Kinetics and Drug Stability

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Questions and Answers

What is the role of a catalyst in a chemical reaction?

To permanently change the reactants into products

To permanently modify the products created in the reaction

To decrease the activation energy required for the reaction (correct)

To absorb light and provide energy for the reaction

Which of the following is a common method to prevent oxidation in pharmaceuticals?

Using chelating agents (correct)

Storing drugs at high temperatures

Increasing the pH of the formulation

Exposing drugs to light

What initiates fast chain reactions in catalysis?

Chemical complexes formed during the reaction

The permanent alteration of reactants

High temperatures that speed up molecular movement

Free radicals produced by the catalyst (correct)

What is a requirement for photochemical reactions to occur?

Absorption of light energy of sufficient energy and proper frequency (D) Signup and view all the answers

How should the pH be adjusted to ensure stability in the hydrolysis of pharmaceuticals?

To align with the minima in the stability-pH profile (B) Signup and view all the answers

What best describes the rate law for a zero-order reaction?

Rate is constant and independent of reactant concentrations. (D) Signup and view all the answers

What is true about the half-life (t0.5) in zero-order reactions?

It remains constant regardless of initial concentration. (B) Signup and view all the answers

How is shelf life (t90%) calculated for zero-order reactions?

It is concentration dependent and measures degradation to 90%. (C) Signup and view all the answers

In first-order reactions, what can be said about the rate of reaction?

Rate depends on the first power of a single reactant. (C) Signup and view all the answers

What is the main difference in concentration dependence between zero-order and first-order reactions?

First-order reactions have a half-life that varies with concentration, while zero-order does not. (D) Signup and view all the answers

When converting the rate law of a first-order reaction to logarithmic form, what is the resulting equation?

log(Ct) = log(Co) - kt (C) Signup and view all the answers

What happens to the concentration of reactants over time in a first-order reaction?

Concentration decreases exponentially with time. (A) Signup and view all the answers

How does the rate constant (k) influence reaction rates?

The value of k indicates the reaction's sensitivity to concentration and temperature. (B) Signup and view all the answers

What is one of the major factors affecting the rate of a chemical reaction?

Nature of reactants (C) Signup and view all the answers

Which factor does NOT affect the stability of a drug product?

Physical appearance (A) Signup and view all the answers

How is the rate of reaction defined?

Change in concentration of a reactant over time (C) Signup and view all the answers

What does the overall order of a reaction indicate?

The sum of exponents in the reaction rates (D) Signup and view all the answers

In a first-order reaction, what is the value of the exponent?

1 (C) Signup and view all the answers

Why is it important to study the rates of drug degradation?

To determine the proper shelf lives and storage conditions (B) Signup and view all the answers

What could happen if a drug's active ingredient is chemically degraded?

Decreased therapeutic effect (A) Signup and view all the answers

What is the order with respect to one reactant in a reaction?

The exponent of that concentration term (A) Signup and view all the answers

What is the defining characteristic of a first order degradation reaction?

The rate of degradation is proportional to the concentration of the drug. (D) Signup and view all the answers

What happens to the reaction rate in an apparent zero-order reaction involving a drug suspension?

The rate remains constant until all suspended particles are dissolved. (D) Signup and view all the answers

According to the Arrhenius equation, which factor primarily influences the specific reaction rate (k)?

The temperature, absolute energy of activation, and frequency factor. (B) Signup and view all the answers

How does temperature affect reaction rates in general?

Higher temperatures tend to increase the number of molecular collisions. (C) Signup and view all the answers

What is the result when all suspended particles of a drug have dissolved in solution?

The reaction becomes first-order kinetics. (B) Signup and view all the answers

What does the frequency factor (A) in the Arrhenius equation represent?

The likelihood of reaction occurrence at a given temperature. (C) Signup and view all the answers

Which statement is true regarding the influence of temperature on reaction rates?

Each 10° rise in temperature can double the reaction rate. (C) Signup and view all the answers

What role does the gas constant (R) play in the Arrhenius equation?

It helps in the calculation of the reaction rate at different temperatures. (A) Signup and view all the answers

What is the primary role of activation energy (Ea) in a chemical reaction?

It acts as the energy threshold that must be exceeded for a reaction to occur. (C) Signup and view all the answers

Which of the following is NOT a limitation of accelerated stability testing?

Can be used for reactions that are thermal phenomena. (B) Signup and view all the answers

How can the best estimation of the Arrhenius constant and activation energy be obtained?

By performing the reaction at three different temperatures or sometimes two. (D) Signup and view all the answers

What is the consequence of a negative catalyst in a chemical reaction?

It decreases the reaction rate. (D) Signup and view all the answers

What is the difference between shelf life and expiry date?

