Question

A fresh zinc-air cell is weighed on an analytical balance before being placed in a hearing aid for use. (i) Asthecelloperates,doesthemassofthecellincrease,decrease,orremainthesame? (ii) Justify your answer to part (b)(i) in terms of the equation for the overall cell reaction. (c) The zinc-air cell is taken to the top of a mountain where the air pressure is lower. (i) Willthecellpotentialbehigher,lower,orthesameasthecellpotentialatthelowerelevation? (ii) Justify your answer to part (c)(i) based on the equation for the overall cell reaction and the information above. (d) Metal-air cells need to be lightweight for many applications. In order to transfer more electrons with a smaller mass, Na and Ca are investigated as potential anodes. A 1.0 g anode of which of these metals would transfer more electrons, assuming that the anode is totally consumed during the lifetime of a cell? Justify your answer with calculations. (e) The only common oxide of zinc has the formula ZnO. (i) Write the electron configuration for a Zn atom in the ground state. (ii) From which sublevel are electrons removed when a Zn atom in the ground state is oxidized?

Answer 1

i)

The chemical equations for the zinc–air cell are:

Anode: Zn + 4OH⁻ → Zn(OH)₄²⁻ + 2e⁻ (E₀ = －1.25 V)

Fluid: Zn(OH)₄²⁻ → ZnO + H₂O + 2OH⁻

Cathode: 1/2 O₂ + H₂O + 2e⁻ → 2OH⁻ (E₀ = 0.34 V pH＝11)

Overall: 2Zn + O₂ → 2ZnO (E₀ = 1.59 V)

ii)

Because the cathode does not change properties during discharge, terminal voltage is quite stable until the cell approaches exhaustion.

Power capacity is a function of several variables: cathode area, air availability, porosity, and the catalytic value of the cathode surface. Oxygen entry into the cell must be balanced against electrolyte water loss; cathode membranes are coated with (hydrophobic) Teflon material to limit water loss.

Since O2 is less at higher level,...cell potential can be lower at higher elevation

d) Na and Ca as anode: reaction will be Na > Na+ + 1 e- , Ca+2 + 2 e-

e)

i) zn has atomic no 30

its E.C is   ^,1s² 2s² 2p⁶ 3s² 3p⁶4s² 3d¹⁰.

ii) E.C of Zn is also : take this configuration: [Ar] 3d10 4s2

and then take away 2 electrons to make this

[Ar] 3d10

remember, you always take away from the number most to the right.