Chemical Reaction

Chemical reactions are part of our day to day life. They are happening all around us every day such as fire, cooking, and respiration and digestion in the body all involve chemical reaction. For example, the food consumed by us is broken down into energy through chemical reactions. Similarly, in respiration, we take oxygen which reacts with glucose in our body and produces carbon dioxide, water and energy as shown in the below reaction;

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

The burning of fuel in the engine that gives power to vehicles to move is also a combustion chemical reaction.

What is a chemical reaction?

In simple words, we can say that a chemical reaction is a process that involves the transformation of one set of substances or chemicals into another set of substances or chemicals. We can say that it transforms one or more substances into new substances with new properties. For example, iron reacts with oxygen gas to form iron oxide as shown below;

Fe + O₂ → FeO₂

In this case, iron oxide is a new substance formed in this reaction. It neither shows the properties of iron nor oxygen. It has its own unique properties that are different from both oxygen and iron.

Features or basic concepts of a chemical reaction

• In a chemical reaction, the molecules or ions are rearranged to form new substances or structures. So, a change in which the starting and ending substances are the same cannot be considered a chemical reaction, for example melting of ice that involves a physical change from solid to liquid is not a chemical change or reaction as it remains the same substance (H₂O). So, in a physical change, the appearance is changed but the molecular structure remains the same.
• In a chemical reaction, the atomic nucleus and its composition remain the same or untouched. Only, the electrons present around the nucleus take part in the chemical reactions. They may be transferred from one atom to another atom or may be shared between two atoms to form chemical bonds. In a nuclear reaction, the composition of the nucleus is altered.
• The number of atoms of each element remains the same before and after the chemical change (reaction) and physical change. In a chemical reaction, atoms rearrange themselves to form new products (molecules or compounds). Whereas, in a physical change the arrangement of atoms does not changes.
• The substances (elements or molecules) that react with each other in a chemical reaction are called reactants. And, the new substances (molecules) produced in a chemical reaction are called products. For example,
  CH₄ + 2O₂ → CO₂ + 2H₂O
In the above reaction, methane and oxygen are reactants, whereas, carbon dioxide and water are products.
• The reactants and products of a chemical reaction are indicated by a chemical equation. The reactants are kept on the left side and products are shown on the right side of the chemical equation and are separated by an arrow that shows the direction in which the reaction proceeds as shown below;
  C + O₂ → CO₂
  Carbon and oxygen are reactants and Carbon dioxide is the product in the above reaction.
• The speed or rate of a chemical reaction is affected by various factors such as temperature, pressure, the concentration of reactants, etc.

Signs of a chemical reaction I Identification of a chemical reaction

We cannot see the changes occurring at the molecular level in a chemical reaction. However, there are certain signs that show a chemical reaction has occurred such as a change in temperature or colour, bubbles, formation of a precipitate, and more. These changes are described as follows;

i) Evolution of a gas: There are many chemical reactions in which the evolution of gas takes place. For example;

a) When sodium carbonate is reacted with hydrochloric acid, carbon dioxide is evolved as shown below;

Na₂CO₃ + 2HCl (dilute) → 2 NaCl (aq) + H₂O (l) + CO₂ (g)

b) When Zinc reacts with dilute sulphuric acid hydrogen gas is produced, see the reaction below;

Zn + H₂SO₄ → ZnSO₄ + H₂

ii) Formation of precipitate: The formation of precipitate also takes place in most chemical reactions. For example;

a) When silver nitrate reacts with sodium chloride, a white precipitate of silver chloride is formed as shown in the below reaction;

AgNO₃ + NaCl → NaNO₃ + AgCl

b) A yellow precipitate of lead iodide is formed when there is a reaction between potassium iodide and lead nitrate.

2KI + PbNO₃ (aq) + → PbI₂ (s) + 2KNO₃

c) When sulphuric acid reacts with barium chloride a white precipitate of barium sulphate is formed.

H₂SO₄ + BaCl₂ → BaSO₄ + 2 HCl

iii) Change in colour: Titrations are commonly performed in chemistry. In titrations, the acids react with bases and its outcome is indicated by a change in colour. Apart from titrations, there are many reactions that are characterized by a change in colour. For example,

a) The purple colour of potassium permanganate disappears when it reacts with citric acid as shown below;

C₆H₈O₇ + 3KM_nO₄ → 3HMnO₄ + C₆H₅K₃O₇

b) When there is a reaction between sulphur dioxide (gas), potassium dichromate and sulphuric acid, the formation of potassium sulphate, chromium sulphate and water take place and the orange colour changes to yellow colour.

