# Chemical Kinetics

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• Created by: LBCW0502
• Created on: 30-11-17 14:35
What are the uses of chemical kinetics in pharmacy? (3)
Biochemical processes (pharmacology, pharmacokinetics), industry processes (rate of synthesis/production of drugs) and pharmaceutics (dissolution, rate of degradation of drugs within formulation, predicting shelf-life of drugs)
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Why do reactions occur?
Atoms/molecules collide with other atoms/molecules, electrons or photons
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The number of collisions per unit time depends on what?
Physical states of the reacting species, their relative concentrations and temperature (affect rate of reaction as well as SA and catalysts)
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What is the activation energy?
Minimum energy required to initiate a chemical reaction
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What is the free energy change of the reaction?
Delta G
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What is the general formula for the rate of a reaction?
Increase in concentration of products over change in time or decrease in concentration of reactants over change in time
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How is the course of a reaction followed?
Using UV spectroscopy - e.g. aspirin hydrolysed to salicylic acid and acetic acid. Loss of intensity is proportional to [sample]. Double bonds in molecules give good UV spectra. Beer's law. Excitation. Aspirin peak (-), salicylic acid (+)
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How do you plot a graph for e.g. degradation of a drug over time?
Drug concentration on the y-axis, time (in days) on the x-axis (include title and appropriate scales)
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How do you calculate the initial rate of reaction?
Draw a tangent at t=0 and measure the gradient of the tangent (determine if there is a positive or negative slope). Instantaneous rate at t=0
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How do you calculate the instantaneous rate of reaction at a particular time e.g. t=2.5?
Draw a tangent at t=2.5 and measure the gradient of the tangent (determine if there is a positive or negative slope)
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How do you calculate the average reaction rate e.g. between t=0 and t=5?
Draw a straight (linear) line between plots at t=0 and t=5 and measure the gradient of the tangent (determine if there is a positive or negative slope)
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What are the units for the rate of degradation of the concentration of a drug over time?
mol.dm^-3.d^-1 (general units - mol.dm^-3.d^-1 or M.t^-1)
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Is the rate defined as a positive or negative quantity?
Positive quantity - the rate of the disappearance of the reactants is a negative quantity
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What is the rate law?
The rate constant, k and reaction orders (which indicate sensitivity of rate of concentration changes in reactants). The rate law provides a direct relationship between the concentration of reactants and the rate equation
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Can the reaction orders be determined by the stoichiometric equation?
No - reaction orders are determined by experimental data (need to carry out reactions)
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What are the general orders of reactions?
0, 1, 2 and 3
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How do you calculate the overall order of a reaction?
Overall order of reaction = sum of orders for each reactant
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What is a zero order reaction?
Increasing the concentration of a reactant does not increase the rate of reaction (other limiting factor involved e.g. absorption of light)
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Give an example of an experiment involving zero order kinetics?
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What is a first order reaction?
Doubling the concentration of a reactant doubles the rate of reaction (e.g. disintegration of radiopharmaceutical sodium pertechnetate)
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What is a second order reaction?
Doubling the concentration of a reactant quadruples the rate of reaction
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What is the general formula used to determine the units for the rate constant (k) depending on the order of reaction?
[molar]^1-n / time (n is the order of reaction)
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What are the units for k in a zero order reaction?
M/t
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What are the units for k in a first order reaction?
1/t
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What are the units for k in a second order reaction?
1/Mt
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What is the integration result for zero order kinetics?
[A] = [A]0 -kt and [B] = kt
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What is the correlation for zero order kinetics when measuring the concentration of the reactant?
Negative correlation (graph of [A] against time)
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What is the correlation for zero order kinetics when measuring the concentration of the product?
Positive correlation (graph of [B] against time)
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What is the integration result for first order kinetics?
ln[A] = -kt + ln[A]0 (exponential form is [A] = [A]0e^-kt)
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What is the correlation for first order kinetics when measuring the concentration of the reactant?
Negative correlation (graph of ln[A] against time). Exponential gives a curve going downwards
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What is a half life?
