This question is about the first 30 elements in the Periodic Table.
Name the element which:
(a) is 78% of clean, dry air
(b) has atoms with an electronic structure of 2,8,1
(c) is extracted from hematite
(d) forms an oxide with a giant covalent structure
(e) is the gas with the slowest rate of diffusion at room temperature
(f) has an anhydrous chloride which turns pink when water is added
(g) has aqueous ions which form a white precipitate when added to aqueous silver ions
(h) forms a blue hydroxide which dissolves in aqueous ammonia
(i) is added to molten iron to remove impurities in the steel making process
(j) is used to galvanize iron.
▶️ Answer/Explanation
(a) Ans: nitrogen
Clean, dry air is composed of approximately 78% nitrogen and 21% oxygen, with trace amounts of other gases.
(b) Ans: sodium
The electronic configuration 2,8,1 indicates 11 electrons, which corresponds to sodium (Na).
(c) Ans: iron
Hematite (Fe2O3) is a primary ore of iron, from which iron is extracted.
(d) Ans: silicon
Silicon dioxide (SiO2) has a giant covalent structure, forming a network of silicon and oxygen atoms.
(e) Ans: chlorine
Chlorine (Cl2) is the heaviest common gas at room temperature, resulting in the slowest diffusion rate.
(f) Ans: cobalt
Cobalt(II) chloride (CoCl2) is anhydrous and blue, turning pink when hydrated with water.
(g) Ans: chlorine
Chloride ions (Cl−) react with silver ions (Ag+) to form a white precipitate of silver chloride (AgCl).
(h) Ans: copper
Copper(II) hydroxide (Cu(OH)2) is blue and dissolves in aqueous ammonia to form a deep blue complex.
(i) Ans: oxygen
Oxygen is blown into molten iron to oxidize impurities like carbon and sulfur during steel production.
(j) Ans: zinc
Zinc is used to galvanize iron by coating it to prevent rusting through sacrificial protection.
Period 3 contains the elements sodium to argon. This question asks about the chemistry of each of the Period 3 elements or their compounds.
(a) Sodium nitrate is a white crystalline solid. When heated it melts and the following reaction occurs.
\(2NaNO_3(l) → 2NaNO_2(l) + O_2(g)\)
A 3.40g sample of sodium nitrate is heated. Calculate the
- number of moles of \(NaNO_3\) used,
- number of moles of \(O_2\) formed,
- volume of \(O_2\) formed, in \(dm^{3}\) (measured at r.t.p.).
(b) Magnesium reacts slowly with warm water to form a base, magnesium hydroxide.
(i) Explain what is meant by the term base.
(ii) Write a chemical equation for the reaction between magnesium and warm water.
(c) Aluminium oxide is amphoteric. It is insoluble in water. Describe experiments to show that aluminium oxide is amphoteric.
(d) Silicon(IV) oxide has a giant structure.
(i) Name the type of bonding in silicon(IV) oxide.
(ii) Give two physical properties of silicon(IV) oxide.
(e) Calcium phosphate is used in fertilisers. The bonding in calcium phosphate is ionic. Calcium phosphate contains the phosphate ion, \(PO_4^{3-}\).
(i) What is ionic bonding?
(ii) Deduce the formula of calcium phosphate.
(f) Sulfur tetrafluoride, \(SF_4\), can be made by combining gaseous sulfur with fluorine.
\(S(g) + 2F_2(g) → SF_4(g)\)
The reaction is exothermic.
(i) Complete the energy level diagram for this reaction. Include an arrow which clearly shows the energy change during the reaction.
(ii) During the reaction the amount of energy given out is 780kJ/mol. The F–F bond energy is 160kJ/mol. Use this information to determine the bond energy, in kJ /mol, of one S–F bond in \(SF_4\).
(g) Chlorine and compounds of chlorine are important in water treatment and in laboratory testing for water.
(i) Chlorine is added to water to make the water safe to drink. Explain why adding chlorine makes water safe to drink.
(ii) A compound of chlorine is used in the laboratory to test for the presence of water. Name the compound of chlorine used in this test and describe the colour change seen in a positive result of this test.
(h) Argon is an unreactive noble gas.
(i) Explain why argon is unreactive.
(ii) Give one use of argon.
▶️ Answer/Explanation
(a)
Moles of NaNO₃: \( \text{Moles} = \frac{3.40}{85} = 0.04 \) mol.
Moles of O₂: From the equation, 2 moles of NaNO₃ produce 1 mole of O₂, so \( \frac{0.04}{2} = 0.02 \) mol.
Volume of O₂: At r.t.p., 1 mole occupies 24 dm³, so \( 0.02 \times 24 = 0.48 \) dm³.
Final Answer: 0.04 mol, 0.02 mol, 0.48 dm³.
(b)(i) A base is a proton (H⁺) acceptor.
(b)(ii) \( Mg(s) + 2H_2O(l) → Mg(OH)_2(aq) + H_2(g) \).
(c) Aluminium oxide reacts with both acids (e.g., HCl) and bases (e.g., NaOH), dissolving in both to form salts and water, demonstrating its amphoteric nature.
(d)(i) Covalent bonding.
(d)(ii) High melting point and poor electrical conductivity.
(e)(i) Ionic bonding is the electrostatic attraction between oppositely charged ions.
(e)(ii) \( Ca_3(PO_4)_2 \).
(f)(i) The energy level diagram should show reactants higher than products with a downward arrow labeled “ΔH = -780 kJ/mol”.
(f)(ii) Total bond energy of \( SF_4 \): \( 780 + (2 \times 160) = 1100 \) kJ/mol. S–F bond energy: \( \frac{1100}{4} = 275 \) kJ/mol.
(g)(i) Chlorine kills bacteria, making water safe to drink.
(g)(ii) Cobalt(II) chloride; color change from blue to pink.
(h)(i) Argon has a complete outer electron shell, making it unreactive.
(h)(ii) Used in lamps for inert atmosphere.