Chemical Kinetics 1 Flashcards
what do chemists try to find out abt a chemical reaction
(a) the feasibility of a chemical reaction which can be predicted by thermodynamics ( as you know that a reaction with DG < 0, at constant temperature and pressure is feasible);
(b) extent to which a reaction will proceed can be
determined from chemical equilibrium;
(c) speed of a reaction i.e. time taken by a reaction to
reach equilibrium.
what is chemical kinetics
the branch of chemistry, which deals with the study
of reaction rates and their mechanisms, called
chemical kinetics.
Kinetic studies not only help us to determine
the speed or rate of a chemical reaction but also describe the
conditions by which the reaction rates can be altered.
what is speed of reaction
the speed of a reaction or the rate of a
reaction can be defined as the change in concentration of a reactant
or product in unit time.
it can be expressed in terms of:
(i) the rate of decrease in concentration of any one of the reactants, or
(ii) the rate of increase in concentration of any one of the products.
Units of rate of a reaction
For example, if concentration is in mol L–1 and
time is in seconds then the units will be mol L-1s
–1. However, in gaseous
reactions, when the concentration of gases is expressed in terms of their
partial pressures, then the units of the rate equation will be atm s–1
instantaneous rate
It can be determined graphically by drawing a tangent at time t on
either of the curves for concentration of R and P vs time t and calculating
its slope
factors affecting rate of reaction
Rate of reaction depends upon the experimental conditions such
as concentration of reactants (pressure in case of gases),
temperature( van’t hoff eqn) and catalyst ( may be +ve,-ve or neutral).
it depends on nature of reacting species, pH, pressure.
what is rate law
state the rate law expression
The
representation of rate of reaction in terms of concentration of the
reactants is known as rate law. It is also called as rate equation or
rate expression.
rate law is the
expression in which reaction rate is given in terms of molar
concentration of reactants with each term raised to some
power, which may or may not be same as the stoichiometric
coefficient of the reacting species in a balanced chemical
equation.
Dependence
of Rate on
Concentration
The rate of a chemical reaction at a given temperature may depend on
the concentration of one or more reactants and products.
rate of a reaction decreases with
the passage of time as the concentration of reactants decrease. Conversely,
rates generally increase when reactant concentrations increase. So, rate of
a reaction depends upon the concentration of reactants.
are the powers of the reactant species always equal to stoichiometric coefficients.
the powers, are usually equal to stoichiometric coefficients, but not always.
Rate law for any reaction cannot be predicted by merely looking at
the balanced chemical equation, i.e., theoretically but must be determined
experimentally.
i) CHCl3+ Cl2 → CCl4 + HCl
ii) CH3COOC2H5+ H2O → CH3COOH + C2H5OH
Rate = k [CHCl3] [Cl2]1/2
Rate = k [CH3COOC2H5]1 [H2O]0
what is order of reactant and order of reacton
Rate = k [A]x [B]y
x and y indicate how sensitive the rate is to the change in concentration
of A and B.
Sum of these exponents, i.e., x + y in gives the overall order of a reaction whereas x and y represent the order with respect
to the reactants A and B respectively.
what is order of the reaction
the sum of powers of the concentration of the
reactants in the rate law expression is called the order of that chemical reaction.
Order of a reaction can be 0, 1, 2, 3 and even a fraction.
A zero order reaction means that the rate of reaction is independent of the concentration of reactants.
what are elementary reactions
The reactions taking place in one step are called elementary
reactions.
what are complex reactions
When a sequence of elementary reactions (called mechanism) gives us the products, the reactions are called complex reactions.
These may be consecutive reactions (e.g., oxidation of ethane to CO2 and H2O passes through a series of intermediate steps in which alcohol, aldehyde and acid are formed), reverse reactions and side reactions
(e.g., nitration of phenol yields o-nitrophenol and p-nitrophenol).
units of rate constant
rate= (concentration)^1-n x (time)^-1
n- order
for zero order, mol/Ls
for 1st order, /s
for 2nd order L/mol s
