Rates of Reaction
Rate of reaction = to find the change in concentration of reactants or products per unit time.
Six factors affecting rate;
- Concentration of reactants (high = fast)
- Surface area of solid reactants (high = fast)
- Pressure of gaseous reactants (high = fast)
- Temperature (high = fast)
- Light (for some photochemical reactions)
Activation Energy = the minimum kinetic energy particles must have in order to react.
1. Particles must collide in the correct orientation - the STERIC FACTOR.
2. The reacting particles must have enough kinetic energy to overcome the activation energy.
These ideas used to explain why conc, pA and surface area increase reaction rates.
1. High conc/pA/surface area means there are more particles available for collisionPER UNIT VOLUME. This means that there are more frequent collisions and so more successful collisions per unit time.
2. High temperatures increase the rate for two reasons:
- particles move faster, therefor collide more frequently.
- More of the particles have enough kinetic energy to overcome the activation energy.
Catalysts = a substance that increases the rate of reaction but is not used up in the process. They work by providing an alternative route with a lower activation energy.
Homogeneous Catalysts = catalysts that are in the same state as the reaction that they catalyse.
Heterogeneous Catalyst = catalysts that in a different state to the reaction that they catalyse.
Working of catalysts can be shown
using the Maxwell-Boltzmann curves
or profile diagrams.
Dynamic Equilibrium = when the rates of forward and reverse reactions are equal so that ll concentrations stay at constant levels. (It always takes time to reach dynamic equilbrium and is a closed system so no molecules can escape)
An equilibrium mixture can contain any react/product ratio. e.g 80% to 20%.
Le Chatelier's Principle
The position of any equilibrium can be moved by changing the surroundings.
When a factor governing equilibrium position is changed, the system acts in a way to oppose the change.
If a bigger concentration of A was added the following equation at equilibrium:
A + B C
The system opposes this change and the concentration decreases, and removes some A by reacting it with B, to form extra C, so therefor, the position moves to the RIGHT.
Typical Exam Question
Explain what is seen when HCl is added to the following system of equilibrium:
Cr O + H O 2CrO + 2H
The system opposes the change, and removes H+ ions therefore the position moves to the RIGHT.
Only effect equilibrium including GASES and when the equation has a different amount of particles on either side.
N + 3H 2NH
The beginning of the equation has 4 particles so it has the higher pressure. Ifthereexternal pressure was increased the system would oppose the change and decrease the pressure by making fewer gas particles, so the position moves to the RIGHT.
Typical Exam Question.
What happens to the position of equilibrium if the pressure is decreased for:
2SO + O 2SO
The system opposes the change, so the pressure increases, more gas particles are made, so the position moves to the LEFT.
In order to predict these effects the sign of deltaH is needed.
A + B C deltaH = -100Kjmol
If the external temperature increases, the system opposed the change and decreases the temperature by moving in the endothermic direction therefore it moves to the LEFT.
Typical Exam Question
What happens to the position of equilibrium if the temperature is decreased:
CaCo CaO + CO deltaH = +210Kjmol
The system opposes the change so the temperature increases so the position moves to the left in an exothermic direction.
IN THEORY: to maximise NH production, the position of equilibrium needs to be as far to the right as possible. The conditions needed are:
1. High pressure that pushes the equilibrium to the right where there are fewer gas particles.
2. Low temperatures that moves the equilibrium to the right in the exothermic direction
3. Catlalyst that has no effect on the position or yield, but does achieve equilibrium faster.
BUT: High pressure reactors are expensive and dangerous so a compromise of 200 Atm is used.
Low temperatures decrease reaction rates of forward and reverse reactions so compromise temperature of 400 degrees is actually used.
This is why a catalyst is so important, as it speeds up the rate the equilibrium is achieved without the cost or need of high temperatures. Catalysts also decrease waste and cut out side-reactions. Finally the NH is removed from the misxture as it forms, so that the system opposes it, so it will form more. The N and the H are then recycled.