order reactions

What are the units of k for each order reaction?

• 0 order: k = concentration · time⁻¹

• 1st order: k = time⁻¹

• 2nd order: k = concentration⁻¹ · time⁻¹

0 order reaction

Rate = k

Rate ∝ [drug]⁰

Rate is constant.

Rate is independent of concentration

1st order reaction

Rate = k[Drug]

Rate ∝ [drug]

Rate is dependent on concentration

2nd order reaction

Rate = k[Drug]²

Rate ∝ [drug]²

Rate is dependent on the square of the concentration

0 order half life equation:

1st Order half life equation:

2nd Order half life equation:

How does increasing initial concentration affect 0 order reactions?

Increasing the initial conc increases the half life

How does increasing initial concentration affect 1st order reactions?

Increasing the initial conc does not change the half life

How does increasing initial concentration affect 2nd order reactions?

Increasing the initial conc decreases the half life

What is removed per unit time in each order reaction?

Zero order: constant amount removed per time
First order: constant fraction removed per time
Second order: rate strongly depends on concentration

Zero order plot

[Drug] against Time

Gradient: -k

1st order plot

ln[Drug] against Time

Gradient: -k

2nd order plot

1/[Drug] against Time

Gradient: +k

What are the units of k for a 0 order reaction?

Concentration per time

e.g.

• mol L⁻¹ s⁻¹

What are the units of k for a 1st order reaction?

Time⁻¹

e.g.

• s⁻¹

• day⁻¹

What are the units of k for 2nd order reactions?

Concentration⁻¹ · time⁻¹

e.g.

• L mol⁻¹ s⁻¹

What does everything stand for?

k = Rate constant

A  = frequency factor

E_a = Activation energy

R = 8.314 J mol^-1 K^-1

T  =  K

What is an Arrhenius plot?

lnk against 1/T

y=mx+c in Arrhenius plot

What does the gradient of an Arrhenius plot equal?

−Ea / R

What does the y-intercept (c) of an Arrhenius plot give?

ln A

BCS Class I

• High solubility

• High permeability

BCS Class II

• Low solubility

• High permeability

BCS Class III

• High solubility

• Low permeability

BCS Class IV

• Low solubility

• Low permeability