Question

construction and working of daniell cell about 4 to 5 page

Answer 1

JOHN FREDRRIC DANIELL, a British chemist & meteorologist, was the inventor of the daniell cell. He invented it in 1836. He wanted to eliminate the hydrogen bubble problem which is found in the voltaic pile, and his solution was to use a second electrolyte to consume the hydrogen produced by the first. The Daniell cell was a great invention over the existing technology used in the early days ofbattery development. The Daniell cell was the first truly practical and reliable electric battery that supported many nineteenth century electrical innovations such as the telegraph. A later variant of the Daniell cell called the GRAVITY CELL or CROWFOOT CELL was invented in the 1860s by a Frenchman named Callaud and became a popular choice forelecctrical telegraphy.

WHAT IS DANIELL CELL? :

The Daniell cell consists of a zinc rod dipping in a solution of zinc sulphate, connected by a wire to a copper rod dipping in a copper sulphate (II) solution. Spontaneous oxidation and reduction reactions generate electric current, with electrons passing from the zinc rod to the wire and from it to the copper rod, originating a current along the wire. During the functioning of the battery the following transformations are observed: the zinc rod goes under corrosion and its mass reduces; consequently, concentration of ions Zn2+ increases in the half cell; the copper rod receives a deposit of molecules of metal and its mass increase. Consequently, the concentration of ions Cu2+ in the solution decreases. The functioning of the battery can be demonstrated by a bulb (that shows electric current passing through the wire) and by the colour of the solution in the cathode cell. Dissolved in water copper sulphate produces a blue coloured solution. During the functioning of the battery, this substance disappears and the liquid becomes colourless.

Reaction example:

Zn(s)→ Zn2+(aq)+ 2e-(cathode).

Cu2+(aq)+ 2e-→ Cu(s)(anode).

The total reaction being:

Zn(s)+Cu2+(aq)→ Zn2+(aq) + Cu(s).

Daniellcellis a kind of Cu-Zn battery which employs a porous

WORKING OF A DANIELL CELL:

The reactions at the electrodes furnish charges that allow the battery to produce electrical current for extended periods. In the Daniell's Cell, the copper strip attracts electrons from the zinc strip. These electrons pass through the wires of our external circuit. As the copper electrode receives electrons, free positive ions in the solution arrive to equalize the charges. Positive copper ions (Cu++) are attracted to the charged copper electrode where they receive two electrons and become neutral and deposit on the electrode in metallic form. The positive zinc ions (Zn++) move to the porous vase. For each copper atom that is deposited on the copper electrode, a zinc atom goes into solution, giving up two electrons to the zinc electrode

The reactions at the electrodes can be represented by this formula:

Zn ==> Zn+++ 2e-

Cu+++ 2e-==> Cu

These reactions result in the dissolution of zinc atoms in their ionic form, which corresponds to the deposition of copper ions in their metallic form:

Zn + Cu++==> Zn+++ Cu

The electrons made available by the zinc atoms pass through the lamp filament, produce light through the joule effect and eventually reach the copper electrode. These electrons account for the current that is produced by the battery and is used by the lamp. If we didn't have the porous vase, the Cu++ ions would go directly to the zinc electrode and pick up free electrons, thereby bypassing the external circuit and stopping the current flow through the wires and lamp. The battery would no longer work. Because the copper electrode attracts electrons from the external circuit, it is considered the positive pole of the battery.

In a battery, there is always a flow of electrons in the external circuit (the electrical circuit or device) and a corresponding flow in the internal circuit (the electrolytic circuit). Like any battery, the Daniell Cell does not last forever, but only as long as there are Cu++ions available and the zinc electrode is not consumed. In reality, the production of current diminishes as the concentration of the electrolyte bathing the zinc electrode increases and that bathing the copper electrode decreases. In fact, the positive ions produced by the zinc electrode need SO4ions to balance the charges. The exact opposite occurs in the copper solution, which becomes scarce of positive ions.

Since the electromotive force of a battery is dependant not only upon the nature it's components, but also upon the concentration of it's electrolytes, the gradient of concentrations that results from the production of electricity causes the battery to generate lower and lower voltages and currents until finally it is considered dead. At the end, Zn++ ions finally reach the copper electrode, surrounding it and blocking any further movement of Cu++ ions by polarizing the electrode.