Analysing electrolysis of molten compounds

Malaysia SPM Form 4 Chemistry, Chapter 6: Electrochemistry.

Electrolysis : Decomposing Using the Passage of Electricity

• There are 2 types of electrochemical cell, namely

1. Electrolytic Cell
2. Voltaic Cell (or sometimes is called Galvanic Cell)

• In electrolytic cell, electric current is flowed through an electrolyte to produce a chemical reaction.
• In electrolytic cell, electrical energy is converted into chemical energy, and the process is called electrolysis.

• In voltaic cell, chemical is used to produce electricity.
• In this cell, chemical energy is converted into electrical energy.

The Electrolytic Cell

• A suitable apparatus for electrolysis is shown in Figure above.
• As we can see, the electrode connected to the positive terminal of the cell is positive electrode and is given a name, anode.
• The electrode connected to the negative terminal of the cell is negative electrode and is called the cathode.

• When electricity is passed through an electrolyte, chemical reaction happens.
• In this reaction, chemical is splitting up into 2 new substances.
• All electrolytes are ionic, which means they are composed of positively and negatively charged ions.
• On passing an electric current through the electrolyte, these ions move towards the oppositely charged electrode.

• Most negatively charged ions are non-metal ions, such as oxide (O^2-, chloride (Cl^-), Iodide (I^-), etc.
• During electrolysis, negatively charged ions move towards the positive electrode(anode). The negative ions lose their electron(s) to the anode, which is positively charged.
• The electron(s) is then move to the cathode through the external circuit (the wire).
• The positively charged ions move towards the negative electrode(cathode').
• These positive ions are metal ions, such as copper (Cu²⁺), silver (Ag⁺), lead (Pb²⁺), etc, or hydrogen (H⁺).
• At cathode, positive ions gain electron(s) from the cathode, which has an excess of electrons and therefore an overall negative charge.

• This process results in the chemical decomposition of the electrolyte. It also allows electrons to travel from the cathode to the anode and hence allows conduction of electricity.
• During the electrolysis, electrical energy is supplied to the system to produce a chemical reaction.
• Therefore, during electrolysis, electrical energy convert into chemical energy.

Example 1:Electrolysis of MOLTEN Lead (II) Bromide

• This is composed of lead(II) ions, Pb^{2 +} , and bromide ions, Br^-. Its chemical formula is therefore PbBr₂.
• A suitable apparatus which could be used to carry out this electrolysis is shown in Figure above.
• The bulb helps to show when electricity is flowing in the circuit, and until the lead(II) bromide is completely molten, the bulb does not light up . This confirms that electrolytes have to be molten for the ions to start to move to the electrodes and thereby conduct electricity.

At the Cathode	At the Anode
Observation When electricity is flowing, a silvery deposit of lead metal forms on the cathode. In fact, as it is molten, it is more likely to drip off in a molten blob.	Observation When electricity is flowing, brown fumes of bromine gas are seen at the anode.
Half equation Pb2+ + 2e ---> Pb	Half equation 2Br- ---> Br2 + e
Explanation The lead(II) ions, as they are positive, move to the negative cathode, where each ion gains two electrons to form a lead atom. Any reaction at a cathode involved is again in electrons. This is called reduction or more exactly, cathodic reduction .	Explanation The bromide ions, as they are negative, move to the positive anode, where each loses an electron to form a bromine atom. Then two of these newly formed atoms combine to form bromine gas. Any reaction at an anode involves a loss of electrons.

• In summary, the lead(II) bromide is split into its component elements :
  

PbBr₂ ---> Pb + Br₂

Example 2: Electrolysis Of Molten Lead(II) Oxide