# Finding it hard to master the concepts of Chemical Kinetics?

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## Core topic in chemical kinetics:

### Collision Theory of Chemical Reactions

The collision theory of chemical reaction states that for a reaction to occur, the reactant molecules must collide with each other and the frequency and the energy of those collisions will decide whether those collisions are effective and thus affect the rate of that reaction. Therefore, the rates of chemical reaction can be predicted by the collision theory of chemical reaction.

### Rate of a Chemical Reaction

The rate of a reaction can be defined as the rate with which the products are formed and the reactants are consumed. The rate can also be defined in concentration terms, the rate of a reaction is concentration of a substance consumed or produced in unit time.

### Integrated Rate Equations

Determining the rate of reaction from the concentration time graph is very difficult. So we integrate the differential rate equation to get a relationship between the concentration at different points. This equation is called the integrated rate equation. Different order reactions have different integrated rate equations.

For a simple first-order reaction an equation relating reactant concentration with time can be given as-

For a second order reaction the equation relating reactant concentration with time can be given as-

Integrated rate law for second order reaction is-

Integrated rate equation for zero order reactions is-

### Pseudo First Order Reaction

A Pseudo first-order reaction is a second-order reaction that behaves like a first-order reaction. In these reactions one reactant is present in excess or is maintained at a constant concentration compared with the other reactant.

These reactions are expected to have second order kinetics but are found to have first order kinetics experimentally.

### Factors Influencing Rate of a Reaction

Following factors can affect the rate of reaction-

Chemical nature of the reactants affect the rate of a reaction. For example, when small pieces of iron and sodium are exposed to air, the sodium reacts completely with air but the iron is barely affected.

The rate of a chemical reaction is also affected by the physical state of the reactants.For example, a log of wood burns slowly but the sawdust burns rapidly.

The temperature also affects the rate of the reaction.Usually, increase in the temperature increases the rate. For example, Food gets spoiled when not kept in the refrigerator.

Concentration of reactants can also affect the rate of the reaction. Increase in concentration of one or more reactants generally increases the rate of the reaction.

Presence of catalysts in a reaction also speeds up the rate of the reaction.

### Temperature Dependence of the Rate of a Reaction

The rate of a reaction increases with increase in temperature. For example, A metal would melt at a much quicker rate at high temperature then at low temperature.

### What is Chemical Kinetics?

Chemical kinetics is the branch of physical chemistry that deals with rates of chemical reactions. Chemical kinetics is very important along with thermodynamics which tells us about the direction in which a reaction would proceed, but not about the rate.

### Rate of Formations and Disappearances

The rate of a chemical reaction is change in concentration over the change in time and can be defined as the rate of disappearance of reactants or the rate of formation of products.

The rate of disappearance of reactants is the measure of the rate of disappearance of the reactants.

The rate of formation of products  is the rate at which the products are formed.

### Average and Instantaneous Rate

At a specific time, the rate at which a reaction is proceeding is known as its instantaneous rate. At the time zero, the instantaneous rate of a reaction is the initial rate. Change in concentration of a reactant or product in a chemical reaction over a given interval of time is known as average rate.

### Factors Affecting The Reaction Rate

Following factors can affect the rate of reaction-

Chemical nature of the reactants affect the rate of a reaction. For example, when small pieces of iron and sodium are exposed to air, the sodium reacts completely with air but the iron is barely affected.

The rate of a chemical reaction is also affected by the physical state of the reactants.For example, a log of wood burns slowly but the sawdust burns rapidly.

The temperature also affects the rate of the reaction.Usually, increase in the temperature increases the rate. For example, Food gets spoiled when not kept in the refrigerator.

Concentration of reactants can also affect the rate of the reaction. Increase in concentration of one or more reactants generally increases the rate of the reaction.

Presence of catalysts in a reaction also speeds up the rate of the reaction.

## Sample questions:

Example 1 Pick the correct statement

• Rate of a reaction decreases with time as the concentration of reactants decreases.

• Rate of a reaction stays the same throughout the reaction.

• Rate of a reaction decreases with the increase in concentration of reactants.

• Rate of a reaction does not depend on temperature change.

Solution- Rate of a reaction decreases with time as the concentration of reactant decreases.

Example 2 Why does more surface area of reactant molecules increase the rate of reaction?

Solution: Larger surface area increases the number of active sites of reactants that can interact with each other to form more products. So a larger surface area will increase interactions between molecules to form more products.

Example 3 Why increase in temperature increases the rate of a reaction ?

Solution: The increase in temperature results in the increase in the kinetic energies of molecules and decrease in the total activation energy of the reaction. As a result, the rate of the reaction increases.

## Chemical Kinetics will no longer be a dreaded topic. Bring in all your doubts and queries at TutorEye:

Does your mind feel boggled by the unpredictability of chemical reactions? Don’t worry! Understanding the different aspects of a chemical reaction can be made easy with the right guidance.

Just like thermodynamics focuses on the relative energy states of the reactants and the products, kinetics deals with the speed at which a reaction occurs and the chemical stages it goes through.

Let us try to break it down for you. The term ‘kinetics’ stands for the rate of change in some entity. So based on the rate, the chemical reactions can be classified as slow, moderate, and fast.

There are various factors that can affect the rate of reactions. Hence, chemical kinetics is all about understanding the rate of reaction and the factors that can influence it.

Any chemical reaction involves the consumption of the reactants and the formation of the products. The rate of the reaction will either depend on the rate at which the reactants are consumed or the rate at which the products are formed.

This rate can be influenced by an alteration in the surface area of the reactants, concentration of the reactants, temperature, catalyst, and the nature of the solvent.

There are some reactions that undergo a single step. These are known as elementary reactions. And there are others that happen in multiple steps. They are referred to as complex reactions.

Chemical reactions are no less than magic. Once you start to understand the concept behind them, you will see how interesting and engaging they can be.

Do you believe you have a scientist hidden somewhere within you? Don’t let your doubts stop you from becoming one.

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### What is the application of chemical kinetics?

Chemical kinetics provides evidence for the mechanisms of different reactions. Reaction mechanisms  are useful in studying and modify them according to our uses. For example drug development.

### How do you calculate chemical kinetics?

The rate of a chemical reaction is change in concentration over the change in time and can be defined as the rate of disappearance of reactants or the rate of formation of products.

The rate of disappearance of reactants is the measure of the rate of disappearance of the reactants.

The rate of formation of products  is the rate at which the products are formed.

### How do you calculate chemical kinetics?

The rate of a chemical reaction is change in concentration over the change in time and can be defined as the rate of disappearance of reactants or the rate of formation of products.

The rate of disappearance of reactants is the measure of the rate of disappearance of the reactants.

The rate of formation of products  is the rate at which the products are formed.

### How to solve chemical kinetics problems?

The rate of a chemical reaction is change in concentration over the change in time and can be defined as the rate of disappearance of reactants or the rate of formation of products.

The rate of disappearance of reactants is the measure of the rate of disappearance of the reactants.

The rate of formation of products is the rate at which the products are formed.