Shelf life is the period after which the product may still be effective, whereas expiry date is the last date for use. (B) Signup and view all the answers

Which factor does NOT affect the validity of accelerated stability testing?

The room temperature at which the product is stored. (D) Signup and view all the answers

What is the primary requirement for a reaction to be predicted using an Arrhenius plot?

The reaction should take place at varying elevated temperatures. (D) Signup and view all the answers

When determining the expiration date of a drug with a concentration degradation observed from 94 units/ml to when it hits 45 units/ml, what should be considered?

The drug's potency and safety threshold. (B) Signup and view all the answers

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Study Notes

Kinetics, Reaction Rates, and Drug Stability

Kinetics is the study of the rate of chemical change. It involves the rate at which reactants are converted to products, influenced by conditions like reactant concentration, products, other chemical species (e.g., solvents), pressure, and temperature.

Factors Affecting Reaction Rates

Nature of reactants: A major factor influencing reaction rates.
Concentration of reactants: Higher reactant concentrations generally lead to faster reaction rates.
Concentration of catalysts: Catalysts speed up reactions without being consumed.
Temperature: Higher temperatures typically lead to faster rates.

Drug Stability and Kinetics

Drug stability is crucial in pharmaceuticals.
The rate of drug degradation is a key factor in determining shelf life and storage conditions for pharmaceutical products.
Problems with stability can affect whether a formulation is accepted or rejected.
Chemical degradation of active ingredients can lead to significant losses.
Instability of the drug can reduce bioavailability.
Chemical degradation can create toxic byproducts.

Rates and Orders of Reactions

Reaction rate (or degradation rate): The velocity at which a reaction occurs.
Reaction rate is expressed as the ratio of the change in concentration of a reactant (or product) to a change in time. This is often represented as dc/dt.
The unit of rate is mole/Liter/second (or mol L⁻¹ sec⁻¹).

Order of a Reaction

The order of a reaction describes how the concentration of a reactant or drug affects the reaction rate.
The order of a reaction can be determined through experiment; it is not directly apparent from the stoichiometric coefficients in the balanced equation.
The order (alpha) of a simple A → B reaction can be expressed as rate = α [A]^α.
The overall order of more complex reactions (e.g., A + B --> C) is the sum of the exponents of the concentration terms (e.g., rate = α [A]^α[B]^β - α + β is the overall order).
Exponents (a and b) aren't directly related to coefficients in the chemical equation.

Rate Constant (k)

The rate constant (k) is a numerical expression of the effect of the nature of reactants and temperature on reaction rate.
k is a proportionality constant

Units of Rate Constants

The units of the rate constant vary depending on the order of the reaction:
- For a zero-order reaction: k = mole/L x sec.
- For a first-order reaction: k = 1/sec.
- For a second-order reaction: k = L/mole x sec.

Zero-Order Reactions

In zero-order reactions, the rate of the reaction is constant and independent of the reactant concentrations.
The equation for zero-order reactions: c= c_o – k_ot.
This means that the drug concentration decreases linearly over time.
Half-life is concentration dependent.

First-Order Reactions

In first-order reactions, the rate of the reaction is proportional to the concentration of a single reactant.
The equation for first-order reactions is c = c_oe^−kt
- (c = remaining concentration, c_o= initial concentration, k = reaction rate constant, t = time)
The graph of the natural log of concentration (ln c) versus time (t) is a straight line.
The half-life is independent of concentration.

Accelerated Stability Testing

Accelerated stability testing is a method used to predict drug shelf life by conducting tests at elevated temperatures or conditions.
The method helps predict the shelf life by calculating rate constants at elevated temperatures, and then extrapolating to the expected storage conditions (room temperature, or other storage temperature).

Catalysis

Catalysts affect reaction rate without changing chemically.
Negative catalysts slow down reactions, while inhibitors slow them down and also undergo permanent change.
Catalysts can speed up reactions by changing the reaction mechanism, hence reducing the activation energy.
Catalysts can be involved in chain reactions, initiating them via free radicals.

Modes of Pharmaceutical Degradation

Hydrolysis: The most common mode, hydrolysis is most important with esters and amides. pH influences these reactions, and formulations should be adjusted to optimal pH for stability.
Oxidation: Substances like ascorbic acid, sodium sulfite/ metabisulfite, and chelating agents can help minimize oxidation reactions.
Photolysis (Light Degradation): Reactions involving light energy. Photochemical reactions are not dependent on temperature. Photodegradation is NOT a form of catalysis.

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Description

This quiz explores the principles of kinetics, focusing on reaction rates and their impact on drug stability. Key factors affecting reaction rates such as concentration, temperature, and the role of catalysts are highlighted. Understanding these concepts is essential in the pharmaceutical field, as they determine drug shelf life and stability.