(a)i. With the aid of an equation, explain briefly why aluminum metal is not affected by air.
(ii) In the extraction of aluminum from bauxite, state the:
- substance used for purifying the ore;
- composition of the mixture electrolyzed.
(b) ZnO is an amphoteric oxide. Write equations to illustrate this statement.
(c)i) List three uses of sodium trioxocarbonate(IV).
(ii) Explain briefly why a solution of trioxonitrate(V) acid turns yellowish on storage for some time.
(ii) Describe briefly how trioxonitrate(V) ions could be tested for in the laboratory.
(d) Write balanced chemical equations for the preparation of hydrogen chloride.
(i) using concentrated H\(_{2}\)SO\(_{4}\):
(ii) by direct combination of its constituent elements.
(iii) State one use of hydrogen chloride.
(a)
(i) Aluminum metal is not affected by air because it is covered with a thin layer of aluminum oxide (Al2O3) that protects the metal from further oxidation. The equation for the formation of aluminum oxide is:
2Al + 3O2 → 2Al2O3
The aluminum oxide layer is very stable and prevents oxygen in the air from reaching the aluminum metal, thus protecting it from further reaction.
(ii) In the extraction of aluminum from bauxite:
- The substance used for purifying the ore is concentrated sodium hydroxide (NaOH) solution, which is used to dissolve the aluminum oxide (Al2O3) present in bauxite, leaving behind impurities.
- The composition of the mixture electrolyzed is a molten mixture of aluminum oxide (Al2O3) and cryolite (Na3AlF6), which is used as a solvent to lower the melting point of Al2O3 and make it easier to extract aluminum by electrolysis.
(b) ZnO is an amphoteric oxide, which means it can react with both acids and bases. The equations to illustrate this statement are:
ZnO + 2HCl → ZnCl2 + H2O
ZnO + 2NaOH + H2O → Na2Zn(OH)4
In the first equation, ZnO reacts with hydrochloric acid (HCl) to form zinc chloride (ZnCl2) and water (H2O). In the second equation, ZnO reacts with sodium hydroxide (NaOH) and water (H2O) to form sodium zincate (Na2Zn(OH)4).
(c)
(i) Three uses of sodium trioxocarbonate(IV) (sodium carbonate, Na2CO3) are:
- It is used in the production of glass, where it acts as a flux to lower the melting point of silica (SiO2) and help it fuse into glass.
- It is used as a water softener in laundry detergents, where it reacts with hard water ions (such as calcium and magnesium) to form insoluble precipitates that can be removed from clothing.
- It is used as a food additive, where it acts as a leavening agent in baked goods to produce carbon dioxide (CO2) gas and make the dough rise.
(ii) A solution of trioxonitrate(V) acid (HNO3) turns yellowish on storage for some time because it slowly decomposes to form nitrogen dioxide (NO2) gas, which is a brownish color. The equation for this decomposition is:
4HNO3 → 2H2O + 4NO2 + O2
(iii) Trioxonitrate(V) ions (NO3-) can be tested for in the laboratory using the Brown Ring Test. In this test, a solution of the sample containing NO3- ions is mixed with freshly prepared iron(II) sulfate (FeSO4) solution and concentrated sulfuric acid (H2SO4). A brown ring will form at the junction of the two layers if NO3- ions are present, due to the formation of a complex between Fe2+ ions, NO3- ions, and H2O molecules.
(d)
(i) The balanced chemical equation for the preparation of hydrogen chloride (HCl) using concentrated sulfuric acid (H2SO4) is:
NaCl + H2SO4 → NaHSO4 + HCl
(ii) The balanced chemical equation for the preparation of hydrogen chloride (HCl) by direct combination of its constituent elements is:
H2 + Cl2 → 2HCl
(iii) One use
(a)
(i) Aluminum metal is not affected by air because it is covered with a thin layer of aluminum oxide (Al2O3) that protects the metal from further oxidation. The equation for the formation of aluminum oxide is:
2Al + 3O2 → 2Al2O3
The aluminum oxide layer is very stable and prevents oxygen in the air from reaching the aluminum metal, thus protecting it from further reaction.
(ii) In the extraction of aluminum from bauxite:
- The substance used for purifying the ore is concentrated sodium hydroxide (NaOH) solution, which is used to dissolve the aluminum oxide (Al2O3) present in bauxite, leaving behind impurities.
- The composition of the mixture electrolyzed is a molten mixture of aluminum oxide (Al2O3) and cryolite (Na3AlF6), which is used as a solvent to lower the melting point of Al2O3 and make it easier to extract aluminum by electrolysis.
(b) ZnO is an amphoteric oxide, which means it can react with both acids and bases. The equations to illustrate this statement are:
ZnO + 2HCl → ZnCl2 + H2O
ZnO + 2NaOH + H2O → Na2Zn(OH)4
In the first equation, ZnO reacts with hydrochloric acid (HCl) to form zinc chloride (ZnCl2) and water (H2O). In the second equation, ZnO reacts with sodium hydroxide (NaOH) and water (H2O) to form sodium zincate (Na2Zn(OH)4).
(c)
(i) Three uses of sodium trioxocarbonate(IV) (sodium carbonate, Na2CO3) are:
- It is used in the production of glass, where it acts as a flux to lower the melting point of silica (SiO2) and help it fuse into glass.
- It is used as a water softener in laundry detergents, where it reacts with hard water ions (such as calcium and magnesium) to form insoluble precipitates that can be removed from clothing.
- It is used as a food additive, where it acts as a leavening agent in baked goods to produce carbon dioxide (CO2) gas and make the dough rise.
(ii) A solution of trioxonitrate(V) acid (HNO3) turns yellowish on storage for some time because it slowly decomposes to form nitrogen dioxide (NO2) gas, which is a brownish color. The equation for this decomposition is:
4HNO3 → 2H2O + 4NO2 + O2
(iii) Trioxonitrate(V) ions (NO3-) can be tested for in the laboratory using the Brown Ring Test. In this test, a solution of the sample containing NO3- ions is mixed with freshly prepared iron(II) sulfate (FeSO4) solution and concentrated sulfuric acid (H2SO4). A brown ring will form at the junction of the two layers if NO3- ions are present, due to the formation of a complex between Fe2+ ions, NO3- ions, and H2O molecules.
(d)
(i) The balanced chemical equation for the preparation of hydrogen chloride (HCl) using concentrated sulfuric acid (H2SO4) is:
NaCl + H2SO4 → NaHSO4 + HCl
(ii) The balanced chemical equation for the preparation of hydrogen chloride (HCl) by direct combination of its constituent elements is:
H2 + Cl2 → 2HCl
(iii) One use
