Presentation on theme: "What is the name of the scientist who investigated the theory of electrolysis in 1830? Definition of Electrolyis Electrolysis is the conduction of electricity."— Presentation transcript:
What is the name of the scientist who investigated the theory of electrolysis in 1830? Definition of Electrolyis Electrolysis is the conduction of electricity by an electrolyte (usually an ionic compound), when molten or dissolved in water, leading to the decomposition of the electrolyte.
Electrolytes conduct electricity because they contain mobile ions. During electrolysis, electrical energy is converted into chemical energy.
Electrolytes Electrolytes is a compound in solution or molten state that conducts electricity with the decomposition at the electrodes as it does so. Electrolytes are usually molten ionic compounds or solutions of ionic salts. Examples: hydrochloric acid, aqueous sodium hydroxide, aqueous sodium chloride
Electrolytes Non-electrolytes Aqueous ethanoic acid Dilute nitric acid Molten lead(II) bromide Copper(II) sulfate solution Lemon juice Sodium carbonate solution Alcohol Sugar solution Pure deionised water Pure ethanoic acid
Why do ionic compounds conduct electricity in molten and aqueous states but not in solid state? In solid state, the ions are held in fixed positions by strong electrostatic forces. In molten or aqueous states, the strong electrostatic forces are weakened/overcome and the ions are now mobile. These mobile ions are able to conduct electricity.
Question for discussion 1. What particles are responsible for electrical conductivity? Mobile electrons present in conductors such as metals & graphite Mobile ions Present in electrolytes such as dilute HCl or molten NaCl
Question for discussion 2.What substance will remain unchanged when an electric current pass through it? Metals and graphite (elements) 3.What substance will decompose when an electric current pass through it? Electrolytes (E.g molten NaCl)
Electrolysis of Molten lead (II) bromide Anode ADD Anode ADD Positive electrode CathodeNegative electrode Electrons flow from anode to cathode
Molten lead(II) bromide contains: Pb 2+ And Br - Positive ions, Pb 2+, are attracted to negative electrode (cathode) Always remember: Cations are attracted to cathode Negative ions, Br -, are attracted to positive electrode (anode) Always remember: Anions are attracted to anode.
Reactions at the electrodes: Reactions at the anode: Br - (l) Br 2 (g) 2 Recall, in balancing ionic equations, charge must also be balanced. -20 + 2e - -2Charge Bromide ions lose electrons to form bromine molecules. Thus in general oxidation occurs at the anode. Remember: An Ox Anode Oxidation Observation at anode: Reddish-brown and pungent gas was observed.
Reaction at cathode, Pb 2+ (l) Pb(l) Charge +20 + 2e - -2 Lead(II) ions gain electrons to form lead atoms. Lead (II) ions are said to be discharged. In general reduction takes place at cathode. Remember: Red Cat Reduction Cathode In the electrolysis of binary molten compounds, a metal is discharged at the cathode; a non-metal is produced at the anode. A binary compound is a compound containing only two elements. Eg NaCl, PbBr 2
Try practice 1 on page 3 of notes Na +, Cl - 2 Cl - (l) Cl 2 (g) +2e - Chlorine gas (yellowish- Green gas) Na + (l) + e - Na(l) Sodium metal Pb 2+, O 2- 2 O 2- (l) O 2 (g) +4e - oxygen gas Pb 2+ +2 e - Pb lead metal Molten electrolyte Ions Eqn at anode Product Eqn at cathode Product NaCl PbO Cal 2 Ca 2+, l - 2 I - (l) I 2 (g) +2e - iodine Ca 2+ +2 e - Ca Calcium metal
Question for discussion (NOT in note) During the electrolysis of molten lead(II) oxide, do you expect the lead form to be solid or liquid? Why? What information is needed? Mp of PbO = 888 C Mp of Pb = 327.5 C Lead formed would be in molten/liquid form.
Aqueous electrolytes or solutions are mixtures of two electrolytes : the compound and water. Water ionises to a very small extent to give a very small amount of H + and OH - H 2 O H + + OH -
Eg 1 : Electrolysis of concentrated hydrochloric acid Ions present : H +, Cl -, OH - From HCl and water From water Ions at cathode:H+H+ Eqn at cathode: 2H + (aq) - H 2 (g) +2 e - Ions at anode: Cl -, OH - There are two anions at anode. Which ion will discharge at anode ?
