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

FeSO₄ Fe₂O₃ +SO₂ + SO₃

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.

2FeSO₄ Fe₂O₃ + SO₂ + SO₃

(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.

2FeSO_4(s) Fe₂O_3(s) + SO_2(g) + SO_3(g)

Since the given example show decomposition reaction thus it require heat to happen, thus mention heat above the arrow, i.e.

2FeSO_4(s) Fe₂O_3(s) + SO_2(g) + SO_3(g)

Problem 2. Identify the reactions which show displacement /double displacement reaction.

(i) CuSO_4(aq) + 2NaOH_(aq) Cu(OH)_2(s) + Na₂SO_4(aq)

(ii) Fe_(s) + CuSO_4(aq) FeSO_4(aq) + Cu_(s)

(iii) Pb(NO₃)_2(aq)+ 2KI_(aq) PbI_2(s) + 2KNO_3(aq)

(iv) 2A1_(S) + Fe₂O_3(s) AI₂O_3(s) + 2Fe_(l)

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) MgO_(s) + Cu_(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) H_{2( g )} + Cl _{2( g )} 2HCl_(g)

(ii) ZnCO₃ ZnO_(s)+CO_2(g)

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)  

AgNO_3(aq) + NaCl_(aq) AgCl_(s) + N_aNO_3(aq)

 

(ii)

(iii)

(iv)