Chemical Cells

Question 1

Explain why unused zinc-carbon cells become useless after they have been stored for several years?

Answer 1

During storage, zinc cup of the cell reacts with the H+(aq) in electrolyte. Zn(s) will be used up eventually even it is not being used.

Zn(s) + H+(aq) →Zn2+(aq) + H2(g)

Question 2

Use of 1. manganese (IV) oxide MnO2
2. powdered graphite
in zinc-carbon cells

Answer 2

1. At graphite (cathode), ——
   (NH4+(aq) accepts e- to form NH3(g) and H2(g).)
   (Eq: NH4+(aq) + 2e- → NH3(g) + H2(g)—–)

• MnO2(s) act as an oxidizing agent which accept e-
  →to removes H2(g)
  →H2(g) is produced & accumulated on surface of graphite rod, as insulator
  +which ↓cell reactions & current produced.
  Eq: 2MnO2(s) + H2(g) → Mn2O3(s) + H2O(l)
  →H2O(l) produced around the graphite rod, ↑area be wet → √conducting

2.↑ electrical conductivity of the mixture

Question 3

Overall equation in zinc-carbon cell

Answer 3

Zn(s) + 2NH4+(aq) + 2MnO2(s) → Zn2+(aq) + 2NH3(aq) + Mn2O3(s) + H2O(l)

Question 4

Use of salt bridge

Answer 4

1. Provide ions→ balance charges in solution of half cells

2. Complete circuit← Allow ions move from 1 electrolyte into the other