Class 10 CHEMICAL EQUATIONS

Topics to be covered

=> Chemical equations
=> Balanced and unbalanced chemical equation
=> Balancing chemical equation

𝐂𝐇𝐄𝐌𝐈𝐂𝐀𝐋 𝐄𝐐𝐔𝐀𝐓𝐈𝐎𝐍𝐒

This description of a chemical reaction in a sentence form is quite long.Therefore we can express it in a shorter form using a word-equation. For eg:

color{red}(underset("(Reactants )")("Magnesium + Oxygen") → underset("(product)")("Magnesium oxide") ) ...............(1.1)

color{green}("𝐑𝐞𝐚𝐜𝐭𝐚𝐧𝐭:") Substances which take part in a chemical reaction are called reactants. For eg: In above reaction Magnesium and oxygen are reactants.

color{green}("𝐏𝐫𝐨𝐝𝐮𝐜𝐭:") New substance formed after chemical reaction are called product. For eg: In above reaction magnesium oxide is the product.

color{green}("𝐂𝐡𝐞𝐦𝐢𝐜𝐚𝐥 𝐞𝐪𝐮𝐚𝐭𝐢𝐨𝐧:") Representation of chemical reaction using symbols of substances is called chemical equation. For eg: color{red}(Mg + O_2 → MgO) ................(1.2)

• A word-equation shows change of reactants to products through an arrow placed between them.

• 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.

𝐁𝐀𝐋𝐀𝐍𝐂𝐄𝐃 𝐀𝐍𝐃 𝐔𝐍𝐁𝐀𝐋𝐀𝐍𝐂𝐄𝐃 𝐂𝐇𝐄𝐌𝐈𝐂𝐀𝐋 𝐄𝐐𝐔𝐀𝐓𝐈𝐎𝐍

color{green}("𝐔𝐧𝐛𝐚𝐥𝐚𝐧𝐜𝐞𝐝 𝐜𝐡𝐞𝐦𝐢𝐜𝐚𝐥 𝐞𝐪𝐮𝐚𝐭𝐢𝐨𝐧:") If the number of atoms of each element in reactants is not equal to the number of atoms of each element in the product , then the chemical equation is called unbalanced chemical equation. For eg:

color{red}(Fe + H_2O → Fe_3O_4 + H_2) .............................(1.3)

color{green}("𝐁𝐚𝐥𝐚𝐧𝐜𝐞𝐝 𝐜𝐡𝐞𝐦𝐢𝐜𝐚𝐥 𝐞𝐪𝐮𝐚𝐭𝐢𝐨𝐧:") When the number of atoms of each element on reactant side is equal to number of atoms of each element on the product side, then the chemical equation is called balanced chemical equation.For eg:

color{red}( Zn + H_2SO_4 → ZnSO_4 + H_2) .....................(1.4)

Here in this equation number of atoms of zinc, hydrogen and sulphate are equal on both the sides. So it is a balanced chemical equation.

According to the law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction. That is, 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.

𝐁𝐀𝐋𝐀𝐍𝐂𝐈𝐍𝐆 𝐓𝐇𝐄 𝐂𝐇𝐄𝐌𝐈𝐂𝐀𝐋 𝐄𝐐𝐔𝐀𝐓𝐈𝐎𝐍

𝐒𝐭𝐞𝐩 𝐈 : To balance a chemical equation, first draw boxes around each formula. Do not change anything inside the boxes while balancing the equation.

color{red}(Fe + H_2O → Fe_3O_4 + H_2) ...............(1.5)

𝐒𝐭𝐞𝐩 𝐈𝐈: List the number of atoms of different elements present in the unbalanced equation (1.5).

𝐒𝐭𝐞𝐩 𝐈𝐈𝐈: 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 color{red}(Fe_3O_4) and the element oxygen in it. There are four oxygen atoms on the RHS and only one on the LHS.

To balance the oxygen atoms –

To equalise the number of atoms, it must be remembered that we cannot alter the formulae of the compounds or elements involved in the reactions. For example, to balance oxygen atoms we can put coefficient ‘4’ as color{red}(4 H_2O) and not color{red}(H_2O_4) or color{red}((H_2O)_4). Now the partly balanced equation becomes –

color{red}(Fe + 4 H_2O → Fe_3O_4 + H_2) ..................(1.6)

𝐒𝐭𝐞𝐩 𝐈𝐕: color{red}(Fe) and color{red}(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 equalise the number of color{red}(H) atoms, make the number of molecules of hydrogen as four on the RHS.

The equation would be –

color{red}(Fe + 4 H_2O → Fe_3O_4 + 4 H_2) .............................(1.7)

𝐒𝐭𝐞𝐩 𝐕: 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.

To equalise color{red}(Fe), we take three atoms of color{red}(Fe) on the LHS.

color{red}(3 Fe + 4 H_2O → Fe_3O_4 + 4 H_2) ..................(1.8)

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

color{red}(3Fe + 4 H_2O → Fe_3O_4 + 4H_2) ....................(1.9)

The numbers of atoms of elements on both sides of Eq. (1.9) are equal. This equation is now balanced. This method of balancing chemical equations is called hit-and-trial method as we make trials to balance the equation by using the smallest whole number coefficient.

𝐒𝐭𝐞𝐩 𝐕𝐈𝐈: Writing Symbols of Physical States Carefully examine the above balanced Eq. (1.9). Does this equation tell us anything about the physical state of each reactant and product? No information has been given in this equation about their physical states.

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 Eq. (1.9) becomes

color{red}(3Fe(s) + 4 H_2O (g) → Fe_3O_4 (s) + 4 H_2 (g)) ...............(1.10)

Note that the symbol (g) is used with color{red}(H_2O) to indicate that in this reaction water is used in the form of steam.

Usually physical states are not included in a chemical equation unless it is necessary to specify them.

The reaction conditions, such as temperature, pressure, catalyst, etc., for the reaction are indicated above and/or below the arrow in the equation. For example –

color{red}(CO(g) + 2H_2(g) overset(340"atm")→ CH_3OH (l)) ............(1.11)

color{red}(6 CO_2 (aq) + 12 H_2O (l) underset("Chlorophyll") overset("Sunlight")→ underset("(Glucose)")(C_6H_(12)O_6 (aq)) + 6O_2 (aq) + 6H_2O(l)) ..................(1.12)