Chemical Kinetics I Flashcards
Reaction Kinetics is…
the study of rates of reaction and factors that influence rate
Explain reaction kinetics;
Most chemical reactions are complex, involving many elementary reactions (steps)
Kinetics provides fundamental information regarding energy changes and molecular interactions
Kinetics can be use to deduce the mechanism of a reaction
Transition State: Highest energy on the energy curve – transient state whereby molecules can return to reactants or proceed to form products
Kinetic Profile:
How the rate of a reaction varies as reaction proceeds
Rate of a Reaction
Measure consumption of a reactant or formation of a product
At t=0, this is initial rate of reaction
Rate at a particular instant: draw tangent and calculate the gradient (rise over run)
Steepness of gradient is how fast the reaction is progressing
Rate of a Reaction
Measure consumption of a reactant or formation of a product
At t=0, this is initial rate of reaction
Rate at a particular instant: draw tangent and calculate the gradient (rise over run)
Steepness of gradient is how fast the reaction is progressing
What is a tangent?
A tangent is a straight line drawn so that it just touches the curve at the point of interest.
What are Units for Rate?
moldm-3s-1 or Pas-1
1 mol : 1 mol – At any time the rate of formation of product =
rate of consumption of reactant
gradient is positive because the concentration _________ with time
increases
List the 5 Factors Affecting Rate of Reaction =
- Temperature of Reactants
- Concentrations of Reactants
- Chemical Nature of Reactants
- State of Subdivision of the Reactants
- Presence of a Catalyst
Elementary Reactions
- single step reactions involving one or two molecules, ions or atoms
- Chemical equation represents the actual changes that take place at a molecular level
- The rate equation expresses the rate of the reaction under specific conditions Rate = k[H∙][Cl2]
- If the temperature is changed, the Rate of the Reaction changes and so k must change
Order of a Reaction
[0, 1st,2nd,3rd]
The power to which the concentration of reactants must be raised to give a rate equation that describes the experimental data
Differential Rate Equations
A rate equation where the rate of consumption of a reactant / formation of a product is written as a differential
What is the different between rate equations and integrated rate equations?
Rate of reaction shows the rate of the reaction at a particular instant during the reaction while integrated shows how the concentration varies with time. (derived from differential rate equation). Equation - linking reactant conc at a particular time to conc at start of reaction.
1st Order Elementary Reaction
A > products
If [A]0 and k are known, the concentration of the reactant / product, at any time can be calculated
If the concentrations during the reaction are measured, k can be calculated
Equation of a straight line: y = mx + c
1st order reaction gradient;
x=y in proportion
1st order Exponential Decay:
The concentration of the reactants decays as a logarithmic exponent with time
A constant fraction of the drug reacts per unit time
2nd Order Elementary Reaction
A + A > products
Note: [A] approaches zero more slowly for 2nd order reactions
2nd Order Elementary Reaction
where A + B > products
Pseudo 1st Order Reaction
A + B > products
This is the method of isolating one reactant by having the other reactant in such a large excess.
If [A] ««< [B] then [B] hardly changed in the reaction, therefore [B] = [B]0
1st Pseudo
RoR = k[A][B]
If k [Bo] then, RoR k’[A]
What is the half life of a reactant?
the time taken for its concentration to fall to half of the initial value
half life - first order
1st Order Reactions:
The half life of a reactant in a 1st order reaction is independent of its initial concentration, so t1/2 is constant through the course of the reaction
half life - 2nd order
2nd Order Reactions:
The half life of the reactant in a second order reaction is inversely proportional to the initial concentration and to the rate constant, so t1/2 is “bigger” if [A]0 is lower in concentration. It is not constant
