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Chemical Equations: Writing & Balancing | Science Class 10 PDF Download

Balancing chemical equations means adding numbers to the chemicals to make sure the same number of each type of atom is on both sides. This is important because a chemical equation must obey the law of conservation of mass and the law of constant proportions, i.e. the same number of atoms of each element must exist on the reactant side and the product side of the equation. 

What is a Chemical Equation?

  • A chemical equation is a symbolic representation of a chemical reaction in which the reactants and products are denoted by their respective chemical formulae.
  • An example of a chemical equation is 2H2 + O2 → 2H2O which describes the reaction between hydrogen and oxygen to form water
  • The reactant side is the part of the chemical equation to the left of the ‘→’ symbol whereas the product side is the part to the right of the arrow symbol.
    Reactant: The substance or substances which takes part in a chemical reaction are called reactants.
    Products: The new substance or substances formed as a result of a chemical reaction are called products.
    Chemical Equations: Writing & Balancing | Science Class 10In the above reaction, P4O10 & water are reactants and Phosphoric acid is the product.

Question for Chemical Equations: Writing & Balancing
Try yourself:Why is it important to balance chemical equations?
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Rules for Writing a Word Equation

  1. The substances taking part in chemical reaction reactants are always written on the left-hand side of arrow.
  2. The substances formed after the chemical reaction, products are always written on the right hand side of arrow.
  3. A plus sign (+) is put in between the reactants or the products. If their number is two or more.
  4. An arrow (→) is put between the reactants and products, the arrow shows the direction of the reaction in which the reaction proceeds. The arrow is read as "to yield" or "to form".

In the word equation when symbols and chemical formulas of the reactants and products are used then it is called a chemical equation.

Example: Na + H2O → NaOH + H2
i.e. A chemical equation is a statement that describes a chemical reaction in terms of symbols and formulas. In this equation the law of conservation of mass is not obeying. Such chemical equation is called skeletal equation or unbalanced equation.
A chemical equation expressed in symbols and formula, such that the number of atoms of different elements towards the side of the reactants is not equal to the number of atoms of the products is called skeletal equation or unbalanced equation.
To make this equation meaningful, this equation is balanced then it is called balanced chemical equation.

Steps to Balance Chemical Equations

A balanced chemical equation is one that contains an equal number of atoms of each element on both sides of the equation. The simple equations are balanced by the "hit and trial method" which is done in the following steps.

Step 1. Count the no. of atoms of various elements on both sides of the equation

Example: Fe + H2O → Fe3O4 + H2

Chemical Equations: Writing & Balancing | Science Class 10

Step 2. Start balancing with the compound which contains maximum number of atoms. It may be a reactant or product. In that compound select the element which has maximum number of atoms.
According to this rule Fe3O4 has maximum number of atoms & oxygen has 4 atoms so it is selected.

Chemical Equations: Writing & Balancing | Science Class 10

So the partly balanced equation is
Fe + 4H2O → Fe3O4 + H2

Step 3. Fe and H are not balanced in the above reaction so the above reaction repeat the above process for both i.e.

Chemical Equations: Writing & Balancing | Science Class 10

Now the equation becomes as
Fe + 4H2O → Fe3O4 + 4H2

Step 4. Balance the Iron atoms similarly.

Chemical Equations: Writing & Balancing | Science Class 10

Now the equation is
3Fe + 4H2O → Fe3O4 + 4H2

Step 5. Finally check the correctness of the balanced equation by counting the number of atoms on both sides of the equation.

Chemical Equations: Writing & Balancing | Science Class 10

Step 6. To make chemical equation more informative physical states of the reactants and products are mentioned as for solid (s), liquid (l), gas (g) and for aqueous solution of reactant or product (aq) is written.
Now the equation becomes as
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

Symbol (g) with water is written to show that water is used in the form of steam in this equation.
If gas is evolved in a reaction it can be shown by the symbol (↑) after the formula i.e. arrow pointing upwards e.g.
2Na(s) + 2H2O(l) → 2 NaOH(aq) + H2(g) or H2 (↑)
The symbol (↓) or ppt is be written for precipitate.
NaCl + AgNO3 → NaNO3 + AgCl (↓) or ppt.
A reversible reaction is represented by the (⇌) symbol and irreversible reaction by the (→) symbol.
The heat evolved in a chemical reaction is written on the right side by putting positive (+) sign and heat absorbed in the chemical reaction is written on the right hand side by putting negative (-) sign.
N2 + 3H2 ⇌ 2NH3 + 22400 calories (Exothermic reaction)
N2 + O2 ⇌ 2NO - 43200 calories (Endothermic reaction)

