L10 Electrolysis (1) PDF - Principles Of Chemistry

Official (Closed) - Non Sensitive Electrolysis PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 1 Official (Closed) - Non Sensitive Learning Objectives Define electrolysis Describe electrolysis as evidence of the structure of ionic compounds in...

Official (Closed) - Non Sensitive Electrolysis PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 1 Official (Closed) - Non Sensitive Learning Objectives Define electrolysis Describe electrolysis as evidence of the structure of ionic compounds in the solid, molten and aqueous states Predict the products of the electrolysis of a molten ionic compound Predict the products of electrolysis of an aqueous electrolyte using the idea of selective discharge of ions ◦ Reactivity series ◦ Concentration effects Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given relevant information. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 2 Official (Closed) - Non Sensitive Introducing electrolysis Electrolysis is the process of using electricity to break down or decompose a compound (usually an ionic compound in the molten state or in aqueous solution). Electrolysis takes place in an electrolytic cell. battery connecting wire electrolyte anode cathode PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 3 Official (Closed) - Non Sensitive Parts of an electrolytic cell Battery Acts as an ‘electron pump’ and draws electrons away from the anode. The anode becomes positively charged. Electrons enter the positive terminal of the battery and are pumped out at the negative terminal. These electrons are supplied to the cathode, which becomes negatively charged. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 4 Official (Closed) - Non Sensitive Parts of an electrolytic cell Electrodes Conduct electricity Usually carbon rods or metal plates Electrode connected to the positive terminal of the battery is called the anode Electrode connected to the negative terminal of the battery is called the cathode PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 5 Official (Closed) - Non Sensitive Parts of an electrolytic cell Electrolyte Conducts electricity Free-moving ions allow electricity to flow through Is a molten ionic compound or an aqueous solution Is decomposed to form positive ions (cations) and negative ions (anions) PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 6  Official (Closed) - Non Sensitive How does electrolysis work? e- e- e- e- At the anode: e - During electrolysis, Anions move to the anode e- e- Anions give up electrons at e- e- - - + the anode e - e - Oxidation occurs at the - + + anode - + PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 7 Official (Closed) - Non Sensitive How does electrolysis work? e- e- e- e- e - At the cathode: During electrolysis, e- e- Cations move to the cathode e- e- Cations receive electrons at - - + e - e - the cathode - + + Reduction occurs at the - + cathode PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 8 Official (Closed) - Non Sensitive How does electrolysis work? e- e- e- e- e - e- e- - e- e- - + e - e - - + + - + Within the electrolyte: During electrolysis, the flow of ions towards the electrodes constitutes the flow of electric current through the electrolyte. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 9 Official (Closed) - Non Sensitive How does electrolysis work? e- e- In the external circuit: e - e- During electrolysis, e - electrons flow from the positive terminal to the e- e- negative terminal of the - e- e- - + battery. e - e - - + + - + PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 10  How does electrolysis work? In the external circuit: Examples: During electrolysis, electrons flow from the Anion : Cl-, OH- positive terminal to the negative terminal of Cation : Na+, H+ the battery. e- e- e- e- At the anode: e - At the cathode: During electrolysis, During electrolysis, Anions move to the anode e- e- Cations move to the cathode Anions give up electrons at e- e- Cations receive electrons at - - + the anode e - e - the cathode Oxidation occurs at the - + + Reduction occurs at the anode - + cathode Within the electrolyte: During electrolysis, the flow of ions towards the electrodes constitutes the flow of electric current through the electrolyte. Official (Closed) - Non Sensitive What happens to ions at the electrodes? Cations receive electrons at the negatively-charged cathode Anions give up electrons at the positively-charged anode. When the cations/anions gain/lose electrons at the electrodes, they form atoms or molecules. We say that the cations or anions are discharged. