C08) RATES AND EQUILIBRIUM
Calculating rates of reactions
Rate of reaction
Collision theory | Chemical reactions can only occur when reacting particles collide with each other with sufficient energy. | Increasing the temperature increases the frequency of collisions and makes the collisions more energetic, therefore increasing the rate of reaction. Increasing the concentration, pressure (gases) and surface area (solids) of reactions increases the frequency of collisions, therefore increasing the rate of reaction. |
Activation energy | This is the minimum amount of energy colliding particles in a reaction need in order to react. |
Factors affecting the rate of reaction | |
Temperature | The higher the temperature, the quicker the rate of reaction. |
Concentration | The higher the concentration, the quicker the rate of reaction. |
Surface area | The larger the surface area of a reactant solid, the quicker the rate of reaction. |
Pressure (of gases) | When gases react, the higher the pressure upon them, the quicker the rate of reaction. |
Quantity | Unit |
Mass | Grams (g) |
Volume | cm3 |
Rate of reaction | Grams per cm3 (g/cm3) HT: moles per second (mol/s) |
Rate of chemical reaction | This can be calculated by measuring the quantity of reactant used or product formed in a given time. | Rate = quantity of reactant used time taken Rate = quantity of product formed time taken |
Factors affecting rates
Collision theory and activation energy
Catalysts
Catalyst | A catalyst changes the rate of a chemical reaction but is not used in the reaction. |
Enzymes | These are biological catalysts. |
How do they work? | Catalysts provide a different reaction pathway where reactants do not require as much energy to react when they collide. |
If a catalyst is used in a reaction, it is not shown in the word equation.
Reversible reactions and dynamic equilibrium
Reversible reactions
Energy changes and reversible reactions
Equilibrium
Changing conditions and equilibrium (HT)
Equilibrium in reversible reactions | 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 exactly at the same rate. |
Le Chatelier’s Principles | States that when a system experiences a disturbance (change in condition), it will respond to restore a new equilibrium state. |
Changing concentration | If the concentration of a reactant is increased, more products will be formed . If the concentration of a product is decreased, more reactants will react. |
Changing temperature | If the temperature of a system at equilibrium is increased:
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Changing pressure (gaseous reactions) | For a gaseous system at equilibrium:
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The relative amounts of reactants and products at equilibrium depend on the conditions of the reaction.
Reversible reactions | In some chemical reactions, the products can react again to re-form the reactants. |
Representing reversible reactions | A + B C + D |
The direction | The direction of reversible reactions can be changed by changing conditions: heat A + B C + D cool |
If one direction of a reversible reaction is exothermic, the opposite direction is endothermic. The same amount of energy is transferred in each case.
For example: endothermic
Hydrated copper Anhydrous copper + Water
sulfate exothermic sulfate