Chapter 4 – Chemical Kinetics
The field of chemistry known as chemical kinetics studies the speeds (or quickness) of chemical reactions, the variables that impact them, and the mechanisms by which the reactions take place.
Rate of reaction:
The change in reactant or product concentration per unit of time is referred to as the rate of reaction.
- A+B -> C is a general reaction.
The negative indicator shows that the level of concentration is waning over time.
The reaction rate is measured in mol L-1s-1.
The reaction’s pace is not a constant value (except for zero order reactions). As the reaction moves ahead, it becomes less and less.
The molar concentration of one or more reactants and the reaction rate are mathematically related in a rate law.
The word “rate law” refers to the depiction of reaction rate in terms of reactant concentration.
The law of mass action states:
In a chemical reaction, the formula is:
- aA + bB products,
where a and b are the reactant’s stoichiometric coefficients.
The overall order of the reaction is represented by m + n, where m and n are experimentally determined and stand for the order of the reaction with regard to A and B, respectively.
Effects of the catalyst:
A catalyst is something that speeds up a reaction without incurring any long-term chemical change.
Determination of reaction rate constants:
The reaction’s rate constant is determined by the concentration of each reacting species at unity. ‘k’ is used to represent it. It is also known as the specific reaction rate or the response velocity constant.
Sum of the exponents:
The sum of the exponents to which the concentration terms are raised in the rate equation (or rate law) of the reaction is the order of reaction. Any full number, a fraction, or zero may be used.
The number of reacting particles (atoms, molecules, or any other species) that collide at the same time to bring about a reaction is known as the modularity of reaction.
It is an abstract idea. It always has a whole number value. Never more than three, ever. It can’t be 0 though.
First order reaction:
A reaction is referred to as first order if the modification of just one concentration term determines the reaction rate.
The reactant concentration is expressed as a function of time in the integrated rate equation.
A first order reaction’s integral rate equation is given as
Second order reaction:
A reaction where the rate law equation’s sum of the powers of the concentration terms is two.
Zero order reactions:
Zero order reactions are those in which the rate of reaction is unaffected by the concentration of the reactants.
The rate law for this kind of reaction is written as.
- Rate = k [A] ° [B] °
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The amount of time needed for the reactant’s starting concentration to be halved.
It has been discovered that the rate constant roughly doubles for chemical reactions when the temperature is raised by 10 °C.
The ratio of a reaction’s rate constants at two temperatures that differ by 10°C is known as the temperature coefficient of a reaction. The two standard readings are 35 °C and 25 °C.
The Arrhenius equation:
where A is a constant termed the frequency factor and Ea is referred to as the energy of activation, can be used to express the variation of rate constants with temperature.
The rate constants at two different temperatures can be connected as follows using the equation above:
The collision theory and the transition state theory are two significant theories of response rates.
Max Trautz and William Lewis devised the collision theory between 1916 and 1918. It is based on the gas kinetic theory. This hypothesis postulates that the reaction happens when molecules hit and assumes that the reactant molecules are hard spheres.
It is defined as the number of collisions between reacting molecules that occur per second per unit volume.
For a bimolecular elementary reaction,
- A + B → Product
- Rate =Z AA e −RTEa
The energy that a molecule has to have in order to have an effective collision is known as the threshold energy.