Cells and Batteries Review Exercises

4.7 Cells and Batteries Review Exercises

1. State the difference between primary and secondary cells.
2. Make a sketch of a zinc-acid cell and label its parts.
3. Explain how polarization and local action are prevented in the Leclanché cell.
4. Draw a diagram of a Leclanché dry cell and label the parts.
5. State two important features of the lithium air.
6. State the three factors which determine the e.m.f. of a cell.
7. Use schematic symbols to show how four dry cells are connected so as to provide double the voltage of one cell.
8. Explain the difference between cells and batteries.
9. State three reasons why lithium ion battery is preferred to power a smartphone than lead-acid battery.

Cells and Batteries Key Points

4.6 Cells and Batteries Key Points

1. An electric cell consists of two electrodes and an electrolyte.
2. Primary cells cannot be recharged, while secondary cells are rechargeable.
3. The zinc-acid cell is not commercially produced because it suffers from polarization and local action.
4. Polarization in cells is the process during which gas bubbles form on an electrode thereby insulating it from the chemical reactions of the cell.
5. Local action is the formation of local cells on the zinc electrode due to iron or lead impurities.
6. The Leclanché cell is the most commonly used wet or dry cell. It consists of a zinc cathode, a carbon anode, sal ammoniac electrolyte and a depolarizer.
7. The dry cell is manufactured in four sizes: D, C, AA and AAA.
8. The mercury cell is constructed small in size for powering small gadgets. It has a very high energy output for its size but due to its high mercury content it has been superseded by silver oxide, zinc air and lithium air cells.
9. Lithium air at the current has the highest shelf life and power output among the so called button or coin cells.
10. The e.m.f. of a cell depends on the materials of its electrodes and electrolyte.
11. Batteries consist of two or more cells connected together to supply large amounts of electric energy.
12. Cells are connected in series to achieve a higher effective voltage source.
13. Cells are connected in parallel to increase the effective current supply.

Secondary cells

4.5 Secondary cells

Secondary cells are rechargeable units. When they are depleted, they can be replenished by charging.

The most common secondary cells are:

1.       The lead-acid cells which make the car battery.

2.       The alkaline cells, such as nickel-cadmium cells.

3.       The lithium ion cell, which makes the cellphone battery and portable computer batteries are most recent.

Panasonic Lithium Ion Batteries

Because they are rechargeable, they are used as standby and emergency power supplies for hospitals, factories, schools, cellphones, tablets pcs and many others. They are also important for situations where higher currents are required, for example, in motor vehicles, aircrafts and mobile equipment used in remote areas.

Cells in parallel and series

4.2.3 Cells in parallel and series

Cells are connected in parallel and series to increase both the voltage and the current rating.  Below four cells are arranged in series to increase the effective to 6 volts and thereafter the series set are connected in parallel to increase effective current.  This increases current 3 times the single bank.

Activity

• How would modify the above arrangement and achieve 9.0 V?
• Draw the schematic symbol of the resultant diagram.

Parallel circuit diagram

4.4.2 Parallel circuit diagram

Parallel circuit diagram of cells

If a higher current is required than that which a single cell can supply, a number of cells are connected in parallel.  When connecting cells in parallel, all the positive terminals are interconnected and all the negative terminals are connected to each other. This has the effect of adding the surface areas of the cell electrodes. The larger surface area can allow more chemical action and, therefore, more electric energy.

Connecting the cells in parallel increases the current rating, but the voltage remains the same if the cells are identical.  If however, the cells have different Emfs then effective Emf of the connection is equivalent to the highest value of the arrangement as show in the parallel arrangement below.

Parallel circuit diagram of cells2

Cells connected in series

4.4.1 Cells connected in series

This is done to increase the effective voltage output of the battery.  The battery output is the arithmetic sum of each individual cell.

Cells connected in series

When four cells each of 1.5 volts are connected in series, the total voltage is (4 x 1.5 V) or 1.5 + 1.5 + 1.5 + 1.5 = 6V.

Circuit Diagram Symbols

4.4 Circuit Diagram Symbols

In a circuit diagram, the cells are represented by the symbols shown below. 

Electric cell symbol

Two cells in series symbol

Cells in series symbol

The long, thin stroke represents the positive electrode; while the short, thick stroke represents the negative electrode of the cell. These are conventional symbols and the polarity marks (+ or -) may be omitted.

Most electric cells source are 1.5 volts when new. This electric potential is too weak to deliver a large amount of electric energy. When this is needed, two or more cells are used. For example, a torch requires two size D cells to give enough light. But more powerful torches require three or four such units. Similarly, small radios require three size D cells, while the large ones require as many as twelve.

In principle, the term battery refers to more than one cell when used or connected together. Consequently, the two dry cells in a torch form a battery just as the six dry cells in the radio. The individual dry cell does not make a battery.

At times, manufacturers connect many cells and house them together to form powerful batteries. The 12-volt car battery consists of six lead-acid cells connected and housed in the same casing. In doing this the battery has enough power to start a car engine. Individual cells or batteries may be connected in various ways to achieve the required voltage and current supply.

