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GCSE C 6
Topic 6 - The Rate and Extent of Chemical Changes
| Term | Definition |
|---|---|
| Activation energy | The minimum amount of energy that particles must collide with to react. |
| Catalyst | Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy. They are not used up during the reaction. |
| Collision theory | According to this theory, chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. |
| Effect of changing concentration on equilibrium | If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again. If the concentration of a product is decreased, more reactants will react until equilibrium is reached again. |
| Effect of changing pressure on equilibrium | An increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules. A decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules. |
| Effect of changing temperature on equilibrium | If the temperature of an equilibrium system is increased then the relative amount of products at equilibrium increases for an endothermic reaction and decreases for an exothermic reaction. |
| Effect of concentration on reaction rate | Increasing the concentration of reactants in solution means the reacting particles will be closer together. This means they will collide more often so there will be a higher rate of successful collisions and a faster rate of reaction. |
| Effect of pressure on reaction rate | Increasing the pressure of gaseous reactants means the reacting particles will be closer together. This means they will collide more often so there will be a higher rate of successful collisions and a faster rate of reaction. |
| Effect of surface area on reaction rate | Increasing the surface area of the reactants means there are more exposed reacting particles. This means there are more frequent successful collisions so the rate of reaction increases. |
| Effect of temperature on reaction rate | Increasing the temperature means the particles will have more kinetic energy and so will move faster and collide more often. a larger proportion of the particles will have at least the activation energy. |
| Equilibrium | When a reversible reaction occurs in apparatus which prevents the escape of reactants and products, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate. |
| Le Chatelier’s Principle | If a reaction at equilibrium is subjected to a change in concentration, temperature or pressure, the position of equilibrium will move to counteract the change. |
| Rate of reaction | The measure of the amount of product formed or reactant used over time. The units of rate of reaction may be given as g/s, cm 3 /s or mol/s . |
| Reversible reaction | Reactions in which the products from the reaction can react together to form the original reactants. The direction of reversible reactions can be changed by changing the conditions. |
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