Wednesday, April 28, 2021

1. Chemical Reactions and Equations | Class 10 CBSE | Web Notes | Part 1 | Chemical Equations

CHEMICAL REACTIONS & EQUATIONS

Chemical reaction is a process in which one or more reactants are converted to one or more products.


During a chemical reaction, chemical change occurs.

The substances that undergo chemical change are called reactants. The new substance formed is called product.

Examples for chemical reactions

  • Burning of a clean magnesium ribbon with a dazzling white flame to form white powder (magnesium oxide). It is due to the reaction of magnesium with oxygen in air.

  • Take lead nitrate solution in a test tube. Add potassium iodide solution. A yellow precipitate of lead iodide appears at the bottom.
  • Take few zinc granules in a conical flask or test tube. Add dilute HCl or H2SO4. Bubbles are observed around zinc granules due to release of hydrogen. Conical flask becomes hot.

The following observations helps to determine whether a chemical reaction has taken place.

  • Change in state.
  • Change in colour.
  • Evolution of a gas.
  • Change in temperature.

CHEMICAL EQUATIONS


The description of a chemical reaction can be written in a shorter form. The simplest way is word-equation. E.g.

Magnesium + Oxygen → Magnesium oxide
                                                        (Reactants)                      (Product)

The reactants are written on left-hand side (LHS) with a plus sign (+) between them. Products are written on the right-hand side (RHS) with a plus sign (+) between them.

The arrowhead points towards the products, and shows the direction of the reaction.


Writing a Chemical Equation

Chemical equations can be simplified by using chemical formulae. A chemical equation represents a chemical reaction. E.g.

Mg + O2 → MgO (skeletal chemical equation)

If the number of atoms of each element is same on LHS & RHS, the equation is balanced. If not, it is unbalanced. It is called a skeletal chemical equation.


Balanced Chemical Equations

According to the law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction. i.e., total mass of the elements present in the products is equal to the total mass of the elements in the reactants.

Number of atoms of each element remains the same before and after a chemical reaction. So, skeletal chemical equation must be balanced.

E.g. Word-equation of a chemical reaction is given:

Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

It is represented by the following chemical equation:

Zn + H2SO4 → ZnSO4 + H2

Element

Number of atoms in reactants (LHS)

Number of atoms in products (RHS)

Zn

1

1

H

2

2

S

1

1

O

4

4


Thus it is a balanced chemical equation.


Steps of balancing a chemical equation

Balancing a chemical equation using least whole number coefficient is called hit-and-trial method.

The steps are given below:

Fe + H2O → Fe3O4 + H2

Step I: Draw boxes around each formula. Do not change anything inside the boxes.


Fe + H2OFe3O4 + H2

Step II: List the number of atoms of different elements.

Element

Number of atoms in reactants (LHS)

Number of atoms in products (RHS)

Fe

1

3

H

2

2

O

1

4


Step III: Select the compound (reactant or product) having maximum number of atoms (Fe3O4). In that, select the element having maximum number of atoms (oxygen).

Atoms of oxygen

In reactants

In products

(i)   Initial

(ii) To balance

1 (in H2O)

1 x 4

4 (in Fe3O4)

4


Fe + 4 H2OFe3O4 + H2

Step IV: Balance the number of hydrogen atoms.

Atoms of hydrogen

In reactants

In products

(i)   Initial

(ii) To balance

8 (in 4 H2O)

8

2 (in H2)

2 x 4


Fe + 4 H2OFe3O4 + 4 H2 (partly balanced)

Step V: Balance the number of iron atoms.


Atoms of iron

In reactants

In products

(i) Initial
(ii) To balance

1 (in Fe)
1×3

3 (in Fe3O4)
3


3 Fe + 4 H2OFe3O4 + 4 H2

Step VI: Count atoms of each element on both sides of the equation to check the correctness.


3Fe + 4H2O → Fe3O4 + 4H2 (balanced equation)

Step VII: If necessary, physical states such as gaseous (g), liquid (l), aqueous (aq) and solid (s) states are included in a chemical equation. Aqueous (aq) means the reactant or product is present as a solution in water.

3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

H2O (g) indicates that water is used in the form of steam.

Sometimes, reaction conditions (temperature, pressure, catalyst, etc.) are indicated above and/or below the arrow in the equation. E.g.


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