Lithium air battery

4.2.7 Lithium air battery

Lithium air battery offers the best energy-to-size ratio compared to mercury, silver oxide and zinc air so far.  Similar in physical appearance as the named cells.

Button cell

Merits

It has a very long shelf life of up to 5 years to reduce capacity to 90% before being de-energized,

It has high e.m.f. of about 3.8 volts

Uses

Backup batteries for low-power computer memories, commonly known as CMOS battery

Portable devices where they are soldered into their circuits to last several years.

Nb. This battery should not be confused with the lithium ion battery used in cell phones which are secondary battery.  This is covered in KCSE Electricity Form 3 course work under storage batteries.

Voltage and Current Capacity of Cells

4.3 Voltage and Current Capacity of Cells

The e.m.f. of a cell depends on the materials of its electrodes and the electrolyte. On the other hand, the amount of current an electric cell can produce in a given span of time depends on the quality of electrodes and the electrolyte it is made of.

The table below shows the e.m.f. of some electric cells and the materials they are made of. 

	Cell	e.m.f.	Positive Electrode	Negative Electrode	Electrolyte
1	Simple cell (Zinc-acid)	1.5	Copper	Zinc	Dilute sulphuric acid
2	Leclanché	1.5	Carbon	Zinc	Sal ammoniac
3	Mercury	1.35	Steel	Zinc	Potassium hydroxide
4	Silver oxide	1.5	Silver oxide	Zinc	potassium or sodium hydroxide
5	Zinc air	1.4	Oxygen gas	Zinc	potassium hydroxide
6	Lithium air	3.8	Oxygen gas	Lithium	Organic carbonate
7	Lead-acid	2.2	Lead dioxide	Lead	Dilute sulphuric acid
8	Nickel-cadmium	1.25	Cadmium	Nickel hydroxide	Potassium hydroxide
9	Lithium ion	3.6	Lithium cobalt oxide	carbon	Lithium salt in organic solvent

Zinc-air cell

4.2.6 Zinc-air cell

Looks similar to mercury cell but have about twice the capacity. It uses air in its chemical sequences, and are usually supplied sealed; pulling a tab off energizes the cell by letting air in. 

Zinc-air cell

Merits

It has a very long storage or shelf life before being de-energized,

It has a high e.m.f. of about 1.4 volts, and

It is relatively cheap.

Silver oxide cell

4.2.5 Silver oxide cell

Similar in appearance to mercury cell, silver oxide cell provides a very stable and slightly higher e.m.f., 1.5 volts as compared to 1.35 volts of the mercury cell. Silver oxide cell’s cathode is made of silver oxide, with potassium or sodium hydroxide, both strong alkalis, as the electrolyte. The capacity of the silver oxide cell, size for size, is considerably better than that of the mercury cell. 

Silver oxide cell

Because silver is expensive, this cell’s application is thus limited for applications where size and stability of voltage is of supreme importance, such as in watches and hearing aids.

Mercury Cell

4.2.4 Mercury Cell

This cell is also m common use and consists of:

an outer steel Container as the positive electrode,

a cylinder of compressed zinc powder in the centre of the cell as the negative electrode

potassium hydroxide as the electrolyte surrounding the zinc plate, and

manganese dioxide and carbon as the depolarizing mixture.

The cell is constructed so small that it is commonly used in watches, hearing aids, and miniature calculators. Its voltage is 1.35 V.

Mercury cell

The most important features of the mercury cell are:

It can be stored for long periods without losing its charge through local action.

It has a very high energy output for its size

Its drawbacks are:

It is toxic due to high mercury content

It is expensive

This cell has been replaced with a safer silver oxide cell.

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.

Atomic Structure Definition Key Points

3.4 Atomic Structure Definition Key Points

1.       Matter is anything that occupies space and has mass.

2.       The smallest particle of matter that can take part in a chemical reaction is called an atom.

3.       An atom consists of protons, neutrons and electrons.

4.       The protons and neutrons form the nucleus of the atom.

5.       Protons and neutrons are collectively known as nucleons.

6.       The atomic number of an element indicates the number of protons in the nucleus of its atom.

7.       The number of electrons in the outermost orbit is the valence electrons.

8.       Electrons carry a negative charge; while protons carry a positive charge. Charged atoms or molecule are called ions.

9.       Most metals contain free electrons which, at room temperatures, roam the spaces between the atoms.

10.    In most substances, atoms team up to form molecules.

11.    Electrons moving in a given direction give rise to an electric current. An electric circuit is a closed path through which an electric current may flow.

12.    Electrically, materials can be grouped as:

a.        conductors,

b.       semi-conductors, and

c.        insulators.

13.    Conductors are those materials which have free electrons. The most common conductors are made of copper and aluminium.

14.    Semi-conductors have few free electrons at low temperatures, but have many free electrons at higher temperatures.

15.    The most commonly used semi-conductors are silicon and germanium. They are used for making diodes, transistors and integrated circuits.

16.    Insulators have no free electrons and do not conduct an electric current.