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].
Explanation
(a)(i) - Covalent compounds contain only molecules while electrovalent compounds do not contain molecules - Covalent compounds usually dissolve in covalent organic Products compounds but not in water but electrovalent compounds are soluble in water - Covalent compounds are usually volatile with low melting and boiling points but electrovalent compounds usually have high melting and boiling points. (ii) \(_{12}\)K has an electronic structure of 1s\(^2\)2\(^2\)2p\(^6\)3s\(^2\); L has the structure of 1s\(^2\)2s\(^2\)2p\(^4\); belongs to group II and L to group VI (iii) Alkali metal M cannot be extracted by reduction of the oxide with oxide coke because of its high stability and reactivity.
(b)(i) The half-life of a radioactive element is the time it takes for one half of any quantity of a radioactive molecule to decay. It is therefore also equal to the time for the amount of the radioactive nuclide present to fall from one half to one quarter, from one quarter to one eight, etc. (ii) The radioactive isotope normally used in dating archaelogical specimens is carbon - 14. (iii) \(^{28}_{13}Al\) -> \(^{28}_{14} Si\) + \(^0_{-1}Q\)
(c)(i) - electropositive - form electrovalent chlorides - Replace the hydrogen in acid if more electropositive than hydrogen -Form metallic hydrides/ionic hydrides. (ii) Duralumin is preferred to steel in aircraft manufacture because duralumin is lighter and more resistant to corrosion. (iii) The ability of metals to be hammered into thin sheet is described as malleability. (iv) To calculate the number of mole of electrons involved in the oxidation of 2.8g of iron filing to Fe\(^{2+}\) + Fe —> Fe\(^{2+}\) + 2e\(^{-}\) ; 56g of Fe loses 2 mole of electrons. 2.8 of Fe would lose \(\frac{2.8 \times 2}{56}\) = 0.10 mole
