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Chemical Kinetics

Chemical Kinetics

  • Chemical Kinetics is the branch of science that deals with rate of reaction, the factors affecting the rate of reaction mechanism.

  • Chemical reactions need varying length of time for completion depending upon nature of reactants and products and conditions under which reaction is run.

  • Many reactions such as explosive reaction take place in a fraction of a second while the rusting of a wrecked ship may take centuries to complete.

  • Similarly ionic reactions in solution such as that between HCl (aq) and AgNO3 (aq) giving a white precipitate of silver chloride within twinkling of eye. In chemical kinetics we actually study the rates of only those reactions which are in between the above two extremes, i.e. neither so fast nor so slow.

  • A chemical reaction involves breaking of bonds in reactant molecules and making of bonds in product molecules.

  • A reaction that involves breaking of weak bond(s) is faster than one involving breaking of strong bond at a given temperature.

  • Ionic compounds remain completely ionised in solution and in any ionic reaction no bond is to be broken. This is why an ionic reaction is so fast. Different reactions differ in respect of the strength of the bonds to be broken and hence they occur at different rates. Reactions which involve less bond rearrangements are generally faster than those which involve considerable bond rearrangements, at a given temperature.

  • Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes.

  • Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition states, as well as the construction of mathematical models that can describe the characteristics of a chemical reaction.

In 1864, Peter Waage and Cato Guldberg pioneered the development of chemical kinetics by formulating the law of mass action, which states that the speed of a chemical reaction is proportional to the quantity of the reacting substances.

Concept of order of reaction and how to determine order of reaction along with integrated rate laws are the most important topics of this chapter. Repeatedly questions appear in IITJEE and AIEEE from these topics.

Thermodynamics deals with the feasibility of a chemical change. The free energy change, ?G, of a reaction helps us to understand whether the reaction will occur or not.

Even though there may be decrease in free energy but reactants do not always form the products instantaneously and actual rate of the reaction may vary from extremely slow to very fast. Thermodynamics is concerned only with initial and final states of reacting systems but offers no explanation about the various stages through which the reactants pass to reach the final state. This leads to following questions concerning chemical changes.

  • How fast do the chemical reactions go?

  • How can the speed of the reaction change?

  • What intermediate steps, the reactants follow to reach the final state of products, i.e., the mechanism of the reaction.

The branch of physical chemistry which deals with the rate at which the chemical reactions take place and the influence of various factors such as concentration, temperature, pressure catalyst, etc., on the reaction rates, is called the chemical kinetics.

Different chemical reactions occur at different rates. On the basis of rates, the chemical reactions are broadly divided into three categories :

Very Fast or Instantaneous Reactions

These reactions are so fast that they occur as soon as the reactants are bought together. Generally, these reactions involve ionic species and thus known as ionic reactions. These reactions take about 10-14 to 10-16 seconds for completion. It is almost impossible to determine the rates of these reactions. Some such examples are:

  • Precipitation of AgCl when solutions of silver nitrate and sodium chloride are mixed. AgNO3 + NaCl → AgCl + NaNO3

  • Precipitation of BaSO4 when solutions of barium chloride and sulphuric acid and mixed. BaCl2 + H2SO4 → BaSO4 + 2HCl

  • Neutralisation of an acid with a base when their aqueous solutions are mixed.HCl + NaOH → NaCl + H2O

Very Slow Reactions

There are certain reactions which are extremely slow. They make take months together to show any measurable change at room temperature. It is also difficult to study the kinetics of such reactions.

Some examples are:

  • Reaction between hydrogen and oxygen at room temperature
  • Reaction of atmospheric H2S on basic lead acetate.
  • Reaction between carbon and oxygen:  Carbon and oxygen are thermodynamically less stable than CO2 at 298 K, yet coke does not spontaneously catch fire in air and remains unreacted even for years. 
  • Reaction between carbon monoxide and hydrogen: The reaction is thermodynamically feasible at 298 K bur in actual practice the reaction proceeds infinitesimally slowly.

Moderate Reactions

Between the above two extremes, there are a number of reactions which take place at moderate and measurable rates at room temperature and it is these reactions which are studied in chemical kinetics. Mostly these reactions are molecular in nature. Some common examples of such type are given below:

  • Decomposition of hydrogen peroxide: 2H2O2 → 2HO + O2 

  • Decomposition of nitrogen pentoxide: 2N2O5 → 2N2O4 + O2 

  • Hydrolysis of an ester:  CH3COOC2H5 + NaOH → CH3COONa + C2H5OH

  • Inversion of cane sugar in aqueous solution.: C12H22O11 + H2O → C6H12O6 + C6H12O6

  • Reaction between nitrogen dioxide and carbon monoxide.NO2 + CO → NO + CO2 

  • Reaction between ferric chloride and stannous chloride.2FeCl3(aq) + SnCl2(aq) → 2FeCl2(aq) + SnCl4(aq) 

  • Decolourisation of acidified potassium permanganate with sodium oxalate.

  • Reaction between nitric oxide and chlorine: NO + Cl2 → NOCl2

The chemical reactions can be slowed down or speeded up by changing conditions under which they occur. For example, very slow reaction. CO + 2H2 → CH3OH, can be speeded up by maintaining temperature around 400oC, pressure about 300 atmosphere and using a catalyst containing ZnO and Cr2O3. The decay of food articles can be slowed down by reserving them in refrigerators. 

 Principal reasons for studying chemical kinetics.

There are two principal reasons for studying chemical kinetics.

  • To predict the rate of a particular reaction under specified conditions: The conditions can be adjusted to make the reactions to go at a desired rate, either rapidly or slowly or moderately. The field of chemical kinetics is useful in industry as the conditions for maximum yields of industrial products can be ascertained. 

  • To predict the mechanism of the reaction:The intelligent guess regarding various elementary processes responsible for the formation of products can be made which should be consistent with experimental data.