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 12 Official (Closed) - Non Sensitive Remember An Ox Red Cat PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 13  Electrolysis as evidence Official (Closed) - Non Sensitive of the structure of ionic compounds Molten : liquefied by heat Ionic compounds can only conduct electricity in the molten state and in aqueous solution. This can be proven by electrolysis. When solid NaCl is used, no current flows. No products are formed at the electrodes. This is evidence that in the solid state, ions are held in place in the lattice structure. Thus, they cannot conduct Solid NaCl electricity. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 14  Electrolysis as evidence Official (Closed) - Non Sensitive of the structure of ionic compounds When solid NaCl is heated until it melts, a current flows through the circuit. Products are formed at the electrodes. This is evidence that in the molten state, ions are free to move and can conduct electricity. Molten NaCl PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 15  Electrolysis as evidence Official (Closed) - Non Sensitive of the structure of ionic compounds When solid NaCl is dissolved in water to form an aqueous solution, a current flows through the circuit. Products are formed at the electrodes. This is evidence that in aqueous solution, ions are free to move and can conduct electricity. Aqueous NaCl PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 16 Official (Closed) - Non Sensitive Electrolysis of molten ionic compounds When a molten binary compound undergoes electrolysis, a metal and a non-metal are formed as products. (Binary compound = compound containing only 2 elements) Molten compound does not contain water molecules PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 17  Official (Closed) - Non Sensitive Electrolysis of molten sodium chloride At the anode: Negatively-charged Cl- e- e- ions are attracted to Carbon the anode. anode Carbon Cl- ions lose electrons cathode to form chlorine gas. They are oxidised. Cl- ions are said to be discharged. 2Cl-(l)  Cl2 (g) + 2e- Molten NaCl PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 18  Official (Closed) - Non Sensitive Electrolysis of molten sodium chloride e- e- At the cathode: Carbon Carbon Positively-charged Na+ ions are anode cathode attracted to the cathode. Each Na+ ion gains one electron to form a sodium atom. Na+ is reduced. Na+ ions are said to be discharged. Na+(l) + e-  Na(l) Molten NaCl The overall reaction is: 2NaCl (l)  2Na (l) + Cl2 (g) PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 19 Official (Closed) - Non Sensitive Inert electrodes During the electrolysis of molten NaCl, the chlorine produced is very reactive. Inert carbon electrodes are used to prevent chlorine from reacting with the electrode. Inert electrodes are electrodes that do not react with the products of electrolysis or with the electrolyte. Carbon (graphite) and platinum electrodes are considered inert because they are rarely involved in electrolytic reactions. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 20 Official (Closed) - Non Sensitive Question In this setup, molten lead (II) bromide is electrolysed. a) Indicate the direction of the electron flow b) Indicate the movement of lead (II) ions and bromide ions c) Identify the electrodes where oxidation & reduction take place. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 21 Official (Closed) - Non Sensitive Question Molten copper (II) bromide is electrolysed using carbon electrodes. a) Name the ions present in molten copper (II) bromide. b) Predict the products obtained at the anode and cathode during electrolysis. c) Write equations for the reactions occurring at the carbon electrodes. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 22 Official (Closed) - Non Sensitive Electrolysis of aqueous solutions An aqueous solution of a compound is a mixture of the compound with water. In an aqueous solution, more than 1 type of cation or anion are present in the electrolyte. E.g. aqueous sodium chloride contains, Na+, H+, Cl-, OH-. However, only 1 cation and 1 anion are preferentially discharged during electrolysis. This is known as selective discharge of ions. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 23 Official (Closed) - Non Sensitive Selective discharge of ions If inert electrodes are used during electrolysis, the ions discharged depend on 3 factors: 1. Selective discharge of cations 2. Selective discharge of anions 3. Effect of concentration on the selective discharge of anions. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 24 Official (Closed) - Non Sensitive Selective discharge of cations The more reactive a metal, the greater its tendency to form ions. Hence, in electrolysis, the ease of discharge of cations depends on the position of the metal in the reactivity series. Potassium ion (K+) Sodium ion (Na+) Calcium ion (Ca2+) H+ ions are discharged in Magnesium ion (Mg2+) preference to ions of metals Zinc ion (Zn2+) above hydrogen in the Iron (II) ion (Fe2+) reactivity series. Lead (II) ion (Pb2+) Hydrogen ion (H+) Ease of Copper (II) ion (Cu2+) Ions of metals below hydrogen discharge Silver ion (Ag+) in the series will be discharged increases during electrolysis. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 25 Official (Closed) - Non Sensitive Selective discharge of anions Sulfate (SO42-) and nitrate (NO3-) Sulfate ion (SO42-) ions remain in the solution and are not discharged during Nitrate ion (NO3-) electrolysis. Chloride ion (Cl-) Bromide ion (Br-) Iodide ion (I-) OH- ions give up electrons most readily during electrolysis to Ease of Hydroxide ion (OH-) form water and oxygen. discharge 4OH-(aq)  2H2O (l) + O2 (g) + 4e- increases PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 26 Effect of concentration Official (Closed) - Non Sensitive on the selective discharge of anions An anion in higher concentration is more readily discharged. E.g. in the electrolysis of concentrated NaCl solution: ◦ Cl- and OH- ions are attracted to the anode ◦ According to relative ease of discharge, OH- ions should be discharged preferentially. ◦ However, in concentrated sodium chloride solution, there are more Cl- ions than OH- ions. Therefore, Cl- ions are discharged. 2Cl-(aq)  Cl2 (g) + 2 e- How about chemical reaction at cathode? (Na+, H+) 2H+ + 2e-  H2 (g) PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 27  Official (Closed) - Non Sensitive Predicting the products ME MO R I SE of electrolysis 1. Identify the cations & anions in the electrolyte Remember that aqueous solution also contains H+ and OH- 2. Determine the anion that is discharged at the anode ◦ OH- will be discharged, unless electrolyte contains a high concentration of Cl -, Br- or I- anions. 3. Determine the cation that is discharged at the cathode ◦ From the metal reactivity series 4. The cations & anions remaining in the solution after electrolysis form the product remaining in the solution. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 28 Official (Closed) - Non Sensitive Electrolysis of dilute NaCl solution PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 29 Official (Closed) - Non Sensitive Electrolysis of dilute NaCl solution Ions present: At anode: Na+(aq), Cl-(aq), H+(aq), OH-(aq) OH- and Cl- ions are attracted to the anode. At cathode: OH- ions give up electrons more readily than Cl- ions. Therefore, H+ and Na+ ions are attracted OH- ions are preferentially to the cathode. discharged to form water and Sodium is higher in the oxygen gas. reactivity series. Therefore, H+ 4OH-(aq)  2H2O(l) + O2(g) + 4e- ions are preferentially discharged as hydrogen gas. Cl- ions remain in solution 2H+ (aq) + 2e-  H2(g) Na+ ions remain in solution Overall reaction: 2H2O(l)  2H2(g) + O2(g) PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 30 Official (Closed) - Non Sensitive Electrolysis of concentrated NaCl solution PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 31 Official (Closed) - Non Sensitive Electrolysis of concentrated NaCl solution Ions present: At anode: Na+(aq), Cl-(aq), H+(aq), OH-(aq) OH- and Cl- ions are attracted to the anode. At cathode: Since Cl- ions are more numerous than OH- ions, Cl- ions H+ and Na+ ions are attracted are discharged as chlorine gas. to the cathode. Sodium is higher in the 2Cl-(aq)  Cl2(g) + 2e- reactivity series. Therefore, H+ OH- ions remain in solution ions are preferentially discharged as hydrogen gas. 2H+ + 2e-  H2(g) Summary: (aq) One volume of H2 gas is given off at the cathode Na+ ions remain