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Rates of Reaction 1
The smallest amount of energy required by particles to react is called the activation energy.

Rates of Reaction 1

Many factors affect rates of chemical reactions - pressure of gases, temperature, surface area of solids, concentration and if there is a catalyst. Anything that will change the probability of particles colliding or change the energy of the collisions will affect the rate of a reaction. This is the first of three GCSE Chemistry quizzes looking at these factors.

The rate of a reaction is a measure of how quickly a chemical reaction progresses. Some reactions can be very slow, for example rusting can take weeks or even years, whilst others, such as neutralisation or an explosion are over very rapidly. The rate of reaction is very important in industrial situations; take explosions for instance - these are incredibly fast reactions. An explosive is a solid that turns into a huge amount of very hot gas VERY quickly! This makes them useful for several purposes - most notably in warfare - though a more peaceful use is in the quarrying for resources.

At a quarry, they drill holes with a diameter of a few centimetres down into the rocks. They then drop sticks of explosives into the holes along with a detonator. The space above the explosives is then filled with something like sand or fine gravel. When the explosive is ignited (this is an increase in energy, one of the factors which affects the rate of a reaction), it vapourises in a fraction of a second, turning into a huge quantity of gas. The hole that was drilled is far too narrow for the gas to escape quickly. The trapped gas creates a huge pressure since it is confined in such a small volume and this pressure is enough to fracture the rocks, sending them flying into the air. You may have seen video footage of the effect of this fast reaction during your chemistry lessons. Before explosives, quarrying was a much slower and less spectacular process since the quarry workers used hammers and wedges to split the rocks apart. Where there were no natural cracks, they had to make their own holes by hand before they could drive the wedges into the rocks.

There are many different ways that you can follow the rate of reaction, you will have used some of them and seen others demonstrated during your GCSE studies. It all comes down to finding ways of measuring how fast the reactants are used up or how quickly the products are made. If one of the products is a gas that can safely be released into the air, life is easy! You can simply let it escape from the reaction vessel and measure how the mass decreases as the reaction proceeds. A better method is to collect the gas and measure how much is produced every few seconds. In a reaction between liquids, if an opaque precipitate or suspension is produced, less and less light will be transmitted through the liquids as the reaction progresses. You could stand the reaction vessel on a piece of paper with a cross on it and time how long it takes for the cross to be obscured. For a more technical solution, use data logging and a light sensor to measure how much light is transmitted through the mixture over the course of the reaction.

When measuring the rate of a reaction, what you can't do is just measure a factor at the start or at the end. This does not work, you need to measure something continuously during the reaction, in other words, your measurements must include time. It is best to start timing at the moment that the reactants are mixed and to continue measuring for as much of the reaction as possible. You will then be able to collect sufficient data for it to be both reliable and valid, from which you can plot one or more graphs to help you to draw your conclusions.

Have a go at this quiz and see how well you understand how to measure the rates of chemical reactions and the factors which will affect them.

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1.
A catalyst is...
a substance that slows down a chemical reaction, and is used up in the reaction
a substance that speeds up a chemical reaction, and is used up in the reaction
a substance that speeds up a chemical reaction, but is not used up in the reaction
a substance that slows down a chemical reaction, but is not used up in the reaction
Make sure that you know this definition off by heart for your GCSE
2.
Which piece of laboratory equipment would be useful for measuring the volume of gas given off?
Beaker
Gas syringe
Conical flask
Burette
This would be the most convenient and accurate method of measuring the volume of gas given off, although theoretically you could use any of the other pieces of equipment
3.
Which of the following methods is NOT a valid way to measure the rate of a chemical reaction?
Measuring the mass of a reaction mixture
Measuring the volume of gas given off
Measuring the light transmitted through a solution
Measuring the volume of liquid at the start of the reaction
If you just measured the volume of a liquid at the start, it tells you nothing about how fast the reaction happened after that point in time. Any method of measuring the rate of reaction MUST be measurable during the whole time of the reaction
4.
In order to accelerate the rate of a chemical reaction, there are two things which we can do. Pick the correct combination.
Decrease the chances of particle collision
Increase the energy the particles collide with
Increase the chances of particle collision
Increase the energy the particles collide with
Increase the chances of particle collision
Decrease the energy the particles collide with
Decrease the chances of particle collision
Decrease the energy the particles collide with
Particles will only react if they collide with each other so increasing the chances of collisions will increase the chances of a reaction occuring. Increasing the energy of collisions gives a better chance that they will collide with an energy greater than the activation energy
5.
Which of the following is NOT a factor that affects the rate of a chemical reaction?
Temperature
Concentration
Surface area
The speed at which the chemicals are mixed
For a slow reaction, even if the chemicals are mixed instantaneously, it will still be a slow reaction and vice-versa
6.
One method of increasing the energy the particles react with is to...
increase pressure
increase temperature
use a catalyst
decrease concentration
Increasing the temperature requires an input of heat energy which makes the particles move around faster, collisions will therfore involve higher energies
7.
One method of increasing the chances of particles colliding during a reaction between gases is to...
increase pressure
decrease temperature
use a catalyst
decrease concentration
Increasing pressure needs the gas mixture to be compressed. If it is compressed, the particles will be closer together and therefore more likely to collide
8.
If 80 cm3 of gas was given off in a reaction in 20 seconds, what was the average rate of reaction?
4 cm/s
4 cm3/s
1,600 cm3/s
0.25 cm3/s
Rate of reaction = amount of reactant used up (or amount of product formed) divided by the time taken
9.
Chemical reactions can only occur when...
particles come together with sufficient energy
particles come together for a sufficient amount of time
particles mix together
particles are able to separate
The amount of energy required is called the activation energy
10.
The smallest amount of energy required by particles to react is called...
chemical energy
reaction energy
enabling energy
activation energy
If particles collide with less than the activation energy, a reaction will not take place
Author:  Kate Gardiner

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