Topic 9 + 16: Kinetics I, II Flashcards
What is rate of reaction?
Change in concentration / time
What is activation energy
This minimum amount of energy required for the reaction to take place
What is collision theory
Particles must collide with sufficient energy and in the correct orientation in order for a successful reaction to occur
What factors affect the rate of reaction
- Nature of reactants - particle size
- Temperature
- Concentration
- Presence of a catalyst
Describe the effect of particle size on rate of reaction
In a solid molecules can only react at the surface (where molecules are exposed). If there is a higher surface area (i.e. smaller particle size) then more molecules are able to react and the frequency of collisions increases.
Describe the affect of temperature on reaction rates
Increasing the temperature means more particles have a higher kinetic energy. Therefore more collisions will have sufficient energy and there will be more frequent collisions. Therefore there are more frequent successful collisions and a faster rate of reaction.
The reverse would be true for decreasing
Describe the affect of concentration on rate of reaction
If the solution is more concentrated then there are more molecules in the same volume. This means there are more molecules closer together. Therefore there are more frequent collisions. Also more particles with the required minimum energy are added to the system.
Describe the affect of catalysts on rate of reaction.
Decrease the activation energy so that a higher proportion of molecules have sufficient energy for successful collisions. Therefore faster rate of reaction (more frequent successful collisions).
What is the most probable energy on a Maxwell-Bolztmann distribution?
The energy at the peak (maximum point) of the graph.
What is the average energy on a Maxwell-Bolztmann distribution
The average energy lies slightly to the right of the most probable energy (the peak).
What does the area under the curve of a Maxwell-Bolztmann distribution represent?
The number of molecules
Explain the shape of a Maxwell-Bolztmann Distribution
Due to many collisions taking place between molecules, there is a spread of molecular energies and velocities.
* No particles have zero energy/velocity
* Some have very low and some have very high energies
* Most have intermediate energies
How does increasing the temperature change the Maxwell-Bolztmann Distribution?
- Shift to higher energies
- Curve gets broader and flatter due to greater spread of values
- Area under the curve stays constant
The opposite is true for decreasing the temperature
What is the affect on the Maxwell-Bolztmann distribution of increasing concentration
Area under the curve increases. Distribution of energies stays roughly the same (it almost looks like another curve the same just copied slightly above the original).
Homogeneous catalyst
Catalyst is in the same phase as the reactants
Heterogeneous catalyst
The reactants and the catalyst are in different phases
Give examples of heterogeneous catalysts
- Zeolite in cracking
- Iron in the Haber process
- Ni/Pt/Pd in hydrogenation of alkenes
Give the 3 stages of heterogeneous catalysis
- Adsorption (molecules bond to active sites on catalyst)
- Reaction (weakened bonds allow for molecules to react with each other forming new bonds and breaking old ones)
- Desorption (products released from surface of the catalyst).
What are the units of rate of reaction
moldm^-3s^-1
What method would you use to measure the rate of reaction of a reaction which produces a gas
- measure volume of gas produced using a gas syringe
- weigh reaction mixture on a balance at time intervals -> look for change in mass
What method would you use to measure the rate of reaction of a reaction which has an acidic or basic reactant or product
- Titration; sample reaction mixture at time intervals, quench and titrate samples
- measure the pH using a pH probe meter
What method would you use to measure the rate of reaction of a reaction which has a coloured reactant or product
Colorimetry -> spectroscopic method
Coloured compounds frequently include transtition metal compounds, aqueous bromine/iodine, etc.
Why is the initial reaction rate often quoted
As the reaction proceeds,
* the concentration of reactants decreases and it can be difficult to measure the changes accurately
* reverse reaction may occur introducing error
What is initial rate of reaction
Rate of reaction when reactants are first mixed
What is instantaneous rate
Rate of reaction at a specific time
What is the general rule for the average rate of the reaction:
aA + bB -> cC + dD
Rate = -1/x * dX/dt
where x = a,b,c or d
What can be said about the rate of reaction with respect to A and B in:
2A -> B
The rate of disappearance of A is twice as fast as the appearance of B
What is the general method for determining rate law
For a reaction involving several reactants;
1. Keep concentration of one reactant constant
2. See how the rate changes as the concentration of the other reactant changes
what is the general rate equation?
A is 2nd order
B is 1st order
rate = k [A]^2[B]
What is the effect of temperature on the rate constant?
Experimentally find that rates of most chemical reactions increase with increasing temperature.
Rate constant approximately doubles for every 10K rise in temp.
Describe conc-time graph for zero-order reaction
linear, negative gradient
describe conc-time graph for first order
non-linear, convex curve (negative gradient increasing)
Describe conc-time graph for second order
non-linear, convex curve (negative gradient increasing)
Steeper gradient initially than first order and flattens out faster
what does a rate-conc graph look like for a zero order reaction
y = constant (i.e horizontal line)
what does a rate-conc graph look like for a first order reaction
y = x (linear relationship)
what does a rate-conc graph look like for a second order reaction
starts at 0, convex curve with increasing gradient (“exponential”)
How doe half lives change for a 0 order reactant
Successive half lives decrease with time
How do half lives of a first order reactant change
Half lives remain constant
How do half lives of a second order reactant change
They increase with time
What is half life
The time required for the concentration of a reactant to decrease to half its original value (can identify from concentration time graph)
What is the relationship between the rate equation and the rate determining step
The rate determining step is the slowest step of the reaction. The reactants involved (or needed to produce the reactants in previous steps) in the slow step appear in the rate equation.
