Sealed type Ni – Cd batteries (“Nicad”) are widely used in portable devices such as cordless power t — Analytical Chemistry Chemistry Question
Electrochemistry: Nicad batteries
Sealed type Ni – Cd batteries (“Nicad”) are widely used in portable devices such as cordless power tools, cellular telephones, camcorders, portable computers, etc. Ni – Cd batteries are cost–effective and have high cycle lives and excellent low or high–temperature performance. They require no maintenance and can be recharged up to 2000 times. A typical sealed type Ni – Cd cell consists of the following two half–cells:
Cd(OH)2(s) + 2 e– ⇌ Cd(s) + 2 OH– E°1 = – 0.809 V
2 NiO(OH)(s) + 2 H2O + 2 e– ⇌ 2 Ni(OH)2(s) + 2 OH– E°2 = – 0.490 V
where E°1, E°2 are standard reduction potentials at 25 °C.
Which reaction occurs at the cathode? Give the corresponding Nernst equation for the potential.
Model Answer
2 NiO(OH) (s) + 2 H2O + 2 e– ⇌ 2 Ni(OH)2 (s) + 2 OH–
E_c = E°_c - (RT / 2F) ln [OH–]^2 = –0.490 V - (RT / 2F) ln [OH–]^2
Which reaction occurs at the anode? Give the corresponding Nernst equation for the potential.
Model Answer
Cd (s) + 2 OH– ⇌ Cd(OH)2 (s) + 2 e–
E_a = E°_a - (RT / 2F) ln (1 / [OH–]^2) = 0.809 V - (RT / 2F) ln (1 / [OH–]^2)
Write a balanced equation that shows the reaction that occurs spontaneously as the cell discharges.
Model Answer
Cd(s) + 2 NiO(OH)(s) + 2 H2O ⇌ 2 Ni(OH)2 (s) + Cd(OH)2 (s)
Calculate the electromotive force E of the cell at 25 °C.
Model Answer
E = E_a – E_c = E°_a – E°_c = 0.809 V – (– 0.490 V) = 1.299 V
What is the mass of Cd contained in a cellular telephone Ni – Cd battery of a nominal capacity of 700 mAh?
Model Answer
700 mAh = 0.700 A × 3600 s = 2520 C
2520 C moles of Cd = 2520 / (2 × 96485) = 0.013 moles of Cd
m(Cd) = 0.013 × 112.4 = 1.47 g Cd