Types Of Battery - Primary cell & Secondary cell - Practice Questions & MCQ

Primary Cells
In such cells, redox reaction occurs only once so cells can not be recharged again. The cell becomes dead after some time as electrode reactions cannot be reversed. For example, dry cells and mercury cells.

• Dry Cell
  • It is a compact form of the Lechlanche cell.
  • It has anode of Zn-container and cathode of graphite rod surrounded by MnO₂ + carbon.
  • Here a paste of NH₄Cl and ZnCl₂ is filled in between the electrodes.
    
    Cell Reactions
    At anode:
    $\mathrm{Zn}(\mathrm{s}) \rightarrow \mathrm{Zn}^{+2}(\mathrm{aq})+2 \mathrm{e}^{-}$
    At cathode:$2 \mathrm{MnO}_2(\mathrm{~s})+2 \mathrm{NH}_4^{+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Mn}_2 \mathrm{O}_3(\mathrm{~s})+2 \mathrm{NH}_3(\mathrm{~g})+\mathrm{H}_2 \mathrm{O}$
  • Zn⁺² combines with NH₃ to form diammine Zn(II) cation.
  • Dry cell has short life as NH₄Cl (acidic) corrodes the Zn-container even if the cell is not in use.
  • The cell potential is 1.25 to 1.5 volt
     
• Mercury Cell
  • In commonly used mercury cell the reducing agent is zinc and the oxidizing agent is mercury(II) oxide.
    
    Cell Reactions
    At anode:
    $\mathrm{Zn}(\mathrm{Hg})+2 \mathrm{OH}^{-} \rightarrow \mathrm{ZnO}(\mathrm{s})+\mathrm{H}_2 \mathrm{O}+2 \mathrm{e}^{-}$
    
    At cathode:
    $\mathrm{HgO}+\mathrm{H}_2 \mathrm{O}+2 \mathrm{e}^{-} \rightarrow \mathrm{Hg}(l)+2 \mathrm{OH}^{-}$
    
    The overall reaction is:$\mathrm{Zn}(\mathrm{Hg})+\mathrm{HgO}(\mathrm{s}) \rightarrow \mathrm{ZnO}(\mathrm{s})+\mathrm{Hg}(l)$
  • The cell potential is approximately 1.35 V and remains constant throughout its life as overall reaction does not involve any ion, whose concentration can change during its life time. It is used in hearing aids, watches etc.         

It can be recharged by passing current to use again as electrode reactions are reversible. Example, lead storage battery, Ni-Cd storage cell. Electrochemical cell used as battery. The voltage provided by the battery is sum of individual voltage of cells.

Types of Batteries: Batteries are of following types:

• Lead Storage Batteries: Six cells are connected in series, each cell is provided 2V so the total volt provided by the battery is 12V. The anode, a series of lead grids packed with spongy lead and cathode, a series of grids packed with lead dioxide 38% by weight H₂SO₄ act as an electrolyte.
  
  
  
  Cell reaction
  At anode:
  $\begin{aligned} & \mathrm{Pb}(\mathrm{s})+\mathrm{HSO}_4^{-}(\mathrm{aq}) \rightarrow \mathrm{PbSO}_4(\mathrm{~s})+2 \mathrm{H}^{+}+2 \mathrm{e}^{-} \\ & \mathrm{E}^{\circ}=0.296 \mathrm{~V}\end{aligned}$
  
  At cathode:
  $\begin{aligned} & \mathrm{PbO}_2(\mathrm{~g})+3 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{HSO}_4^{-}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow 2 \mathrm{PbSO}_4(\mathrm{~s})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \\ & \mathrm{E}^{\circ}=1.628 \mathrm{~V}\end{aligned}$
  Net Reaction
  $\begin{aligned} & \mathrm{Pb}(\mathrm{s})+\mathrm{PbO}_2(\mathrm{~s})+2 \mathrm{H}^{+}+2 \mathrm{HSO}_4^{-}(\mathrm{aq}) \rightarrow 2 \mathrm{PbSO}_4(\mathrm{~s})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{s}) \\ & \mathrm{E}^{\circ}=1.924 \mathrm{~V}\end{aligned}$
• Nickel-Cadmium Cell:
  A rechargeable nickel-cadmium cell is a jelly roll arrangement and separated by a layer soaked in moist sodium or potassium hydroxide.
  
  

The overall cell reaction during the discharge is 

$\mathrm{Cd}(\mathrm{s})+2 \mathrm{Ni}(\mathrm{OH})_2(\mathrm{~s}) \rightarrow \mathrm{CdO}(\mathrm{s})+2 \mathrm{Ni}(\mathrm{OH})_2(\mathrm{~s})+\mathrm{H}_2 \mathrm{O}(\mathrm{l})$