Kinetics Flashcards
Collision theory
Particles must collide in the correct orientation
Particles must have sufficient minimum energy when colliding
Rate of reaction
Dependent on frequency of effective collisions
Factors affecting rate of reaction
- Temperature
- Concentration
- Surface area
- Catalyst
Concentration of aq or g reactants affecting rate
higher conc, more collisions per second per volume, higher probability of effective collisions/ higher frequency of effective collisions, rate of reaction increase
Surface Area of solid reactant affecting rate
smaller particle size, higher SA of contact between particles, more collisions, higher frequency of effective collisions, rate of reaction increase
Boltzmann distribution
- Temperature
2. Catalyst
Temperature affecting rate
temp decrease, ave KE of reacting particles decrease,fraction of particles with KE> or equal to Ea decrease, less collisions, lower frequency of effective collisions, rate of reaction decreases
Catalyst
speeds up rate of reaction
provides an alternative path with lower Ea for reaction to occur
remains chemically unchanged after the end of a reaction
exerts no effect on enthalpy change of reaction
Catalyst affecting rate
provides alternative pathway with lower Ea, fraction of molecules with KE> or equal to Ea increase, more collisions, frequency of effective collisions increase, rate of reaction increase
Energy profile diagram
- shows stability of reactants/products
- number of steps
- Magnitude of Ea in reaction
Steps in reaction
- Single
2. Two
Single step
Reaction takes place in a single step
there will only be a single transition state (hump)
Two steps
Reaction takes place in 2 steps
there will be two transition states (2 humps)
a reaction intermediate is formed and is short lived as it is consumed in the subsequent step. Stable enough to be isolated for a short period of time
Activation energy
Minimum amount of energy required to initiate a chemical reaction. A reaction which occurs in a different pathway will have different activation energy
Transition state
Most unstable state hence appears at the maximum energy. at this stage it is equally likely to form reactants and products.
Types of catalyst
- Heterogeneous
- Homogeneous
- Auto
- Biological
Heterogeneous
Catalyst and reactants are in a different phases
usually involving solid catalyst and gaseous reactants. Increases rate of reaction by weakening existing bonds in all reactant particles by bringing them closer hence providing alternative pathway of lower Ea
usually one step reaction
Examples: Fe or Fe2O3 (Haber process)
Pt, Pd or Rh (catalytic converters) remove harmful gases and converts them to unharmful gases through redox reactions
Mode of action in heterogenous catalyst
- Adsorption
- Reaction
- Desorption
Adsorption
reactant particles adsorbed onto surface of catalyst (stick to) weak interactions formed between reactant and catalyst
weakening existing bonds in all reactant particles by bringing them closer hence providing alternative pathway of lower Ea
Reaction
occurs at faster rate on surface of catalyst as reactants brought closer to one another and existing interactions in reactant molecules weakened lowering Ea
Desorption
Products desorbed from catalyst surface catalyst regenerated and other reactants can be adsorbed again
Slow step
step which has higher Ea than other steps therefore determining overall rate of reaction
when aqueous catalyst is involved in slow step, its conc matters as opposed to solid catalyst the more the aq catalyst used the higher the rate of rxn
Homogeneous catalysts
Catalysts and reactants are in the same phase
functions by reacting with one of the reactants to form a reactive intermediate (step 1) reactive intermediate then reacts with remaining reactant in later stage to complete reaction (2)
usually 2 step rxn
alternative pathway will require one additional step to complete reaction as only a maximum of two colliding particles is possible in a step for effective collisions <2 particles, effective collision hardly likely
Example: Concentrated H2SO4 in ester formation
NO2 catalysing conversion of SO2 to SO3
Autocatalyst
catalysis of a reaction by one of its products
autocatalysed rxn slow at first becomes more rapid as catalyst is produced in the reaction
Example: Redox reaction between manganate (VII) and ethanedioate ions
Hydrolysis of ester
Biological
AKA enzymes
contain active site which reactant particles of particular size and shape will fit making them highly specific in action
attractive forces between reactant molecule (substrate) and enzymes weakens bonds within substrate molecules hence lowering Ea and increasing rate of reaction
Example: Decomposition of H2O2 by catalase/iodide ion
effect of substrate concentration on enzyme activity
at high substrate levels, all active sites of enzyme are saturated an increase in substrate concentration will not increase rate of reaction but increase in enzyme concentration will. ROR no longer depends on substrate conc and becomes 0 order with respect to it
Average rate
overall rate of reaction with no consideration of rate variation during reaction
Calculated by: 1.rate= d(products)/d(time) (increase in concentration of products per unit time )or
2. rate= d(reactants)/d(time) (decrease in concentration of reactants per unit time)
Initial rate
Rate at the start of the reaction corresponds to the slope of the tangent when t=0
At initital rate rate of reaction is the highest due to greatest amount of reactant consumed
Instantaneous rate
rate at a particular point of time can be determined by finding the gradient of the tangent at any one point of the concentration time graph
Rate equation
k{reactant]^x
[reactant]= reactants involved in slow step k= rate constant to show dependence of rate of reaction on Ea and temperature constant with collision theory x= order of reaction wrt to reactant/ shows number of reactants in slow step
can only be determined experimentally
derived from slow step
Overall order of the reaction
power of the concentration of a specific reactant in rate eqn
sum of the powers of the reactants involved
[A]^x[B]^y
x+y= overall order
Order
- Zero
Rate of rxn not affected by this reactant - First
Rate of rxn is directly proportional to concentration of reactant - Second
Rate of rxn is directly proportional to square of the concentration of the reactant
Factors affecting rate constant
1. Increase in concentration of a reactant rate of rxn increase, no change in k 2. Increase in temperature rate of rxn increases, k increases 3. Addition of a catalyst rate of rxn increases, k increases
units of k
depends on overall order of reaction
units for rate is mol dm^-3t^-1’
Mechanism
a reaction involving 3 or more reacting particles is unlikely to combine all reactants in one step so usually take place in multiple steps wiTh a max of 2 reacting particles per step.
