Reaction Kinetics Flashcards
What is rate of reaction
Types of rates
Relationship between time interval average rate and initial rate
Change in concentration of a particular reactant or product per unit time
Instantaneous rate- rate at particular time
Initial rate- instantaneous rate when time = 0
Average rate- change in concentration of reactant/ product over time interval
Smaller time interval means average rate is closer to initial rate
5 different ways to determine reaction rate
- amount of reactant/ product
- volume of gas evolved/ mass of gas evolved
- intensity of colour usng colorimetry
- electrical conductivity
- change in pressure of gaseous system
Continuous method vs clock reaction
Continuous method- monitor concentration of reactant or product species continuously over time
Clock reaction- monitor time taken for stated change to occur {species responsible for change is the ‘clock’)
Sampling and titration steps
- Mix reactants, start stopwatch simultaneously
- At suitable time, withdraw aliquot (small portion) from reaction mixture into conical flash by pipette
- Stop reaction mixture in withdrawn sample by adding large volume of ice-cold water (dilute + cool mixture) or add excess quenching agent (react with reactant/ catalyst)
- Titrate quenched sample to determine concentration of reactant remaining/ product forming at time
- Repeat sampling-quenching-titration steps at suitable time intervals
- Plot graph of volume of titrant used against time
- Determine instantaneous rate at any specified time by finding gradient of tangent drawn to curve at that time
Measuring colour intensity at regular time intervals steps
- Prepare calibration curve of colour intensity against known concentration of coloured species, by preparing at least 5 solutions with different but known concentration, measuring colour intensity with colorimeter, to plot data forming a calibration curve
- Measure colour intensity of reaction mixture at regular time intervals using colorimeter
- Rate of reaction determined from change in colour intensity over specified time
- Use calibration curve determine concentration of coloured species in reaction mixture at various time intervals
Clock reaction steps
- Add known values of reactants and fixed value of catalyst
- Start stopwatch simultaneously
- Measure time taken for a colour change/ colour formed can obscure a cross drawn on piece of white paper
- Repeat steps 1 to 3 by using same volume of catalyst but varying volumes of reactants, and add water to make sure total volume of reaction mixture is constant, and use the same beaker,
Why vary volume of water?
Why same beaker?
Keep total volume of reaction mixture constant so initial concentration of each reactant is directly proportional to volume used
Ensure depth of reaction mixture remains the same to ensure same amount of product is precipitated
Factors affecting rate of reaction
- Physical states of reactants
- Concentration of reactants
- Temperature
- Catalyst
Physical states of reactants
(solid, covalent bond breaking, acid base, surface area)
Slower in solid form
Activation energy of a reaction involving breaking of covalent bond is high, heating required to increase temperature of reaction mixture so more reactant particles can have energy greater than/ equal to activation energy of reaction to increase rate of reaction
For acid-base reaction, both strong acids and base will dissociate completely and form oppositely charged ions which have natural tendency to attract each other, hence reaction occurs readily
More finely divided a solid reactant, greater surface area per unit volume, more contact it makes with other reactant and reaction occurs faster
Concentration of reactant
When concentration increases, reactant particles come closer together, increasing frequency of collisions, increasing probability of collision having correct collision geometry and sufficient energy for reaction to occur, hence increase frequency of effective collisions, increasing reaction rate
Increase of pressure, will cause reactant particles to come closer together leading to higher frequency of effective collisions
Rate law
Rate = k [A]^m [B]^n
[A]= concentration of reactant A
[B]= concentration of reactant B
k= rate constant
m= order of reaction with respect to A
n= order of reaction with respect to B
m+ n= overall order of reaction
Rate equation
Rate constant
Order of reaction with respect to reactant
Overall order of a reaction
Half life of reaction
Mathematical expression that relates rate of reaction to concentration of each reactant raised to appropriate power
Constant of proportionality in rate equation of reaction (larger rate constant means reaction is faster, k increases with increasing temperature or with a catalyst)
Power to which concentration of reactant is raised
Sum of powers of concentration terms in rate equation
Reaction half life is time taken for concentration of reactant to decrease to half of its initial value
First order reactions (Rate, half life)
Graphs (rate against reactant, reactant against time, product against time)
rate = k [A]
Half life is ln2/k, half life is constant at constant temperature, independent of initial concentration of reactant
Rate against [reactant] is straight line passes through origin and positive gradient
[reactant] against time is doing down curving inwards (time taken for reactant to decrease from A to A/2 = time taken for reactant to decrease from A/2 to A/4)
[product] against time is going up curving inwards (time taken for product to increase from 0 to B/2 = time taken for product to increase from B/2 to 3B/4)
Second order reactions (rate, half life)
Graphs (rate against reactant, reactant against time, product against time)
rate = k [A]^2
Half life of reaction is not constant
Rate against [reactant] increase quadratically
Rate against [reactant]^2 is straight line
[Reactant] against time is going down curving in
Zero order reaction
Graphs (rate against reactant, reactant against time, product against time)
Independent of concentration of reactant, rate unaffected by changes in concentration of reactant
Rate against [reactant] is straight line horizontally
[Reactant] against time is straight line going down
[Product} against time is straight line going up from origin
Pseudo-order reactions
- Presence of a large excess of reactant
- If larger excess, concentration of it will hardly change during reaction relative to change in other reactant
- Concentration can be regarded as effectively constant throughout reaction, rate quation can be modified as rate = k’ [other reactant], where k’ = k [reactant]
- Order of that specific reactant would be the pseudo-mth-order reaction - Reactant is solvent
- Solvent would be present in excess and concentration remains essentially constant - Presence of catalyst
- Increases rate of reaction but not consumed by reaction, concentration of catalyst can be regarded as constant during reaction
To find order of reaction
Compare experiments _ and _,
when initial [X] is same and initial [Y] doubles, initial rate doubles, hence order of reaction with respect to Y is 1
Compare experiments _ and _,
when initial [X] is same and initial [Y] doubles, initial rate x4, hence order of reaction with respect to Y is 2
Find order of reaction from concentration time graph
From graph,
first half life of reaction = time taken for A to drop from _ to _ is _ min
second half life of reaction = time taken for A to drop from _ to _ is _ min
Since half life of reaction is constant, reaction is first order with respect to A.
Reaction mechanism
Elementary step
Molecularity
Intermediate
Collection of elementary steps in proper sequence showing how reactant particles are converted into products
Distinct step in reaction mechanism describing a single molecular event involving breaking or making bonds
Molecularity of elementary step is number of reactant particles taking part in that step (probability of three particles colliding simultaneously with required activation energy and correct orientation is small, so termolecular step is hard
Species formed in one step of reaction mechanism and consumed in subsequent step
Rate-determining step
Slowest step in reaction mechanism of multi-step reaction and determine overall reaction rate, step with highest activation energy
