Table of contents | |
What is a Chemical Reaction? | |
Chemical Equations | |
Types of Chemical Reactions | |
Have You Observed the Effects of Oxidation Reaction in Everyday Life? |
In our daily lives, physical changes involve alterations in appearance or state without creating new substances, like melting ice. On the other hand, chemical changes result in the formation of entirely new substances, such as cooking food or rusting iron. Recognizing these changes helps us understand when a chemical reaction occurs.
A chemical reaction is a process in which one or more substances, the Reactants, are converted to one or more different substances, the Products.
Chemical Equation is representation of chemical reaction using symbols and formulae of the substances. A chemical equation represents a chemical reaction.
The substances that undergo chemical change in the reaction, Magnesium and Oxygen, are the Reactants. When magnesium burns in air, it combines with oxygen to form magnesium oxide.
Chemical equations can be made more concise and useful if we use chemical formulae instead of words. Example: The chemical formula for water is H2O, which indicates that it is made up of two hydrogen atoms and one oxygen atom.
Representation of a chemical reaction in terms of symbols and chemical formulae of the reactants and products is known as a chemical equation.
The reactants are on the left (LHS) of the arrow, while the products are on the right (RHS). A plus sign (+) links the different reactants and products together.
A balanced account of a chemical transaction is a complete chemical equation, which symbolically depicts the reactants, products, and their physical states.
The Law of Conservation of Mass states that in a chemical reaction, atoms can't be created or destroyed. This means that the total number of atoms for each element in the starting materials (reactants) must be the same as in the end products, keeping the overall mass the same.
(ii) Balanced chemical equation
A chemical equation is considered balanced when the number of atoms for each element on the reactant side is identical to the product side.
Steps for Balancing Chemical Equations
Here are the steps to write a balanced chemical equation: Fe + H2O → Fe3O4 + H2
Step 1: Write the number of atoms of elements present in reactants and in products in a table as shown here.
Step 2: Balance the atom which is maximum in number on either side of a chemical equation.
Start by balancing the number of oxygen atoms, which has the maximum count on the right-hand side (RHS).
To balance oxygen, multiply the number of oxygen atoms on the left-hand side (LHS) by 4.
Fe + 4 × H2O → Fe3O4 + H2
Step 3: Balancing oxygen created an imbalance in the number of hydrogen atoms. There are now eight hydrogen atoms on the LHS and two on the RHS.
To balance hydrogen, multiply the number of hydrogen atoms on the RHS by 4.
Fe + 4 × H2O → Fe3O4 + 4 × H2
Step 4: There is only one iron atom on the LHS and three on the RHS.
To balance iron, multiply the number of iron atoms on the LHS by 3.
3 × Fe + 4 × H2O → Fe3O4 + 4 × H2
The equation is now balanced, and the same number of atoms of each element is present on both sides.
After balancing, the above equation can be written as follows:
3Fe + 4H2O → Fe3O4 + 4H2
Considering various factors, chemical reactions are classified into several categories.
This is the equation of a combination reaction.
Example:
(i) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.
CaO(s) + H2O(l) → Ca(OH)2 (aq) + Heat
(ii) A solution of slaked lime produced by the reaction is used for white-washing walls. Calcium hydroxide reacts with carbon dioxide in the air to form calcium carbonate. Calcium carbonate is formed after two to three days of whitewashing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO3.
Note: Reactions that release heat when products are formed are known as exothermic chemical reactions.
Other Examples of Exothermic Reactions
(a) Burning of Coal: When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen.
C(s) + O2 (g) → CO2 (g)
(b) Formation of H2 and O2
2H2 (g) + O2 (g) → 2H2O (l)
(c) Burning of natural gas
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
Note: Do you know that respiration is an exothermic process?
We need energy to live, and we get it from the food we eat. When we digest food, it's broken down into simpler forms. For instance, rice, potatoes, and bread have carbohydrates, which are turned into glucose. This glucose then mixes with oxygen in our cells to produce energy.
