a)(i) State three differences between covalent compounds and electrovalent compounds. (ii) Two elements represented as K and L have atomic numbers of 12 and...
Question 1 Report
a)(i) State three differences between covalent compounds and electrovalent compounds.
(ii) Two elements represented as K and L have atomic numbers of 12 and 8 respectively. Write their electronic structures and state the group to which each belongs in the Periodic Table.
(iii) If an alkali metal M exists naturally as the oxide, state with reason whether or not M can be extracted by reduction of the oxide with coke.
(b)(i) What is meant by the half-life of a radioactive element?
(ii) Mention the radioactive isotope used in dating archaeological specimens.
(iii) Balance the following equation and identify Q,
\(^{28}_{13}Al\) —> \(^{24}_{13}Si + \(^{0_1Q\)
(c)(i). Give three chemical properties of metals.
(ii) State two reasons why duralumin preferred to steel in aircraft manufacture.
(iii) What term is used to describe the ability of metals to be hammered into thin sheets?
(iv) Calculate the number of mole of electrons involved in the oxidation of 2.8g of iron filings to iron (II) ions. [Fe = 56].
Answer Details
(a)(i) Differences between covalent and electrovalent compounds
| Covalent compounds | Electrovalent (ionic) compounds |
|---|---|
| Formed by sharing of electrons | Formed by transfer of electrons |
| Usually have low melting/boiling points | Usually have high melting/boiling points |
| Do not conduct electricity (non-electrolytes) | Conduct electricity when molten or in solution (electrolytes) |
| Generally soluble in organic solvents | Generally soluble in water |
(a)(ii) Electronic structures and groups
- K (atomic number 12): 2, 8, 2 — Group II.
- L (atomic number 8): 2, 6 — Group VI.
(a)(iii) No. An alkali metal is very electropositive and reactive; it holds oxygen far more strongly than carbon does, so its oxide cannot be reduced by coke (carbon). Such metals are extracted by electrolysis of their molten salts.
(b)(i) Half-life: the time taken for half of the atoms (nuclei) in a given sample of a radioactive element to decay.
(b)(ii) Carbon-14 (\(^{14}_{6}\text{C}\)).
(b)(iii) Balancing mass and charge numbers, aluminium-28 decays by beta emission to silicon-28:
\[ ^{28}_{13}\text{Al} \to\ ^{28}_{14}\text{Si} + ^{\ \ 0}_{-1}\text{Q} \]
Q is a beta particle (electron), \(^{\ \ 0}_{-1}e\).
(c)(i) Three chemical properties of metals
- React with oxygen to form basic oxides.
- React with dilute acids to liberate hydrogen gas.
- Displace less reactive metals from their salt solutions (they act as reducing agents, losing electrons).
(c)(ii) Duralumin is light (low density) yet strong, and it is resistant to corrosion.
(c)(iii) Malleability.
(c)(iv) Moles of electrons
\[ n(\text{Fe}) = \frac{2.8}{56} = 0.05\ \text{mol} \]
Fe \(\to\) Fe2+ + 2e-, so:
\[ n(e^-) = 2 \times 0.05 = 0.10\ \text{mol} \]
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