10 Questions
In a pseudo-first-order reaction, why does the reaction appear as second order?
One reactant is in great excess or maintained at constant concentration.
How does the determination of the order type in a reaction differ between the substitution method and the graphical method?
The substitution method substitutes equations to find the order, while the graphical method involves plotting data.
Why is water considered an excess reactant in the hydrolysis of aspirin?
It participates in the reaction but its concentration remains essentially constant.
Why does concentration of water essentially remain constant during the hydrolysis of aspirin?
Water participates in the reaction but is present in great excess.
What factor influences the rate of degradation in a pseudo-first-order reaction?
Initial concentration of the main reactant.
What does it signify when calculated k values remain constant in a kinetic study?
The reaction is first order.
How can you determine if a reaction is first order using kinetics data?
By observing a linear relationship between k values and concentrations.
'Pseudo' in pseudo-first-order reaction implies:
The appearance differs from the actual order behavior.
What role does maintaining water at constant concentration play in pseudo-first-order reactions?
It eliminates water's influence on degradation rate.
What information does a plot of kinetic data help to determine?
The order of the reaction based on k values variation.
Study Notes
Reaction Kinetics
- Chemical reaction kinetics deals with the rates of chemical processes
- Rate of reaction is the velocity or speed of a reaction
- Rate of reaction can be expressed as:
- The increase in concentration per unit time of one of the products
- The decrease in concentration per unit time of any of the reactants
- Rate of reaction is given by the expression ±dc/dt, where dc is the increase or decrease of concentration over a time interval dt
Order of Reaction
- Order of reaction expresses the experimentally determined dependence of the rate upon the reactants concentration
- It is the manner in which the rate of reaction is influenced by the initial concentration of the reactants
- Types of order of reaction:
- Zero-order reaction: rate is independent of the concentration of any of the reactants
- First-order reaction: rate is dependent on the concentration of one of the reactants
- Second-order reaction: rate is determined by the concentrations of two reacting species
Zero-Order Reaction
- Rate equation: -dc/dt = k
- Rate is constant
- Examples: Photolysis of vitamin A, Photolysis of cefotaxime, Loss in color of multi sulfa product
- Half-life (t1/2): time taken for the concentration of the substance to fall to half of its initial value
- For reactions following zero-order kinetics, the half-life can be calculated using the equation: Co – Ct = k t
First-Order Reaction
- Rate equation: -dc/dt = kc
- Rate is not constant
- Examples: Dilute aqueous solution of procaine or aspirin, Catalytic decomposition of hydrogen peroxide
- Half-life (t1/2): time taken for the concentration of the substance to fall to half of its initial value
- For reactions following first-order kinetics, the half-life can be calculated using the equation: log Ct = log Co - kt/2.303
Pseudo-Zero Order Reaction
- Reaction appears as first-order, but behaves as zero-order
- Examples: Solid state decomposition of many drugs, Pharmaceutical suspensions
- In suspensions, the reaction appears to be dependent upon the initial concentration of the drug, but actually the rate of the reaction depends upon the amount of the drug dissolved in the vehicle of the suspension
- The reaction rate becomes pseudo-zero order due to the presence of the suspended drug reservoir that ensures constant concentration
Pseudo-First-Order Reaction
- Reaction appears as second-order, but behaves as first-order
- Examples: Dilute aqueous solution of aspirin, Hydrolytic decomposition of drugs
- In this case, one reacting material is present in great excess or is maintained at constant concentration as compared with other substances
- The rate of degradation will then depend only upon the concentration of the drug
Determination of Order Type
- Substitution Method: substitute equations to find the order
- Graphical Method: plot the data to determine the order
- Half-life Method: use half-life to determine the order
This quiz covers the fundamentals of reaction kinetics and the rate of chemical reactions in the context of physical pharmacy. Topics include understanding the velocity of reactions, expressing rate of reaction, and interpreting chemical processes based on concentration changes over time.
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