23. (i) CaCO3 (s) -> CaO (s) + CO2 (g) 1 mark
(ii) Because the reaction is reversible. As CO2 is formed it combines with CaO and CaCO3 is reformed CaO(s) + CO2(g) -> CaCO3(s) 1 mark
(iii) When the top of the chimney was kept opened the gas CO2 escaped and CaO was left .Thus the reaction continued in the forward direction. 1 mark
(iv) Awareness of chemical principles is helpful in industry. 1 mark
24. (a) Correct statement 1 mark
Correct definition 1 mark
(b) 6C + 3H2 -> C6H6 ∆fHO=? (1) ½ mark
C6H6 + 15/2 O2 -> 6CO2 + 3H2O ∆cHo = -3267 KJ mol-1 (2) ½ Mark
C + O2 -> CO2 ∆ fHO = -393.5 KJ mol-1 (3) ½ mark
H2+ ½ O2 -> H2O fHO = -285.83 KJ mol-1 (4) ½ mark
Eqn (3) x 6 + eqn (4) x3 – eqn (2) . we get 1 ½ mark
6C + 3H2 -> C6H6 , Δ fHO = 48.51 KJ mol-1 ½ mark
Free Energy – it is the net energy available to do useful work . 1 mark
Bond Enthalpy – Its is the change in enthalpy when one mole of covalent bond of a gaseous covalent compounds is broken to form products in the gas phase. 1 mark
∆GO = -2.303 RT log K 1 mark
= – 2.303 x 8.314 J K-1 x 300 K x log 10 1 mark
=55.27 KJ mol -1 1mark
25. (i) (a) Due to small size and high electronegativity , carbon forms Pπ- Pπ double bonds with O- atoms to form CO2 molecules .These molecules are held together by weak vander waals forces.
Du to larger size & lower electronegativity than carbon silicon forms four single covalent bonds with O-atoms which are tetrahedrally arranged around it leading to the formation of a three dimensional network structure.
( b) Diamond is the hardest substance.
(c ) on a weight to weight basis .The electrical conductivity of aluminum is twice that of copper.
(ii ) (a) 2BF3 + 6Li H -> B2H6 + 6 LiF
( b) B2H6 + 6H2O -> 2H3BO3 + 6 H2 1*3+1*2=5 mark
(a) H3BO3 – — Δ( HBO2 -Δ–(B2O3
(b ) Al2O3. 2H2O (s) + 2NaOH (aq) ( 2NaAlO2 + 3H2O
Sodium metal aluminate
(c ) CO + ZnO -> Zn + CO2
Inert pair effect – The reluctance of ns2 electron pair to participate in bond formation. Catenation – The self-linking property of atoms of an element.
26. (i) (a) Correct Chemical reaction 1 mark
(b) Correct Chemical reaction 1 mark
(c ) Correct Chemical reaction 1 mark