Unit 3 - Controlling the rate Flashcards
Why must chemists controll the rate of reaction?
Because if ROR is too low then manufactoring will not be economically viable but if ROR is too high there is a risk of explosion.
For a chemical reaction to occur what must happen.
reactant molecules must collide
Why must the energy required be at sufficient levels?
so that the bonds in the reactant molecules can be broken, and then new chemical bonds can form to create product molecules.
What is the criteria for a successful collision?
- Particles must collide with sufficient energy.
- Particles must collide with correct alignment.
Effect of Surface Area on ROR (for solids)
if the particle size is smaller then there is a larger number of particles on the surface of the solid that are able to take part in collisions.
Effect of Pressure on ROR (for gases)
if the pressure is increased, then there will be more reacting particles in a given volume, which means that more collisions will take place between these reaction particles.
Effect of concentration on ROR
at higher concentrations there are more reacting particles in a given volume which means more collisions will take place and therefore the reaction is faster.
equation for relative rate of reaction
1/time
Exothermic reaction
Causes heat to be released to the surroundings and increases temperature
Endothermic reaction
causes heat to be taken in from the surroundings and lowers temperature
Enthalpy
energy content or ‘chemical potential energy’
enthalpy units
Joules (J) or KiloJoules (kJ)
molar enthalpy units
kilojoules per mole (kJ mol -1)
How do you find the enthalpy change
by subtracting the enthalpy content of the reactants from that of the products
What enthalpy change do exothermic reactions have
Negative because the products have less energy than the reactants
What enthalpy change do endothermic reactions have
Positive because the products have more energy than the reactants
Activation energy
the minimum energy required for a reaction to occur (to form an activated complex)
the activated complex
it is an unstable arrangement of atoms formed at the maximum of the potential energy diagram for a reaction
how does the activated complex form
can form as a result of a successful collision between reactant molecules.
What is happening on the Maxwell-Boltzmann diagram?
The number of particles that don’t have enough energy to react is represented by the unshaded large curve, and the shaded area represents the particles that have high-enough energies to react.
How do you speed up the reaction?
you need to increase the number of the very energetic particles, those with energies equal to or greater than the activation energy.
temperature
the measure of the average kinetic energy of the particles of a substance.
Catalyst
a substance which alters the rate of reaction but is chemically unchanged at the end of a reaction.
what does the catalyst do in the reaction
provides an alternitave route for the reaction which has a lower activation energy.
Alterinative path - Adsorption
- Molecules bond to catalyst at active sites
- Weaking the intra-molecular bonds
Alterinative path - Reaction
- Reaction takes place on the catalyst surface.
- More favourable collision geometry.
- Less activation energy needed to break weakened bonds
Alterinative path - Desorption
- Product molecules leave active sites vacant allowing another reaction cycle.
