The Language of Chemistry — ICSE Class 9 Chemistry Important Questions

Answer: (b) $Al_2(SO_4)_3$Al_2(SO_4)_3.

Aluminium is $Al^{3+}$Al^3+ and sulphate is $SO_4^{2-}$SO_4^2-. Cross-multiplying the valencies gives two aluminium ions and three sulphate ions: $Al_2(SO_4)_3$Al_2(SO_4)_3.

Answer: (c) $Na$Na.

The symbol of sodium comes from its Latin name natrium, so it is $Na$Na. $S$S is sulphur and $Sn$Sn (stannum) is tin.

Answer: (c) $3$3.

Each hydrogen has a valency of $1$1, and three hydrogen atoms combine with one nitrogen atom. Hence nitrogen exercises a valency of $3$3 in $NH_3$NH_3.

Answer: (a) $3,4,4$3,4,4.

Balancing iron needs $3Fe$3Fe; the four oxygen atoms in $Fe_3O_4$Fe_3O_4 need $4H_2O$4H_2O; that supplies $8$8 hydrogen atoms, giving $4H_2$4H_2. Balanced equation: $3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$3Fe + 4H_2O → Fe_3O_4 + 4H_2.

Answer: (a) A balanced equation obeys the law of conservation of mass, which states that matter is neither created nor destroyed; therefore R correctly explains A.

(i) Valency: The combining capacity of an element, i.e. the number of hydrogen atoms that combine with or replace one atom of that element. Example: the valency of oxygen is $2$2 (as in $H_2O$H_2O).

(ii) Radical: An atom or a group of atoms of elements that carries a charge and behaves as a single unit in a chemical reaction. Example: the sulphate radical $SO_4^{2-}$SO_4^2-.

(i) Ammonium carbonate: ammonium is $NH_4^{+}$NH_4^+ and carbonate is $CO_3^{2-}$CO_3^2-. Criss-crossing gives $(NH_4)_2CO_3$(NH_4)_2CO_3.

(ii) Calcium hydrogen carbonate: calcium is $Ca^{2+}$Ca^2+ and hydrogen carbonate (bicarbonate) is $HCO_3^{-}$HCO_3^-. Criss-crossing gives $Ca(HCO_3)_2$Ca(HCO_3)_2.

(i) $4Al + 3O_2 \rightarrow 2Al_2O_3$4Al + 3O_2 → 2Al_2O_3

(ii) $2KClO_3 \rightarrow 2KCl + 3O_2$2KClO_3 → 2KCl + 3O_2

(iii) $2Pb(NO_3)_2 \rightarrow 2PbO + 4NO_2 + O_2$2Pb(NO_3)_2 → 2PbO + 4NO_2 + O_2

In each case the number of atoms of every element is now equal on both sides, satisfying the law of conservation of mass.

Balanced equation:

$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)\uparrow$Zn(s) + H_2SO_4(aq) → ZnSO_4(aq) + H_2(g)

Information the symbolic equation adds:

1. The exact chemical formulae of reactants and products.

2. The relative number of atoms/molecules (ratio) taking part and the physical states of each substance.

(It also shows, by the $\uparrow$, that hydrogen escapes as a gas.)

Each formula is obtained by balancing the positive and negative valencies of the constituent radicals.

(a) Information conveyed:

1. The reactants and products with their correct formulae.

2. The relative number of molecules and, hence, the mole and mass ratios of the substances.

3. The physical states and reaction conditions.

Limitations:

1. It does not tell the speed (rate) of the reaction or the time taken.

2. It does not indicate the concentration of reactants, whether the reaction is reversible, or the heat/energy change unless specially stated.

(b) $Fe_2(SO_4)_3 + 6NaOH \rightarrow 2Fe(OH)_3 + 3Na_2SO_4$Fe_2(SO_4)_3 + 6NaOH → 2Fe(OH)_3 + 3Na_2SO_4

(c) $2Pb(NO_3)_2 \xrightarrow{\Delta} 2PbO + 4NO_2\uparrow + O_2\uparrow$2Pb(NO_3)_2 → (Δ) 2PbO + 4NO_2 + O_2

(a) Variable valency is the property of certain elements of showing more than one valency in their compounds. The lower valency takes the suffix -ous and the higher takes -ic.

Iron: $Fe^{2+}$Fe^2+ (ferrous) and $Fe^{3+}$Fe^3+ (ferric). Copper: $Cu^{+}$Cu^+ (cuprous) and $Cu^{2+}$Cu^2+ (cupric).

(b)

• Ferrous chloride: $FeCl_2$FeCl_2

• Ferric chloride: $FeCl_3$FeCl_3

• Cuprous oxide: $Cu_2O$Cu_2O

• Cupric oxide: $CuO$CuO

(c) $Cu + 4HNO_3 \rightarrow Cu(NO_3)_2 + 2NO_2\uparrow + 2H_2O$Cu + 4HNO_3 → Cu(NO_3)_2 + 2NO_2 + 2H_2O

(i) Skeletal equation: $Fe_2O_3 + Al \rightarrow Fe + Al_2O_3$Fe_2O_3 + Al → Fe + Al_2O_3

(ii) Balanced equation:

$Fe_2O_3 + 2Al \rightarrow 2Fe + Al_2O_3$Fe_2O_3 + 2Al → 2Fe + Al_2O_3

(iii) Aluminium gains oxygen, so aluminium is oxidised; ferric oxide loses oxygen, so $Fe_2O_3$Fe_2O_3 is reduced.

(iv) Balancing is based on the law of conservation of mass — atoms are neither created nor destroyed, so the number of atoms of each element must be equal on both sides.