Chemical Equations

We cover:
1. What is Chemical Equation?
2. How to write a Chemical Equation?
3. Balancing a Chemical Equation.

CHEMICAL EQUATIONS

What is Chemical Equation?

A chemical reaction is the processes by which new substances with new properties are formed. For eg. A magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide.

The important characteristics of chemical reactions are:

(i)   Evolution of a gas
(ii)  Formation of a precipitate
(iii) Change in colour
(iv)  Change in temperature
(v)  Change in state

(i) Evolution of a Gas: Some chemical reactions are characterized by the evolution of a gas. The chemical reaction between zinc and dilute sulphuric acid is characterized by the evolution of hydrogen gas.

(ii) Formation of a precipitate: Some chemical reactions are characterized by the formation of a precipitate. A precipitate is a ‘solid product’ which separates out from the solution during a chemical reaction. The chemical between potassium iodide and lead nitrate is characterized by the formation of a yellow precipitate of lead iodide.

(iii) Change in colour: Some chemical reactions are characterized by a change in colour. The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterized by a change in colour from purple to colourless.

(iv) Change in temperature: Some chemical reactions are characterized by a change in temperature. The chemical reaction between quicklime and water to form slaked lime is characterized by a change in temperature.

(v) Change in State: Some chemical reactions are characterized by a change in state. When wax is burned (in the form of wax candle), then water and carbon dioxide are formed. Now, wax is a liquid whereas carbon dioxide is a gas. This means that during the combustion reaction of wax, the physical state changes from solid to liquid and gas.

How to write a Chemical Equation?

The simplest way to do this is to write it in the form of a word-equation.

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

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

The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.

Mg + O2 → MgO

This equation is a skeletal chemical equation for the burning of magnesium in the air as the number of atoms of each element on the LHS is not equal to the number of atoms in the RHS.

Balancing a Chemical Equation

As per Law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. So, the number of atoms of each element remains the same, before and after a chemical reaction. Hence, we need to balance a skeletal chemical equation.

Let us learn about balancing a chemical equation step by step. The only thing to remember is:

Element Number of atoms in Number of atoms reactants (LHS) in products (RHS)

Let us try to balance the following chemical equation –

Fe + H2O → Fe3O4 + H2

Step I: List the number of atoms of different elements present in the unbalanced equation. To equalize the number of atoms, it must be remembered that we cannot alter the formulae of the compounds or elements involved in the reactions.

Step II: It is often convenient to start balancing with the compound that contains the maximum number of atoms. It may be a reactant or a product. In that compound, select the element which has the maximum number of atoms.

Using these criteria, we select  Fe3Oand the element oxygen in it. There are four oxygen atoms on the RHS and only one on the LHS. To balance the oxygen atoms, we can put coefficient ‘4’ as 4 H2O and not H2O4 or (H2O)4. Now the partly balanced equation becomes –

Fe + 4 H2O → Fe3O4 + H2

Step III: Fe and H atoms are still not balanced. Pick any of these elements to proceed further. Let us balance hydrogen atoms in the partly balanced equation. To equalize the number of H atoms, make the number of molecules of hydrogen as four on the RHS.

Step IV: Examine the above equation and pick up the third element which is not balanced. You find that only one element is left to be balanced, that is, iron.

Step V: Finally, to check the correctness of the balanced equation, we count atoms of each element on both sides of the equation.

3Fe + 4H2O → Fe3O4 + 4H2

The numbers of atoms of elements on both sides of Equation are equal. This equation is now balanced. This method of balancing chemical equations is called a hit-and-trial method.

Step VI: Writing Symbols of Physical States To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae. The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively. The word aqueous (aq) is written if the reactant or product is present as a solution in water.

The balanced Equation becomes

3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

Note that the symbol (g) is used with H2O to indicate that in this reaction water is used in the form of steam.

Worksheet on Balancing Chemical Equations with Answers – HERE

Download the complete NOTES  in PDF.

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