The time taken for the concentration of a reactant to be reduced by half (reactant decomposition)
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What is the integration result for second order kinetics when [A] = [B]?
1/[A] = kt + 1/[A]0 (also other integration for [A] not = [B], see lecture notes)
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What type of reaction usually involves second order kinetics?
Hydrolysis (e.g. aspirin). Catalysed by the presence of hydronium ions or hydroxide ions. Common reaction in drug degradation
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Why is hydrolysis pseudo first order kinetics?
In buffered solutions, the concentration of hydronium or hydroxide ions remains constant, so the rate of reaction depends solely on the concentration of the drug (also true for hydrolysis in solution where water is in excess)
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What is different about the k constant in pseudo first order kinetics?
Rate constant is known as the apparent rate constant (k app)
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In which reaction does pseudo zero order kinetics apply to?
Hydrolysis of poorly soluble drug in suspension. Water is in excess (constant). Drug slowly dissolves in suspended crystals as it is being degraded so the concentration of the solution remains constant
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What are the methods for determining the order of a reaction (integration equation methods)?
Determine change in reactants/time using experiments, obtain rate equation (consistent value for k at time intervals/closely to order. Determine half life (log graph - slope is 1-n where n is the order of reaction)
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What method is used to determine the order of a reaction (kinetics)?
Kinetics of drug composition studied at different concentrations and the half life was determined (results used to determine order of reaction and rate constant)
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What method is used to determine the order of a reaction (differential equation)?
Measure initial rates of reaction for several different concentrations of reactant(s). Plot graph of log initial rate against log initial concentration of reactant(s). Slope gives reaction order n
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Integration
For zero, first and second order kinetics (see lecture notes)
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What is a reversible reaction?
When the equilibrium is reached, the forward and reverse rates are equal. The higher the equilibrium constant (K) the further the reaction goes in the forward direction (integration and graphs)
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Give an example of a first order reversible reaction
Epi-tetracycline (minimal effect) converted to tetracycline (antibacterial)
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What is a parallel reaction?
Both reactions happen at the same time e.g. hydrolysis of the antibiotic cefotaxime (potential for more than one pathway for degradation) - equations and graphs
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What is a consecutive reaction?
Conversion of one structure to another (two rate constants, not reversible - equations and graphs) e.g. hydrolysis of cefotaxime leading to lactone formation
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In a solid state, are the concepts of concentration and order of reaction significant?
No
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The decomposition of a solid follow which shape on a graph?
Sigmoidal shape
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What is the nucleation theory?
Decomposition occurs rapidly around active nuclei in crystal (equation)
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Describe features of a catalyst
Speeds up the rate of a reaction by lowering the activation energy. No net change in quantity before or after reaction. Used in nature and industry. Can do nothing for reactions that are not thermodynamically feasible
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How does temperature affect reaction rates?
As the temperature increases, there is an increase in kinetic energy (increase probability of collisions). Exceed activation energy. Reaction rates may double for each 10 K increase in temperature (Maxwell-Boltzmann distribution curve)
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Which equation gives the relationship between rate of reaction and temperature?
Arrhenius equation
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Describe an Arrhenius plot
T^-1 (temperature change moves from hot to cold) against ln k. Straight line. Negative correlation. Y-intercept is ln A and gradient is -Ea/R
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What can be interpreted from an Arrhenius plot?
Measure stability early in process (higher temperature). Faster change in concentration in hot region (easy to measure degradation). Slow change in cold region. Measure at high temperature and extrapolation to cold temperature
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What is shelf-life prediction?
Time taken for 5% degradation (or time taken for 95% formation)
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What is non-isothermal kinetics?
Reaction rate and activation energy determined in a single experiment where the temperature is varied over time (use Arrhenius equation) - graph and extrapolation
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## Other cards in this set

### Card 2

#### Front

Why do reactions occur?

#### Back

Atoms/molecules collide with other atoms/molecules, electrons or photons

### Card 3

#### Front

The number of collisions per unit time depends on what?

#### Back ### Card 4

#### Front

What is the activation energy?

#### Back ### Card 5

#### Front

What is the free energy change of the reaction?

#### Back 