Reaction Kinetics Flashcards
Defn. Rate of reaction
The change in concentration of reactants or products with time
Defn. Rate equation
An experimentally determined equation that links the rate of a reaction to the concentration of each reactant and/or catalyst raised to a specific power
Defn. Order of reaction
The order of reaction w.r.t. a given reactant / catalyst is the power to which the concentration of that reactant / catalyst is raised in the experimentally determined rate equation
At molecular level, order of reaction..
indicates the actual number of respective molecules colliding to from products in the rate-determining step
Defn. Half-life of a reaction. t1/2
The time taken for the concentration of a reactant to reduce to half of its initial value
Defn. Rate constant, k
The proportionality constant in the rate equation
Collision Theory
For a chemical reaction to occur, the reactant particles must collide with the correct orientation and sufficient energy
Arrhenius Equation
k = Ae^ (-Ea/RT)
Defn. Activation energy, Ea
The minimum energy which the colliding molecules must possess before a collision will result in a reaction
Defn. Catalyst
a substance which increases the rate of reaction by providing a different reaction pathway with a lower activation energy without itself undergoing any permanent chemical change
Factors affecting rate constant, k
temperature and catalyst only
Temp affecting rate constant, k
When the temperature increases, Ea/RT decreases, hence k increases
Catalyst affecting rate constant, k
In the presence of a catalyst, Ea/RT decreases, hence k increases
Factors affecting rate of reaction
Temperature, concentration, catalyst
Temp affecting rate
When the temperature increases,
- Average kinetic energy of reactant molecules increases
- More molecules have energy greater than or equal to Ea
- Hence, the frequency of effective collisions increases
- Rate constant (k = Ae ^(-Ea/RT)) increases, hence rate increases
USE MAXWELL-BOLTZMANN DISTRIBUTION DIAGRAM
(No. of molecules against KE)
Catalyst affecting rate
When a catalyst is added,
- Catalyst provides an alternative reaction pathway with a lower activation energy
- More molecules have energy greater than or equal to Ea
- Hence, the frequency of effective collisions increases
- Rate constant (k = Ae ^(-Ea/RT)) increases, hence rate increases
USE MAXWELL-BOLTZMANN DISTRIBUTION DIAGRAM
(No. of molecules against KE)
How to determine order of rxn?
- non-continuous/ initial rates method
- continuous/ graphical method
Non-continuous/ initial rates method
- Many separate experiments are conducted with different initial concentrations and the initial rate for each experiment is determined
- By comparing change in initial concentration used for each reactant and change in initial rate, one can determine the order of rxn w.r.t. each reactant
Continuous/ graphical method
- Only one experiment is conducted with many readings taken at timed intervals
- Timing starts when reaction begins and the amount of reactants/ products present at various timings are determined
- By plotting a graph of [reactant] or [product] against time, we can determine the order of rxn w.r.t the reactant
Defn. Rate-determining step (r.d.s)
The slowest step in the rxn mechanism
Zero order reactions
- rate is independent of [A]
- the r.d.s. in the mechanism does not involve any A molecules
First order reactions
- rate is directly proportional to [A]
- the r.d.s. in the mechanism involves the reaction of one molecule of A
Second order reactions
- rate is directly proportional to [A]^2
- the r.d.s. in the mechanism involves the reaction of 2 molecules of A
How to find rate constant, k, from an overall 1st order reaction from the constant half-life?
k = (ln2) / (t1/2)