C7.1 – Rate of Reaction

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1. Describe the effect of concentration, particle size, catalysis and temperature on the rates of reactions.

Rate of reaction is the speed of a reaction. It is calculated using the following formula:

rate of reaction = amount of product formed/reactant used up ÷  time

Here are four factors that affect this rate:

• concentration (the amount of particles in a given space): as the concentration of reactants increase, the rate of reaction increases because; and vice versa
• particle size: when the surface area of reactants increases, the rate of reaction increases because there is more surface available for the reaction; and vice versa.
• catalysts: catalysts are substances that speed up chemical reactions.
• temperature: when temperature increases, more energy is gained by the reactants which increase the rate of reaction; and vice versa.

2. Describe a practical method for investigating the rate of a reaction involving gas evolution.

If a gas is given off in a reaction, you can collect it in a gas syringe (with measurements on it) attached to the flask. As gas is produced, the syringe will be pushed back. The amount of gas produced can be recorded easily, using the volume markers on the syringe. Use a clock to measure the time it takes to produce the gas.

Rate of reaction = volume of gas produced ÷ time

3. Devise a suitable method for investigating the effect of a given variable on the rate of a reaction.

You can measure what effect a change in temperature, concentration and surface area and a presence/absence of catalyst can have on the rate of reaction.

Concentration: use different concentrations of solution (eg: acid) and record the rates of reactions to see how it changes when concentration changes.

Temperature: Place the experiment in different temperatures (a cold room, in the sunlight etc) and measure the rate of reactions in each place.

Catalysis: Repeat experiment twice, once with a catalyst and once without and record the difference in the rates of reactions.

Surface area: Use different sized reactants (for example, magnesium can be in the form of strips,  chips and powder). See how the rate changes as the surface area changes.

4. Interpret data obtained from experiments concerned with rate of reaction.

When you do an experiment multiple times with different variables, make sure you record the same amount of product produces in the same time and that the other variables are all constant. Only the variable you’re experimenting with should change between the experiments, otherwise, the results will be inaccurate and unreliable. Once you’ve found the rates, you should be able to state how the speed has changed with the change in the variable.

5. Describe the application of the above factors to the danger of explosive combustion with fine powders (e.g. flour mills) and gases (e.g.mines).

6. Describe and explain the effects of temperature and concentration in terms of collisions between reacting particles

As mentioned above, increasing temperature and concentration increases the rate of reaction. But how does that happen?

Temperature: Increasing the temperature of reactants will give the particles more kinetic energy, so the frequency of collisions between particles will increase and so there is more reaction occurring- the rate of reaction increases.

Concentration: Increasing the concentration means that there are more reactant particles in each cm³ of space, so there will be more frequent collisions between particles. The rate of reaction increases.

7. Define catalyst as an agent which increases rate but which remains unchanged.

Catalysts are substances that increase the rate of reaction but remain unchanged. They themselves do not undergo any chemical reaction and will still remain in the flask/beaker even after the reaction has taken place.

Notes submitted by Lintha

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