Reaction rates
QUICK POINTS
The graph of the course of a reaction shows that the reaction is fast at the beginning and gradually slows down with time until it finally stops.
The slope of the graph gets steeper when concentration and temperature are increased.
The use of catalysts also increases the rate of a reaction by lowering the activation energy of the reaction.
Reaction rates are also affected by the size of the solid particles used. Smaller pieces of solid or a powdered form will increase the amount of surface area available for reaction. This results in a faster reaction rate.
QUESTION 1
A student investigated the rate of the reaction between zinc and dilute sulphuric acid.
(a) Write a balanced equation for this reaction.
(b) The student carried out two experiments using the same mass and pieces of zinc in both cases. The results are shown in the table below. (i) Explain why the reaction rate is different in both experiments by referring to the collision of particles.
ANSWERS
(a) Zn (s) + H2SO4 (aq) ZnSO4 (aq) + H2 (g)
(b) The reaction rate is different in experiments 1 and 2. The results show that experiment 1 was carried out at 20°C, using acid at a concentration of 0.5mol/dm3. This reaction was slow compared to experiment 2, which used acid that was three times more concentrated and carried out at 40°C. The more concentrated acid in experiment 2 would have more reactant particles, which would lead to more collisions that increase the reaction rate. The higher temperature would give the particles more energy, allowing them to move about and collide more. This also resulted in an increased reaction rate.
QUESTION 2
Dilute hydrochloric acid reacts with marble chips (calcium carbonate), releasing carbon dioxide as one of its products. Three different experiments were carried out using large pieces of calcium carbonate, small pieces of carbonate and powdered calcium carbonate. York Castle High’s Schools’ Challenge Quiz team. The volume of carbon dioxide released in five minutes for each experiment is shown in the table. (a) Write a balanced equation for the reaction. (b) Explain the results obtained by noting the effect of surface area on reaction rate.
ANSWERS
(a) CaCO3 (s) + 2 HCl (aq) CaCl2 (aq) + H2O (l) + CO2 (g)
(b) In the three experiments, different sizes of calcium carbonate were used. The volume of carbon dioxide doubled when small pieces were used, compared to large pieces of calcium carbonate. However, when powdered calcium carbonate was used, the reaction increased by more than 15 times.
This occurred as the powdered calcium carbonate had a much larger surface area than the large and small pieces used. The larger surface area allowed a greater chance for the calcium carbonate particles to collide with the particles from the hydrochloric acid. More collisions resulted in an increase in the reaction rate, as more carbon dioxide gas is produced.