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1. Chemical Kinetics
Chemical Kinetics
Chemical Kinetics y Study of rate of reactions and factors affecting it like temperature, pressure, catalyst and radiations is called chemical kinetics. y On the basis of rate, reactions can be of slow, moderate and very fast type. y Very fast reactions take 10–14^ to 10–16^ seconds for their completion, for example, ionic reactions, such as the one shown below: NaCl + AgNO 3 → AgCl +NaNO 3
y Fast reactions are not studied under kinetics. These are studied by flash-photolysis and spectrographic methods. y Blackening of white lead,Rusting of iron, formation of water at room temperature burning of coal and are not studied under kinetics. y Reactions with moderate speed, like molecular reactions, are studied under kinetics. For e.g., hydrolysis of ester or sugar.
Rate of Reaction Rate of reaction is defined as the rate of change of concentration of a reactant or a product per unit time.
Rate of reaction 2 1 2 1
[C ] [C ]
t t
y Rate of reaction decreases with time because concentration of the reactant also decreases. This can be observed from the following reaction. 2NO 2 + F 2 →2NO F 2 Here, rate of reaction is measured by the decrease in concentration of F 2. y Unit for rate of reaction is mol L–1^ time–1^ or atm time–1 (gaseous substance).
Rack your Brain
Can we study the rate of neutralisation reaction of a strong acid and base using chemical kinetics?
Concept Ladder
Chemical Kinetics basically tells us about how fast or slow a reaction is. Reactions with moderate speed are studied using chemical kinetics.
Previous Year’s Questions
For the reaction, N 2 3 H 2 2 NH 3 , if d dt mol L s
[NH ] 3 4 1 1
2 10 ^ ^ ^ , the
value of −d dt
[H ] (^2) would be
[AIPMT]
(1) 4 × 10-4^ mol L-1^ s- (2) 6 × 10-4^ mol L-1^ s- (3) 1 × 10-4^ mol L-1^ s- (4) 3 × 10-4^ mol L-1^ s-
Chemical Kinetics 2.
Average Rate of Reaction As rate varies greatly with time so we use average reaction rate. By dividing the total change in concentration of reactant or product by a time interval, we actually get the average rate of the reaction.
Instantaneous Rate of Reaction Instantaneous rate of reaction is equal to very small change in concentration (d[x]) during a very small interval of time (dt). It is given as d[x] dt
Definition
The instantaneous rate of reaction i.e., rate of reaction at any instant of time is the rate of change of concentration of any one of the reactants or products at that instant of time.
Concept Ladder
Average rate of reaction is termed as the rate of reaction measured over the long time interval. Avg rate x t
R t
P t
. [ ]^ [^ ]^ [ ]
^
Previous Year’s Questions
For the chemical reaction, N 2 (g) + 3 H 2 (g) 2 NH (g) 3 the correct option is [NEET-2019] (1) 3 d^2 dt
d dt
[H ] [NH ]
d 2 3 dt
d dt
[H ] [NH ]
(3) (^) d dt
d dt
[N ] 2 [NH ] 3
(4) (^) d dt
d dt
[N ] 2 1 [NH ] 3
Chemical Kinetics 4.
y For reactants, negative sign indicates decrease in concentration. y For a reversible reaction at dynamic equilibrium, the net reaction rate is zero.
Features of Rate of Reaction y Rate of reaction is always a positive quantity. y It is proportional to the active masses of the reactant. y It is measured by measuring active mass or molar concentration of a reactant or a product with respect to time. y It varies with time, therefore it is better to use average reaction rate.
Specific Reaction Rate or Velocity Constant Consider, nA Product
dx dt or dx dt
k
n
n
[A]
[A]
Rate law equation If [A]n^ = 1 then dx (^) k (At a particular temperature) dt
k = Rate constant or specific reaction rate y At concentration value of unity, rate of reaction is equal to the value of rate constant. y Rate constant, k ∝ Temperature (rate constant always Increases with increase in temperature). y Rate constant does not depend upon presence of a catalyst or on nature of reactant. y Rate constant does not depend upon pressure, volume or concentration. y Unit of rate constant is represented as (mole/ litre)1–n^ time–1^ (Here n = Order of reaction).
Rack your Brain
Find out the units of rate constant for a 3rd^ order reaction.
Definition
According to the law of mass action, it states that the rate of chemical equation is directly proportional to the product of the active mass of the reactants, with each active mass term raised to the power equal to its respective stoichiometric coefficient in a balanced equation with a constant temperature which was later discarded.
