Chemical Kinetics

Question 1

Using the arrhenius equation - if we are given - temperature, the rate constant at that temperature, and Ea and we have to work out the rate constant at a different temp - what equation is used?

Answer 1

modified Arrhenius

lnK1 - LnK2 = Ea/R x 1/T2 - 1/T1

Question 2

how is product storage life calculated - T10% and T90%

Answer 2

T10%- want to know 10% decay so 90% remains which is 0.9
ln(0.9) = -kt (t = time)

T90% - want to know 90% so 10% remains, which is 0.1
ln(0.1) = -kt

Question 3

For concentration - time graphs - what graphs are used for first and second order? sketch these also

Answer 3

The curves are not useful so
First order uses ln[A]
Second order uses 1/[A]

Question 4

what is half life for first order?

Answer 4

Constant -

T1/2 = ln2 / k

Question 5

How would you determine the value of k from the first order graph?

Answer 5

reaction slope - gradient = -k

Question 6

what is rate constant k dependent on?

Answer 6

temperature (independent of concentration)

Question 7

What are the 2 types of second order reaction

Answer 7

homomolecular - Rate = k[A]2

Heteromolecular - Rate = k[A][B]

Question 8

what are the linear graph gradients for first and second order reactions

Answer 8

first order - ln[A] Vs time. Slope = -k

Second order - 1/[A] Vs time. Slope = k

Question 9

Define pseudo first order

Answer 9

a reaction that appears to follow first order kinetics with respect to one reactant even though the reaction is usually second order or higher. Typically due to the concentration of one reactant being constant whilst the other is consumed.

Question 10

What is the rate mechanism?

Answer 10

We use the rate law to suggest rate mechanisms. The rate determining step (slow) relates to the stoichiometry of the rate law
The sum of all elementary reactions = the overall balanced equation

Question 11

Give 3 limitations of predicting drug shelf life from accelerated stability testing

Answer 11

• high stress reaction may not occur in low stress conditions
• order may change after a given storage time
• decomposition may be via complex reactions

Question 12

what are the conditions for accelerated stability testing

Answer 12

controlled, high stress conditions

Question 13

What is a zero order reaction? give an example

Answer 13

Rate is completely independent of concentration
Eg drug release from an ideal controlled release capsule

Conc Vs time graph

Question 14

Define the rate constant

Answer 14

The constant that is independent of concentration but dependent on temperature.
Its units depend on the order of the reaction
K = Rate / reactants

Question 15

define reaction order

Answer 15

the number that relates the rate of a chemical reaction with the concentration of reacting substances

Question 16

A drug degrades by reaction. with water. Under what conditions might this reaction be predicted to follow pseudo first order kinetics and why? (2 marks)

Answer 16

if water is either present in the form of atmospheric moisture or if the drug is dissolved in water - [H2O] will remain constant as degradation reaction proceeds.

With atmospheric moisture - concentration of water will stay constant but low
With drug dissolving in water -concentration of water will stay constant but high

Thus the reaction is pseudo as it will depend only on the drug concentration that falls with time not water conc.

Question 17

What expression describes how a reaction rate varies with temperature?

Answer 17

Arrhenius