C6H12O6 (aq) + 6O2 (aq) → 6CO2 (aq) + 6H2O (l) + energy
There are three types of decomposition reactions:
A decomposition reaction carried out by heating is called thermal decomposition
Example 1: Heating of Ferrous Sulphate
Ferrous sulphate crystals (FeSO4. 7H2O) lose water when heated and the green colour of ferrous sulphate crystals fades. It then decomposes to ferric oxide (Fe203), sulphur dioxide (SO2) and sulphur trioxide (SO3). Ferric oxide is a solid, while S02 and SO3 are gases.
Example 2: Decomposition of Calcium Carbonate
Heating calcium carbonate breaks it down into calcium oxide (lime) and carbon dioxide. This reaction is important in industries, with calcium oxide being used in cement production.
Example 3: Heating of Lead Nitrate Powder
where light is needed for the reaction. An example is the photolytic decomposition of H2O2.
Photolytic Decomposition
Example: Effect of sunlight on Silver Chloride
White silver chloride turns grey in sunlight. This occurs due to the decomposition of silver chloride into silver and chlorine when exposed to light.
where electricity is necessary for the reaction. An example is the electrolytic decomposition of H2O. Electrolysis of water is the decomposition into oxygen and hydrogen gas due to an electric current passed through the water.
Note: What form of energy is causing these decomposition reactions?
Decomposition reactions need energy to break down the substances involved. This energy can come from heat, light, or electricity. These types of reactions, where energy is taken in rather than given off, are called endothermic reactions.
Some Important Reactions are:
(i) Reaction of iron nails with copper sulphate solution.
Fe (s) + CuSO4(aq) → FeSO4(g) + Cu (s)
The iron nail becomes brownish in colour and the blue colour of copper sulphate solution fades. In this reaction, iron has displaced or removed another element, copper from the copper sulphate solution.
(ii) Reaction of lead with copper chloride solution.
Pb (s) + CuCl2(aq) → PbCl2 (aq) + Cu (s)
Note: Zinc and lead are more reactive elements than copper. They displace copper from its compounds.
Example: When the solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.
Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + NaCl (aq)
Note: Double Displacement Reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reactions
(i) Exothermic Reaction: An exothermic reaction is a type of chemical reaction in which energy is released from the reaction system into the surroundings, usually in the form of heat, light, or sound.
Example: Formation of Carbon dioxide
The chemical reaction can be depicted as:
C(s) + O2(g) → CO2(g) + Heat
(ii) Endothermic Reaction: An endothermic reaction is a type of chemical reaction in which energy is absorbed from the surroundings in the form of heat, light, or electricity.
Example: Photosynthesis
Plants absorb heat energy from sunlight to convert carbon dioxide and water into glucose and oxygen.
Reaction as: 6CO2 + 6H2O → C6H12O6 + 6O2
(i) Oxidation is the process in which an atom, ion, or molecule loses one or more electrons. During oxidation, the oxidation state of the atom or molecule increases, since it becomes more positive or less negative.
Some examples of oxidation include the reaction of iron with oxygen to form rust and the reaction of glucose with oxygen in cellular respiration.
(ii) Reduction is the opposite process of oxidation, in which an atom, ion, or molecule gains one or more electrons.
Some examples of reduction include the reaction of silver ions with electrons to form silver metal, and the reaction of hydrogen ions with electrons to form hydrogen gas.
Oxidation of Copper to Copper Oxide
The surface of copper powder becomes coated with black copper(ll) oxide because oxygen is added to copper and copper oxide is formed. If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place, and copper is obtained. During this reaction, the copper(ll) oxide loses oxygen and is reduced. The hydrogen gets oxygen and is oxidized.
Examples:
ZnO + C → Zn + CO
MNO2 + 4HCl → MnCl2 + 2H2O + Cl2
4Fe(s) + 3O2(from air) + xH2O(moisture) → 2Fe2O3.xH2O(rust)
Corrosion of copper:
Cu(s) + H2O(moisture) + CO2(from air) → CuCO3.Cu(OH)2(green)
Corrosion of silver:
Ag(s) + H2S (from air) → Ag2S(black) + H2(g)
Rusting
85 videos|437 docs|75 tests
|
1. What is a chemical reaction and how can it be identified? |
2. How do you write and balance a chemical equation? |
3. What are the main types of chemical reactions? |
4. Can you provide everyday examples of oxidation reactions? |
5. Why is it important to study chemical reactions and equations in chemistry? |
|
Explore Courses for Class 10 exam
|