06: reaction kinetics

Question 1

rate of reaction

Answer 1

the rate of a reaction is defined as the change in concentration of reactants or products per unit time

Question 2

rate equation

Answer 2

for a general reaction equation, aA +bB → products

rate = k[A]^m[B]^n

[ ] is the concentration in moldm-3

where m and n are the order of reaction with respect to reactant A and with respect to reactant B

k is rate constant

Question 3

order of reaction

Answer 3

the order with respect to a reactant is defined as the power to which its concentration term is raised in the rate equation

Question 4

half-life of reaction

Answer 4

the half life, t(1/2), of a reaction is the time taken for the concentration of the reactant to fall to half its original value

Question 5

rate constant

Answer 5

the rate constant, k is a proportionality constant in the rate equation and remains constant when the concentrations of the reactants change

Question 6

activation energy

Answer 6

the activation energy, Ea, is defined as the minimum energy which colliding molecules must posses before a collision will result in a reaction

Question 7

catalysis

Answer 7

catalysis is an increase in rate of reaction due to the addition of a substance (catalyst) which provides and alternative pathway of lower activation energy.

Question 8

collision theory

Answer 8

for a reaction to occur the reactant molecules must:
1. collide with a certain minimum energy
2. collide with the correct orientation

Question 9

heterogeneous catalysis

Answer 9

reactants and catalysis exists in a different phase from the reactants ad increase the rate of a reaction without itself undergoing any permanent chemical change

Question 10

adsorption theory

Answer 10

1. diffusion: the reactant molecules diffuse towards the catalyst surface
2. reactant molecules are chemically adsorbed on an active site of the catalyst and weak temporary bonds are formed, these bonds weakens the intramolecular bonds in the molecule and lowers activation energy
3. the reactant molecules are closer together, increasing their concentration at the catalyst surface.
4. molecules are oriented in the right position for the reaction, increasing number of effective collisions
5. gaseous products are then desorped off the catalyst surface, regenerating the active site

Question 11

enzymes

Answer 11

1. enzymes are homogeneous catalysts that are highly specific
2. and only work well within a narrow pH

Question 12

‘lock- and - key’ mechanism

Answer 12

enzymes work via a lock-and-key mechanism
where the enzyme is the ‘lock’ and substrate is the ‘key’.

the active site is the part of the enzyme that binds the substrate and lowers the activation energy of the reaction

Question 13

[concentration] vs time graph

Answer 13

zero-order : rate is constant (linear)

first-order: half-life is constant (t1= t2)

second-order: half-life not constant (t1!= t2)

Question 14

rate vs [concentration] graph

Answer 14

zero-order : rate is independent (straight line)

first-order: rate is directly proportional (linear)

second-order: rate is proportional to the square of [A] (exponential)

Question 15

initial rate method (given concentration time graph: 1. find reactant on y-axis, 2. find reactant w varying concentration)

Answer 15

1. for reactant on y-axis: find at least two half-lives
2. for reactant with varying concentration:
   find initial rate by drawing tangent (find gradient)
   then comparing change in initial rate vs change in concentration