Conditions Required in Chemical Reaction

Conditions required in Chemical Reaction

Chemical reactions vary with time. Some chemical reactions take place in a very rapid way as soon as they are mixed with reactants while some may take a long time that we feel there is no any reaction taking place. The knowledge of chemical reaction is important in the view of industries as some reactions need to be accelerated while some need to be slow down. The change in the concentration of any one of the reactants per unit time that is needed to complete the reaction is said to be the rate of chemical reaction. There are various factors that affect the rate of chemical reaction. Some of the major conditions or factors are:

1. Surface Area
   The rate of chemical reaction is slow if the surface area of the reactants is small as there is less chance to contact between the reactants. On the other hand, when the surface area is big, the rate of chemical reaction increases. So, in order to increase the reaction rate, the surface area of the reactants should be larger.
   Example:
   When large-sized granulated zinc is treated with dilute hydrochloric acid, they react together rapidly and form hydrogen gas and zinc chloride.
   Zn + dil. 2HCL → ZnCl₂ + H₂ ↑
   
   
2. Catalyst
   A catalyst is a substance that changes the speed of a chemical reaction without undergoing any changes.
   Example:
   Manganese dioxide (MnO₂) acts as a catalyst in the decomposition of potassium chlorate on heating at low temperature. At the end of the chemical reaction, the amount of catalyst remains similar to that of the beginning.
   2KClO₃ \(\xrightarrow{MnO_2/ 240^oC}\) 2KCl + 3O₂ ↑
   
   
3. Pressure
   Some chemical reaction takes place only by applying the pressure. The certain amount of pressure is necessary for them to conduct the reaction.
   Example:
   When sulphur and potassium chlorate are mixed together and placed under high pressure, they explode loudly. Similarly, nitrogen reacts with hydrogen under high pressure to give ammonia gas.
   \(\overset{N2}{(1 vol)} + \overset{3H2}{(3 vol)} \xrightarrow[500^oC, Fe\:and\:Mo]{200-900atm} \overset{2NH3}{(2 vols)}\)
    
4. Heat
   There are various reactions that take place only under the certain temperature. When heat is supplied to the reactants they go under reaction as heat also increases the rate for the chemical reaction.
   Example:
   There is a reaction between copper and sulphuric acid on heating that produces SO₂.
   Cu  + Conc. 2H₂SO₄ \(\xrightarrow {\Delta}\) CuSO₄ + 2H₂O
   
   
5. Light
   Light is also one of the factors that stimulate the rate of reaction. There are some reactions that undergo only in the presence of light.
   Example:
   Hydrogen reacts violently with chlorine in the presence of diffused sunlight to produce hydrogen chloride.
   H₂ + Cl₂ \(\xrightarrow {sunlight}\) 2HCl
   In the presence of sunlight, green plants manufacture food in the form of glucose using carbon dioxide and water as raw material.
                   6H­₂O + 6CO₂ \(\xrightarrow[Chlorophyll]{Sunlight}\) C₆H₁₂O₆ + 6O₂

Catalyst

A chemical substance that changes the rate of chemical reaction without undergoing any permanent changes during the course of the chemical reaction is said to be a catalyst. The process through which a catalyst changes the rate of chemical reaction is said to be catalysis. Catalyst either increases or decreases the rate of chemical reaction. Catalysts are of two types: Positive Catalyst and Negative Catalyst.

Positive catalyst is defined as the catalyst which increases the rate of chemical reaction. For example: Manganese dioxide (MnO₂), a positive catalyst increases the rate of decomposition of hydrogen peroxide.

                2H₂O₂ \(\xrightarrow {MnO_2}\) 2H₂O + O₂ ↑

Negative catalyst is defined as the catalyst which decreases the rate of chemical reaction. For example: Glycerin [C₃H₅(OH)₃],  a negative catalyst that decreases the rate of decomposition of hydrogen peroxide into water and oxygen.

                2H₂O₂ \(\xrightarrow {C­_3H_5(OH)_3}\) 2H­₂O + O₂

Properties of Catalyst

1. Catalyst does not change in its mass and chemical composition during the chemical reaction.
2. Catalyst can be in a small quantity.
3. Catalyst changes the rate of chemical reaction but does not initiate the chemical reaction.

Endothermic Reaction and Exothermic Reaction

Endothermic reaction is the reaction in which heat energy present in the surrounding is absorbed. The compounds which are formed by the absorption of heat from the surrounding are called endothermic compounds. For example:

                CaCO₃ + heat → CaO + CO₂ ↑

                2KClO₃ + heat → 2KCl + 3O₂ ↑

                N₂ + O₂ + heat → 2NO (Nitric Oxide)

Exothermic reaction is the reaction in which heat energy is released in the surrounding. The compounds that are formed by exothermic reactions are said to be exothermic compounds. For Example:

                C(s) + O₂(g) → CO_2(g) + heat

                2H₂(g) + O₂(g) → 2H₂O(l) + heat

                CaO(s) + H₂O(l) → Ca(OH)₂ (aq) slaked lime + large amount of heat

                Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) + heat

Word equation:

The chemical reaction which is expressed in terms of the word (name of reactants and products) is called word equation. For example:

Sodium + Chlorine \(\longrightarrow\) Sodium Chloride

Formula equation:

The chemical equation which is written in terms of symbols by writing their molecular formula is called formula equation. It is of two types:

1. Skeletal equation: The formula equation in which the number of ions, radicals, atoms etc. in reactant and product side is not equal then it is called skeletal equation. It is also sometimes called unbalanced equation.
   
   
   The above equation is skeletal equation as the number of one oxygen atom in reactant and product sides are not equal.
2. Balanced equation: The formula equation in which the number of atoms, radicals, ions, etc is made equal in both reactant and product side is called a balanced equation. For example:
   
   Above equation is now a balanced equation because the number of oxygen is made equal in both reactant and product by keeping the suitable number.

    

   Balancing a chemical equation

   An unbalanced chemical equation can be converted into a balanced equation by using the following steps:

   Step 1: Write the word equation.

   Potassium chlorate \( \overset{Heat}{\longrightarrow}\) Potassium chloride + oxygen

   Step 2: Convert the word equation into formula equation.

   KClO₃\( \overset{\Delta}{\longrightarrow}\)KCl +O₂

   Step 3: Count the number of atoms, ions, etc of the reactant and product and make them equal by using the suitable number in front of them.

   2KClO₃ \( \overset{\Delta}{\longrightarrow}\) 2KCl +3O₂

   Information conveyed by balanced chemical equations

   • It gives symbols, name and the molecular formula of the chemical substances involved as reactants and products.
   • It gives the relative number of atoms and molecules of reactants that take part in the chemical reaction.
   • It gives the relative number of atoms and molecules of products formed in the reaction.
   • It gives the ratio of masses of reactants and products.
   • It shows the type of chemical reaction.

   Limitations of balanced chemical equations

   • The physical states of reactants and products are not known.
   • The concentration of reactants and products, in the end, is not known.
   • The rate of chemical reaction and the time taken for its completion is not known.
   • The conditions under which chemical reaction takes place is not known.
   • Whether the heat is released or absorbed during the reaction is not known.
   • Some reactions may be explosive. It is not revealed by the chemical equation.
   • Whether the reaction is reversible or not.