*WAEC CHEMISTRY*

(3ai)
(PICK ANY ONE)
Enthalpy of neutralization is the heat change that occurs when one mole of water is formed by the reaction between an acid and a base in dilute aqueous solution.

OR

Enthalpy of neutralization is the enthalpy change when one mole of H? ions reacts completely with one mole of OH? ions to form one mole of water.

(3aii)
H?(aq) + OH?(aq) --> H?O(l)

(3bi)
W = Sodium propanoate (C?H?COONa)

X = Ethyl propanoate (C?H?COOC?H?)

Y = Propan-1-ol (CH?CH?CH?OH)

Z = Propene (CH?CH=CH?)

(3aii)
CH?CH?COOH + CH?CH?OH ? CH?CH?COOCH?CH? + H?O

(3ci)
(PICK ANY ONE)
An acid anhydride is an oxide of a non-metal that reacts with water to form an acid.

OR

An acid anhydride is a compound which on addition of water produces the corresponding acid.

(3cii)
(I) H?SO? --- SO?

(II) HNO? -- N?O?

(III) H?CO? -- CO?

(3di)
(PICK ANY ONE)
Aluminium is extracted from purified alumina (Al?O?) by the Hall-Héroult process. Alumina is dissolved in molten cryolite to lower its melting point and increase electrical conductivity. The molten mixture is electrolysed using carbon electrodes. During electrolysis, aluminium ions gain electrons at the cathode to form molten aluminium, while oxide ions lose electrons at the anode to form oxygen. The molten aluminium collects at the bottom of the cell and is tapped off periodically.

OR

Purified aluminium oxide is dissolved in molten cryolite and electrolysed in a steel tank lined with carbon. On passing electric current, Al³? ions are discharged at the cathode to produce aluminium metal, while O²? ions are discharged at the anode to produce oxygen gas. The molten aluminium settles at the bottom and is removed regularly.

(3dii)
Graphite electrodes

(3diii)
The carbon anode reacts with the oxygen produced during electrolysis to form CO? (and some CO), causing the anode to be gradually consumed; therefore it must be replaced periodically.

(3ei)
By Graham's law:

Rate of diffusion ? 1/?M

Given that gas X diffuses twice as fast as oxygen,

2 = ?(32/M?)

4 = 32/M?

M? = 32/4

M? = 8

Relative molecular mass of X = 8

(3eii)
Volume of gas = 5 dm³

Molar volume at s.t.p. = 22.4 dm³ mol?¹

Number of moles = 5/22.4 = 0.223 mol

Mass = number of moles × molar mass

= 0.223 × 8

= 1.79 g

Mass of gas X = 1.79 g
? 1.8 g