Reaction Kinetics

Question 1

Defn. Rate of reaction

Answer 1

The change in concentration of reactants or products with time

Question 2

Defn. Rate equation

Answer 2

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

Question 3

Defn. Order of reaction

Answer 3

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

Question 4

At molecular level, order of reaction..

Answer 4

indicates the actual number of respective molecules colliding to from products in the rate-determining step

Question 5

Defn. Half-life of a reaction. t1/2

Answer 5

The time taken for the concentration of a reactant to reduce to half of its initial value

Question 6

Defn. Rate constant, k

Answer 6

The proportionality constant in the rate equation

Question 7

Collision Theory

Answer 7

For a chemical reaction to occur, the reactant particles must collide with the correct orientation and sufficient energy

Question 8

Arrhenius Equation

Answer 8

k = Ae^ (-Ea/RT)

Question 9

Defn. Activation energy, Ea

Answer 9

The minimum energy which the colliding molecules must possess before a collision will result in a reaction

Question 10

Defn. Catalyst

Answer 10

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

Question 11

Factors affecting rate constant, k

Answer 11

temperature and catalyst only

Question 12

Temp affecting rate constant, k

Answer 12

When the temperature increases, Ea/RT decreases, hence k increases

Question 13

Catalyst affecting rate constant, k

Answer 13

In the presence of a catalyst, Ea/RT decreases, hence k increases

Question 14

Factors affecting rate of reaction

Answer 14

Temperature, concentration, catalyst

Question 15

Temp affecting rate

Answer 15

When the temperature increases,

1. Average kinetic energy of reactant molecules increases
2. More molecules have energy greater than or equal to Ea
3. Hence, the frequency of effective collisions increases
4. Rate constant (k = Ae ^(-Ea/RT)) increases, hence rate increases

USE MAXWELL-BOLTZMANN DISTRIBUTION DIAGRAM
(No. of molecules against KE)

Question 16

Catalyst affecting rate

Answer 16

When a catalyst is added,

1. Catalyst provides an alternative reaction pathway with a lower activation energy
2. More molecules have energy greater than or equal to Ea
3. Hence, the frequency of effective collisions increases
4. Rate constant (k = Ae ^(-Ea/RT)) increases, hence rate increases

USE MAXWELL-BOLTZMANN DISTRIBUTION DIAGRAM
(No. of molecules against KE)

Question 17

How to determine order of rxn?

Answer 17

1. non-continuous/ initial rates method
2. continuous/ graphical method

Question 18

Non-continuous/ initial rates method

Answer 18

1. Many separate experiments are conducted with different initial concentrations and the initial rate for each experiment is determined
2. 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

Question 19

Continuous/ graphical method

Answer 19

1. Only one experiment is conducted with many readings taken at timed intervals
2. Timing starts when reaction begins and the amount of reactants/ products present at various timings are determined
3. By plotting a graph of [reactant] or [product] against time, we can determine the order of rxn w.r.t the reactant

Question 20

Defn. Rate-determining step (r.d.s)

Answer 20

The slowest step in the rxn mechanism

Question 21

Zero order reactions

Answer 21

• rate is independent of [A]
• the r.d.s. in the mechanism does not involve any A molecules

Question 22

First order reactions

Answer 22

• rate is directly proportional to [A]
• the r.d.s. in the mechanism involves the reaction of one molecule of A

Question 23

Second order reactions

Answer 23

• rate is directly proportional to [A]^2
• the r.d.s. in the mechanism involves the reaction of 2 molecules of A

Question 24

How to find rate constant, k, from an overall 1st order reaction from the constant half-life?

Answer 24

k = (ln2) / (t1/2)

Question 25

Experimental techniques (non-continuous/ initial rates method)

Answer 25

7. Observing precipitation and noting the time taken for a small and fixed amount of precipitate to be formed (sulfur clock)
8. Observing colour change and noting the time taken for a small and fixed amount of reactant/ product to be formed (iodine clock)

Question 26

Experimental techniques (continuous/ sampling method)

Answer 26

1. Titration
2. Measuring volume of gas produced
3. Colour intensity
4. Monitoring electrical conductivity
5. Monitoring optical activity
6. Pressure measurement (for gas only)

AT REGULAR TIME INTERVALS

Question 27

Defn. Heterogenous catalyst

Answer 27

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

Question 28

Heterogenous catalyst mode of action

Answer 28

1. Reactant molecules are adsorbed on the active site of the catalyst via weak temporary bonds
2. Reactant molecules are brought close together, thus increasing their concentration at the catalyst surface
3. The bonds in the reactant molecules are weakened, thus lowering the activation energy
4. The molecules are orientated in the right positions for reaction, increasing the number of effective collisions
5. The product is desorbed from the catalyst surface, regenerating the active site on the catalyst

Question 29

Haber process

Answer 29

• Heterogenous catalysis
• finely divided iron ( FE(s) ) acts as catalyst
• N2 (g) + 3H2 (g) -> 2NH3 (g)

Question 30

Catalytic converter

Answer 30

• 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

Question 31

Defn. Homogenous catalyst

Answer 31

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)

Question 32

Reaction between I- and S2O8 2-

Answer 32

• 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

Question 33

Oxidation of atmospheric sulfur dioxide

Answer 33

• 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)

Question 34

Enzymes

Answer 34

• 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

Question 35

Autocatalysis

Answer 35

• 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

1. Initially the rxn is slow,
2. but it speeds up as Mn2+ ions are produced and catalyses the reaction. As [Mn2+] increases, rate of rxn increases
3. Rate of reaction will then decrease as the concentration of reactants decreases

Question 36

Criteria of rxn mechanism

Answer 36

1. The elementary steps must add up to the overall balanced equation
2. The reaction mechanism must be consistent with the experimental rate equation
3. The elementary steps must be physically reasonable.