Factors in deciding which ion is discharged are: Its position in electrochemical series Concentration of the ion Type of electrode
Electrochemical Series In an electrochemical series, metals or non-metals are arranged in order of their reactivity. Metallic ions K + (Group I) Na + (Group I) Ca 2+ (Group II) Mg 2+ (Group II) Al 3+ (Group III) H+H+ Cu 2+ Ag + Product at cathode Hydrogen (from water) (Ions formed by metals above hydrogen in the electrochemical series are not discharged.) Hydrogen copper silver Reference point Least readily discharged Most readily discharged Zn 2+ Pb 2+ (Depends on concentration) Fe 2+
Product at anode Oxygen (from water) (Sulfate and nitrate ion are not discharged.) Non-metallic Ions SO 4 2- NO 3 - Cl - Br - I - OH - oxygen O 2 if electrolyte is Dilute; respective Halogens if Electrolyte is concentrated Note: When the non-metallic ions are close in their position in the electrochemical series, order of discharged is influenced by the concentration of anion. Least readily discharged Most readily discharged
E.g In the electrolysis of concentrated HCl, both OH - & Cl - migrate to the anode. But concentration of Cl - is larger than that of OH -. Hence Cl - is preferentially discharged instead. 2 Cl - (l) Cl 2 (g) +2e -
Eg 2Electrolysis of dilute sulfuric acid Ions present in dilute sulfuric acid: H +, OH -, SO 4 2- Ions migrating to cathode: Hydrogen gas Equation representing reaction at cathode: H + (aq) H 2 (g) 2 +2 e - Products at cathode: Observation: Effervescence of colourless and odourless gas. H+H+
Equation representing reaction at anode: OH - (aq) H 2 O(l)+ O 2 (g) 24 +4e - Reason: sulfate ion which is high up in the electrochemical series is very stable. A large amount of energy is required for sulfate to give up its electrons, hence it is not discharged. Thus, hydroxide ions, OH -, is preferentially discharged to give oxygen gas. Product at anode:oxygen gas (and water) Eg 2Electrolysis of dilute sulfuric acid Ions migrating to anode: OH - SO 4 2- Observation: Effervescence of colourless and odourless gas.
The volume of H 2 collected during electrolysis of dilute sulfuric acid is twice that of oxygen. Why? Anode: 4OH - 2H 2 O + O 2 + 4e - Cathode: 2H + + 2e - H 2 For 1 mole of oxygen produced at the anode, 4 moles of electrons are released. These 4 moles of electrons will reduce hydrogen ions at the cathode to produce 2 moles of hydrogen gas. Thus volume of hydrogen is twice that of oxygen. Overall equation can be represented as: 2H 2 O 2H 2 + O 2 As water molecules are removed/decomposed, concentration of sulfuric acid increases.
Eg 2Electrolysis of aqueous copper(II) sulfate using graphite electrodes Ions present : H+H+ Cu 2+ SO 4 2- Ions migrating to cathode: Copper (brown/pink solid deposited on cathode) Equation representing reaction at cathode: Cu 2+ (aq) + 2e - Cu(s) Products at cathode: H+H+ OH - Cu 2+ [link]
Eg 2Electrolysis of aqueous copper (II) sulfate using graphite electrodes Ions migrating to anode: Equation representing reaction at anode: Products at anode: oxygen and water OH - SO 4 2- OH - (aq) H 2 O(l)+ O 2 (g) 24+4e -
Eg 2Electrolysis of aqueous copper (II) sulfate using graphite electrodes What happens to the electrolyte after some time? The blue electrolyte fades away and eventually becomes colourless. This is due to the reduction of copper (II) ion to copper atoms, leading to a decrease in concentration of copper (II) ions in solution over time. What happens to the pH of the electrolyte? Why? The pH of electrolyte decreases. As hydroxide ions are discharged, concentration of hydroxide ions decreases and is less than that of hydrogen ions, hence making the solution acidic.
