CHEM 1405 - Chapter 9
CHEMICAL EQUATIONS CALCULATIONS
“Law of Conservation of Mass” by Antoine Lavoisier
The law states that mass are conserved in a chemical reaction. It follows that atoms are
neither created nor destroyed during any chemical reaction.
Balancing of Chemical Equation. Equalizing the number of atoms of each element on either
side of the equation is called balancing of chemical equations. Reactants are written on the
left hand side and products are written on the right hand side of the equation. Balancing of
equations is in accordance with the Law of Conservation of Mass.
For balancing, we need to adjust the stoichiometric coefficients by putting suitable
coefficients. Never change the subscripts while balancing the equation.
CH4 + 2O2
CO2 + 2H2O
2C2H6(g) +
7O2(g)
4CO2(g) + 6H2O(g)
Chemical equation must have an equal # of atoms of each element on either side.
Stoichiometry in chemical reactions.
This is the quantitative study of reactants and products in a chemical reaction.
Chemical equation gives a description of chemical reactions. There are two parts of a
chemical equation: reactants (left of the arrow) and products (right of the arrow):
2H2 + O2
H2O
Read the + sign as “reacts with” and the arrow as “produces”.
Numbers in front of the formulas are called Stoichiometric coefficients. This gives
number of moles (or molecules) of reactants and products.
Numbers in the formulas (they appear as a subscripts); give number of atoms in a
molecule.
Example: Consider the reaction of methane with oxygen:
CH4 + O2
CO2 + H2O
Atoms in the reactants: 1 C, 4 H, and 2 O.
Atoms in the products: 1 C, 2 H, and 3 O.
Quantitative information from balanced chemical Equations
Consider the balanced reaction:
2H2(g)
+
O2(g)
2H2O(g)
Moles
2 moles
1 mole
2 moles
Molecules 2(6.02 x 1023)
6.02 x 1023
2(6.02 x 1023)
Mass
4 x 1.008g
2 x 16.00g
2 x 18.02g
Volume
2L
1L
2L
Mole – Mole Relationship
2 moles hydrogen
1 mole oxygen
2 moles hydrogen
2 moles water
1 mole oxygen
2 moles water
The Limiting reactant Concept
The one or more reactants that are completely used up in a chemical reaction
are called limiting reactants.
Consider 10 H2 molecules mixed with 7 O2 molecules
According to balanced chemical equation, the stoichiometric ratio of H2 to O2 is 2 : 1
2H2(g) + O2(g)
2H2O(l)
i.e. 2 molecules of hydrogen react completely with 1 molecule of oxygen
This means that 10 H2 molecules require 5 O2 Molecules. Since we have 7 O2
molecules, the limiting reactant is Hydrogen as it is completely consumed. The O2 is
present in excess by 2 molecules.
2CO + O2
2CO2
We know that 2 moles of CO( 2 x28.01g) react with 1 mole of Oxygen (2 x 16.00g) to
form 2 moles of Carbon dioxide( 2 x 44.01g). If we mix 1 mole of CO with 2 moles of
O2, CO is completely used up in the reaction leaving O2 excess. Thus CO is the
limiting reagent.
Percent Yield
Actual yield
Percent yield
x 100
=
Theoretical yield
Theoretical yield is the amount of product that will form if all the limiting reactant
is reacted.