Reaction Kinetics Flashcards
Rate of Reaction
(& Unit)
A decrease in concentration of a particular reactant or an increase in concentration of a particular product over a period of time.
(Unit : mol dm-3 s-1)
Rate equation
An equation showing the relationship b/w the rate constant and the conc. of the species that affect the rate of reaction.
Stoichiometry in rate equation?
Rate equations can only be determined from experimental data. They cannot be found from the stoichiometric equation.
Type of reactions with fracational order of reactions
Reactions involving free radicals
Rate constant
(& Unit)
k = rate / [A] [B]
(Unit : from the equation as no fixed unit; put the unit with the positive index first)
Order of reactions from (x axis ) Reaction rate vs (y axis) Conc. of reactant graph
Zero order : The plot of reaction rate against concentration is a horizontal straight line. The reaction rate does not change with concentration. For a zero-order reaction, k is numerically equal to the reaction rate:
rate = k
This is because any number to the power of zero = 1
1st order : The plot of reaction rate against concentration is an inclined straight line going through the origin. The rate is directly proportional to the concentration of reactant.
rate = k [A]
2nd order : The plot of reaction rate against concentration is an upwardly curved line. In this case, reaction rate is directly proportional to the square of the concentration of reactant.
rate = k[A]^2
Order of reactions from (x axis ) Conc. of reactant vs (y axis) Time graphs
For first- and second-order reactions, the graph is a curve. The curve for the second-order reaction is much deeper than for a first-order reaction. It also appears to have a relatively longer ‘tail’ as it levels off. We can also distinguish between these two curves by determining successive half-lives of the reaction.
Half lives of orders of reaction
The successive half-lives are more or less constant for first order reaction. In a first-order reaction like this, the half-life is independent of the original concentration of reactant.
A zero-order reaction has successive half-lives which decrease with time.
A first-order reaction has a half-life which is constant.
Finding rate constant K from half live equation
k = 0.693 / t 1/2
What does tangent or gradient of conc. of reactant/time graph represent?
Rate of reaction
In terms of Boltzmann distribution curve and collisions between particles what effect does the temperature have on the rate constant of a reaction?
Increase in temperature increases value of rate constant [1]
The maximum of Boltzmann distribution curve shifts to the right when temperature increases [1]
Greater proportion of particles have energy greater than the activation energy [1]
The rate constant is proportional to the fraction of molecules with energy equal to or greater than the activation energy.
Rate determining step
It’s the slowest step in a reaction mechanism. If a substance does not appear in the overall rate equation it does not take part in the rate-determining step.
p.s. Remember that the overall stoichiometry of a chemical equation does not tell us anything about the reaction mechanism. A suggested mechanism should be consistent with the rate equation not the stoichiometric equation.
Unimolecular and Bimolecular
If there is only a single species (atom, ion or molecule) in the rate-determining step we call the reaction unimolecular. If two species (which can be the same or different) are involved in the rate-determining step,
we say that the reaction is bimolecular. Mechanisms that involve a trimolecular step are rare. This is because it is unlikely that three species will collide at the same time.
Which elements act as best catalysts?
Ions of transition elements are often good catalysts because of their ability to change oxidation number.
The iodine–peroxodisulfate reaction
pg no. 481, 482
The peroxodisulfate and iodide ions both have a negative charge. In order to collide and react, these ions need considerable energy to overcome the repulsive forces when like charges approach each other. Fe 3+ (aq) ions catalyse this reaction. In the energy level diagram the catalysed reaction has two energy ‘humps’ because it is a two-stage redox reaction.
