electrolysis revision

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  • 1. ELECTROLYSIS +- + - + - + - + - + - + +Module C2 Splitting up ionic compounds (F) Molten compounds Ionic solutions & discharge rules Q = It and OILRIG Brine and purifying copper

2. SPLITTING UP IONIC COMPOUNDS 1 Cl- ION Na+ IONIonic compounds (eg sodium chloride) are made from: POSTIVE IONS (atoms which LOST negative electrons) NEGATIVE IONS (atoms which GAINED negative electrons) As these ions have OPPOSITE CHARGES they attract each other strongly to form IONIC BONDS 3. SPLITTING UP IONIC COMPOUNDS 2 2 ways to split up the ions:MELT+- + - + - + - + - + - + +800C- + + - + + --+-DISSOLVE+-+ - + ++ ---+20CH2O 4. SEPARATING THE IONS 1+_Metal ELECTRODEELECTRON Battery pulls electrons off one electrode and pushes them onto the otherThis IS SHORT OF electrons so becomes POSITIVELY CHARGED ANODEThis HAS EXTRA electrons so becomes NEGATIVELY CHARGED CATHODE 5. SEPARATING THE IONS 2+ MOLTEN IONIC COMPOUND-+ -+ ANODE++-+-+++- CATHODEWhen the battery is switched on, the + IONS move to the CATHODE the IONS move to the + ANODE This gives a way to SPLIT UP IONIC COMPOUNDS: ELECTROLYSIS 6. Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)- =Cl-chloride ION, extra 1 electronClchlorine ATOM, Cl Cl NEUTRALCl2 molecule+Cl Cl-ClClClCl- chloride IONS lose their extra electrons and turn into neutral chlorine ATOMSClClAt ANODE: Clthen: Cl + Cle- + Cl Cl2 (gas)Both together:2Cl- 2e- + Cl2 7. Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt) + = Na+sodium ION, missing1 electronNasodium ATOM, NEUTRAL+ + sodium IONS gain an extra electron and turn into neutral sodium ATOMSAt CATHODE: Na+ + e-Na Na+Na Na+Na Na+Na Na+Namolten sodium metal sinks to bottom 8. Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt) - CATHODE+ ANODEELECTRONSSODIUM metal NaCHLORINE gas Cl2Cl-MOLTEN SODIUM CHLORIDEAt ANODE: Cle- + Cl Cl + ClCl2 (gas)Na+At CATHODE: Na+ + eNa 9. Example 2: Splitting up MOLTEN LEAD BROMIDE PbBr 2 - CATHODE+ ANODEELECTRONSLEAD Metal PbBROMINE gas Br2Br-MOLTEN LEAD BROMIDEAt ANODE: Bre- + Br Br + Br Br2 (gas)Pb2+At CATHODE: Pb2+ + 2eBoth together:2Br- 2e- + Br2Pb 10. What happens when the ionic compounds are dissolved in water? Here, water molecules break up into HYDROGEN IONS, H+ and HYDROXIDE IONS OH-H2O H+ + OHSo, in an ionic solution (eg sodium chloride solution), there will be FOUR types of ion present: TWO from the ionic compound and TWO from the water ( H+ SODIUM CHLORIDE SOLUTION NaCl (aq) H+ OH-ClNa+OHH+H+Na+ Cl--OHNa+ Cl-+ OH-) 11. IONIC SOLUTIONH+ OHOH-Cl-+NaH++NaCl-H+ Na+OHCl-Which ions gain or lose electrons (get discharged) and which stay in solution? 12. IONIC SOLUTIONS: At the CATHODE Na+sodium ION, missing 1 electronH++hydrogen ION, missing 1 electronAs HYDROGEN is LESS REACTIVE than SODIUM, it is discharged. The sodium ions stay in solution.At CATHODE: 2H+ + 2e-HHydrogen ATOM, NEUTRALwhich ions?Na+H+H H+Na+H2 13. IONIC SOLUTIONS: At the CATHODE halogen compounds Cl-chloride ION, extra 1 electronhydroxide ION, O H from water extra electronchlorine ATOM, NEUTRALCl+HOO HCl Cl-HClClOO H ClClwhich ions?ClClAt ANODE:If the ion is a HALOGEN (Cl, Br, I) it is discharged and chlorine (or Br or I) is given off and the OH - ions stay in solution2Cl-2e- + Cl2 14. IONIC SOLUTIONS: CATHODE non halogen compounds nitrate ION, extra NO3 1 electronhydroxide ION, OHO H from water, extra electronOOxygen