Chemical Kinetics Class 12 Notes
Chemical Kinetics is a medium that helps us to understand the nature of the reaction and how they occur. The phrase kinetics is derived from the Greek phrase ‘kinesis’ which means movement.
Rates of any Reaction
Ionic reactions occur very fast whereas some reactions such as rusting of iron are very slow and gradual processes. Inversion of cane sugar and hydrolysis of starch on the other hand happens at a medium pace.
The change in concentration of a reactant or product in unit time can be characterised as the speed or rate of a reaction. It can be expressed in terms of
- the rate of reduction in the concentration of any one of the reactants, or
- the rate of increase in the concentration of any one of the products, to be more specific
Average Rate of Reaction
The average rate is determined by the time it takes for a change in concentration of reactants or products to occur. It is measured in the units of mol L-1s–1. However, for the gaseous state, it will be atm s–1. The mathematical formula for average rate of a reaction is follows:
Factors affecting the rate of a reaction
The concentration of one or more reactants and products can affect the pace of a chemical reaction at a given temperature. The rate law is a depiction of the rate of reaction in terms of the concentration of the reactants. It’s also known as a rate expression or a rate equation.
Rate law in a balanced chemical equation is the expression in which reaction rate is expressed in terms of the molar concentration of reactants with each term raised to a power, which may or may not be the same as the stoichiometric coefficient of the reacting species.
What is an order of a reaction?
In the reaction equation
The sum of powers of the concentration of the substances in the above equation is called the order of the reaction.
Molecularity of A Reaction
The total number of atoms, molecules, and ions involved in an elementary reaction, which collide to cause a chemical reaction, is called the molecularity of a reaction. In simple words, it is the total number of moles involved in any reaction.
Order Vs. Molecularity
- The order is an empirical quantity. It can be 0 and even a fraction, but calculating the number of moles cannot be zero or less than an integer.
- The order applies to both basic and complex reactions while molarity only applies to basic reactions. For a complex reaction, the number of moles is meaningless.
- For a complex reaction, the order of the slowest step and the number of moles of the slowest step is the same as the order of the overall reaction.
Zero Order Reaction
Zero-order reaction means that the rate of the reaction is proportional to the power of the concentrations of the reactants. Zero-order reactions are relatively rare, but they occur under special conditions. Some reactions catalyzed by enzymes and reactions occurring on metal surfaces are some examples of zero-order reactions. The decomposition of ammonia gas on a hot platinum surface is a zero-order reaction at high pressure. Another example of a zero-order reaction is the thermal decomposition of HI on the gold surface.
Plotting the zero-order graph, it appears like this
First Order Reaction
The rate is proportional to the first power of the concentration of the reactant.
All the natural and artificial radioactive decay of unstable nuclei takes place by first-order kinetics. Hydrogenation of Ethene also falls under this category. N2O5 and N2O decomposition are also examples. In the case of gaseous phase reaction, the equation modifies as follows, where p stands for the pressure of gaseous reactant:
Half Life of A Reaction
The half-life of a reaction is the time it takes for the concentration of a reactant to drop to half of its initial concentration. It is represented as t1/2. It is clear that t1/2 for a zero-order reaction is directly proportional to the initial concentrations of the reactants and inversely proportional to the rate constant. For a zero-order reaction, t1/2 is directly proportional to R0. For the first-order reaction, t1/2 is independent of R0. For the zero-orde reaction, half time o]can be found out by using the following equation:
For the first-order reaction, the formula for finding half time of reaction is as follows
The order of a reaction is sometimes altered by conditions. There are many reactions that obey the first-order rate law even though they are higher-order reactions. A pseudo-first-order reaction can be defined as a second-order reaction or a two-molecular reaction that behaves like a first-order reaction. This reaction occurs when one reactant is present in large excess or is maintained at a constant concentration relative to the other.
Does the rate of reaction depend on Temperature Change?
The answer is Yes and was first tried to attempt by the Dutch chemist J.H. van’t Hoff but the Swedish chemist Arrhenius provided its true physical justification and explanation. The temperature dependence of chemical reaction rates can be precisely explained by the Arrhenius equation.
A= Arrhenius Constant/ frequency factor.
It has been found that for a chemical reaction when the temperature is increased by 10°, the rate constant almost doubles. It is also known as the exponential factor. It is a constant that characterizes a particular reaction. R is the gas constant and Ea is the activation energy measured in joules/mol (J mol −1).
According to Arrhenius, the reaction can only take place when two different molecules of different reactants collide to form an unstable intermediate. It lasts for a very short time and then decomposes to form two molecules of a new compound. The energy required to form this intermediate, called the activator complex (C), is called the activation energy (Ea).
Most Probable Kinetic Energy
Ludwig Boltzmann and James Clark Maxwell used statistics to predict the behavior of many molecules. Therefore, the kinetic energy distribution can be explained by plotting the given kinetic energy (E) and the numerator ratio of the kinetic energy (NE / NT) (Fig. 4.8). Where NE is the number of molecules of energy E and NT is the total number of molecules. The peaks of the curve correspond to the most likely kinetic energy. H. Kinetic energy of the largest part of the molecule. The number of molecules with energies higher or lower than this value is decreasing.
Do Catalysts have an effect of Catalyst on the rate of reaction?
Catalyst is simply defined as the substance used in the reaction to speed up the process without being consumed. So, this statement itself gives the answer to the question.
The catalyst is believed to provide an alternative pathway or reaction mechanism by reducing the activation energy between the reactants and the product, thereby lowering the potential energy barrier. A small amount of catalyst can catalyze a large number of reactants. The catalyst does not change the free energy ΔG of the reaction. It catalyzes spontaneous reactions, but not spontaneous reactions. that is it can catalyze forward as well backward reactions to the same extent. Also, the catalyst does not change the equilibrium constant of the reaction, but it helps to reach equilibrium more quickly. That is, it catalyzes both forward and reverses reactions to the same extent, so the equilibrium remains the same, but arrives faster.
What is Collision Theory?
Developed by Max Trautz and William Lewis in 1916, Collision Theory 18 provides better insight into the energetic and mechanical aspects of the reaction. The equilibrium remains the same but reacts to the same extent to reach faster. It is based on the kinetic theory of gas. According to this theory, reactant molecules are assumed to be rigid spheres, and reactions occur when the molecules collide with each other. The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z).
Z= the collision frequency of reactants, A and B, and e–Ea /RT represents the fraction of molecules having energies greater than or equal to Ea.
This chemical kinetics class 12 notes is a wholesome gist of the topic covered in the Std. 12 Syllabus. It should give you a fair idea. the subject matter.
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