SO₂ + K₂ Cr₂ O₇ + 3H₂ SO₄ → K₂ SO₄ + Cr₂ (SO₄)3 + 3H₂ O

iv) Change in temperature: Some reactions involve a change in temperature. If the heat is evolved, it is known as an exothermic reaction and if the heat is absorbed, it is called an endothermic reaction. For example,

a) Heat is evolved, when quick lime reacts with water to form slaked lime as shown below;

CaO + H₂O → Ca (OH)₂

b) Heat is also evolved when zinc granules react with sulphuric acid as shown below;

Zn + H₂SO₄ → ZnSO₄ + H₂

c) Heat is absorbed when barium hydroxide and ammonium chloride react to form barium chloride, ammonia and water.

Ba (OH)₂ + NHCI → BaCI₂ + NH₃ + H₂O

v) Change in State: In some chemical reactions, a change in physical state occurs. For example, when ammonia gas reacts with hydrogen chloride gas, it forms crystals of ammonium chloride, which shows a change from the gaseous state to solid state. The reaction is shown below;

NH₃ (gas) + HCl (gas) → NH₄Cl (s)

Reaction Rate

It is defined as the speed at which the reactants turn into products. All chemical reactions do not occur at the same rate. Some occur quickly such as explosions and some take more time such as rusting of iron. Some even take hundreds or thousands of years such as when the plants and fishes become fossils.

There are various factors that affect the rate of a reaction, which means may increase or decrease the reaction. Some of them are described below;

i) Concentration of reactants: The rate of a chemical reaction tends to increase when the concentration of one or more reactants is increased. This is because the higher concentration of one or more reactants results in more collisions between reactants per unit time. So, the reaction rate is usually high in the beginning when the concentration of reactants is highest and it tends to slow down with time as the amount of reactants decreases.

For example, wood burns slowly in the open air, which contains 20% oxygen, but burns more rapidly in pure oxygen (100% oxygen). The higher concentration of oxygen increases the speed at which the wood burns.

ii) Temperature: It also increases the speed of a reaction. It increases the kinetic energy of reactants that causes more collision per unit time. Generally, most of the reactions (not all) occur at double speed for each 10 degree centigrade increase in temperature.

iii) Pressure: It mostly affects the rate of reactions of gaseous reactants. When pressure is increased, the molecules get less space to move that increases the number of collisions per unit area and thus speed of reaction increases. The reaction rate of solids and liquids is nearly unaffected by an increase in pressure as they are nearly incompressible.

iv) Exposure to radiations: There are certain radiations that may increase the speed of reactions. The photons of the radiations break the bonds in reactants that increase the reaction rate.

For example; the reaction of hydrogen and chlorine is very slow in the absence of light but becomes fast in the presence of light.

v) Nature of the reactants and products: The nature of substances also affect the reaction rate. The old bonds are broken and new bonds are formed in a chemical reaction. The substances whose bonds break easily will react more rapidly. So, the reaction rate in the gaseous state is more than the liquid state and in a liquid state, it is higher than in solid state. The bonds in solid do not break easily so they have a low reaction rate.

For example; the oxidation of nitric oxide to nitrogen dioxide occurs rapidly as compared to the oxidation of carbon monoxide to carbon dioxide.

2NO2 + O2 → 2NO2 (fast reaction)

2CO + O₂ → 2CO2 (slow reaction)

vi) Presence of catalyst: Catalyst is a substance that participates in a chemical reaction and affects its rate or speed without undergoing any chemical change. Catalysts provide an alternate pathway with lower activation energy and thus help speed up the reaction. Activation energy is the minimum amount of energy required to activate the reactants. There are many reactions that need catalyst to occur.

For example, H₂ does not react with O₂ at room temperature. But, when the finely divided platinum, which acts as a catalyst, is added to the mixture, the reaction becomes vigorous.

vii) Surface area: The rate of reaction increases with an increase in surface area. For example, if the surface area of a solid reactant is increased by dividing the solid into pieces, it reacts rapidly. We can say that finely divided solid reactants react faster than large substances.

For example, a log of wood burn slowly but when it is cut into pieces it burns rapidly. This is because the surface area of small pieces of wood is greater than the surface area of the log of wood.