CHEMICAL KINETICS CLASS- XII. VINAY KUMAR PGT CHEMISTRY KV NTPC KAHALGAON PATNA REGION. It is the branch of physical chemistry which deals with the study of the rate of a chemical reaction and the mechanism by the reaction occur.
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KV NTPC KAHALGAON
It is the branch of physical chemistry which deals with the study of the rate of a chemical reaction and the mechanism by the reaction occur.
Rate of reaction =Decrease in the concentrationof R
Or Increase in the concentrationof P
Unit of rate is Mol L-1 S-1 or atm S-1 (For gaseous reaction)
For a general chemical reaction
aA + bB cC + dD
Rav. = -1 A] = -1 B] = 1 C] = 1 D]
a t b t c t d t
Rinst. = -1 dA] = -1 dB] = 1 dC] = 1 dD]
a dt b dt c dt d dt
RATE LAW:- It is experimentally determined expression which relates the rate of reaction with the concentration of reactants.
For a hypothetical reaction
A + B Products
Rate A]m B]n
Rate = k A]m B]n
Where k is the rate constant .
If A] = B] = 1 Mol L-1 than Rate = k
Thus rate constant is the rate of reaction when concentration of each reactant in the reaction is unity.
ORDER OF REACTION:- It may be defined as the sum of the power of the concentration of reactants in the rate law expression. Order of chemical reaction can be 1,2 or 3 and even may be fractional.
Consider a general zero order reaction
1. Graphical Method:-
This method is applicable to those reactions wherein only one reactant is involved.
2. Initial rate Method:-
This method is used for those reactions where more than one reactant is involved.
In this method we carried out some series of experiments.
We change the one reactant’s concentration and determine the rate of reactions by keeping the constant concentration of each other reactants and compare the rate from initial concentration rate.
Most of the chemical reactions are accelerated by increase in temperature. For example, in decomposition of N2O5, the time taken for half of the original amount of material to decompose is 12 min at 50oC, 5 h at25oC and 10 days at 0oC. We also know that in a mixture of potassiumpermanganate (KMnO4) and oxalic acid (H2C2O4), potassium permanganate gets decolourised faster at a higher temperature than that at a lower temperature.
It has been found that for a chemical reaction with rise in temperature by 10°, the rate constant is nearly doubled.
k = A e -Ea /RT 1
where A is the Arrhenius factor or the frequency factor. It is also called pre-exponential factor. It is a constant specific to a particular reaction. R is gas constant and Ea is activation energy measured in joules/mole (J mol –1).
2H2(g) + I2(g)→ 2HI(g)
It exists for a very short time and then breaks up to form two molecules of hydrogen iodide. According to Arrhenius, this reaction can take place only when a molecule of hydrogen and a molecule of iodine collide to form an unstable intermediate.
The energy required to form this intermediate, called activated complex (C), is known as activation energy (Ea). Reaction coordinate represents the profile of energy change when reactants change into products. Some energy is released when the complex decomposes to form products. So, the final enthalpy of the reaction depends upon the nature of reactants and products.
Ludwig Boltzmann and James Clark Maxwell used statistics to predict the behaviour of large number of molecules. According to them, the distribution of kinetic energy may be described by plotting the fraction of molecules (NE/NT) with a given kinetic energy (E) vs kinetic energy. Here, NE is the number of molecules with energy E and NT is total number of molecules.
There aredecreasing number of molecules with energies higher or lower than this value. When the temperature is raised, the maximum of the curve moves to the higher energy value and the curve broadens out, i.e., spreads to the right such that there is a greater proportion of molecules with much higher energies.
slope = – Ea/ R and intercept = ln A. So we can calculate Ea and A using these values. At temperature T1, equation (1) is
ln k1 = – Ea/RT1 + ln A (2)
According to intermediate complex theory a catalyst participate in a chemical reaction by forming temporary bonds with the reactants resulting in a intermediate complex.
A + B →Products