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C8: Rates and Equilibrium (Altering Conditions (If the forward reaction…
C8: Rates and Equilibrium
Rate of Reaction
There are two ways to work out the rate of reaction:
Find out how quickly the reactants are used up
Find out how quickly the products are made
Measuring the decreasing mass of a reaction mixture :
Since the mass of the reaction decreases as the reaction takes place, you measure the mass at regular intervals to work out the rate
Measuring the decreasing light passing through a solution
You can measure the rate at which the precipitate (insoluble solid) appears (it will make the solution go cloudy)
Measuring the increasing volume as a gas is given off
You can measure the volume of gas given of at regular intervals
Mean rate of reaction
=
quantity of reactant used
/
time
or
quantity of product formed
/
time
Collision Theory and Surface Area
Four factors affect the ROR:
Pressure/concentration
- More particles are moving in the same space so they are more likely to collide
The presence of a catalyst
- Lowers the activation energy of the reaction
The surface area of a solid
- More particles can react. The smaller the size of an object the larger its surface area to volume ration
Temperature
- Particles collide more often with more energy
The minimum amount of energy that particles must have before they can react is called the
activation energy
Reactions only take place when the reacting particles collide with sufficient energy. This is
collision theory
Reactions are more likely to occur if you:
Increase the frequency that the particles collide
Increase the energy they have when they collide
The Effect of Temperature
When a substance is heated up, energy is transferred to its particles which means the particles move (or vibrate) faster. As a result, they collide more often
Quicker moving particles have more energy when they collide so a larger percentage of the collision results in the reaction occurring.
Particles colliding with more energy have a greater effect than particles colliding frequently
The Effect of Catalysts
Reduces the activation enregy so more particles have sufficient energy to react (more effective collisions)
Mostly used in the form of powders, pellets or fine gauzes which gives them the biggest surface are to volume ratio
Different catalyst are needed for different reactions e.g iron is used to make ammonia
Using catalyst reduces energy costs so less fossil fuels are burnt. This reduces the amount of carbon dioxide in the air.
Catalysts do
NOT
get used up in a reaction. However they eventually become 'poisoned' and stop working
Reversible Reactions
Reversible reaction
- when the products can react to make the original reactants
Left hand of the equation = reactants
Right hand of the equation = products
Litmus paper is blue in the presence of an alkali an red in the presence of an acid
Energy and Reversible Reactions
Energy cannot be created or destroyed so the same amount of energy taken in when the reaction goes in one direction is the same amount given out when the reaction goes in the opposite direction
If the forward reaction is endothermic then the backward reaction must be exothermic
Dynamic Equilibrium
In a closed system, the reactants will eventually start making products at the same rate that products are making reactants
Equilibrium
- When the rate of the forward reaction matches the rate of the backward reaction
Le Chatelier's Principle
- The position of equilibrium shifts to oppose the change
Altering Conditions
If the forward reaction produces
more
molecules of gas:
An
increase
in pressure
decreases
the amount of product formed
A
decrease
in pressure
increases
the amount of product formed
(The opposite is also true)
If you increase the pressure in the reaction vessel the equilibrium will shift to the side with fewer gas molecules
When there are equal numbers of gas molecules on either side of the equation the changing the pressure has no effect on the equilibrium
When the gas molecules are equal increasing the pressure will speed up the reaction in both directions
If the forward reaction is
exothermic
:
An
increase
in temperature
decreases
the amount of product formed.
A
decrease
in temperature
increases
the amount of product formed
(The opposite is also true)
If the temperature is increased the equilibrium shifts as if to reduce the temperature and vice versa
If the concentration is increased the position will shift to the left/right to reduce the concentration and oppose the change