in solution One volume of Cl2 gas is produced at the anode PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 32  Official (Closed) - Non Sensitive Electrolysis of CuSO4 solution (inert electrodes) PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 33  Official (Closed) - Non Sensitive Electrolysis of CuSO4 solution (inert electrodes) Ions present: Cu2+(aq), SO42-(aq), H+(aq), OH-(aq) PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 34  Official (Closed) - Non Sensitive Electrolysis of CuSO4 solution (inert electrodes) At anode: At cathode: OH- and SO ions are 4 2- H+ and Cu2+ ions are attracted attracted to the anode. to the cathode. SO ions are not discharged. 2- Copper is lower than 4 Instead, OH- ions are hydrogen in the reactivity preferentially discharged to series. Therefore, Cu2+ ions form water and oxygen gas. are preferentially discharged as copper metal. 4OH- (aq)  2H2O(l) + O2(g) + 4e- Cu2+ (aq) + 2e-  Cu(s) SO 2- ions remain in solution 4 H+ ions remain in solution Summary: Copper metal is deposited at the cathode and oxygen gas is given off at the anode. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 35 Official (Closed) - Non Sensitive Questions 1. Two compounds, potassium sulfate & copper (II) sulfate, are dissolved in water and electrolyzed using platinum electrodes. a) Name the ions present in the solution. b) Predict which ion will be discharged at the cathode. c) Predict which ion will be discharged at the anode. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 36 Official (Closed) - Non Sensitive Questions 2. An aqueous solution of sodium sulfate is electrolyzed using carbon electrodes. Write the: a) Ionic half equation at the cathode b) Ionic half equation at the anode c) Equation for the overall reaction PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 37 Official (Closed) - Non Sensitive Question 3. A concentrated solution of potassium iodide is electrolyzed using platinum electrodes. a) What would you observe? b) Write ionic half equations for the reactions at the cathode & anode. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 38 Official (Closed) - Non Sensitive Industrial applications of electrolysis Electrolysis can be used to Purify metals ◦ E.g. Pure copper (99.9% purity) is obtained from crude copper using electrolysis. Deposit a layer of metal on another substance (electroplating) ◦ Decorative and/or to prevent corrosion ◦ E.g. Silver or gold plating PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 39  Official (Closed) - Non Sensitive Electrolysis of CuSO4 solution (copper electrodes) In this case, copper electrodes are reactive electrodes. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 40 Official (Closed) - Non Sensitive Electrolysis of CuSO4 solution (copper electrodes Ions present: Cu2+(aq), SO42-(aq), H+(aq), OH-(aq) PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 41 Official (Closed) - Non Sensitive Electrolysis of CuSO4 solution (copper electrodes) At reactive copper anode: At reactive copper cathode: OH- and SO ions are 2- H+ and Cu2+ ions are attracted 4 attracted to the anode. to the cathode. Since copper is not an inert Copper is lower than electrode, the following hydrogen in the reactivity reaction occurs: series. Therefore, Cu2+ ions are preferentially discharged. Cu (s)  Cu2+(aq) + 2e- Cu2+ + 2e-  Cu(s) The anode decreases in mass. (aq) The cathode is coated with a layer of copper. The cathode Summary: increases in mass. The Cu2+ ions discharged at the cathode come mainly from the copper anode. No net loss of Cu2+ ions from the CuSO4 solution. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 42 Official (Closed) - Non Sensitive Purification of impure copper To refine impure copper, the impure copper is used as the anode, while a thin sheet of copper is used as the cathode. The electrolyte must be an aqueous solution of copper. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 43 Official (Closed) - Non Sensitive Electroplating The object to be electroplated is made the cathode, and the anode is the source of the plating metal. The electrolyte is an aqueous solution of a salt of the plating metal. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 44 Official (Closed) - Non Sensitive Question Draw a labelled diagram to show how a steel spoon can be electroplated with silver. PRINCIPLES OF CHEMISTRY, PFP, LSCT, NGEE ANN POLYTECHNIC 45