Chemical Reactions and Equations – Solved Problems
Problem 1. Write down a chemical equation for the following statements and then balance it?
Ferrous sulphate crystals on heating lose water and then decompose to form ferric oxide, sulphur dioxide and sulphur trioxide.
Answer.
The above reaction can be written as
Now in order to balance this equation we have to follow, the following steps,
(i) First of all count the number of all the reactants and products.
Total no. of Fe – 1 (LHS) – 2(RHS)
Total no. of O – 4 (LHS) – 8(RHS)
Total no. of S – 1 (LHS) – 2(RHS)
(ii) As O has maximum no. of atoms, therefore start from it. To balance the no of O atoms on RHS & LHS multiply FeSO4 by 2, we get.
(iii) Now if we calculate the total no of atoms of reactants and products on both sides. We find that
Total no. of Fe on the both side —> 2
Total no. of S on the both side —- > 2
Total no. of O on the both side —> 8
Thus this is a balanced equation,
(iv) The above equation can be more informative if physical states of the reactants and products are mentioned along with their chemical formula.
2FeSO4(s)
Since the given example show decomposition reaction thus it require heat to happen, thus mention heat above the arrow, i.e.
2FeSO4(s)
Problem 2. Identify the reactions which show displacement /double displacement reaction.
(i) CuSO4(aq) + 2NaOH(aq)
(ii) Fe(s) + CuSO4(aq)
(iii) Pb(NO3)2(aq)+ 2KI(aq)
(iv) 2A1(S) + Fe2O3(s)
Answer.
Option (ii) and (iv) show displacement reaction. Since in displacement reaction one element takes the place of another element and in option (ii) & (iv) we find same whereas option (i) and (iii) show double displacement reactions. In a double displacement reaction two compounds react by exchange of ions to form two new compounds and in reaction (i) & (ii) we find same.
Problem 3. In the given reaction
CuO(s) + Mg(s)
identify the substance which
(i) is reduced
(ii) acts as oxidising agent
(iii) is oxidised
(iv) acts as reducing agent
Answer.
(i) CuO is reduced to Cu
(ii) Mg is oxidised to MgO
(iii) CuO acts as oxidising agent as it supplies O to Mg
(iv) Mg acts as reducing agent as it accepts O provided by CuO.
Problem 4. From the reactions given below.
(i) H2( g ) + Cl 2( g )
(ii) ZnCO3
Which is a combination and which is a decomposition reaction?
Answer.
(i) In this reaction H2 and Cl2 gas react to form a single product HCl thus it is a combination reaction.
(ii) In this reaction a single reactant ZnCO3 form ZnO and CO2. Thus this is a decomposition reaction.
Problem 5. Give one example of each.
i. A double displacement reaction which neither show oxidation nor reduction
ii. A combination reaction which shows oxidation and reduction
iii. A decomposition reaction which shows oxidation and reduction
iv. A displacement reaction which shows oxidation and reduction
Answer.
(i)
AgNO3(aq) + NaCl(aq)
(ii)