Conc NaCl solution ElectrolyteIons Eqn at cathode Product Eqn at anode Product Dilute aqueous potassium sulfate Na +, Cl -, H +, OH -, 2 H + (aq) +2e - H 2 (g) Recall Na + not dishcarged in aq. form. Hydrogen gas 2Cl - (aq) Cl 2 (g) +2e - Chlorine gas K +, SO 4 2 - H +, OH - 2 H + (aq) +2e - H 2 (g) Hydrogen gas 4OH - (aq) 2 H 2 O(l) + O 2 (g) +4e - Oxygen and water Dilute NaCl solution Na +, Cl -, H +, OH -, 2 H + (aq) +2e - H 2 (g) Recall Na + not dishcarged in aq. form. Hydrogen gas 4OH - (aq) 2 H 2 O(l) + O 2 (g) +4e - Oxygen and water
Electrodes Inert electrodes (Electrode has no reaction with the electrolyte. Eg graphite and platinum) Reactive electrodes (Electrode can dissolve in the electrolyte. Eg copper, silver nickel)
Copper, a metal, has the greatest tendency to lose electrons and is most easily oxidised to copper (II) ions. Hence at the anode, Cu electrode dissolves. Eqn at anode: Cu (s) Cu 2+ (aq) + 2e - Hence anode loses weight and becomes smaller/thinner. Copper electrodes aq. CuSO 4 Ions present in aqueous copper(II) sulfate: Cu 2+, H +, OH -, SO 4 2- Go to cathode Go to anode Substances at anode: OH -, SO 4 2- and copper anode. Which substance is most easily oxidised? Electrolysis of aqueous copper(II) sulfate using copper electrodes
At the cathode, brown/pink copper is deposited: Eqn at cathode: Cu 2+ (aq) + 2e - Cu (s) Hence cathode gains weight and becomes thicker. The concentration of the electrolyte, copper(II) sulfate, remains unchanged. What happens at the cathode? Substances at cathode: H +, Cu 2+, Copper cathode Which substance is most easily reduced? Copper(II) ions which are below hydrogen in the electrochemical series has the greatest tendency to gain electrons. Electrolysis of aqueous copper(II) sulfate using copper electrodes
What happens to the concentration of copper(II) sulfate? Why? Cu 2+ (aq) + 2e - Cu (s)Cathode Cu (s) Cu 2+ (aq) + 2e - Anode The concentration of the electrolyte, copper(II) sulfate, remains unchanged. For every mole of copper(II) ion reduced at the cathode, there is equal no. of mole of copper(II) ions being formed in solution due to oxidation of copper at anode.
+ - pure Cu (cathode) aq. CuSO 4 Impure Cu (anode) + - pure Cu (cathode) Impure Cu (anode) Impurities The electrolysis of copper(II) sulfate using copper electrodes is used to refine (purify) copper. The impure Cu is the anode. A piece of pure Cu is used at the cathode. Impurities on the anode will fall off. They contain silver and gold.
Reaction at anode: Cu (s) Cu 2+ (aq) + 2e - Each copper atom loses 2 electrons to form a copper(II) ion. Hence anode slowly dissolves. It loses weight/reduces in size and becomes thinner. Reaction at cathode: Cu 2+ (aq) + 2e - Cu (s) At the cathode, each copper(II) ion gains 2 electrons to form a copper atom. Hence cathode gains weight and becomes thicker. Concentration of electrolyte remains unchanged.
Electroplating Electroplating is the process of coating a metallic electrolysis object with another metal by electrolysis. Electroplating is used for protection against corrosion and for decoration. At the anode, each copper atom loses 2 electrons to form copper(II) ions, hence copper electrode dissolves. Cu(s) Cu 2+ (aq) + 2e - At the cathode, each copper(II) ion gains 2 electrons and is discharged to form solid copper. Copper metal is plated on the object. Cu 2+ (aq) + 2e - Cu (s) - + copper anode object at cathode copper (II) sulfate solution The overall change is for copper to be transferred from the anode to the object. Video clip
In electroplating, the object to plated is the cathode. the metal used for electroplating is made the anode. the electrolyte is a solution of ions of the metal. Electroplating Different types of electroplating 1. Tin plating 2. Solder plating 3. Chromium plating 4. Nickel plating 5. Silver / gold plating