration or temperature; use of larger pieces of solid.

QUESTION 2

A student investigated the reaction between magnesium ribbon and dilute hydrochloric acid. The volume of gas produced was measured. The reaction was carried out at 25°C. The results are shown below. Use the graph to answer the following questions:

(a) What does the flat line on the graph represent?

(b) What volume of gas is produced after 20 seconds?

(c) How long did it take for 30cm3 of gas to be produced?

(d) How would the shape of the graph change if the reaction was carried out at 15°C? (e) The student repeated the reaction using magnesium powder instead of magnesium ribbon. How would this affect the rate of the reaction?

(f) When magnesium reacts with hydrochloric acid, a temperature change is seen. In an experiment the temperature change was as follows:

Temperature at start = 21°C

Temperature at end = 62°C

(i) What conclusion can be made about the type of reaction from these results? (ii) What observation would the student make to support this conclusion?

ANSWERS

(a) The flat line on the graph suggests that no more gas was being produced so the reaction has stopped.

(b) The volume of gas produced after 20 seconds is40 cm3.

(c) 30cm3 of gas was produced after 14 seconds.

(d) If the reaction was carried out at a lower temperature (15°C), the shape of the graph would change. The slope of the graph would be less (not as steep) as the reaction rate would be decreased.

(e) If the reaction is repeated using magnesium powder, the reaction rate would increase, resulting in a graph with a steeper slope (gradient).

(f) The temperature of the reaction increased from 21°C to 62°C. This suggests that the reaction was exothermic, as heat was given off. The student would observe the reaction vessel becoming warm as heat is produced. Francine Taylor-Campbell is an independent contributor. Send feedback to kerry-ann.hepburn@gleanerjm.com.