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Module 3 Revision notes - Rates of Reaction & Collision Theory

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What is the “rate" of a reaction?

The rate of a reaction is the speed of the reaction. It is not “how much” of a product is made, but instead “how quickly” a reaction takes place.

How can we measure the rate?

If we consider a reaction

e.g. zinc + hydrochloric acid —> zinc chloride + hydrogen

then there are two possible ways of measuring the rate:

1) measure how quickly one of the products (e.g. the hydrogen) is made
2) measure how quickly one of the reactants (e.g. the zinc) disappears

So we could, for example, measure the volume (in ml.) of hydrogen made every 10 seconds or the loss in mass (of the zinc and hydrochloric acid as they change into hydrogen gas escaping from a beaker) every 10 seconds.

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Collision theory

The theory that we use to explain how different variables change the rate of reaction is called the collision theory.

For a reaction to take place, the particles of the substances that are reacting have to collide. If they collide, with enough energy then they will react. The minimum amount of kinetic (movement) energy that two particles need if they are going to react when they collide is called the activation energy.

There are therefore two main ways of increasing the rate of a reaction:

1) increase the number of collisions
2) increase the amount of movement (kinetic) energy so that more collisions lead to a reaction

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A thought experiment on collision theory

A group of students are taken into the school hall and blindfolded. They are asked to move around the hall. If two students crash into each other and both students fall over, then they stay lying on the ground - they have "reacted".

How can we increase the rate at which students fall down? There are two obvious choices:

1) Put more students into the hall. This will lead to more collisions between students.

2) Ask the students in the hall to run more quickly. This will also lead to more collisions and to a greater chance of a collision leading to the students falling over.

We will use this "thought experiment" to try and explain rates of reaction.

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What can change the rate of a reaction?

The four main variables that can change the rate of a reaction are:

1. The concentration of the substances reacting
2. The temperature
3. The size of the particles
4. A catalyst being present

We will now look at these factors in turn.

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The effect of concentration

The concentration of a solution is how strong the solution is. For example, if we consider the reaction between marble chips (calcium carbonate) and hydrochloric acid:

calcium carbonate + hydrochloric acid —> calcium chloride + water + carbon dioxide

Then a stronger acid contains more acid particles and less water particles than a weaker acid.

Increasing the concentration of a solution leads to more collisions (greater frequency of collisions) so the rate of the reaction goes up.

Looking back to the "students in the Hall" thought experiment, we are putting more students into the Hall.

In a less concentrated acid, the number of collisions is low, so the rate of the reaction is slower. (The water particles aren't shown):

With a more concentrated acid, the number of acid particles is greater, so the number of collisions is greater and the rate of the reaction is higher (faster):

Changing the concentration of the acid does not change how quickly the particles are moving (i.e. it doesn't increase the amount of energy they have).

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The effect of temperature

When we increase the temperature at which a reaction is taking place, the particles move more quickly. This has two effects:

1) More collisions take place

2) When a collision occurs, there is more chance that the collision will lead to a reaction, because the amount of energy is more likely to be greater than the minimum amount of energy needed (the activation energy)

Looking back to "students in the Hall" though experiment, we are now asking the students to run more quickly around the Hall.

At a lower temperature, the number of collisions is lower because the particles are moving more slowly. Also when a collision occurs, there is less chance of a reaction taking place because the movement energy in the particles is less (in the animation successful collisions flash yellow, unsuccessful collisions flash blue):

At a higher temperature, the number of collisions is greater because the particles are moving more quickly. When a collision occurs, there is more chance of a reaction taking place because the movement energy in the particles is greater:

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The effect of particle size

Solids with a smaller particle size (e.g. powders or small chips) react more quickly than solids with a larger particle size (e.g large chips). Here is why:

Look at this diagram

The perimeter of the large chip is 12 units. The acid particles can only collide with the edge of the chip. However, if we break up the large chip into 9 smaller chips:

then the perimeter around each chip is 4 units, but there are 9 of them so the total perimeter is 4 x 9 = 36 units. Notice how the acid in the second diagram can reach what used to be the centre of the large chip.

Reducing the size of particles increases the rate of a reaction because it increases the surface area available for collisions to take place. This increases the number of collisions. It has no effect on the energy of the particles.

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Adding a catalyst

A catalyst is a substance that speeds up a reaction without being used up itself. Some reactions have catalysts that can speed them up, but for many reactions there is no catalyst that works.

Here is an example of a reaction with a catalyst. Hydrogen peroxide decomposes (falls apart) to form water and oxygen gas:

hydrogen peroxide —> water + oxygen

This reaction only occurs very slowly unless we add the compound manganese oxide that acts as a catalyst for this reaction. When the catalyst is added the reaction speeds up greatly but the manganese oxide never runs out.

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How does a catalyst work?

1) A catalyst provides a surface on which the reaction can take place. This increases the number of collisions between the particles of the substances that are reacting.

2) A catalyst lowers the activation energy (the minimum amount of energy needed for a reaction to take place). This means that the particles can react with less energy than they needed before the catalyst was added. (Think about it: if the government lowered the legal age to buy cigarettes, then more people could legally buy cigarettes. Similarly, if we lower the amount of energy needed for particles to react, then more particles can react).

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