The symbolic representation of a chemical reaction is called a chemical equation. The reactants in a chemical equation are the substances that initiate the chemical reaction, and the products are the substances that are formed during the chemical reaction. Reactants are always written on the left of the equation and products on the right, with an arrow between them used to show the direction of the chemical change.
For writing a chemical equation, follow these steps:
Step 1: Identify reactants and products and write a word equation for the reaction. Represent the chemical equation as follows:
Reactants → Products
Step 2: Write the symbols and formulae of reactants and products. Indicate their physical states in parentheses. Use s for solid, l for liquid, g for gas, and aq for aqueous.
The burning of coal can be represented as follows:
Coal + Oxygen → Carbon Dioxide
C + 02 → CO2
Represent the following chemical reactions by chemical equations:
1. Burning of hydrogen (H2) to produce water.
2. Burning of magnesium (Mg) to produce magnesium oxide (MgO).
A chemical reaction only changes the arrangements of atoms. The number of atoms remains the same. Count the number of atoms of each type in the following equation:
Reactants: C + O2
Products: CO2
1 C-atom on both sides
2 O-atoms on both sides
Note that the number of atoms of each type is the same on the reactant side and the product side. Such a chemical equation is called a balanced chemical equation.
If the chemical equation is unbalanced, consider the following reaction:
Reactants: CH4 + O2
Products: CO2 + H2O
1 C-atom on both sides
4 H-atoms on the left and 2 H-atoms on the right
2 O-atoms on the left and 3 O-atoms on the right
Balance one element at a time using coefficients. Always start with the lowest coefficient. Remember that you should not change subscripts in a chemical formula.
Step 1: Put a coefficient 2 before H2O to balance H-atoms:
CH4 + O2 → CO2 + 2H2O
Step 2: Balance O-atoms. There are 4 O-atoms on the right side, so put a coefficient 2 before O2:
CH4 + 2O2 → CO2 + 2H2O
Step 3: Check the equation again. It is now balanced.
A chemical equation in which substances dissolved in water are written as individual ions is called an ionic equation. For example:
Reaction: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Write the substances that are soluble in water in their dissociated form:
H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq) → Na+(aq) + Cl–(aq) + H2O(l)
Remove common ions from both sides. These ions do not actually take part in the chemical reaction and are called spectator ions. Write the net ionic equation:
H+(aq) + OH–(aq) → H2O(l)
1. Mg + H2SO4 → MgSO4 + H2
2. H2SO4 + Ba(OH)2 → BaSO4 + 2H2O
Transform the following chemical equations into ionic equations:
1. AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
2. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)