Presentation on theme: "Changes at the electrodes"— Presentation transcript:
1 Changes at the electrodes 6.2 Redox ReactionsChanges at the electrodesEXAM TIPS:You will be expected to write half equations for the reactions at the electrodes.In half equations, the number of electrons MUST balance the number of charges on the ion.
2 Objectives: I will understand • That electrons are transferred during electrolysis.• That electrolysis can be represented in half equations.• That water affects electrolysis.Outcomes: I will demonstrate I understand at Grade C by• Recall the transfer of electrons at the anode and cathode.• Recognise oxidation and reduction at electrodes.• Predict the products of electrolysis.Outcomes: I will demonstrate I understand at Grade A by• Explain the transfer of electrons in electrolysis.• Construct half equations• Explain how water affects the products of electrolysis.
3 Do you remember what goes where? heatNegative ions moveto the positive electrode (the anode) and lose electrons (Ox)Positive ions moveto the negative electrode (cathode) and gain electrons (Red)
5 Oxidation What is reduction? Oxidation is the gain of oxygen Rust is an example of oxidationWhat is reduction?
6 Reduction Metal Displacement reactions are an example of reduction. + Reduction is the loss of oxygenMetal Displacement reactions are an example of reduction.oxygen removedreductionlead oxide+carbonleadcarbon monoxideoxygen addedoxidation
8 Why is this type of reaction called a redox reaction? Reduction and oxidation always take place together.Why is this type of reaction called a redox reaction?redox = reduction and oxidationYOU SHOULD ALWAYS WRITE THIS NEXT TO THE QUESTIONOILRIGOxidationIsLoss of electronsReductionIsGain of electrons
9 + magnesium + oxygen magnesium oxide 2Mg(s) O2(g) 2MgO(s) Magnesium burns in oxygen to form Magnesium oxide.The Magnesium obviously is oxidised but what happens to the oxygen?magnesium + oxygen magnesium oxide2Mg(s)O2(g)2MgO(s)+
10 Mg is oxidised (lost electrons) O is reduced (gained electrons) Redox Reactions can also be explained in the terms of losing or gaining electronsoxidized(electrons lost)reduced(electrons gained)MgMg2+O2-O+Mg is oxidised (lost electrons)O is reduced (gained electrons)
11 In terms of oxygen In terms of electrons Copy and Complete the table...In terms of oxygenIn terms of electronsOxidationLoss of electronsReductionLoss of oxygen
12 Half Equations + magnesium + oxygen magnesium oxide 2Mg (s) O2 (g) Redox reactions involve the transfer of electrons.Equations written to show what happens to the electrons during oxidation and reduction are called half-equations.magnesium + oxygen magnesium oxide2Mg (s)O2 (g)2MgO (s)+oxidation: Mg Mg e-reduction: O2 + 4e- 2O2-
15 Draw a flow diagram Boardworks GCSE Additional Science: Chemistry ElectrochemistryDraw a flow diagram15
16 Electrolysis of molten PbBr2 – redox equations Boardworks GCSE Additional Science: ChemistryElectrochemistryElectrolysis of molten PbBr2 – redox equationsWhat redox processes occur at the electrodes during the electrolysis of molten lead bromide (PbBr2)?At the negative electrode:Pb2+ + 2e- Pb (reduction)At the positive electrode:2Br- Br2 + 2e- (oxidation)What is the overall equation for the electrolysis of molten lead bromide with state symbols?lead bromide lead + brominePbBr2 (l) Pb (l) + Br2 (g)16
17 Can you role-play what happens at the electrodes? We need anAnode (to take electrons)Cathode (to give out electrons)Pb and Br ions.Can you role-playwhat happens atthe electrodes?
18 Your Turn – Complete these equations with states 2FeCl3(aq)Anode(+)?Cathode(-)?2Fe(s) + 3Cl2(g)2Cl-(aq) Cl2(g) + 2e-OXIDATIONFe3+ (aq) + 3e Fe (s)REDUCTION
19 How does water effect the products of electrolysis? Water contains hydrogen and hydroxide ions.When solutions of ions in water are electrolysed, hydrogen may be produced at the positive electrode.This happens if the other positive ions are metals more reactive than hydrogen.Whats the name of the positive electrode?
20 Summary notesWhen positively charged ions reach the negative electrode they gain electrons to become neutral atoms. Gaining electrons is called reduction, so the positive ions have been reduced. Ions with a single positive charge gain one electron and those with a 2+ charge gain 2 electrons.At the positive electrode, negative ions lose electrons to become neutral atoms. This is oxidation. Some non-metal atoms combine to form molecules, for example bromine forms Br2.We can represent the changes at the electrodes by half equations. For lead bromide.At the negative electrode: Pb2+ + 2e- PbAt the positive electrode: 2Br- Br2 + 2e-Water contains hydrogen and hydroxide ions. When solutions of ions in water are electrolysed, hydrogen may be produced at the positive electrode. This happens if the other positive ions are of metals more reactive than hydrogen.Oxidation is loss (OIL) reduction is gain (RIG).In half equations the number of electrons must balance the number of charges on the ion.
21 Check your understanding What happens in electrolysis to copper ions, Cu2+, at the negative electrode?What happens in electrolysis to chloride ions, Cl-, at the positive electrode?Why are ions of metals always reduced in electrolysis?CLICK AGAIN FOR THE ANSWERSANSWERS:They are discharged, gain electrons and are reduced to form copper atoms.They are discharged, lose electrons and are oxidised to form chlorine atoms which then bond to form chlorine molecules (diatomic molecules)All metals form positive ions so they are attracted to the negative electrode where they gain electrons.
23 Boardworks GCSE Additional Science: Chemistry ElectrochemistryTeacher notesAppropriately coloured voting cards could be used with this classification activity to increase class participation.23
24 Boardworks GCSE Additional Science: Chemistry ElectrochemistryTeacher notesThis completing sentences activity provides the opportunity for some informal assessment of students’ understanding of redox reactions.24
25 Boardworks GCSE Additional Science: Chemistry ElectrochemistryTeacher notesThis drag and drop activity provides the opportunity for informal assessment of students’ understanding of the products of electrolysis.25
26 Boardworks GCSE Additional Science: Chemistry ElectrochemistryTeacher notesThis drag and drop activity could be used to check students’ understanding of the products of electrolysis.26
27 Boardworks GCSE Additional Science: Chemistry ElectrochemistryTeacher notesAppropriately coloured voting cards could be used with this classification activity to increase class participation.27