Kinetics

Question 1

Collision theory

Answer 1

Particles must collide in the correct orientation

Particles must have sufficient minimum energy when colliding

Question 2

Rate of reaction

Answer 2

Dependent on frequency of effective collisions

Question 3

Factors affecting rate of reaction

Answer 3

1. Temperature
2. Concentration
3. Surface area
4. Catalyst

Question 4

Concentration of aq or g reactants affecting rate

Answer 4

higher conc, more collisions per second per volume, higher probability of effective collisions/ higher frequency of effective collisions, rate of reaction increase

Question 5

Surface Area of solid reactant affecting rate

Answer 5

smaller particle size, higher SA of contact between particles, more collisions, higher frequency of effective collisions, rate of reaction increase

Question 6

Boltzmann distribution

Answer 6

1. Temperature

2. Catalyst

Question 7

Temperature affecting rate

Answer 7

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

Question 8

Catalyst

Answer 8

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

Question 9

Catalyst affecting rate

Answer 9

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

Question 10

Energy profile diagram

Answer 10

1. shows stability of reactants/products
2. number of steps
3. Magnitude of Ea in reaction

Question 11

Steps in reaction

Answer 11

1. Single

2. Two

Question 12

Single step

Answer 12

Reaction takes place in a single step

there will only be a single transition state (hump)

Question 13

Two steps

Answer 13

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

Question 14

Activation energy

Answer 14

Minimum amount of energy required to initiate a chemical reaction. A reaction which occurs in a different pathway will have different activation energy

Question 15

Transition state

Answer 15

Most unstable state hence appears at the maximum energy. at this stage it is equally likely to form reactants and products.

Question 16

Types of catalyst

Answer 16

1. Heterogeneous
2. Homogeneous
3. Auto
4. Biological

Question 17

Heterogeneous

Answer 17

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

Question 18

Mode of action in heterogenous catalyst

Answer 18

1. Adsorption
2. Reaction
3. Desorption

Question 19

Adsorption

Answer 19

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

Question 20

Reaction

Answer 20

occurs at faster rate on surface of catalyst as reactants brought closer to one another and existing interactions in reactant molecules weakened lowering Ea

Question 21

Desorption

Answer 21

Products desorbed from catalyst surface catalyst regenerated and other reactants can be adsorbed again

Question 22

Slow step

Answer 22

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

Question 23

Homogeneous catalysts

Answer 23

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

Question 24

Autocatalyst

Answer 24

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

Question 25

Biological

Answer 25

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

Question 26

effect of substrate concentration on enzyme activity

Answer 26

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

Question 27

Average rate

Answer 27

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)

Question 28

Initial rate

Answer 28

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

Question 29

Instantaneous rate

Answer 29

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

Question 30

Rate equation

Answer 30

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

Question 31

Overall order of the reaction

Answer 31

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

Question 32

Order

Answer 32

1. Zero Rate of rxn not affected by this reactant 2. First Rate of rxn is directly proportional to concentration of reactant 3. Second Rate of rxn is directly proportional to square of the concentration of the reactant

Question 33

Factors affecting rate constant

Answer 33

``` 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 ```

Question 34

units of k

Answer 34

depends on overall order of reaction | units for rate is mol dm^-3t^-1'

Question 35

Mechanism

Answer 35

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

Question 36

Intermediates

Answer 36

Cannot appear in rate equation | replace them the reactants that reacted together to form them. needed for reaction to proceed

Question 37

deducing the rate equation

Answer 37

1. 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 2. 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 1. rate - conc graph 2. comparing degree of rate change due to change in conc

Question 38

experiments to determine rate equation

Answer 38

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 2. Gravimetric method measure the decrease in mass over time (applicable for reactions with gaseous product) 3. Monitor change pH using data logger/meter 4. 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 6. Changes in pressure pressure change is due to number of moles of gaseous reactants being different from number of moles of gaseous products

Question 39

Titrimetric

Answer 39

reactions that do not have colour or other means by which physical measurement can be taken to track rxn progress 1. Start rxn with knwn amounts of reactants 2. Extract fixed portions of reaction mixture at regular interval 3. Quench 4. Determine reactant concentration by titration

Question 40

Quenching

Answer 40

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 1. rapid cooling in ice bath 2. removing the catalyst 3. by adding large volume of cold water which lowers reaction temperature and dilutes reaction

Question 41

Clock

Answer 41

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 1. Appearance of colour 2. Disappearance of colour

Question 42

Appearance of colour

Answer 42

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

Question 43

Disappearance of colour

Answer 43

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

Question 44

data processing methods to deduce rate equation

Answer 44

1. Inspection/ Mathematical method- for initial rate | 2. Graphical method- for both initial rate and continuous

Question 45

Inspection/mathematical

Answer 45

inspection : compare conc vs rate between suitable pair of experiments Mathematical: use rate eqn to compare suitable pair of experiments

Question 46

Graphical

Answer 46

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

Question 47

Half life

Answer 47

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

Question 48

Determining half life from graph

Answer 48

1. 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) 2. 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

Question 49

Combination of initial rate and continuous method

Answer 49

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

Question 50

Aqueous species

Answer 50

have the potential to affect to ensure a particular reactant seemingly exerts no effect on rate has to be 1. present in large excess (much larger conc) 2. kept the same between experiments