breakdown and retails of how and what reacts at each elementary step in a multi step rxn known as rxn mechanism
Intermediates
Cannot appear in rate equation
replace them the reactants that reacted together to form them. needed for reaction to proceed
deducing the rate equation
- Continuous
A single experiment
conc of reactant/ prod tracked over period of time
conc remaining determined through experiment
conc time graph is plotted for analysis half life can be determined to determine if rxn 1st or 2nd order - Initial rate
Multiple experiments
compare the change in reaction rate when concentration of a reactant is systematically changed between the experiments
time taken in each experiments needs to taken to determine reaction rate
order of reaction can be determined by - rate - conc graph
- comparing degree of rate change due to change in conc
experiments to determine rate equation
Continuous
1. Titrimetric
Initial
1.Clock reaction
Physical( for continuous or initial rate)
1. Gas collection method for reaction that produces gas
measure increase in volume at regular time interval under constant pressure
- Gravimetric method
measure the decrease in mass over time (applicable for reactions with gaseous product) - Monitor change pH using data logger/meter
- Monitor change in electrical conductivity for reactions involving change in number of ions from reactants to products
5.Colorimetry method
measure change in colour intensity over time
intensity of colour proportional to concentration of coloured substance
- Changes in pressure
pressure change is due to number of moles of gaseous reactants being different from number of moles of gaseous products
Titrimetric
reactions that do not have colour or other means by which physical measurement can be taken to track rxn progress
- Start rxn with knwn amounts of reactants
- Extract fixed portions of reaction mixture at regular interval
- Quench
- Determine reactant concentration by titration
Quenching
Slowing rxn abruptly at measured time from start of rxn. To allow analysis of content in rxn mixture at point of time so conc of reactants/products can be determined with time
can be achieved
- rapid cooling in ice bath
- removing the catalyst
- by adding large volume of cold water which lowers reaction temperature and dilutes reaction
Clock
for rxns accompanied by prominent visual changes
measures the time taken for a sudden colour change to happen
Rate of reaction= change in conc of reactants or products/ time taken for sudden observable change
for
- Appearance of colour
- Disappearance of colour
Appearance of colour
amount of product formed is fixed
determining time taken for fixed amount of I2 to be formed
rate= change in fixed amt of coloured products/time taken for observable change AKA
rate proportional to 1/time taken for sudden observable change
Disappearance of colour
measuring time taken for colour of reactant of varying concentration to disappear
rate = change in conc of reactant (NOT fixed)/ time taken for reactant to disappear
amt of coloured reactant can vary between experiments
data processing methods to deduce rate equation
- Inspection/ Mathematical method- for initial rate
2. Graphical method- for both initial rate and continuous
Inspection/mathematical
inspection : compare conc vs rate between suitable pair of experiments
Mathematical: use rate eqn to compare suitable pair of experiments
Graphical
Initial rate method
A rate- conc graph is usually plotted
order of rxn deduced from shape
Continuous method
A conc-time graph usually plotted
order of reaction deduced through shape and gradient
know how to draw rate conc graphs for 0,1,2
conc-time graphs for 0,1,2
Half life
Used to differentiate 1 order conc-time grph from 2 order conc time graph
t1/2 defined to be the time taken for the concentration of reactant to fall to half of its initial concentration
if t1/2 constant rxn - 1st order
if t1/2 is not constant (change in t1/2 is significant double or triple) - 2nd order
Half life can be used to determine the rate constant
t1/2= ln2/k
Determining half life from graph
- Reactant time graph
first half life is the time taken for conc to fall from 1 moldm-3 to 0.5 (half of initial)
second half life is time taken for conc to fall from 0.5moldm-3 to 0.25moldm-3 (halved) - Product time graph
assuming initial reactant conc to be 1.0moldm-3 and mole ratio to be 1:1
first half life- from 0 to at 0.5moldm-3
second half life at 0.75 moldm-3
need to know how to draw graph
Combination of initial rate and continuous method
When rxn involves two types of reactants, overall rate of rxn could depend on either one or both types of reactants
to find rate eqn need to find order wrt reactants,done through combination of continuous and initial rate method
given k[A]^x[B]^y
Step 1. Continuous method (finding order wrt A)
Keep [B] constant by adding large excess of B on;y small amt of B consumed so conc of B relatively constant any change in rate is not cuz of B
Rate= k’[A] k’ = k[B] (a constant)
Step 2. Initial rate method (find order wrt B)
keep [A] constant
Change [B] but still keeping B in large excess
Compare time taken for both reactants to reach the same [A] left. Use inspection method to deduce order of reaction
Aqueous species
have the potential to affect to ensure a particular reactant seemingly exerts no effect on rate has to be
- present in large excess (much larger conc)
- kept the same between experiments