Concept Ladder
The rate of reaction is directly proportional to the product of concentration of reactant and each concentration is raised to some power which may or may not be equal to stereochemistry experimentally.
5. Chemical Kinetics
Factors Affecting Rate of Reaction Physical state of reactants
Gaseous state liquid state solid state Decrea sing order of rate of reaction
^ >^ > →
Temperature For every rise of 10°C in temperature, rate or dx/ dt increases by two to three times or rate increases with temperature as the number of effective collisions increases.
Temperature coefficient (^)
^
k C k
t C t C
The value of temperature coefficient lies in between 2 to 3.
Increase in Rate of Reaction with Rise of Temperature We know that all the molecules of the reactants do not have the same energy. According to Maxwell distribution curve for energies, if energies of the reacting molecules are plotted against the corresponding fractions (ΔN/N) at a particular temperature T, a curve of the type shown in figure given below is obtained. Suppose the threshold energy is attained at the point Et. Here and beyond this point Et , all collisions will be effective and hence, a chemical reaction can take place. The shaded area under the curve AB gives the fraction of molecules having energies equal to or greater than the threshold energy. (F)
If the temperature is raised from T K to (T +
- K, the distribution curve shifts towards right, as shown, but the magnitude of the threshold energy will remain the same. Now, the shaded area beyond Et becomes almost double (in some cases triple also) and the fraction of the molecules having energy equal to or greater than
Concept Ladder
Concentration of reactant, surface area, temperature, nature of reactant are some basic factors which affects the rate of reaction.
Rack your Brain
Why are reactions of higher order less in number?
Previous Year’s Questions
The decomposition of phosphine (PH 3 ) on tungsten at low pressrue is a first-order reaction. It is because the [NEET-2016] (1) rate is proportional to the surface coverage (2) rate is inversely proportional to the surface coverage (3) rate is independent of the surface coverage (4) rate of decomposition is very slow.
7. Chemical Kinetics
Concentration Rate of reaction (dx/dt) ∝ Concentration.
Pressure For gaseous reactants dx (^) Pressure (as P C) dt ∝ ∝
Surface area In the case of heterogenous reactions, greater is the surface area, faster is the rate of reaction due to presence of more number of active sites. dx (^) Surface area dt
Example (1) Coal dust burns fast than large coal piece (2) Wood shavings burn fast than a log of Wood
Nature of reactant and product For ionic reactants, rate of reaction is fast, as activation energy is zero for these. Molecules have a slower reaction rate as they require higher activation energy.
Radiation For a photochemical reaction dx (^) Intensity of radiation dt
Rate of reaction for photochemical reactions depends upon the intensity of light radiations and it increases by absorption of photons. Positive catalyst Positive catalyst increases rate of a reaction by providing an alternative path which requires a lesser activation energy. y A negative catalyst decreases the rate of reaction. y Catalysts are more effective in ‘solid powdered form’, which provides a greater surface area, that is, more number of active sites.
Concept Ladder
The rate of a reaction can be increased by increasing the temperature, concentration, surface area, pressure, intensity of radiation or by adding a catalyst.
Rack your Brain
Why boiling of an egg in an open vessel takes more time in a hill station?
Previous Year’s Questions
For the reaction, A + B → Products, it is observed that (i) on doubling the initial concentration of A only, the rate of reaction is also doubled and (ii) on doubling the initial concentration of both A and B, there is a change by a factor of 8 in the rate of the reaction. The rate of this reaction is [AIPMT] (1) rate = k[A][B]^2 (2) rate = k[A]^2 [B]^2 (3) rate = k[A][B] (4) rate = k[A]^2 [B]
Chemical Kinetics 8.
Chemical Kinetics 10.
11. Chemical Kinetics
y According to this theory, rate of reaction is given as Rate = f Z where, f = Fraction of molecules with potential energy greater than threshhold energy. f =e−E /RT^ a P = Orientation factor Z = Collision frequency y Activated state or transition state theory or absolute rate theory was given by Henry Erying. C A + B → ABC Activated complex → AB +Pr oductC
Activation Energy It is the minimum amount of additional external energy needed to change a reactant into product, or to make potential energy of the reactants equal to the threshold energy. It is denoted by Ea. Ea = Threshold energy – Potential energy of reactants or Average kinetic energy of reactants
Rate of Reaction a
1 ∝E
Hence, lower the value of activation energy, faster will be the reaction.
