Unit 2 - Controlling The Rate Flashcards
Why Must Reaction Rates Be Controlled In Industrial Processes?
If the rate of the reaction is too low, the manufacturing process will not be economically viable.
If the rate is too high, there is a risk of explosion.
Relative Rate of Reactant
For all reactants, the rate of reaction, is indirectly proportional to the time taken i.e if rate of reaction is high, the time taken is small.
When time is measured in seconds, relative rate is measured in s-1.
Collision Theory
Reactions occur when particles of reactants collide, provided they collide with a certain minimum kinetic energy. The more collisons between particles in a given time, the faster the reation. The higher the energy of the colliding particles, the greater the chance of an effective collision (one that results in a reaction).
Collisions have to have the correct energy (activation energy) and correct genometry/orrientation to occur.
Concentration
Increasing the concentration, increases the rate of the reaction. At higher concentrations, there are more reacting particles present in a given volume so more collisions take place between these reactant particles so the reaction is faster.
Particle Size
In any reaction involving solids, decreasing the particle size (i.e increasing the surface area) increases the rate of reaction.
Reactants with a smaller particle size have a larger number of particles on the surface of the solid able to take part in the collisions.
Pressure
If pressure is increased (for gaseous reactants) there are more reactant particles for a given volume so there will be more collisions.
Increasing pressure increases the rate of reaction
Temperature
Temperature is a measure of the average kinetic energy of the particles of a substance.
If the temperature of a reaction is raised the particles have a higher kinetic energy so move faster and collide more often.
At higher temperature, as the particles have more energy, the collisions are more likely to result in a reaction.
Increased T increases:
* frequency of collisions
* energy of collisions
Activation Energy
The activation energy is the minimum kinetic energy required by colliding particles before a reaction may occur.
In general, the lower the activation energy, the faster the reaction.
Catalytic Effect
A catalyst provides an alternative pathway which lowers the activation energy. Therefore more particles now have enough energy to react when they collide so the rate of reaction increases greatly.
Reaction Pathways
Energy changes
During an exothermic reaction (e.g burning methane) some of the chemical potential energy in the reactants is released as heat energy. This means that the products contain less potential energy than the reactants.
During an endothermic reaction heat energy is absorbed from the surroundings and so the products have more potential energy than the reactants.
Enthalpy Change
The enthalpy change is the energy difference between products and reactants. The units are normally in kilojoules (kJ)
ΔH is always negative for an exothermic reaction.
ΔH is always positive for an endothermic reaction.
Activated Complex
An activated complex is an unstable arrangement of atoms formed at the maximum of the potential energy barrier during a reaction. The activation energy is the energy required by colliding particles to form an activated complex.
Values for activation energy are usually given in kJ mol-1
Reaction Pathways
Effect of a Catalyst
A catalyst provides an alternative reaction pathway with a lower activation energy. The activation energy is lowered for both the forward and reverse reactions. Catalysts have no effect on the potential energy of the reactants or products i.e enthalpy change.
