Q: A drug in aqueous solution degrades following first-order kinetics. If the initial concentration is $$5.0 \times 10$$^{-3}$$ g/cm^3$ and after 24 months it drops to 3.48$$ \times $$10^{-3} g/cm$^3$$, what is the rate constant $$k (in $$month$$^{-1}$$)?

$$1.5 \times 10$$^{-2}$$ month$$^{-1}$$

Question 1

Which of the following best describes the rate law for a reaction that is first order with respect to reactant A?

Accepted Answer

$$R = k[A]$$

Question 2

A drug in aqueous solution degrades following first-order kinetics. If the initial concentration is $$5.0 	imes 10$$^{-3}$$ g/cm^3$ and after 24 months it drops to 3.48$$ 	imes $$10^{-3} g/cm$^3$$, what is the rate constant $$k (in $$month$$^{-1}$$)?

Accepted Answer

$$1.5 	imes 10$$^{-2}$$ month$$^{-1}$$

Question 3

Which of the following statements about zero-order reactions is correct?

Accepted Answer

The degradation rate is independent of the reactant concentration

Question 4

For a first-order reaction, which of the following plots will yield a straight line?

Accepted Answer

A plot of $$\ln[A] vs $$time

Question 5

A drug suspension decomposes by zero-order kinetics with a rate constant of $$2 mg mL$^{-1}$$ month$$^{-1}. If $$the initial concentration is $$100 mg mL$^{-1}$$, what is the shelf life ($$t_{0.9}$$)?

Accepted Answer

4 months

Question 6

Which of the following is true for a second-order reaction of the type $$2A \rightarrow$$ products?

Accepted Answer

The half-life may be represented by $$t_{1/2} = [A]_0/2k$$

Question 7

A reversible reaction $$A \rightleftharpoons B$$ has forward and backward rate constants $$k_1$$ and $$k_{-1}$$, respectively. At equilibrium, which expression is correct?

Accepted Answer

$$k_1$$[A]_{eq} = k_{-1}[B]_{eq}$$

Question 8

Which of the following best describes the relationship between the rate constant $$k$$ and absolute temperature $$T as $$predicted by the Arrhenius equation?

Accepted Answer

A plot of $$\ln k$$ against $$1/T$$ will be linear

Question 9

The Arrhenius equation is $$k = Ae$$^{-E_a/(RT)}$$. What is the physical meaning of E_a$ in this equation?

Accepted Answer

It is the minimum energy required for a reaction to occur

Question 10

A plot of $$1/[A]$$ versus time for a reaction yields a straight line. What is the order of the reaction?

Accepted Answer

Second order

Question 11

A pharmaceutical product has a shelf-life ($$t_{0.9}) $$defined as the time for 10% decomposition of the active drug. For a first-order reaction, which formula gives the shelf-life?

Accepted Answer

$$t_{0.9} = \frac{0.105}{k}$$

Question 12

Which of the following factors does NOT generally increase the rate of a chemical reaction?

Accepted Answer

Decreasing the surface area of a solid reactant

Question 13

A drug degrades by first-order kinetics with a rate constant $$k = 0.062$$ year$$^{-1}. $$What percentage of the original drug remains after 2 years? (Use $$[A]_t = [A]_0 e$$^{-kt}$$)

Accepted Answer

83%

Question 14

Which of the following is the correct unit for the rate constant $$k in a $$second-order reaction?

Accepted Answer

concentration$$^{-1}.$$time$$^{-1}$$

Question 15

In a drug release system, a polymer matrix controls the release of the drug. Which kinetic model is most likely if the rate of drug release is constant over time?

Accepted Answer

Zero-order kinetics

Question 16

Which of the following statements about the half-life of a first-order reaction is correct?

Accepted Answer

It is given by $$t_{1/2} = \frac{0.693}{k}$$

Question 17

A reaction has a rate constant $$k$$ that increases with temperature. According to the Arrhenius equation, what is the effect of increasing $$T on$$ $$k$$?

Accepted Answer

$$k$$ increases exponentially

Question 18

In the context of drug metabolism, which process is NOT part of ADME?

Accepted Answer

Precipitation

Question 19

Which of the following is a correct statement about the effect of a catalyst on a chemical reaction?

Accepted Answer

It decreases the activation energy

Question 20

A plot of $$\ln k$$ versus $$1/T$$ for a reaction yields a straight line with a slope of $$-4000 K. $$What is the activation energy $$E_a (in J/$$mol)? (Use $$R = 8.314 J K$^{-1}$$ mol$$^{-1})$$

Accepted Answer

$$E_a = 33256 J/$$mol

Elementary Chemical Kinetics and Reaction Rate Laws: Study Notes

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