Previous Year’s Question
An increase in the concentration of the reactants of a reaction leads to change in [NEET-2014] (1) heat of reaction (2) threshold energy (3) collision frequency (4) activation energy
Concept Ladder
A catalyst speeds up the reaction by lowering the potential energy barrier and thus by decreasing the activation energy.
The rate of a particular reaction quadruples when the temperature changes from 293 K to 313 K. Calculate activation energy.
K 2 /K 1 = 4, T 1 = 293 K, T 2 = 313 K
log .
K
K
E
R T T
2 a 1 2 303 2 1
^1 ^1
Thus, on calculating and substituting values, we get Ea = 52.86 KJ mol-
Q
Sol.
13. Chemical Kinetics
Graph Between log 10 k and 1/T When a graph is plotted between log 10 k and 1/T, a straight line with a negative slope is obtained. Here, slope is equal to –Ea /2.303R, that is, Εa = –2.303 R. Here, intercept is equal to log 10 A.
In Exothermic Reactions Potential energy of reactant > potential energy of product Ea of backward reaction > Ea of forward reaction.
Rack your Brain
Can activation energy have zero or negative values? If not, why?
Elementary and Complex Reactions It is interesting to know that a balanced chemical equation never shows us a true picture of how the reaction is taking place as very rarely a reaction gets completed in only one step. y The reaction which occurs in one step is known as an elementary reaction and when a sequence of such reactions (mechanism) provides us with the desired products then these reactions are termed as complex reactions.
Previous Year’s Questions
The rate of reaction between two reactants A and B decreases by a factor of 4 if the concentration of reactant B is doubled. The order of this reaction with respect to reactant B is [AIPMT] (1) 2 (2) – (3) 1 (4) –
Chemical Kinetics 14.
y Such reactions may be consecutive, reverse or side reactions. For example, in the complete combustion of an alkane the final products are always carbon dioxide and water. However, during this reaction a series of elementary reactions take place during which alcohols, aldehydes and acids are formed. Molecularity y Molecularity is the number of particles (atoms, ions or molecules) that must collide with one another simultaneously,which results in a chemical reaction. y It is a theoretical value. y It is always a whole number. y It never determines the rate of reaction. y It never depends upon external factors like temperature and pressure. y Molecularity cannot be more than three as there is nearly no chance for more than three molecules to undergo an effective collision at a time. y In a multistep reaction, molecularity is determined separately in each step. y In case of complex reactions, (reactions taking place in a number of steps), the slowest step is called the rate determining step or rate limiting step. y The overall molecularity of a complex reaction is equal to the molecularity of the slowest step. y If molecularity of a reaction is 1 the reaction is called unimolecular as in the following reaction: PCl 5 → PCl 3 +CI 2 y If molecularity is 2, the reaction is called bimolecular. C 12 H 22 O 11 + H O 2 → C HGlucose 6 12 O 6 +C HFructose 6 12 O 6
y If molecularity is 3, the reaction is called termolecular 2FeCI 3 + SnCI 2 → 2FeCI 2 +SnCI 4
Concept Ladder
Reactions of higher order are very rare as the chances for the molecules to come together in a proper orientation to undergo effective collisions are very less.
Rack your Brain
The specific reaction rate is equal to the rate of reaction. Reaction is of which order?
Concept Ladder
Molecularity of a reaction is mostly between 1 to 3, but never zero. It is rare that molecularity exceeds 3.
Chemical Kinetics 16.
Zero-order Reaction In zero-order reactio. concentration of the reactants. dx (^) [Concentration] 0 dt A P dx (^) k (A) k dt
°
On integrating it, we get x = k.t + c If c = 0 then x = k.t k x t
y Unit of is mol L–1^ time–
Examples of a zero-order reaction y Photochemical reactions
h 2 2
2 2 2
3 2 2 3 3 2 3 2 2 2
H CI 2HCI
(studied over watch surface) N O N 1 O 2 (studied over platinum surface) 2NH N 3H CH COCH I CH COCH I HI 2HI H I (studied over gold surface)
y Iodination of acetone. y Enzyme catalysed reactions. Features of a zero-order reaction y The concentration of the reactant decreases linearly with time [A]t = [A] 0 – kt y Rate = k (at all concentrations) y t1/2 ∝ a 0 (initial concentration) y Here rate and concentration of reactants do not vary with time.
Concept Ladder
In the plot concentration vs time for a zero order reaction, the slope gives the value of rate constant and intercept gives the initial concentration.
