reaction kinetics

Question 1

define rate of reaction

Answer 1

change in concentration of reactant or product with time

Question 2

define rate equation

Answer 2

it relates the rate of the reaction to the concentration of reactants raised to the appropriate power

Question 3

define order of reaction

Answer 3

power to which the concentration of that reactant is raised to in the rate law

Question 4

define rate constant, k

Answer 4

k is proportionality constant in the rate equation that relates the concentration of the reactants to the rate of reaction

Question 5

define half-life

Answer 5

it is the time taken for the reactant concentration to decrease to half its original value

Question 6

define rate-determining step

Answer 6

it is the slowest step in the sequence of steps leading to the formation of the product

Question 7

define activation energy, Ea

Answer 7

the minimum amount of energy that the reactant particles must possess to overcome the energy barrier in order for effective collisions to result in a chemical reaction

Question 8

define catalyst

Answer 8

a substance that increases the rate of a chemical reaction by providing an alternative pathway with lower activation energy. it remains chemically unchanged at the end of the reaction

Question 9

what does it mean when ‘equal amounts of reactants are used’ ?

Answer 9

concentration is halved

Question 10

what are the properties of enzymes?

Answer 10

1. they remain chemically unchanged at the end of reaction
2. small qtys
3. affected by pH(best 5-8), temperature(20-25C)[denatured at high temp], substrate concentration, enzyme concentration
4. Highly specific in their function

Question 11

Why proposed mechanism is chosen?

Answer 11

1. Slow step - - > stoichiometric coefficients of reactants agree to the orders of reaction in the rate equation
2. Sum of steps in the proposed mechanism gives the balanced equation

Question 12

Factors affecting rate of reaction: presence of catalyst

Answer 12

1. In presence of catalyst, Ea is lowered
2. No. of reacting particles with E >= Ea’(cat) increases
3. Frequency of effective collisions increases
4. since rate of reaction is proportional to the frequency of effective collisions, rate of reaction increases

Question 13

Factors affecting rate of reaction: conc/pressure

Answer 13

1. orders of reaction wrt to (reactants name) are non-zero
2. Higher means no. Of reactant particles per unit volume increases
3. Frequency of effective collisions increases
4. since rate of reaction is proportional to the frequency of effective collisions, rate of reaction increases

Question 14

Heterogeneous catalysis

Answer 14

1. Adsorption
   a) reactant molecules diffuse towards the catalyst(s) surface and are absorbed onto the active sites at the surface
   b) They become chemisorbed ie. weak chemical bonds are formed between molecules on the active sites thereby weakening the bonds in the adsorbed reactant molecules
   c) The adsorption process increases the concentration of the reactant molecules on the catalyst surface
2. Reaction at the surface
   a) Intramolecular bonds within the reactant molecules are weakened, lowering the Ea
   b) Reactant molecules are brought into close contact and are correctly orientated for reaction
3. Desorption
   a) Product molecule desorbs and diffuses away from the catalyst surface.
   b) This frees up the active sites on the catalyst surface for adsorption with new reactant molecules

Question 15

Factors affecting rate of reaction: temperature

Answer 15

1. At higher temperature, total number of reactant particles remain unchanged but average KE of the reactant particles increases
2. no. of reactant particles with energy >= Ea increases.
3. Frequency of effective collisions increases
4. since rate of reaction is proportional to the frequency of effective collisions, rate of reaction increases

Question 16

suggest why step 1 is the slow step:

step1: (CH3)3CBr –> (CH3)C+ + Br-
step2: (CH3)C+ +H2O –> (CH3)CH2O+
step3: CH3)CH2O+ + Br- –> CH3)COH +HBr

Answer 16

-In step 1, energy is absorbed to break the C-Br bond to form a reactive intermediate that is unstable relative to (CH3)3CBr.

• Step 2 is a bond formation process which releases energy and Step 3 involves lower activation energy due to favourable bond formation between 2 oppositely charged species

-Since the activation energy is highest in Step 1, it is the slowest step

Question 17

Why is this mechanism not plausible?
Step1: NO2 +O3 –> NO + 2O2 (slow)
Step2: NO2 + NO + O2 –> N2O5 (fast)

Answer 17

The fast elementary step involvign reaction among 3 molecules is unlikely to occur cuz the probability for 3 molecules to collide effectively with correct orientation and sufficient KE AT THE SAME TIME is low.

Question 18

Effect of pH on stucture of enzyme catalase

Answer 18

The optimum pH at which catalase works best is __. Above and below this value, the rate of decomposition decreases as the structure of the enzyme changes leading to a decline in its performance. At extreme pH, the enzyme is denatured with loss in shape of its active sites, leading to a much lower rate of decomposition.

Question 19

Account for the shape of the graph. (as [H2O2] increases, rate of reaction increases at decreasing rate until it becomes constant)

Answer 19

1. At low [H2O2], there are plenty of active sites available on each enzyme molecule. The rate of reaction increases proportionately with increasing [H2O2] OR rate of reaction is first-order wrt H2O2.
2. At intermediate [H2O2], rate of reaction is of mixed order wrt H2O2
3. At high [H2O2], all the active sites are fully occupied. Increasing [H2O2] no longer increases reaction rate as there are no more active sites available for H2O2 to attach to and react. Reaction becomes zero order wrt H2O2