TOPIC 9 - KINETICS I + II Flashcards
rate of reaction
= change in concentration (of product or reactant) / time for change to happen = change y/change x
DRAW TANGENT
initial rate t=0
rate at end = 0
Collision theory
and steric hinderance
for chemical reaction to occur PARTICLES MUST COLLIDE and have SUFFICIENT ENERGY, =/> Ea to overcome the Ea for this reaction. Particles MUST HAVE RIGHT ORIENTATION (gradient).
- If atom or groups of atoms that are large can prevent reaction with other molecules so reaction suffers steric hinderance.
What do we need to find out to work out rate of reaction?
Either:
- how fast one of the reactants is being used up or
- how fast one of the products is being formed
Activation energy, Ea
If Ea did not exist then exothermic reactions would take place spontaneously - such as fuels combusting spontaneously.
Minimum energy that colliding particles need for a reaction to occur. E.g. heat, light, friction, flash.
Why does increasing concentration speed up a reaction?
increase the concentration of reactants in solution, there are more particles closely together in a given volume of the solution and they are closely packed together, so there is an increase chance to collide more frequently.
Curve A’s gradient is higher and levels of before B due to ROR being greater due to higher concen of HCl for calcium carbonate - limiting reactant. if other way round REACTION WOULD NOT OCCUR.
DOES NOT CHANGE MAXWELLL-BOLTZMANN CURVE NOR DOES VOLUME CHANGE as it does not change energy of individual particle.
Why does increasing pressure speed up a reaction?
If any of the reactants are gases, there are more molecules of the gas pushed closer together in the volume so there are more successful collisions. Ea is overcome - ROR increases
p = n/V RT
p directly proportional to concentration
DOES NOT CHANGE MAXWELLL-BOLTZMANN CURVE as it does not change energy of individual particle.
Why does increasing temperature speed up a reaction?
The molecules have more kinetic energy, so the frequency of successful collisions increases. The proportion of molecules that have Ea», increases ROR.
Effect of surface are
increase in SA, increase frequency of successful collisions in total per second, more exposure of surfaces, increase ROR.
DOES NOT CHANGE MAXWELLL-BOLTZMANN CURVE as it does not change energy of individual particle.
Catalyst
A substance that increases the rate of a chemical reaction without being used up in the reaction it provides an alternate route that is lower in activation energy.
On Max Boltzmann curves and REACTION PROFILES - (shows the progress of the reaction over time, with the energy of the reactants and products), the Ea will be LOWERED DUE TO CATALYST and moves to left.
- catalysts are later regenerated
Catalysts allow
- low temps and pressure used
- give higher atom economy
- quicker product produced
- less product seperation
- lowers energy demands
Heterogenous cataylst
A catalyst that is in a different phase to the reactants.
Catalytic converters - exhaust gases in contact with catalyst coated on honeycomb struc to increase SA.
Increasing pressure has limited effect on the rate of heterogenous catalyzed reactions because the reaction takes place on surface of the catalyst. The active sites on the catalyst surface are already saturated with reactant molecules so increasing pressure wont have an effect.
Max Boltzmann curves
show the distribution of energy of particles in a reaction
Max Boltzmann curves
Area under the curve represents total no. of particles in a sample.
ORIGIN - molecules have 0 energy.
Emp -(most probable energy) the point at which curve peaks - most particles have energies around this value at any given time.
Mean energy ( under little dip) - area under the curve on either side is equal.
Ea - area to the right of this represents the no. of particles with at least this energy.
T2 compared to T1
T2 is at a higher temperature, which means a greater distribution of particles have a higher energy, compared to T1; this will result that fraction of the molecules that has the required energy to react. So Ea line is same.
The peak of T2 is to the right and lower than the peak of T1 and line crosses over and above where it levels off.
Solid catalyst
provides SA in which gas molecules can adsorb then react and in correct orientation, this increases APPROXIMITY weakens the covalent bond, increases ROR. Then product molecules desorb from surface.
