Question

Translate the following statement into the chemical equation and then balance the equation:
(i) Phosphorus burns in oxygen to give phosphorus pentoxide.
(ii) Aluminium metal replaces iron from Ferric oxide ($F{e_2}{O_3}$) giving aluminium oxide and iron.
(iii) Carbon disulphide burns in air to give carbon dioxide and sulphur dioxide.
(iv)Barium chloride reacts with zinc sulphate to give zinc chloride and barium sulphate.

Answer 1

We have to remember that a balanced chemical equation contains the same number of atoms for each element that takes place in the chemical reaction. We have to calculate the number of atoms for a given element by multiplying the coefficient of any formula having that element by the subscript of the element in the formula. If we notice an element is seen in more than one formula on a given side of the equation, we have to compute the number of atoms and then add them together.

Complete answer:
To write the chemical equations and to later balance them, the first step is to understand the statement and write the equation using their respective symbols and follow certain rules to balance the equation. The two very important rules to be followed are as follows:
Identify each element found in the equation. The number of atoms of each type of atom must be the same on each side of the equation once it has been balanced.
The net charge must be the same on each side of the equation once it has been balanced.
Keeping the rules in mind, let us proceed in translating the given statements into chemical equations and balancing them.
(i) Phosphorus burns in oxygen to give phosphorus pentoxide.
Phosphorus + Oxygen → Phosphorus pentoxide
${P_4}(s) + {O_2}(g) \to {P_2}{O_5}(s)$Unbalanced
We know that in the reactant side there are four moles of phosphorus so in order to balance the equation we need two moles of phosphorus pentachloride on the product side. So, put 2 before phosphorus pentoxide in the product side. We can write the balanced chemical equation as,
$4P(s) + 5{O_2}(g) \to 2{P_2}{O_5}(s)$
(ii) Aluminium metal replaces iron from Ferric oxide (Fe2O3) giving aluminium oxide and iron.
Aluminium + Ferric oxide → Aluminium oxide + Iron
$Al(s) + F{e_2}{O_3}(s) \to A{l_2}{O_3}(s) + Fe(s)$(Unbalanced)
We need two moles of Aluminium on the reactant side and two moles of Fe on the product side to get a balanced chemical equation. So put 2 before Al in the reactant side and put 2 before Fe in the product side. We can write the balanced chemical equation as,
After balancing equation,
$2Al(s) + F{e_2}{O_3}(s) \to A{l_2}{O_3}(s) + 2Fe(s)$
(iii) Carbon disulphide burns in air to give carbon dioxide and sulphur dioxide.
Carbon disulphide + Oxygen → Sulphur dioxide + Carbon dioxide
$C{S_2}(s) + {O_2}(g) \to S{O_2}(g) + C{O_2}(g)\;$(Unbalanced)
We need three moles of oxygen on the reactant side and two moles of SO2 on the product side to get a balanced chemical equation. So put 3 before oxygen in the reactant side and put 2 before SO2 in the product side. We can write the balanced chemical equation as,
$C{S_2}(s) + 3{O_2}(g) \to 2S{O_2}(g) + C{O_2}(g)$
(iv) Barium chloride reacts with zinc sulphate to give zinc chloride and barium sulphate.
Barium Chloride + Zinc sulphate → Zinc chloride + Barium sulphate.
$BaC{l_2}(aq) + ZnS{O_4}(aq) \to ZnC{l_2}(aq) + BaS{O_4}(s)$
This equation is already balanced.

Note: In order to solve questions of these types, identifying the elements and the rules to balance them must be kept in mind. Not just identifying the elements will help to write the chemical formula, but knowing the valencies of each element helps to write the chemical formula, Furthermore, understanding the chemical state of the compound, i.e, whether it is in solid, aqueous or gaseous state is a bonus. We have to remember that the balancing chemical equations also helps to determine the stoichiometric relationship between the substances/elements/compounds.