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) Nitrogen (N) – It constitutes 78% of Earth’s atmosphere by volume.
(b) Sodium (Na) – Its electron configuration (2,8,1) matches the given structure.
(c) Iron (Fe) – Hematite (Fe2O3) is a major ore of iron.
(d) Silicon (Si) – Silicon dioxide (SiO2) has a giant covalent structure.
(e) Chlorine (Cl2) – Heavier diatomic gases diffuse slower; chlorine is the heaviest among common gases.
(f) Cobalt (Co) – Anhydrous cobalt(II) chloride (CoCl2) is blue but turns pink when hydrated.
(g) Chlorine (Cl) – Chloride ions (Cl−) form a white precipitate (AgCl) with silver ions.
(h) Copper (Cu) – Copper(II) hydroxide (Cu(OH)2) is blue and dissolves in ammonia to form a deep blue complex.
(i) Oxygen (O2) – It oxidizes impurities like carbon and sulfur in the Bessemer process.
(j) Zinc (Zn) – Galvanization involves coating iron with zinc to prevent rusting.
The Periodic Table is a method of classifying elements.
(a) Identify the element which is in Group VI and Period 4.
(b) Calcium is in Group II and chlorine is in Group VII of the Periodic Table.
Explain, in terms of number of outer shell electrons and electron transfer, how calcium atoms and chlorine atoms form ions. Give the formulae of the ions formed.
(c) Group V chlorides are covalent molecules. The boiling points of some Group V chlorides are shown.
(i) Suggest the approximate boiling point of \(PCl_3\).
(ii) Explain the trend in boiling points in terms of attractive forces between particles.
(iii) Complete the dot-and-cross diagram to show the electron arrangement in a molecule of \(PCl_3\). Show outer electrons only.
(d) \(PCl_3\) reacts with chlorine, \(Cl_2\), to form \(PCl_5\). This reaction is exothermic and reaches an equilibrium.
\(PCl_3(g) + Cl_2(g) \leftrightarrow PCl_5(g)\)
(i) Describe two features of an equilibrium.
(ii) State the effect, if any, on the position of this equilibrium when the following changes are made. Explain your answers.
(iii) Explain, in terms of particles, what happens to the rate of the forward reaction when the reaction mixture is heated.
(e) \(PCl_5\) reacts with lithium fluoride, LiF, to form \(LiPF_6\).
\(PCl_5 + 6LiF \rightarrow LiPF_6 + 5LiCl\)
Calculate the mass of LiF needed to form 3.04g of \(LiPF_6\) using the following steps.
- Calculate the number of moles of \(LiPF_6\) formed. [Mr: \(LiPF_6\), 152]
- Deduce the number of moles of LiF needed.
- Calculate the mass of LiF needed.
(f) Lithium fluoride has ionic bonding.
(i) What is an ionic bond?
(ii) Give two physical properties of ionic compounds.
▶️ Answer/Explanation
(a) The element in Group VI and Period 4 is selenium (Se).
(b) Calcium has 2 outer electrons and chlorine has 7. Calcium loses 2 electrons to form \(Ca^{2+}\), while chlorine gains 1 electron to form \(Cl^-\). The ionic bond forms due to electron transfer.
(c) (i) The boiling point of \(PCl_3\) is approximately 75°C (range: 72–129°C).
(ii) The boiling points increase down Group V due to stronger van der Waals forces as the molecular size increases.
(iii) The dot-and-cross diagram for \(PCl_3\) shows phosphorus with 3 bonding pairs (shared with Cl) and 1 lone pair, while each chlorine has 3 lone pairs and 1 bonding pair.
(d) (i) At equilibrium, the rates of forward and reverse reactions are equal, and concentrations remain constant.
(ii) Increasing temperature shifts equilibrium left (reverse reaction is endothermic). Increasing pressure shifts equilibrium right (fewer gas moles on the right).
(iii) Heating increases particle energy, leading to more frequent and successful collisions, thus increasing the forward reaction rate.
(e) Moles of \(LiPF_6 = \frac{3.04}{152} = 0.02\). Moles of LiF needed = \(0.02 \times 6 = 0.12\). Mass of LiF = \(0.12 \times 26 = 3.12 \text{ g}\).
(f) (i) An ionic bond is the electrostatic attraction between oppositely charged ions.
(ii) Ionic compounds have high melting/boiling points and conduct electricity when molten/dissolved.