2. Simple ELECTRIC cell
graphic
A Simple cell or an Electric cell is a device that converts chemical energy into electrical energy (e.g. the battery in your torchlight converts chemical energy into electrical energy. Note: a battery consists of simple cells) Thus, do take note that the operation and uses of a simple electric cell are DIFFERENT from those of an electrolytic cell. In electrolysis, an electric current is passed into an electrolyte and we are keen to study and investigate the chemical reactions that occur in the electrolytic cell and the type of products that are formed at the electrodes.   

Case Study 1
Let's begin by performing a very simple experiment. If we were to place a copper rod and a zinc rod into dilute sulfuric acid contained in a beaker, what do you think will occur ? From your study of the Reactivity Series of metals in Secondary 3,  you would know that the zinc rod will dissolve in the dilute sulfuric acid while the copper rod will not dissolve. Zinc is a reactive metal - it will react with sulfuric acid to form zinc(II) sulfate solution and hydrogen gas. Bubbles of gas (hydrogen) will also be observed at the zinc rod. On the other hand, copper will not react with dilute sulfuric acid.
Case Study 2 - The Simple Electric Cell
If the experiment in case study 1 was repeated and if we were to connect both the copper and zinc rods by a wire (as shown by the red arrow line in the diagram above), the voltmeter registers a reading ( potential difference exists between the zinc and copper rods and a current flows) and bubbles of hydrogen gas would now appear at the copper rod instead of the zinc rod. Strange isn't it ? Well, the explanation is as follows:
a) Zinc is more reactive than copper. Thus, it is also more electropositive than copper - that means, zinc loses electrons more easily than copper. As a result, oxidation occurs at the zinc rod (the anode) and zinc metal loses electrons to become zinc ions, that is, Zn (s) - 2e
èZn2+(aq)

The electrons then flow from the zinc rod to the copper rod through the external circuit. At the copper rod, reduction occurs - the hydrogen ions in solution accept these electrons to form hydrogen gas; 2 H+(aq) +2e
è H2 (g) This explains why bubbles of gas are produced at the copper rod when the two rods are connected by a wire.

IMPORTANT   There are two points that you must take note of.

First, in a SIMPLE CELL, the MORE REACTIVE metal / electrode is ALWAYS designated the NEGATIVE electrode. Thus, in this example here, the zinc electrode is the negative electrode while the copper electrode is the positive electrode
( DO NOT GET CONFUSED - although zinc is designated the negative electrode in a simple cell, it still functions like an Anode ; Oxidation - the loss of electrons- occurs at the anode only. This fact remains )

Second, the electrons in a simple cell will ALWAYS flow from the NEGATIVE electrode (made of the MORE reactive metal) to the POSITIVE electrode. So in our example here, the electrons will flow out of the zinc rod, pass through the voltmeter V and finally, into the copper rod through the wire in the external circuit ( see diagram above - the direction of the arrow head shows the direction of the electron flow )
REMARKS
1. The magnitude(size) of the voltage(potential difference) is related to the positions of the two metals in the reactivity series. The further spaced the two metals are, the larger will be the potential difference(voltage) produced. For example, let's say that the voltage generated in our Zn-Cu cell is 2 V. If we were to replace the zinc rod with magnesium metal now, the voltage generated will perhaps be 3.5V or 4V now. However, no potential difference exists if the two rods are made of the same metal.

2. What do you think will occur if the electrolyte, dilute sulfuric acid is now replaced with copper(II) nitrate solution ? ( assume that copper and zinc rods are still used )
Answer - the reaction taking place at the zinc rod and the direction of the electron flow will remain the same. The only change would be the reaction occurring at the positive copper rod (cathode). Instead of hydrogen ions gaining electrons, copper(II) ions from the electrolyte, copper(II) nitrate solution, will now gain electrons to form metallic copper; Cu2+(aq) +2e
è Cu (s) The mass of the copper electrode increases.

QUIZ
Another type of a simple cell construction involves the use of a "salt bridge" and two different aqueous electrolytes instead of one - zinc electrode immersed in a beaker of zinc nitrate solution, copper immersed in a beaker of copper(II) nitrate solution and both electrodes connected by a wire.
1.State your observations at the electrodes, explain your observations and write ionic equations for the reactions that occur at the electrodes.
2. Explain the function of the "salt bridge".
Hint: Click on the link below for an explanation (animated tutorial)
http://www.coolschool.ca/lor/CH12/unit5/U05L01.htm
compiled by Alex Teoh / 2 Dec 2006 / 5.33 pm

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