Chapter 7, Problem 27Q

Describe the importance of a separator in primary and secondary batteries. What would happen if the anode and cathode were allowed to touch inside a battery? Explain.

Interpretation Introduction

Interpretation:

• • The importance of a separator in primary and secondary batteries has to be described.
• • What happens if the anode and cathode were allowed to touch inside a battery has to be explained.

Concept Introduction:

Electrochemical cell is a device capable of converting chemical energy into electrical energy or induces chemical energy by electrical energy. In the galvanic cell spontaneous redox reaction takes place. Redox reaction means the reaction in which the oxidation state of the atoms changes due to the occurrence of both reduction and its complementary oxidation reactions. The element under oxidation reaction loses electron whereas the element in the reduction gains electron.

Primary battery: Primary batteries are irreversible transform chemical energy to electrical energy. When the initial supply of reactants is exhausted, energy cannot be readily restored to the battery by electrical means.

Secondary battery: This can be recharged, that is they can have their chemical reaction reversed by supplying electrical energy to the cell restoring their original composition.

Explanation of Solution

Primary battery:

These type batteries are single-used, because they cannot be recharged. A common primary battery is the dry cell. Let us consider the zinc-carbon battery the zinc can serves as both container and the negative electrode. The positive electrode is rod made of carbon that is surrounded by the paste of $Mn{O}_2$, ammonium chloride, carbon powder and small amount of water.

The reaction at the anode can be represented as the simple oxidation process.

  $Zn(s)\underset{}{\to }Z{n}^{2+}(aq)+2e^{-}$   

Cathode reaction is,

  $2Mn{O}_2(s)+2N{H}_{4}Cl(aq)+2e^{-}\stackrel{}{\to }M{n}_2{O}_3(s)+2N{H}_3(aq)+H_{2}O(l)+2C{l}^{-}$

Hence, the overall reaction is,

  $\begin{array}{l}2Mn{O}_2(s)+2N{H}_{4}Cl(aq)+Zn(s)\stackrel{}{\to }\\ Z{n}^{2+}(aq)+M{n}_2{O}_3(s)+2N{H}_3(aq)+H_{2}O(l)+2C{l}^{-}\end{array}$

If the anode and cathode were allowed to touch inside battery, the voltage delivered by a battery is the same regardless of the size of battery. The reason is D,C,A,AA and AAA batteries all have the same voltage rating. However, larger batteries can deliver more moles of electrons.

A battery is a device consisting of one or more cells that can be produce a direct current by converting by chemical energy into electrical energy.

Secondary batteries:

Nickel-Cadmium $(Ni-Cd)$ battery: This is a rechargeable battery is one in which the oxidation-reduction reaction can be reversed with the input of energy.

It is useful for powering small applications, such as garden tools and mobile phones. The basic galvanic cell in a $(Ni-Cd)$ battery contain a cadmium anode an nickel-hydroxide cathode and an alkaline electrode.

  $\begin{array}{l}Cd(s)+2O{H}^{-}(aq)\underset{}{\to }Cd{(}_O+2e^{-}---anode\\ Ni{O}_2(s)+2H_{2}O(l)+2e^{-}\stackrel{}{\to }Ni{(}_O(s)+2O{H}^{-}(aq)--cathode\\ ------------------------------\\ Cd(s)+Ni{O}_2(s)+2H_{2}O(l)\underset{}{\to }Cd{(}_O+Ni{(}_O(s)--overallreaction\\ ------------------------------\end{array}$

The positive and negative plates, which are prevent from shorting by the separator are rolled together and put into the case. This is a jelly-roll design and allows the $Ni-Cd$ cell to deliver much current than a similar-sized alkaline battery.