Edexcel IGCSE Chemistry: Rates of Reaction

Describe an experiment to show the effect of surface area on the rate of the reaction.

A flask is stoppered with cotton wool and contains marble chips. The Cotton wool is to allow the carbon dioxide to escape, but to stop any acid spraying out. The measuring cylinder contains dilute hydrochloric acid. Everything is placed on a balance, set to zero. The mass of carbon dioxide lost is measured at intervals, and a graph is plotted. The steeper the slope of the line, the faster the reaction.
Repeat the experiment using the exact same quantities of everything, but smaller marble chips. The reaction now happens faster.

Describe an experiment to show the effect of changing the concentration of acid has on a reaction.

A flask is fitted with a gas syringe and contains marble chips. The measuring cylinder contains dilute hydrochloric acid. The syringe is set to zero and then the acid to added to the marble chips. The volume of carbon dioxide produced is measured at intervals, and a graph is plotted. The steeper the slope of the line, the faster the reaction.
Repeat the experiment, but with hydrochloric acid only half as concentrated as before. The reaction will be slower.

Describe an experiment to show the effect that changing the temperature has on a reaction.

A concical flask is filled with sodium thiosulphate, a measuring cylinder contains hydrochloride acid. The concical flask and solutions inside is heated to a specific temperature using a water bath. Once at temperature the hydrochloride acid is added to the concical flask and the flask is then placed on top of a black cross. Measure the time it takes for you to no longer be able to see the cross.
Repeat the experiment at a higher temperature. The reaction will get faster as the temperature increases.

Describe an experiment to show the effect that using a catalyst has on a reaction.

We observed the very slow decomposition of hydrogen peroxide to water and oxygen.
When the experiment was completed using a catalyst, the reaction will be faster.

What effect does increasing the concentration have on the rate of a reaction, and why?

There are more particles in a given volume. Particles collide more often, therefore frequency of collisions increases - the rate increases.

What effect does increasing the temperature have on a reaction, and why?

The particles have more energy. More particles have the required energy, resulting in more successful collisions. As the particles have more energy they move around quicker resulting in a greater frequency of collisions- the rate increases.

What effect does increasing the surface area have on a reaction, and why?

More particles in contact with the other reactant. Particles collide more often, therefore frequency of successful collisions increases - the rate increases.

What effect does using a catalyst have on the rate of reaction, and why?

Catalysts provide an alternative pathway for the reaction. This alternative pathway has a lower activation energy than the original pathway. This means that more of the particles have the required energy, hence, there will be more successful collisions.

What effect does changing the pressure of a gas have on the rate of reaction, and why?

If you have a fixed amount of gas, you increase the pressure by squeezing it into a smaller volume. This forces the particles closer together, so they collide more frequently.

What is the activation energy?

The colliding particles must have sufficient energy to react. This energy is the activation energy.

What is rate of reaction?

The 'amount' of product produced per unit of time.

Why do reactions usually slow down as they progress?

As a reaction occurs, the reactant particles are used up because they are turned into products. This means that the reactant concentration (or surface area) decreases and so there is a lower frequency of collisions so the rate goes down.