Rates of reaction

Rates of reaction

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  • Created by: Aman
  • Created on: 21-02-11 16:55

How fast?

·         The rate of a chemical reaction tells us how fast the reactants turn into products.

·         There are two ways to measure the rate of reaction;

·         Measure how quickly the reactants are used up to make products,

·         Or we can measure the rate at which the products of the reaction are made

·         We can summaries these methods of working out the rate of reaction using this equation;

Rate of reaction = amount of reactant or product formed/time

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Collision Theory

·         There are four main factors which affect the rate of reaction;

·         Temperature,

·         Concentration of pressure,

·         Surface area,

·         Presence of a catalyst

·         Reactions take please when particles that make up the reactants come together. They must collide with enough energy otherwise they will not react. This is the idea of the collision theory.

·         The smallest amount of energy the particles need to react is known as the activation energy.

·         If we increase the chance of individual particles colliding, we are increasing the rate of reaction.

·         When a solid reacts with a solution, the size of the (pieces) of the solid makes a big difference to the rate of reaction. The smaller the pieces, the surface area is very small so the rate of reaction increases (faster).

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The effect of temperature

·         There are two reasons why the increase of temperature in a reaction will also increase the rate of reaction

·         Particles collide more often – when we heat up a substance, energy is transferred to the particles that make up the substance. This means that they move faster. And when particles move faster they have more collisions.

·         Particles collide with more energy – Particles that are moving quickly have more energy which means that the collisions they have are much more energetic.  

·         So overall, when we increase the temperature, the particles have more collisions and energy therefore the particles collide more often and with more energy.

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The effect of concentration

·         Increasing the concentration of the reactants in a solution, increases the rate of reaction because there are more particles of the reactants moving around in the same volume. The more ‘crowded’ together the reactant particles are, the more likely it is they will collide and a reaction will occur.

·         Increasing the pressure of a reaction involving gases has the same effect. It squashes gas particles more closely together. This increases the chance that they will collide and react and so speeds up the rate of reaction. 

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The effects of catalysts

·         A catalyst is a substance which increases the rate of a chemical reaction but not affected chemically itself at the end of the reaction. It is not used up in the reaction, so it can be used over and over again to speed up the conversion of reactants to products.

·         A catalyst is usually used when the previous methods are impossible to use or it is too expensive to increase the pressure or temperature.

·         We need to use different catalysts with different reactions. Many of the catalysts used are transition metals or there compounds. For example, Iron is used in the Haber process.

·         Catalysts are often very expensive because they are made of precious metals. But it is often cheaper to use than to pay for all the energy needed to increase the temperature or pressure of a reaction.

·         Usually, catalysts provide a platform/surface for the reacting particles to come together. They lower the activation energy needed for the particles to react. This means that more of the collisions between particles result in a reaction taking place.

·         Catalysts are usually powdered so they have a bigger surface area. 

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