Sometimes the reaction conditions, such as temperature, pressure, catalyst etc. are written above or below the arrow in the equation e.g.
N2 + 3H2Chemical Equations: Writing & Balancing | Science Class 10 2NH3 + 22400 calorie heat
450°C (Fe-Mo)

Exothermic and Endothermic Reactions:Chemical Equations: Writing & Balancing | Science Class 10

(i) Exothermic reaction: The reaction in which heat is liberated (or given out) is called an exothermic reaction.
Example: C(s) + O2(g) → CO2(g) + Heat (393 kJ/mol)
(ii) Endothermic reaction: The reaction in which heat is absorbed (or taken in ) is called an endothermic reaction.
Example: C(s) + 2S(g) → CS2(g) - Heat (92 kJ/mol)

The reaction with + Heat term on the product side is called exothermic reaction, while those with - Heat term on the product side are called endothermic reactions.
During respiration, the digested food gets oxidised and the energy is released. That is why, it is considered as an exothermic reaction.
Balancing of a chemical equation is necessary because no matter (hence, no atom) is lost or gained during a chemical reaction.

Question for Chemical Equations: Writing & Balancing
Try yourself:
How can a chemical equation be balanced?
View Solution

Writing the Symbols of Physical States of Substances

By writing the physical states of substances a chemical equation becomes more informative.

  • Gaseous state is represented by symbol ‘g’
  • Liquid state is represented by symbol ‘l’
  • Solid state is written by symbol ‘s’
  • Aqueous solution is written by symbol ‘aq’

Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.

Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.

Solved Examples


Q.1. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Ans.
(a) N2(g) + 3H2(g) Chemical Equations: Writing & Balancing | Science Class 10 2NH3(g)
(b) 2H2S + 3O2 → 2H2O + 2SO2
Chemical Equations: Writing & Balancing | Science Class 10
(d) 2K (s) + 2H2O (1) → 2KOH + H2 (g)

Q.2. Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
Ans.
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O

Q.3. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride.
Ans.

Chemical Equations: Writing & Balancing | Science Class 10
Chemical Equations: Writing & Balancing | Science Class 10
Chemical Equations: Writing & Balancing | Science Class 10
Chemical Equations: Writing & Balancing | Science Class 10

Q.4. Write the balanced chemical equation for the following and identify the type of reaction in each case -
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Ans.
(a) 2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(aq)
This reaction is a double-displacement reaction.
(b) ZnCO3(s) → ZnO(s) + CO2(g)
This reaction is a decomposition reaction

Q.5. Why should a magnesium ribbon be cleaned before burning in air?
Ans.
Magnesium reacts with the constituent gases of the atmosphere to form various compounds which get deposited over its surface. The ribbon is cleaned before burning to remove the layer of these compounds so that pure magnesium can burn in air.

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FAQs on Chemical Equations: Writing & Balancing - Science Class 10

1. What is a chemical equation?
Ans. A chemical equation is a symbolic representation of a chemical reaction, showing the reactants on the left side and the products on the right side, with the chemical formulas and coefficients of each substance involved.
2. What are the rules for writing a word equation?
Ans. The rules for writing a word equation include using appropriate chemical names for all substances, writing the reactants on the left and products on the right, and ensuring the equation is balanced in terms of the number of atoms of each element.
3. What are the steps to balance chemical equations?
Ans. The steps to balance chemical equations include writing the unbalanced equation, adjusting the coefficients of the substances to ensure the same number of each type of atom on both sides, and checking the equation to confirm it is balanced.
4. Why is it important to include the symbols of physical states of substances in a chemical equation?
Ans. Including the symbols of physical states (such as (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous) in a chemical equation provides additional information about the physical state of each substance involved in the reaction.
5. Can you provide a solved example of writing and balancing a chemical equation?
Ans. Sure. For example, the reaction between hydrogen gas (H2) and oxygen gas (O2) to form water (H2O) can be written and balanced as: 2H2(g) + O2(g) → 2H2O(l)
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