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CHEMISTRY FORM 2


1. STRUCTURE OF THE ATOM, AND THE PERIODIC TABLE
2. CHEMICAL FAMILIES AND PATTERNS IN PROPERTIES
3. CHEMICAL BONDING AND STRUCTURE
4. SALTS
5. EFFECT OF AN ELECTRIC CURRENT ON SUBSTANCES
6. CARBON AND SOME OF ITS COMPOUNDS
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Effect of an electric current on substances: Electrolysis

5.0 Effect of an electric current on substances


5.4 Electrolysis


So far we have learnt about the movement of electrons and ions towards electrodes. What happens to ions as they arrive at the electrodes?


electrolysis of various substances, high school chemistry , form 2

Figure 5.4(a) Set-up for electrolysis of various substances


Electrolysis of molten lead (II) bromide


Observe what happens at the electrodes as molten lead (II) bromide conducts electricity.


(courtesy Youtube-The electrolysis of lead bromide by David Read)


Questions 5.4a

  1. What are the charges on the cathode, and which particles carry them?

  2. When lead (II) ions (Pb2+) reach the cathode, they combine with the charges there and become neutralized (or discharged). Write the ionic equation for the reaction.

  3. The product deposits on the cathode. What is observed on the cathode? Name the substance responsible for this observation.

  4. When bromide (Br-) ions reach the anode, they lose their negative charge to the electrode (anode). Write the ionic equation for the reaction.

  5. What is observed on the anode?

  6. In this process, called electrolysis, molten lead (II) bromide decomposes to new substances as it conducts electricity. Define electrolysis.

  7. A substance that behaves in a similar manner as molten lead (II) bromide is called an electrolyte. Define electrolyte.


Answers to Questions 5.4a


Electric cell, as a power source, urges electrons in the solid parts of the circuit (wires and anode) to move to the cathode. This makes the cathode negative. The anode automatically becomes positive, having lost some electrons to the cathode. This flow of electrons is anticlockwise in Figure 5.4 and only occurs along the solid conductors.


Positive ions in the electrolyte are then attracted to the cathode, where they gain (combine with) the negative electrons and become neutralized (discharged). Remember, unlike charges attract and combine with each other.


Example at the cathode: Pb2+(l)  +   2e   ⟶   Pb(s)


Negative ions are attracted to the positive anode where they lose their negative charge and become neutralized.


Example at the anode: 2Br-(l)  ⟶   Br2(g)  +   2e


Overall, conduction in solids is by electrons; but in electrolytes, it is by ions (Figure 5.4(b).


Movement of ions and electrons during electrolysis,high school chemistry

Figure 5.4(b) Movement of ions and electrons during electrolysis


Analogy

Electrons (-) are like cars, which need a ferry to cross a water channel (likened to electrolyte) but move freely once on land (likened to solid conductor). In Figure 5.4(b) Br- is like a ferry-with-a-car.


Electrolysis of molten sodium chloride, copper (II) oxide and aluminium oxide


Electrolysis of molten binary electrolytes is similar to that of molten lead (II) bromide. Binary electrolytes are those consisting of two elements only (a metal and non-metal).

Questions 5.4b

  1. Complete the following table for electrolysis of molten sodium chloride, copper (II) oxide and aluminium oxide.
    high school chemistry

  2. For the electrolysis of molten sodium chloride, describe the observations made at the anode and cathode separately.
  3. For the electrolysis of molten aluminium oxide, describe the observations made at the anode and cathode separately.

Answers to Questions 5.4b


In electrolysis, the electrode used in electrolysis should be a good conductor which does not react with the electrolyte, because this would interfere with the process. For that reason, graphite is more commonly used. Such an electrode is said to be inert.


Electrolysis of aqueous solutions

Observe the demonstration of substances which conduct electricity in aqueous solutions.


Questions 5.4c

  1. Which of the following aqueous solutions conduct electricity?

    Sugar solution

    Solution of ethanol in water

    Copper (II) sulphate solution

    Sodium chloride solution

    Sodium hydroxide solution

    Sulphuric acid solution

  2. As a conclusion, which types or groups of substances conduct electricity in aqueous solutions?

  3. Does pure water conduct electricity?



Answers to Questions 5.4c


All soluble acids, bases, and salts dissociate in water to form ions; so their aqueous solutions are electrolytes. But, unlike molten electrolytes, they have more than one type of positive and negative ions (at least four) which compete for discharge at the electrodes. These will be discussed in later modules.