We have learnt that ionic compounds conduct electricity when molten or aqueous. When an electric current passes through such compounds, the compounds are decomposed in a chemical reaction. This is known as electrolysis. The ionic compound is called an electrolyte. In this lesson, we will learn about the electrolysis of Molten ionic compounds Aqueous ionic compounds ( Introduction ) ( Introduction )( Molten electrolyte )( Aqueous electrolyte )( Summary )  ( Worksheet ) ( Exit )

Let us look at the electrolysis of molten lead (II) bromide. The experimental setup is shown below. Molten lead(II) bromide Carbon electrodes Heat Clay triangle Porcelain crucible ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  ( Worksheet ) ( Exit )

Electrodes are usually made of carbon or platinum, as they are unreactive or inert. They do not react with the compounds in electrolysis. ELECTRODES The electrode attached to the positive terminal of the cell. Anions are attracted to it. The electrode attached to the negative terminal of the cell. Cations are attracted to it. AnodeCathode ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  ( Worksheet ) ( Exit )

At the cathode The lead ions take electrons from the cathode to become lead atoms (reduction). Pb 2+ e-e- Pb + e-e- Pb e - Pb Lead forms molten globules at the bottom of the crucible. We say the lead ions have been discharged. ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  ( Worksheet ) ( Exit )

At the anode The bromide ions donate electrons to the anode to become bromide molecules (oxidation). The bromine is seen as a reddish-brown gas around the anode during the electrolysis. Br - Br e-e- e-e- + 2Br - Br 2 + 2e - ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  ( Worksheet ) ( Exit )

Bromide ions are attracted to the anode The overall chemical reaction is PbBr 2 Pb + Br 2 Lead ions are attracted to the cathode anode ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  cathode Pb 2+ Br - Molten lead (II) bromide ( Worksheet ) ( Exit )

Bromide ions move near to anode Lead ions move near to cathode anodecathode Pb 2+ Br - Molten lead (II) bromide ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  ( Worksheet ) ( Exit )

Bromide ions donate electrons anodecathode Pb 2+ Br - e-e- e-e- ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  Molten lead (II) bromide ( Worksheet ) ( Exit )

Bromide ions donate electrons anodecathode Pb 2+ Br - Electrons flow from anode to battery e-e- e-e- e-e- e-e- ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  Molten lead (II) bromide ( Worksheet ) ( Exit )

Bromide ions donate electrons anodecathode Pb 2+ Br - e-e- e-e- e-e- e-e- Battery pumps electrons ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  Molten lead (II) bromide e-e- ( Worksheet ) ( Exit )

Bromide ions donate electrons anodecathode Pb 2+ Br - Electrons flow from battery to cathode e-e- e-e- e-e- e-e- ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  Molten lead (II) bromide e-e- e-e- ( Worksheet ) ( Exit )

Bromide ions donate electrons anodecathode Pb 2+ Br - Electrons flow from battery to cathode Lead ions accept electrons e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  Molten lead (II) bromide e-e- e-e- e-e- ( Worksheet ) ( Exit )

Lead atoms are produced Bromine atoms combine to form bromine gas Mg Cl anodecathode Pb Br ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte )  Molten lead (II) bromide ( Worksheet ) ( Exit )

QUIZ 1 1. The electrolysis of molten iron (III) chloride yields iron metal and a gas. What is the colour of this gas produced? A. Reddish-brown. B. Colourless. C. Yellowish-green. ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte ) Click on the correct answer  Next question ( Worksheet ) ( Exit )

QUIZ 1 2. With reference to the previous question, at which electrode is the gas formed? A. Anode. B. Cathode. ( Introduction )( Aqueous electrolyte )( Summary ) ( Molten electrolyte ) ( Molten electrolyte ) Click on the correct answer  ( Worksheet ) ( Exit )

Solutions can be electrolysed using the apparatus shown below. to battery test-tubes carbon electrodes electrolyte (solution) - + ( Introduction ) ( Aqueous electrolyte ) ( Aqueous electrolyte )( Summary )( Molten electrolyte )  ( Worksheet ) ( Exit )

Let us look at the electrolysis of dilute hydrochloric acid. The experimental setup is shown below. test-tubes carbon electrodes dilute hydrochloric acid to battery - + ( Introduction ) ( Aqueous electrolyte ) ( Aqueous electrolyte )( Summary )( Molten electrolyte )  ( Worksheet ) ( Exit )

At the cathode The hydrogen ions take electrons from the electrode to become hydrogen atoms (reduction). ( Introduction )( Molten electrolyte ) ( Aqueous electrolyte ) ( Aqueous electrolyte )( Summary ) H+H+ H + e-e- H + + e - H H + H HH H + H H 2  When two of the newly formed atoms combine, hydrogen gas is produced. ( Worksheet ) ( Exit )

At the anode There are two anions in the electrolyte: hydroxide and chloride ions. Since hydroxide ions are easier to discharge, oxygen gas is produced at the anode. OO + e-e- e-e- e-e- e-e- H H O H H O OH - + 4OH - O 2 + 2H 2 O + 4e - ( Introduction )( Molten electrolyte ) ( Aqueous electrolyte ) ( Aqueous electrolyte )( Summary )  ( Worksheet ) ( Exit )

The electrolysis of dilute hydrochloric acid is will result in the production of hydrogen gas and oxygen at the cathode and anode respectively. dilute hydrochloric acid oxygen carbon electrodes - + hydrogen ( Introduction )( Molten electrolyte ) ( Aqueous electrolyte ) ( Aqueous electrolyte )( Summary )  ( Worksheet ) ( Exit )

What gas do you think will be found at the anode when aqueous copper(II) sulphate is electrolysed? Sulphur dioxide? No, not quite. Oxygen gas is evolved at the anode instead and copper metal is deposited at the cathode. How do you explain this phenomenon? The products can come from the electrolyte or from the water present. The product that is discharged depends on the nature of the ions. ( Introduction )( Molten electrolyte ) ( Aqueous electrolyte ) ( Aqueous electrolyte )( Summary )  ( Worksheet ) ( Exit )

Negative ions from the electrolyte are discharged if they are chloride, bromide or iodide ions. For sulphates and nitrates, oxygen from water is discharged. Positive ions from the electrolyte that are below Ni 2+ (aq) in the electrochemical (or reactivity) series are discharged at the negative cathode. If the positive ions are those of reactive metals above Ni 2+ (aq) (e.g.. Na +, K + and Ca 2+ ), hydrogen gas from water is discharged. ( Introduction )( Molten electrolyte ) ( Aqueous electrolyte ) ( Aqueous electrolyte )( Summary )  ( Worksheet ) ( Exit )

K + (aq) Na + (aq) Ca 2+ (aq) Mg 2+ (aq) Zn 2+ (aq) Fe 2+ (aq) Pb 2+ (aq) H + (aq) Cu 2+ (aq) Ag + (aq) SO 4 2- (aq) NO 3 - (aq) Cl - (aq) Br - (aq) I - (aq) OH - (aq) SO 4 2- (aq) AnionsCations Difficulty of discharge decreases ( Introduction )( Molten electrolyte ) ( Aqueous electrolyte ) ( Aqueous electrolyte )( Summary )  ( Worksheet ) ( Exit )

QUIZ 2 1. When we electrolyse a solution, what takes place at the cathode? A. Oxidation. B. Reduction. ( Introduction )( Molten electrolyte ) ( Aqueous electrolyte ) ( Aqueous electrolyte )( Summary )  Click on the correct answer Next question ( Worksheet ) ( Exit )

QUIZ 2 2. During the electrolysis of sulphuric acid, hydrogen gas and another gas are produced. What is the other gas? A. Oxygen. B. Sulphur dioxide. ( Introduction )( Molten electrolyte ) ( Aqueous electrolyte ) ( Aqueous electrolyte )( Summary )  Click on the correct answer ( Worksheet ) ( Exit )

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The decomposition of a substance by electricity is called electrolysis. An electrolyte is an ionic compound, in the molten or aqueous state, that conducts electricity and is decomposed by the current. The rods through which the direct current enters and leaves the cell are known as electrodes. Electrodes are usually inert. ( Introduction )( Molten electrolyte )( Aqueous electrolyte ) ( Summary ) ( Summary )  ( Worksheet ) ( Exit )

The anode is the electrode which is connected to the positive terminal of a cell. Anions are attracted to it. Oxidation occurs at the anode. The cathode is the electrode connected to the negative terminal of the cell. Cations are attracted to the cathode. Reduction occurs at this electrode. ( Introduction )( Molten electrolyte )( Aqueous electrolyte ) ( Summary ) ( Summary )  ( Worksheet ) ( Exit )

When a molten electrolyte is electrolysed, a metal (from the positive ions) is discharged at the cathode. a non-metal (from the negative ions) is discharged at the anode. ( Introduction )( Molten electrolyte )( Aqueous electrolyte ) ( Summary ) ( Summary )  ( Worksheet ) ( Exit )

When a aqueous electrolyte is electrolysed, the products come from either the electrolyte or water present. The product at the cathode is a metal or hydrogen gas. The product at the anode is a non-metal. ( Introduction )( Molten electrolyte )( Aqueous electrolyte ) ( Summary ) ( Summary )  ( Worksheet ) ( Exit )

Reactive metals are not discharged at the cathode. Instead, hydrogen from water is evolved. Sulphate and nitrate ions are not discharged at the anode. Instead, oxygen from water is produced. ( Introduction )( Molten electrolyte )( Aqueous electrolyte ) ( Summary ) ( Summary )  ( Worksheet ) ( Exit )

Redesigned and edited by Adeline Yong Yean Pin Supervised by Mr Ashley Tan Prepared by Anita Anusha and Tang Yun Kwee Rosemary 20 October 1999 DIVISION OF INSTRUCTIONAL SCIENCE national institute of education copyright 1999 return to micro lessons ( Introduction )( Molten electrolyte )( Aqueous electrolyte )( Summary ) ( Exit )  Credits ( Worksheet )