Manganese-alkaline cell

4.2.3 Manganese-alkaline cell

The manganese-alkaline cell is commonly available, under several brand-names, as the 'popular long life battery' e.g. trade names Energizer, Duracel etc.

Its construction is more complicated than that of a Leclanché cell as below.

Manganese-alkaline cell

The positive terminal at the top of the cell is connected to a dense layer, near the outside of the cell, consisting of compressed manganese dioxide and graphite. An absorbent separator cylinder is followed (working inwards towards the middle) by a paste of zinc mixed with potassium hydroxide. This is connected to the bottom of the cell by an internal post, riveted or welded to the bottom of the cell.

The e.m.f. of a manganese alkaline cell is 1.5V.

This cell’s capacity, or service life, is many times better than that of Leclanché cell in most applications and secondly, it can be stored for a long time, up to 3 years, without losing its original capacity.

Dry Leclanché and manganese-alkaline cells are manufactured in different sizes for use in torches, radios, remote control devices and other portable electronic gadgets as shown below.

Relative sizes of dry cell

Dry Cell

4.2.2.2 Dry Cell

The dry Leclanché cell or simply dry cell is useful because it is compact and portable. It is constructed with the same materials as the wet cell, except that the electrolyte is in the form of a paste or jelly. The zinc electrode is in the form of a can, as shown below. 

Dry cell

The chemical action is the same as in the wet cell and its e.m.f. is 1.5 V, too.

Leclanché cell

4.2.2 Leclanché cell

The most common type of primary cell in use is the Leclanché cell. In its wet form, it consists of a glass jar containing:

1. a saturated solution of sal ammoniac (ammonium chloride) as the electrolyte,
2. a zinc rod as the negative electrode, and
3. a plate of carbon as the positive electrode.

4.2.2.1 Wet Leclanché Cell

Wet Leclanché cell

A mixture of manganese dioxide and powdered carbon is pressed around the carbon rod and then enclosed in a porous pot which the solution can soak through. The manganese dioxide acts as the depolarizer, while the powdered carbon gives greater conductivity.

The e.m.f. of the Leclanché cell is 1.5 V and the internal resistance of a normal size cell is 1 Ohm. The wet cells are now going obsolete.  They were once a majorly used to supply power to land-line telephone installations in remote areas.

Defects of a simple Electric Cell

4.2.1.1 Defects of a simple Electric Cell

The practical value and performance of simple cells is limited by the following defects:

(a)          Polarization

In the reaction in the simple cell, hydrogen gas is evolved. It collects in bubbles around the positive electrode and-eventually insulate the positive electrode from the solution.  This stops the reaction. This process is known as polarization.

It is minimized by use of a depolarizer.  This is a chemical which reacts with the hydrogen to produce water e.g. manganese dioxide.

(b)          Local Action

Impurities such as iron and lead embedded in the zinc electrode form small local cells. The impurity acts as the positive electrode and zinc as the negative. The formation of these local cells between the impurities and the zinc electrode is referred to as local action. It tends to wear the zinc electrode and the electrolyte; this happens even when the cell is not in use.

Local action may be minimized by using pure zinc, but zinc in its pure state is very expensive. Instead, a cheaper option is used alloying the zinc electrode with mercury. This process is referred to as amalgamation and resultant alloy is called zinc amalgam.

Primary Cell

4.2 Primary Cell

It is not rechargeable.  After it is exhausted or depleted, it is discarded. The reason is that the chemical action that takes place in it is not reversible.

The most common primary cell types are the zinc acid cell, Leclanché cell, Manganese-alkaline cell, mercury cell, silver oxide and lithium-air cell.

4.2.1 The Zinc-acid Cell

The diagram below illustrates a zinc-acid cell also referred to as the simple cell. It consists of:

(a)        zinc as the negative electrode,

(b)        copper as the positive electrode, and

(c)        dilute sulphuric acid as the electrolyte.

Simple cell

The chemical reaction that takes place between zinc and sulphuric acid is:

Zinc + Sulphuric acid  à Zinc sulphate + Hydrogen + Electric energy.

The hydrogen gas collects in bubbles around the copper elec­trode. The e.m.f. of this cell is 1.5V. This cell has many defects and for these, it not viably produced commercially.

Cells and Batteries

4 Cells and Batteries

4.0 Introduction

Cells and batteries are portable sources of electrical energy. They are used in areas where a normal electrical supply is not available. Generally, in the rural areas, people use dry cells for their torches. Cells are of two types, the primary cell and the secondary cell.

Due to advancement in technology looking at cell does not guarantee it is primary or secondary but rather evaluating if it rechargeable or not gives a precise answer.  Therefore torch dry cells belong to the primary cells if they are not rechargeable; while the car battery is made up of secondary cells.

Secondary cells are more expensive than primary cells, but they last longer. Therefore, they are used in very remote areas for special jobs such as in cellphones, mobile radio transmitters and telephone exchanges.

When two or more cells are connected together, they form a battery. A battery is, therefore, capable of producing more electrical energy than a single cell.

Batteries are used in hospitals, laboratories and many other places to operate standby generators for providing emergency power where an electrical source of energy is essential at all times. In such places, they are called backup supplies.

Their uses come handy when normal electrical supply fails. 

4.1 Electric Cell

A cell comprises an arrangement of chemically active materials whose reaction produces electric energy when the external electric circuit is completed.

Basic parts of an electric cell are:

(a)        a positive electrode (anode),

(b)        a negative electrode (cathode),

(c)        an electrolyte (active reagent).

The electrolyte reacts with either one or both electrodes to produce electric energy. Reaction stops when the electric circuit is opened.

There are two types of cells:

Primary Cells

Secondary Cells

These are rechargeable. After they are depleted, they can be recharged by connecting them to a battery charger. In the process, a current is passed into the cell in the reverse direction. This reverses the chemical reaction.

Atomic Theory Definition Review Exercises

3.5 BASIC ATOMIC THEORY REVIEW EXERCISES      

1.       What is an atom?                                         

2.       State the characteristics of an atom.

3.       How do atoms of different elements differ?

4.       List ten elements.

5.       Define the following: Molecule, element and compound.

6.       Deduce electronic configuration and draw the atomic structure of a neutral atom whose atomic number is 14 with 14 neutrons.

7.       What is the principal characteristic of good conductors of electricity?

8.       List four good conductors, starting with the best.

9.       Explain the meaning of the term "poor conductor" and give five examples of such materials.

10.    How do insulators differ from the conductors of electricity?

11.    What are the characteristics of a pure semi-conductor?

12.    Name the two most common semi-conductor materials and state their main applications in industry.