Chemical Reactions - Part IV

4. Reaction rates  
While observing the time taken by chemicals to react with each other, we will observe that some reactions are slow and some are fast. The reactions therefore have a certain rate at which they proceed. The rate of a chemical reaction can be defined as the change in the concentration of reactants or products in a unit time.

When the concentration of the products is increasing with time, the reaction is called a forward reaction. There are cases when the products themselves interact to give back the reactants. These reactions are called reversible reactions. In reversible reactions, the rate of the reaction in the forward direction increases at first; then after a time, the products start reacting, so the rate at which the products are combined reduces. There will come a time when the rate of reaction in the forward and the backward direction will become equal. At this time the reaction is said to be in equilibrium. Such a reaction is called balanced reaction.

For example, if calcium oxide is kept in close contact with carbon dioxide, the two substances slowly unite to make calcium carbonate.

  CaO    +    CO₂          CaCO₃

This a forward reaction.  
Now if you heat CaCO₃  you will get back CaO and CO₂  . This is a reverse reaction.

The two equations can be combined as  

                CaCO₃         CaO  + CO₂

The horizontal arrows show that the reaction proceeds in both the directions. In such types of reactions an equilibrium is achieved. Equilibrium is obtained when the rate of the two reactions is same.

Reaction rate, in order of their importance, depends on the following conditions :

• Nature of the reactants

• Size of the particles

• Concentration

• Temperature

• Catalyst

Nature of reactants : We have seen that some elements, because of the way their electronic configuration occurs,  are very reacting. For example Na, K, Rb readily give off their electrons. F, Cl, Br are elements which quickly accept electrons. On the other hand noble gases like He, Ne, Ar, Kr are completely chemically inert. Non-metals like C, N, O react but not as vigorously as F, Cl, Br, etc.  In most cases valence of the reactants will give you a rough estimate about the reaction rates.

Examples of fast reactions :

Na  +   Cl       NaCl.

2Na  +   2H₂O         2 Na(OH)  + H₂                      

Examples of slow reactions :

2Fe  +  3O₂        Fe₂O₃  

CaO  + CO₂       CaCO₃

Size of the particles : The reaction rates are faster for reactants whose particle size is small. The reason for this is very simple. In a chemical reaction, each substance is changing at the atomic level. In smaller particles, the surface area for reaction on the microscopic level is larger. In other words, if the number of collisions between the reacting molecules is large, the reaction rate would be higher.

Example to see the difference in rates of reaction with respect to the particle size :

Take two test tubes.  In one add some iron filings and in the other add a piece of iron like a nail. Let the weight of the iron filings and the nail be the same. Add hydrochloric acid in both the test tubes. Watch the reaction proceed. You will observe that the iron filings dissolve much faster than the nail. The reaction that is taking place is

2Fe   +   6HCl    2FeCl₃  +   3H₂

In another activity, take a piece of marble (CaCO₃) in a test tube. In another add powdered CaCO₃.  Take equal quantities by weight in both the test tubes. Add HNO₃ in both the test tubes. In both the test tubes the reaction starts, but in the case of the powder you will see the powder disappear quickly. The reaction that is taking place is

CaCO₃    +    2HNO₃        Ca(NO₃) ₂  + H₂O  + CO₂ 

Another example of effect of particle size on chemical reaction in our everyday lives is dissolving of sugar in water. If the sugar crystals are small, they dissolve quickly. If the sugar crystals are larger, they take more time to dissolve in water.

Concentration : Larger the number of atoms or molecules reacting together, faster will be the reactions. In the above examples, see the difference in time of reaction when you use dilute and concentric acids. The reactions will be much faster in case of concentric acids. In other words, larger the concentration of the reactants, larger will be the probability of the reactants coming together, hence faster would be the reaction rates. In case of gases, the gas pressure will be a parameter in place of concentration. Higher the gas pressure faster is the reaction.

Temperature : In the example of iron filings and HCl given above, heat the HCl and then pour it into the test tube containing the Fe filings. You will observe that the reaction is faster than before. This indicates that the reaction is faster when the temperature is higher. It is easy to see why : as the temperature rises, the atoms or molecules of the reactant will start jittering around more. This will increase the probability of collision between the reacting atoms or molecules. As the probability increases, the reaction rate will increase too.  

Example of everyday life would again be the dissolution of sugar in water. You will notice that sugar dissolves more quickly in hot water than in cold water.

Catalyst : There are certain substances, which by their mere presence enhance the rate of reaction. These substances are called catalysts. Catalysts themselves do not participate in the reaction and remain unchanged. For a reversible and balanced reaction, a catalyst helps the reaction to be more favorable in one direction than the other.

There are many examples of catalyst in chemical reaction. For example if you heat KClO₃, it will start to decompose at a very high temperature. If a bit of MnO₂ is added, the decomposition of KClO₃ is faster and at a lower temperature. Chlorophyll in leaves act as a catalyst for the plants to convert energy from the sun to food. Platinum is used as a catalyst for reactions where acids are manufactured.

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