RATES OF REACTION
Mean rate of reaction
Mean rate of reaction = quantity of product/time
In a graph:
Steep slope=faster rate
Becoming less steep=shows us the rate is slowing down
Flat line= all of the reactants have been used up
Using tangents
The slope is constantly changing, which means that the rate is constantly changing. This is overcome by drawing straight lines called tangents.
A triangle is drawn and the gradient of this triangle equals the rate of reaction.
Effect of concentration
Collision theory - chemical reactions can only take place when reacting particles collide, and have enough energy. The rate of reaction is determined by frequency of collisions.
Rate increases with concentration, so it is PROPORTIONAL
Effect of surface area
The particles in a solution can only react on the surface on the solid
With 2 blocks, there is a greater surface area so there are more collisions. This means that the rate of reaction increases with surface area, so these are also PROPORTIONAL.
Effects of temperature
When a product has less energy than what it started with, it means this reaction is exothermic. The energy increases before the reaction completes and this is called the ACTIVATION ENERGY.
Temperature increase gives particles energy, causing more collisions. This means that the rate of reaction increases with temperature.
Catalysts
Increase the rate of reaction without being used up or increasing temperature
Increase the rate by providing different pathways, lowering the activation energy
Reversible reactions
We can change the rate by changing the conditions e.g. heating and cooling
If it is exothermic in one direction, it is endothermic in the other
EQUILLIBRIUM - the forward and backwards reactions are taking place at exactly the same rate
Concentration and reversible reactions
If we carry out a reversible reaction in a closed system, none of the reactants or products can escape
If we change the concentration of the reactants or products the reaction is no longer at equillibrium,
If the concentration of the reactant changes, more product will be formed until it reaches equilibrium again and vice versa
Temperature and reversible reactions
If the temperature of the reactants or products is changed, it will no longer be at equillibrium
When the temperature increases, equilibrium shifts to the left in order to reduce it and vice versa
Pressure and reversible reactions
Pressure affects reactions involving gases
If pressure is increased, equilibrium will shift to the side with the larger number of molecules
If the forward reaction is exothermic and the backward reaction is endothermic: