Post on 08-Apr-2015
Embed Size (px)
definitions of terms uses of electrolysis learn to predict products of electrolysis molten compounds aqueous solutions construct ionic equations for the rxns describe electrolysis of copper(II) sulphate, as means of purification electroplating of metals electricity from simple cells
Introducing Electrolysis Electrolysis is the decomposition of an ionic compounds by passing electricity through molten compounds or aqueous solutions of compounds. Electricity is used to produce chemical changes. The apparatus used for electrolysis is called an electrolytic cell.
Uses of Electrolysis Some important uses of electrolysis:
Terms Used in Electrolysis An electrolyte is an ionic compound which, when molten or in aqueous solution, conducts an electric current and is decomposed in the process. An electrode is a rod or plate where electricity enters or leaves an electrolyte during electrolysis. Reactions occur at the electrodes (and not inside the electrolyte). (inert/reactive) The anode is the positive electrode connected to the positive terminal of the d.c. power source. Oxidation occurs at the anode.
20.2 Introducing Electrolysis Terms Used in Electrolysis The cathode is the negative electrode connected to the negative terminal of the d.c. power source. Reduction occurs at the cathode. An anion is an ion with a negative charge. During electrolysis, it is attracted to the anode. A cation is an ion with a positive charge. During electrolysis, it is attracted to the cathode.
In electrolysis, a compound in the molten state or in aqueous solution, conducts electricity and is decomposed by it.
REVISION Sec 3 topics Properties of ionic compounds IONIC COMPOUNDS conduct electricity in molten state or when dissolved in water In these 2 states; the ions are mobile to conduct electricity but not as solids since the ions in solids are immobile
The diagram shows the electrolysis of lead(II) bromide using inert electrodes. Why does the bulb only light up when the lead(II) bromide is melted? A. Bromine atoms in lead(II) bromide are converted to ions when it is melted B. Electrons flow through the lead(II) bromide when it is melted C. The ions in lead(II) bromide are free to move when it is melted D. There are no ions in solid lead(II) bromide
Exceptions acids such as HCl and H2SO4 as they form H+ ions in water they behave as an electrolyte In general, strong acids and alkali are strong electrolytes weak acids and alkalis are weak electrolytes
In the circuit, the ammeter showed that no current was flowing. However after a few drops of dilute sulphuric acid were added to liquid X, the ammeter showed that a current was flowing. What is liquid X? A. Aqueous sodium chloride B. Aqueous sodium hydroxide C. Dilute sulphuric acid D. Distilled water E. Tetrachloromethane
Conduction of electricity by conductor: the conductor remains unchanged
Conduction of electricity by electrolyte: the electrolyte is decomposed
The experiment was set up, the bulb light up but there was no decomposition products at the electrodes. What is X? A. Aqueous sodium chloride B. Liquid bromine C. Molten sodium chloride D. Mercury
+ + + + + + ++
- - - - - - - - - - + Cathode (-) + +
Electrolysis of Molten compounds(using inert electrodes)
Lead(II) bromide PbBr2 The ions present : Pb2+ and Br In the molten electrolyte, charged ions are free to move When current is passed through, chemical changes occur at both the electrodes
Electrolysis of PbBr2
At the anode, the power supply pulls the electrons out of the anode. The anode is now positively charged. The anode attracts Br- ions. When an ion touches the anode, it gives up an electron to the anode; an atom of bromine is formed. Br Br + e 2Br- Br2(g) + 2e-
At the anode brown gas Br2
At the cathode : Power supply pushes the electrons into the cathode ; the cathode is therefore negatively charged. This attracts the Pb2+ ions. When Pb2+ ions touches the cathode, it takes 2 electrons and changes into lead atom. Pb2+ + 2e Pb(l)
At the cathode silvery liquid Pb
In general, lead(II) bromide is split into its component elements PbBr2 (l) Pb (l) + Br2 (g) observations at anode : brown gas at cathode : silvery deposit of molten lead
SUMMARY Electrolysis of molten ionic compoundsSubstance PbBr2 KI CaCl2 anode pdtBrown fumes of bromine gas Purple fumes of iodine gas Yellowish green fumes of chlorine gas
cathode pdtSilvery deposits of molten lead molten potassium Silvery deposits of molten calcium
The Electrolysis of Molten Compounds The Electrolysis of Molten Sodium Chloride At the cathode(-): Na+(l) + eNa(l) reduction At the anode(+): 2Cl-(l) Cl2(l) + 2e- oxidation Overall change: 2NaCl(l) 2Na(l) + Cl2(g)
The Electrolysis of Molten Compounds Electrolysis of Other Molten Compounds When a molten compound is electrolysed, the metal is produced at the cathode and the nonmetal is produced at the anode.
