W7.2_Order Reactions

Question 1

Explain the importance of pharmaceutical kinetics in pharmacists in community and clinical fields.

Answer 1

• Community pharmacist: expiration/light degradation can affect potency/safety of drugs
• Clinical: drug level monitoring is required for drugs with narrow therapeutic range

Question 2

Describe the general equation of reaction rate. Explain rate laws by stating the equation, and relating it to the overall reaction order and properties of k value. How are integrated laws used in determining reaction rate in practical situations?

Answer 2

• Rate of reaction = (+/-)(1/a)*(d[A]/dt) (+ for reactants, - for products, a: coefficient in equations)
• Rate laws: relationships between rate of the reaction and concentration of the reactants and rate constant
• ∴ rate = k[A]^m[B]^n in aA + bB -> …
• Overall reaction order: m+n
• k can be integer/fraction/positive/negative, unit changes with order
• ∵ Measuring the initial rate to determine the kinetics is difficult
• ∴ Use integrated laws to describe reaction rate (dx/dt) (tangent to the curve at t = 0)

Question 3

Refering to the diagram, explain zeroth order reactions and give related examples of it. State the equation of it, how to calculate the amount of remaining drug, and the implications of slope and y-intercept.

Answer 3

• Reactions at constant rate, independent from concentration of reactant
• When [A] doubles, rate stays the same
• ex. elimination of alcohol, suspensions of poorly soluble drugs
• k(0)=-d[A]/dt moldm^-3s^-1
• Remaining drug: A=A-k(0)t
• Slope = -k(0), y-intercept: A

Question 4

Refering to the diagram, explain first order reactions and give related examples of it. State the equation of it, how to calculate the amount of remaining drug, and the implications of slope and y-intercept. What are pseudo first order reactions?

Answer 4

• Reaction rate dependent on concentration of one reactant
• When [A] doubles, rate doubles
• ex. most eliminations of drugs
• k(1)=-d[A]/dt s^-1
• Remaining drug: log(A)=log(A)-(k(1)*t/2.303)
• Slope = -k(1)/2.303, y-intercept = log(A)
• Pseudo first order reactions: involve more than one reactants but still follow first order kinetics due to excess concentration (of water) that becomes negligible (ex. ester hydrolysis of aspirin)

Question 5

Refer to the diagram, explain second order reactions and give related examples of it. State the equation of it, and the implications of slope and y-intercept.

Answer 5

• Either one of the reactants is raised to the second power or two reactants each raised to first power
  rate = k[A]^2 or rate = k[A][B]
  For first case, after integration: 1/A=1/[A])(0)+k(2)*t
  Slope = k(2), y-intercept = 1/A

Question 6

Define half-life of a reaction. What is its equation for different orders? How is the order of reaction determined from a data set?

Answer 6

Definition: time it takes for concentration of reactants to decrease by half of its original value
Zeroth order: t(1/2)=A/(2k(0))
First order: t(1/2)=0.693/k(1)
Second order: t(1/2)=1/(Ak(2))
- Determining order of reaction: modify data -> plot graphs -> confirm when it’s a straight line and starts at initial point