Kinetics HL and SL

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6.2 Collision Theory

• For a reaction to occur

1. particles must collide

2. with enough energy

3. in the correct orientation

• Not all collisions are successful, because not all particles have the activation energy

• The activation energy is the minimum energy required for a reaction to occur

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The Maxwell-Boltzmann Distribution

• In a sample gas or liquid, the molecules are flying about and bumping into each other and the walls all the time.

• When they do this, no energy is lost from the system - they’re elastic collisions.

• So in a sample of gas or liquid at any given time, the energy that molecules have is going to be spread out, with some particles having more or less energy than others.

• If you were to draw a graph of how the energy was spread over different molecules, you’d get the Maxwell-Boltzmann distribution!

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This graph is important if you want to get those easy marks. You have to remember that:

• the area under the curve = the total molecules in the sample, and doesn’t change

• there are no molecules with no energy

• very few molecules have high energies, but

• there is no maximum energy for a molecule, and

• the most probable energy value is where the curve is at its highest, as shown.

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If you increase the temperature of the sample, the distribution changes as shown below. It becomes stretched out and has a lower peak, but the area under the curve remains the same.

The reverse happens when you cool down a sample.

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The rate of a reaction is the change in concentration of a substance in unit time and has units of moldm-3s-1.

The rate of a reaction is affected by

• change in concentration of reagents in solution (or pressure for gases),

• the surface area of a solid,

• the temperature,

• the presence of a catalyst

We will look at a variety of techniques for measuring rates of reaction.

6.1 Reaction Rates

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Concentration

An increase in concentration of a reagent increases the number of particles in a given volume, and so increases the chance of a successful collision. This increases the rate of a reaction.

As a chemical reaction proceeds, the reagents are used up, so their concentration reduces. The rate is at its maximum at the start of a reaction. On a concentration against time graph, the initial gradient is the most negative. The gradient falls to zero at the completion of the reaction.

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Surface Area

When a solid reagent is broken into smaller pieces the surface area of the solid increases. This allows more collisions to occur with particles of the other reagent.

e.g. reaction of dil hydrochloric acid and marble chips

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Temperature

An increase in temperature increases the rate of a chemical reaction.

The particles gain kinetic energy as temperature increases so they move about more quickly and there are more collisions. However, this is not the most important factor.

Particles only react when they have at least the minimum amount of energy. This minimum amount of energy is called the ACTIVATION ENERGY (Ea).

The activation energy is the minimum energy required for a collision to result in a reaction.

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At higher temperatures, the mean energy of the particles is increased.

Ea

The number of molecules with E > Ea is much higher at a higher temperature (area under line).

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The number of successful collisions at higher temperature is much greater.

Even a small temperature increase can lead to large increases in rate.

Some reactions do not occur at room temperature because their activation energy is high

e.g. petrol and oxygen only react if sparked.

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Catalysts

A catalyst is a substance which alters the rate of a chemical reaction without being used up.

Catalysts work by providing an alternative route with lower activation energy.

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The Maxwell-Boltzmann curve shows that by reducing the activation energy there are many more particles that are able to react.

Ea catalyst

Ea