When a molten ionic compound is electrolysed,- a metal (from the positive ions) is produced at the cathode, and - a non-metal (from the negative ions) is produced at the anode.
Molten ionic compounds conduct electricity because the ions can move. Solid ionic compounds do not conduct electricity because the ions cannot move. Inert electrodes do not react in the electrolysis.
Electrolysis of aqueous ionic compounds(using inert electrodes)
In the electrolysis of molten electrolytes, there are only two kinds of ions. In aqueous solutions, there are four kinds of ions; two come from the electrolyte and two from the water. Of the four ions in an aqueous solution, only two are discharged.
1. Electrolysis of aq. H2SO4 The ions present from H2SO4 from water H+ H+ SO42OH-
Factors affecting Electrolysis Electrochemical series Concentration Type of electrode
Electrolysis of aq. H2SO4 At the anode OH- ions are discharged with ease compared to SO42 4OH O2 + 2H2O + 4e oxygen gas is given off
Electrolysis of aq. H2SO4 At the cathode, only H+ ions are present 2H+ + 2e- H2 hydrogen gas is given off
Electrolysis of aq. H2SO4 With electrolysis of aqueous acids or alkalis, the volume of hydrogen collected is always twice the volume of oxygen H2 : O2 = 2 : 1 essentially the elements of water are lost during the electrolytic process electrolysis of aqueous sulphuric acid is the electrolysis of water
Electrolysis of Aqueous Solutions Electrolysis of Dilute Sulphuric Acid
At the cathode: Hydrogen ions are discharged to produce hydrogen gas.2H+(aq) + 2eH2(g)
At the anode: Hydroxide ions are discharged, producing oxygen gas.4OH-(aq) O2(g) + 2H2O(l) + 4e2H2(g) + O2(g)
Changes in solution: Only water decomposes.
Since water is a covalent compound, pure or distilled water is a non-electrolyte. A few drops of ionic compound like dilute sulphuric acid are enough to make the water become an electrolyte
The diagram shows the results of an electrolysis experiment using inert electrodes. Which of the following could liquid X be? A. Aqueous copper(II) sulphate B. Concentrated hydrochloric acid C. Dilute sulphuric acid D. Distilled water E. Ethanol
TYS : The diagram represents an experiment in which an electric current is being passed through dilute sulphuric acid. a) Give the formulae of all the ions present in this solution b) Construct an ionic equation for the reaction at each electrode. c) Describe a test for each of the electrode product.
TYS : The diagram represents an experiment in which an electric current is being passed through dilute sulphuric acid. a) Give the formulae of all the ions present in this solution H+ OH- SO42b) Construct an ionic equation for the reaction at each electrode. Cathode: 2H+ + 2e- H2 Anode: 4OH- O2 + 2H2O + 4ec) Describe a test for each of the electrode product. H2 : It produces a pop sound with a lighted splint O2 : It relights a glowing splint
2. Electrolysis of Concentrated Sodium Chloride solution The ions present : from sodium chloride from water Na+ H+ ClOH-
Electrolysis of concentrated NaCl
At the anode Cl2
At the anode, both Cl- and OH- migrate to the anode the Cl- are preferentially discharged because of their higher concentration 2Cl- Cl2 + 2e- (half-equation) chlorine gas is given off
At the cathode H2
At the cathode, both H+ and Na+ ions migrate to the cathode but H+ is preferentially discharged as H+ is lower than Na+ in the electrochemical series; it accepts electrons more readily 2H+ + 2e- H2 (half-equation) hydrogen gas is given off
Eventually, there will be a depletion of H+ and Clions in the solution the Na+ and OH- however remains so solution will become NaOH which is alkaline !
3. Electrolysis of Copper(II) Sulphate Solution using Carbon Electrodes Ions present:cations From copper(II) sulphate From water Cu2+ (aq) H+(aq) anions SO42- (aq) OH-(aq)
Type of Electrode Electrolysis of aq. copper(II) sulphate using carbon as electrodes the ions present from CuSO4 Cu2+ from water H+ OHSO42-
Electrolysis of aq. CuSO4 using carbon electrodes At the anode OH- being higher in concentration gets discharged 4OH O2 + 2H2O + 4e-
oxygen gas is given off
Electrolysis of aq. CuSO4 using carbon electrodes At the cathode Cu2+ is discharged instead of H+ as Cu2+ is lower than H+ in ecs Cu2+ + 2e- Cu copper metal deposited
Electrolysis of Copper(II) Sulphate Solution using Carbon Electrodes At the cathode: Copper metal is