Question

SECTION A (Answer all questions in this section)
1(a)(i) Define an acid according to the Lewis concept.
(ii) Give one example of a Lewis acid.
(b) Explain salting out in soap preparation.
(c) State the reagent and condition necessary for the conversion: H–C≡C–H → Ag–C≡C–Ag
(d) What is the percentage abundance of an isotope?
(e)(i) Why does the element with atomic number 18 not have an oxide?
(ii) Explain why chlorine (I) oxide has a low melting point.
(f) Describe a test to distinguish between concentrated HNO3 and concentrated H2SO4.
(g) State two differences between an electrochemical cell and an electrolytic cell.
(h) How does the trend in ionization energy affect the reactivity of group 1 elements?
(i) Define the term molecular formula.
(j)(i) State which of the gases H2 and NH3 would deviate more from ideal behaviour.
(ii) Give reasons for the answer stated in 1(j)(i).

Accepted Answer

(a)(i) According to Lewis concept, an acid is a species (atom, ion or molecule) capable of accepting a pair of electrons.
(ii) Example of a Lewis acid: AlCl3 (other examples include BF3, PCl5).

(b) Salting out in soap preparation is the addition of salt (NaCl) to the reaction mixture to enhance the separation/precipitation of soap from the mixture.

(c) Reagent: Ammoniacal silver trioxonitrate(V) [AgNO3/NH3(aq)]. Condition: heat.

(d) Percentage abundance of an isotope is the fraction of a given isotope present in a mixture of isotopes of the same element, expressed as a percentage.

(e)(i) The element with atomic number 18 (argon) has completely filled outermost electron shells, making it stable and unreactive (does not readily form an oxide).
(ii) Chlorine (I) oxide has a low melting point because of the presence of weak (van der Waals) forces between its molecules, so not much heat is required to break these forces.

(f) Test to distinguish conc. HNO3 from conc. H2SO4: Add copper turnings to each. Conc. HNO3 produces a brown fume (NO2) and a deep blue solution of copper (II) nitrate; conc. H2SO4 does not react with copper turnings at room temperature (or: reaction with sugar — H2SO4 chars/dehydrates sugar releasing heat, HNO3 does not).

(g) Differences between electrochemical cell and electrolytic cell:
- In an electrochemical cell, chemical energy is converted to electrical energy; in an electrolytic cell, electrical energy is converted to chemical energy.
- An electrochemical cell generates current spontaneously; an electrolytic cell requires an external power source (non-spontaneous reaction).

(h) Ionization energy decreases down group 1, so it becomes easier to lose an electron going down the group, hence reactivity of group 1 elements increases down the group.

(i) Molecular formula is a formula that shows the actual number of atoms of each element present in a molecule of a substance.

(j)(i) NH3 would deviate more from ideal gas behaviour than H2.
(ii) This is because NH3 has a larger molecular volume and stronger intermolecular forces (hydrogen bonding), causing it to deviate more from ideal gas behaviour.

WAEC Chemistry 2022 Theory — Question 1

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