Question

a. Molar conductivity of substance “A” is 5.9 × 10³ S/m and “B” is 1 x 10^-16 S/m. Which of the two is most likely to be copper metal and why?

b. What is the quantity of electricity in Coulombs required to produce 4.8 g of Mg from molten MgCl₂? How much Ca will be produced if the same amount of electricity was passed through molten CaCl₂? (Atomic mass of Mg = 24 u, atomic mass of Ca = 40 u).

c. What is the standard free energy change for the following reaction at room temperature? Is the reaction spontaneous?

Sn(s) + 2Cu²⁺ (aq) à Sn²⁺ (aq) + 2Cu⁺ (s)

Answer 1

a. “A” is copper, metals are conductors thus have high value of conductivity.

b. Mg²⁺ + 2e^- à Mg

1 mole of magnesium ions gains two moles of electrons or 2F to form 1 mole of Mg 24 g Mg requires 2 F electricity

4.8 g Mg requires 2 x 4.8/24 = 0.4 F = 0.4 x 96500 = 38600C

Ca²⁺ + 2e^- → Ca

2 F electricity is required to produce 1 mole = 40 g Ca

0.4 F electricity will produce 8 g Ca

c. F = 96500C, n = 2,

Sn²⁺ (aq) + 2e^– → Sn(s) –0.14V

Cu²⁺ (aq) + e^- → Cu⁺ (aq) 0.15 V

E°cell = E°cathode – E°anode

= 0.15 – (-0.14) = 0.29V

ΔG° = -nFE°_cell

= -2 x 96500 x 0.29 = -55970 J/mol

ΔG° < 0

therefore, reaction is spontaneous.