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)
Experimental techniques (non-continuous/ initial rates method)
- Observing precipitation and noting the time taken for a small and fixed amount of precipitate to be formed (sulfur clock)
- Observing colour change and noting the time taken for a small and fixed amount of reactant/ product to be formed (iodine clock)
Experimental techniques (continuous/ sampling method)
- Titration
- Measuring volume of gas produced
- Colour intensity
- Monitoring electrical conductivity
- Monitoring optical activity
- Pressure measurement (for gas only)
AT REGULAR TIME INTERVALS
Defn. Heterogenous catalyst
is one that exists in a different phase from the reactants and increases the rate of reaction by providing a different reaction pathway with a lower activation energy without itself undergoing any permanent chemical change
Heterogenous catalyst mode of action
- Reactant molecules are adsorbed on the active site of the catalyst via weak temporary bonds
- Reactant molecules are brought close together, thus increasing their concentration at the catalyst surface
- The bonds in the reactant molecules are weakened, thus lowering the activation energy
- The molecules are orientated in the right positions for reaction, increasing the number of effective collisions
- The product is desorbed from the catalyst surface, regenerating the active site on the catalyst
Haber process
- Heterogenous catalysis
- finely divided iron ( FE(s) ) acts as catalyst
- N2 (g) + 3H2 (g) -> 2NH3 (g)
Catalytic converter
- Heterogenous catalysis
- platinum acts as catalyst
- 2NO (g) -> N2 (g) + O2 (g)
- 2NO (g) + CO (g) -> N2 (g) + 2CO2 (g)
The catalytic converter speeds up the above reactions by getting the pollutants to react with each other to form harmless products.
A honeycomb structure is used to maximise the surface area on which heterogenously catalysed reactions can take place
Cars fitted with catalytic converters must use unleaded petrol to prevent poisoning of the catalyst in the converters
Defn. Homogenous catalyst
is one that exists in the same phase as the reactants and increases the rate of a reaction by providing a different reaction pathway with a lower activation energy without itself undergoing any permanent chemical change
(typically involves redox reactions)
(intermediate is usually formed, both steps have lower activation energy)
Reaction between I- and S2O8 2-
- Homogenous catalysis
- Fe 2+ acts as catalyst
Both reactants are negatively charged and tend to repel each other
Step 1:
2 Fe 2+ (aq) + S2O8 2- (aq) -> 2 Fe 3+ (aq) + 2SO4 2- (aq)
Fe 3+ (intermediate compound)
Step 2:
2 Fe 3+ (aq) + 2 I- (aq) -> 2 Fe 2+ (aq) + I2 (aq)
Fe 3+ is reduced to Fe2+, hence regenerating the Fe 2+ catalyst
Oxidation of atmospheric sulfur dioxide
- Homogenous catalysis
- Oxides of nitrogen acts as catalyst
SO2 is harmful to the environment because it can react with O2 in air to form SO3 -> dissolves in water vapour -> sulfuric acid (acid rain)
- 2SO2 (g) + O2 (g) -> 2SO3 (g)
- catalyst: NO2 (g)
Step 1: SO2 (g) + NO2 (g) -> SO3 (g) + NO (g)
Step 2: NO (g) + 1/2 O2 (g) -> NO2 (g)
Enzymes
- Homogenous catalysts that are highly specific
- Works well over a narrow pH and temp range
- Lock-and-key hypothesis
How substrate conc affects rate of reaction
1. At low substrate concentration, the reaction is first order with respect to substrate
-> active sites on the enzymes are unsaturated
-> as the substrate concentration increases, rate increases proportionally
2. Further increase in substrate concentration does not increase rate of reaction and the reaction becomes zero order with respect to the substrate
-> the active sites on the enzymes are saturated
-> an increase in substrate concentration will not lead to further increase in rate as there are no extra active sites to catalyse the substrate reaction
Autocatalysis
- occurs when the product catalyses the reaction
- reaction is catalysed by Mn2+ formed as a product of the rxn
2 MnO4- + 5C2O4 2- +16 H + -> 2Mn 2+ +10 CO2 + 8 H2O
- Initially the rxn is slow,
- but it speeds up as Mn2+ ions are produced and catalyses the reaction. As [Mn2+] increases, rate of rxn increases
- Rate of reaction will then decrease as the concentration of reactants decreases
Criteria of rxn mechanism
- The elementary steps must add up to the overall balanced equation
- The reaction mechanism must be consistent with the experimental rate equation
- The elementary steps must be physically reasonable.
