Rates of Reactions
Experiment Setup
To measure the effects of changes in surface area, concentration of solutions, temperature and use of catalyst, you can react calcium carbonate marble chips with dilute hydrochloric acid and measure the mass of CO2 produced by weighing the difference in mass of the reactants and the mass of the products (there won’t be any change in mass produced, because the initial mass of reaction will equal the final mass, however, since carbon dioxide gas is formed, this will escape from the flask, and therefore, the amount of mass lost will be the mass of carbon dioxide produced. Plot the results on a graph with mass against time and you’ll get an upward curve.
For the reaction to occur, acid particles must collide with the surface of the marble chips. As the acid particles get used up, the collision rate decreases, so the reaction slows down.
Changes in Surface Area of Solid
You can repeat the above experiment by keeping the same mass of marble chips, just using smaller ones to increase the surface area. The reaction happens faster. You have to remember the graphs. Notice however, that in the end, the amount of carbon dioxide produced is still the same – just that the small chips experiment happens faster.
Why does it happen faster? Because the surface area in contact with the gas or liquid is much greater. Less marble chip particles are hidden away from the acid particles.
Changes in the Concentration of Solutions
Repeat the original experiment but using hydrochloric acid only half as concentrated as before. The graph should look something like this (ignore the 80% line) – in which the reaction happens slower and produces half as much carbon dioxide gas:
In terms of collision theory, if you increase the concentration of reactants, the reaction becomes faster because it increases the frequency of collisions per second.
Changes in the Temperature of the Reaction
Do the original experiment again, but this time, at a higher temperature. Your graph will look like this (ignore the concentration label ‘cause it’s WRONG unless the lower concentration solution is still in excess).
Increasing the temperature means more kinetic energy for the particles, which make them move faster, therefore, making them collide more frequently.
Also, not all collisions make new bonds. Some particles just bounce off each other. In order for a reaction to happen, particles have to collide with a minimum amount of energy called activation energy. Increasing the temperature produces a very large increase in the number of collisions that have enough energy for a reaction to occur.
In the following diagram, a) shows a fail collision and b) shows a successful one.
Changing the Pressure
Changing the pressure of a reaction where the reactants are only solids or liquids makes virtually no difference, so the graphs remain unchanged. Increasing the pressure in a reaction where the reactants are gases does speed the reaction up. This is because it forces the particles closer together, so they hit each other more frequently.
Catalysts and How They Work
Catalysts speed up the rate of reactions but aren’t used up in the process. You can show that manganese (VI) oxide is a catalyst by simply having two conical fasks containing hydrogen peroxide. Hydrogen peroxide decomposes to give oxygen and water. Put the manganese (VI) oxide in one of the flasks. Oxygen would be given off quickly. To check that the manganese (VI) oxide hasn’t been used up, simply filter it out from the solution and weigh it (remember to weigh it before the experiment too!). The graph should look like the pressure graph – in which the rate of reaction increases, but the amount you get at the end is still the same.
So how does it work?
Adding a catalyst gives the reaction an alternative route for reactions with a lower activation energy.