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Balancing Chemical Equations

In this page, we are going to discuss about balancing chemical equations concept. If the number of atoms of an element on both sides of a chemical reaction is equal, then the equation is said to be balanced. For Example, Potassium nitrate decomposes on heating to form Potassium nitrite and Oxygen.

KNO3 $\rightarrow$ KNO2 + O2

Now, count the number of atoms of each element present on both the sides of the equation.

Left Side Right Side
K 1 1
N 1 1
O 3 4

The oxygen atoms are unbalanced. Such an equation is called an unbalanced equation or skeleton equation.

All chemical reactions needs to be balanced. This is because, in a chemical reaction, “matter can neither be created nor destroyed”. This means that the total mass of the reactants must be equal to the total mass of the products. This is the law of conservation of mass. Equations may be balanced by the Hit and Trial method.

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How to Balance Chemical Equations?

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The steps followed to balance chemical equations are illustrated as follows:
  • Examine the number of atoms of different elements on both sides of the equation and decide which atom remains unbalanced.

Mg(s) + HCl(aq) $\rightarrow$ MgCl2(aq) + H2(g)

Left Side Right Side
Mg 1 1
Cl 1 2
H 1 2

The Cl and H atoms are unbalanced. So, we move to the next step. If all the atoms are balanced, there is no need to continue.

  • Start with the most complicated formula and use it to balance atoms other than hydrogen and oxygen. Balance hydrogen and oxygen at the end. In the above equation, MgCl2 is the most complicated formula. The equation is already balanced with respect to Mg and we can balance Cl by putting a 2 in front of HCl.

Mg + 2 HCl $\rightarrow$ MgCl2 + H2 $\uparrow $

Writing HCl2 is incorrect, since it would not be the same compound as HCl.

It is observed that hydrogen is also balanced.

  • Finally, to check the correctness of the balanced chemical equation, count the atoms of each element on both sides of equation.
Left Side Right Side
Mg 1 1
Cl 2 2
H 2 2

Since all atoms are equal, the equation is balanced and would be reported as follows:

Mg(s) + 2 HCl(aq) $\rightarrow$ MgCl2(aq) + H2(g)

  • The smallest whole number coefficients are preferred. If the coefficients are divisible by a common factor, reduce them to their lowest whole numbers.
  • Polyatomic ions can be balanced as units.

For example, if sulphate ion (SO42-) appears on both sides of the equation, treat it as a unit for balancing purposes. These are some of the principles of balancing chemical equations

Balancing Chemical Equations Examples

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$AlCl_{3}(aq) + Ca(OH)_{2}(aq) \rightarrow Al(OH)_{3} + CaCl_{2}(aq)$

Polyatomic ions that change 'partners' can be balanced as units. Hydroxide ions qualify in this case.
  • Count each type of atom or ion.
Left Side Right Side
Al 1 1
Cl 3 2
OH - 2 3
  • The most complicated formula with O and H atoms is Aluminium (III) chloride. The Al atoms are balanced. Put a 3 in front of CaCl2 and 2 in front of AlCl3 to balance the Cl atoms (six on each side). Now, put a 2 in front of Al(OH)3 to rebalance the aluminum. Now, balance calcium by placing a 3 in front of Ca(OH)2. The OH- ions are now balanced.

Information Conveyed by Chemical Equation

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In the following chemical equation, we can see that,
principles of balancing chemical equations
  • It is a highly informative equation, which is balanced.
  • It represents the physical state of the reactants and products.
  • It also illustrates the experimental conditions ie., the presence of catalyst (iron containing molybdenum), the atmosphere pressure and temperature 450oC.
  • The equation conveys that the reaction is reversible.
  • The equation can also be read as, 1 volume of nitrogen gas reacts with 3 volumes of hydrogen gas under a pressure of 900 atmospheres and at a temperature of 450oC, in the presence of catalyst iron containing molybdenum, to give 2 volumes of ammonia gas.
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