Rack your Brain
What will be the plot of the rate of reaction vs time for a zero order reaction?
Previous Year’s Question
In a zero order reaction, for every 10°C rise of temperature, the rate is doubled. If the temperature is increased from 10°C to 100°C, the rate of the reaction will become. [NEET] (1) 256 times (2) 212 times (3) 64 times (4) 128 times
17. Chemical Kinetics
First-order Reaction Here the reaction rate is determined by one concentration variable term only. Here, t = Time period k = Rate constant Ar or a or N 0 = Initial amount A or a – x or N = Amount left after time t
Examples of a first-order reaction
2 2 2 2 4 2 2 2 2 5 2 2 2 2 2 2 6 5 2 6 5 2
H O H O 1 O
NH NO N 2H O
2N O 4NO O
SO CI SO CI
C H N NCl H O C H OH N HCl
∆ ∆ ∆ ∆
All radioactive disintegration processes are examples of Ist^ order.
Features of a first-order reaction y Unit of k is time– y k does not depend upon concentration units or change in concentration. y t1/2 does not depend upon initial concentration (a). It depends only upon rate constant.
1/ t 0. k
Concept Ladder
0 0 t 0 10 0 t
If a P (a x) 2P P k 2.303log P t 2P P
Concept Ladder
If the amount of substance left after one half life is [A 0 ]/2, then the amount of substance left after n- half lives is [A 0 ]/2n.
Rack your Brain
How the plot of [A] vs. t for a first order reaction looks like?
19. Chemical Kinetics
Second-order Reaction Examples of a second-order reaction
3 2 3 2 5 3 2 5 2 2 2 2 2
2 O 3 O CH COOC H NaOH CH COONa C H OH 2 Cl O 2 Cl O 2NO 2NO O
Features of a second-order reaction y Unit of k is L mol–1^ time–
1/ t 1 a
y When concentrations are same
k 1 x t a(a x)
y When concentrations are different
10 k 2.303^ log b(a^ x) t(a b) a(b x)
y The change in the unit of concentration, changes the value of k here.
Third-order Reaction
dx (^) [A] (^3) or [A] [B] [C] 1 1 1 dt
Examples of a third-order reaction
2 2 2 3 2 4 2
2NO O 2NO
2NO Cl 2NOCl 2FeCl SnCl SnCl 2FeCl
Concept Ladder
The integrated rate equation for nth order is given by
( ) n^1 0 n^1
K 1 1 1 t n 1 [C] −^ [C ]−
= (^) − − (^)
Where n = 2,3….
Previous Year’s Questions
The given reaction, 2FeCl 3 + SnCl 2 → 2FeCl 3 + SnCl 4 is an example of [AIPMT] (1) third order reaction (2) first order reaction (3) second order reaction (4) none of these
What is the order of reaction whose rate constant has the same units as the rate of reaction?
Zero order
Q
Sol.
Chemical Kinetics 20.
Features of a third-order reaction
2 2
1/2 (^2)
k 1 1 1 2 t (^) (a x) a t 1 a
y The change in the unit of concentration changes the value of A here. y Unit of k is L^2 mol–2^ time–
Pseudo First-order Reactions Pseudo first-order reactions are the reactions which are not truly of the first order but under certain conditions become reactions of the first order. Here molecularity is more than 1, but order of reaction is always one. Few example of pseudo first-order reactions are as follows:
Hydrolysis of ethyl acetate
3 2 5 2 3 2 5
0 10 t
0 t
CH COOC H H O CH COOH C H OH
Rate cons tan t (k) is given as 2.303 V^ V k log t V V Here V Volume at inf inite V Initial volume V Volume at time t
∞ ∞ ∞
Hydrolysis of sugar C 12 H 22 O 11 + H O 2 → C HGlucose 6 12 O 6 +C HFructose 6 12 O 6
Rate constant in this case is given as 0 10 t
2.303 r^ r k log t r r
∞ ∞
Here r 0 = Initial optical rotation r∞ = Rotation at infinite dilution rt = Rotation at t time
Rack your Brain
The half life of a reaction is inversely proportional to the square of initial Concentration, what is the order of reaction?
Concept Ladder
If one of the reactants is present in excess, its concentration does not change much with time through out the reaction, hence the reaction is considered as a pseudo first order reaction.
Rack your Brain
Hydrolysis of ethyl acetate with HCl is a first order reaction, why?