atom+ NO3 H-ONO3HOwhich ions?NO3-O H NO3-If the ion is NOT a halogen (eg nitrate, sulphate etc) then the HYDROXIDE ions from the water are discharged to make WATER and OXYGEN gas. The other ions stay in solution.O HAt CATHODE: 4OH-2H2O + O2 + 4e- 15. RULES FOR IONIC SOLUTIONS + ANODE Attracts ions (Anions)- CATHODE Attracts + ions (Cations)If ions are HALOGENS ieIf + ions (metals) are MORE REACTIVE than hydrogenchloride Clbromide BrK, Na, Ca, Mg, Zn, Fe-iodide Ithe HALOGEN is produced. If ions are NOT HALOGENS Eg sulphate SO4 , 2-Then HYDROGEN is produced If + ions (metals) are LESS REACTIVE than hydrogennitrate NO3-Cu, Ag, Aucarbonate CO32-Then the METAL is producedOXYGEN is produced. 16. (REACTIVITY: K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe3+ H+ Cu2+ Ag+ Au3+ ) IonsCathode (-) Anode (+)potassium chloride moltenK+ Cl-potassiumchlorinealuminium oxidemoltenAl3+ O2-aluminiumoxygencopper chloridesolutionCu2+ Cl- H+ OH-copperchlorinesodium bromidesolutionNa+ Br- H+ OH-hydrogenbrominesilver nitratesolutionAg+ NO3- H+ OH-silveroxygenpotassium chloride solutionK+ Cl- H+ OH-hydrogenchlorinezinc sulphateZn+ SO42- H+ OH-hydrogenoxygenCompoundStatesolution(REACTIVITY: K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe3+ H+ Cu2+ Ag+ Au3+ ) 17. ELECTROLYSIS makes a CIRCUIT Complete electric circuit: Current carried by:ELECTRONS in electrodes/wires + +-IONS in the electrolyte -To DOUBLE the MASS of substance discharged at electrodes: 2 x CURRENT (2x batt. voltage) 2 x TIME current flows for (Q = I t) 18. OILRIGCl-Cl-Cl-Cl-Oxidation is loss, reduction is gain OILRIGNa+Na+Na+Na+- ions LOSING electrons + ions GAINING electrons to become atoms is called to become atoms is called OXIDATION REDUCTION (even though oxygen may not be involved) 19. INDUSTRIAL USES OF ELECTROLYSIS 1. To extract reactive metals such as ALUMINIUM, sodium, magnesium etc from their compounds. This is EXPENSIVE due to the large amounts of electrical energy needed. Aluminium is extracted from bauxite (Al2O3). 2. Electrolysis of BRINE (salt solution) to produce see below CHLORINE (for disinfectants and plastics) HYDROGEN (for ammonia fertilisers, margarine) SODIUM HYDROXIDE (for soap and cleaning agents) 3. Purifying copper. The copper for wiring etc needs to be more pure than that produced in a blast furnace. see below Electrolysis is used to convert impure copper to pure copper 20. INDUSTRIAL ELECTROLYSIS OF BRINE Chlorine gasHydrogen gasBRINE (NaCl solution)CATHODE H+ and Na+ANODE OH- and Cl2Cl-2H+ + 2e-2e + Cl2 -OH- left in solution so concentration growsSodium chloride solution (neutral) slowly changed to sodium hydroxide solution (alkaline)H2Na+ left in solution so concentration grows 21. Industrial chlorine production from electrolysis of brine 22. PURIFYING COPPER IMPURE COPPER ANODECopper atoms from impure copper are OXIDISED to copper ionsPURE COPPER CATHODECopper sulphate CuSO4 solution Cu2+Cu Cu2+Cu2+CuCopper ions transported from anode to cathodeCopper ions from impure copper are REDUCED to copper atomsAs the atoms of the impurities are not transported, the copper that builds up on the anode is extremely pure. 23. IMPURE COPPER ANODEPURE COPPER CATHODEIMPURE COPPER ANODEPURE COPPER CATHODEOver time, the impure anode dissolves away and the impurities sink to the bottom. The pure cathode grows as more pure copper is deposited on it. Why will the concentration of the solution stay the same?