Electrochemical Cells: Galvanic Cell

ELECTROCHEMICAL CELLS: GALVANIC CELL There are TWO types of electrochemical cells: 1. Galvanic (also called a voltaic cell). cell 2. Electrolyticcells The type of chemical reaction taking place in both of these electrochemical cells is a REDOX reaction. Galvanic cell Through into a spontaneous, - sustaining REDOX reaction, chemical energy can be converted self energy. electrical Definition: into Galvanic cell: A cell which chemical in in energy is converted electrical energy. NOTE:A galvanic electrical energy. A spontaneous, cell is battery. Batteries operate - sustaining self exothermic REDOX by converting chemical energy reaction takes place in into a galvanic cell. Voltmeter (Used to measure the cell emf) Structure of the galvanic cell Conducting wire (Used to connect the in the external electrodes circuit) Electrode Electrode [e.g. Cathode (+)] [e.g. Anode(-)] (Conducts charge) (Conducts charge) Beaker (Holds the electrolyte Porous plugs solution) wool or cotton J(e.g., glass Electrolyte (Solution contains the same ion as the metal (e.g., wool) Electrolyte that anode) is (Solution same ion the electrode Salt bridge that made of) the electrode (e.g., (Containsa saturated salt solution). contains the as the metal cathode) is made of) Notes on the structure of a galvanic cell Agalvanic cell consists of (electrolytic solutions), in two electrodes which two are placed separate beakers, forming two in separate electrolytes half- cells. Electrodes The electrodes must be made of a solid that can conduct electricity and therefore the made of a conducting metals. However, one (or both) of the half electrodes are usually if gas [e.g. chlorine gas (Cl,) or hydrogengas (H,)] some cases, graphite (carbon) (C) cells is a or in KMiss Angler 186 the electrode used is platinum (Pt) ELECTROCHEMICAL CELLS: GALVANIC CELL There are TWO types of electrochemical cells: 1.Galvanic (also called a voltaic cell). cell 2.Electrolytic cells The type of chemical reaction taking place in both of these electrochemical cells is a REDOX reaction. Galvanic cell Through into a spontaneous, self - sustaining REDOX reaction, chemical energycan be converted electrical energy. Definition: Galvanic cell: A cell which chemical in in energy is converted into electrical energy. NOTE:A galvanic cell electrical energy. Aspontaneous, self - is battery. Batteries operate sustaining REDOX exothermic by converting chemical energy reaction takes place in into a galvanic cell. Voltmeter (Used to measure Structure of the galvanic cell the cell emf) Conducting wire (Used to connect the in the external electrodes circuit) Electrode Electrode [e.g. Cathode(+)] [e.g. Anode(-)] (Conducts charge) (Conducts charge) Beaker (Holds the electrolyte Porous plugs solution) e.g.. glass Electrolyte (e.g., is same ion (Solution same the electrode that anode) wool) Electrolyte (Solutioncontainsthe as the metal wool or cotton Salt bridge made of) contains the as the metal that the electrode (e.g., (Contains a saturated ion cathode) is made salt solution). of) Notes on the structureof a galvanic cell A galvanic cell consists of (electrolytic solutions), in two electrodes which two are placed separate beakers, forming two in separate electrolytes half- cells. Electrodes The electrodes must be made of a solid that can conduct electricity and therefore the electrodes are usually made of a conducting metals. However, one (or both) of the half cells is a gas (e.g. chloine gas (Cl,) or hydrogen gas (H,)] the electrode used is platinum (Pt) or in some cases, graphite (carbon) (C) if Miss Angler 186 why platinum (Pt)is used: Reasons Platinum 1. a is good conductor of If the half - cell is (it is NOTE:'adsorbs' is not be confused with 'absorbs'. a metal) Adsorbs:To hold 2. Platinum is inert (unreactive) 3. Platinum adsorbs the gas onto NOTE: electricity a liquid, (molecules of a gas or liquid) as a layer on the outside surface surface. its (C)can be used as the graphite solid electrode. Anode and cathode (electrodes) PRO -TIPS Anode Anode: Definition: anode place.The galvanic cell.The where oxidation Electrode AN OX:Oxidation takes the negative electrode (-)in anode decreases in mass. is takes place at the anode. a RED CAT: Reduction takes place at the cathode. Cathode Definition: Cathode: where Electrode reduction takes place. Thecathodeis the positive (+) in a galvanic celL.The NOTE: electrode cathode increasesin mass. The electrodes are connected to each other consisting of conducting Electrolytes Each half and a voltmeter to an external measurethe A Electrolyte: the electrolyte water to give a solution that conducts in solution. e.g., if the electrode which the zinc electrode solution can be used. is is electricity. is placed in a zinc (Zn) electrode, the electrolyte placed, must contain zinc ions (Zn 2• ), for example, zinc nitrate a NOTE:Compounds Salt bridge • Asalt ions bridge is shapedtube an inverted Ua saturated electrolyte(i.e., • emf of which the aqueoussolution contains ions substance that dissolves The electrolyte solution must contain the ion of the metal electrode which in cell contains an electrolyte solution. cell OR a substance solution circuit (potential difference or voltage). Definition: • wires in salt (usually made of glass) that or concentrated solution containing nitrate are usually soluble. is filled with a suitable to allow for the salt solution), movement of ions through the two half cells. The aqueous salt solution is kept in the U shaped tube by porous plugs in each end, such as glass wool or cotton wool, through which the ions in the salt bridge can move The electrolyte,which is placed in the salt bridge must a concentrated or saturatedsalt solution that contains ions that are weaker oxidising agents and reducing agents. Examples of saturated salt solutions that can be used include: saturated KNO, (aq) or NaNO,(aq)or KCL (aq) or NaCl (aq). • KNO, or NaNO,isusually the best option because it contains ions that are soluble in water. Functionsof the salt bridge 1. 2. Completesthe circuit. Maintains electrical 3. electrical neutrality neutrality i.e. it (allows movement of ions betweenthe electrolytes,to ensure acts as an ion exchanger). Separates the electrolytes in the two half - cells so that they Miss Angler 187 do not mix. How does the galvanic cell operate? Flow of electrons anode,through the external Electrons flow from the negative Remember: AN OX to the positive cathode. circuit anode oxidation takes place) cathodereduction takes place) (at the RED CAT (at the neutrality in Flow of ions To maintain electrical bridge to the Anions the half anode and cathodehalf (negative ions) from the Cations (positiveions) from The reasoning behind this will the be -cells, therefore there is a flow of ions from the salt cells: towards the anode. salt bridge move salt bridge move towards explained later in this the cathode. chapte. Standard conditionsunder which the galvanic cell operates Galvanic Two cell with solid electrodes standard conditions under which the galvanic cell operates: • The concentration The temperature Galvanic cell of is all solutions is l mol.dm 25°C or 298 K. with at least one gas electrode Four standard conditions under which the galvanic cell operates: The pressure of all gases is latmosphere(latm) OR 101, 3 kPa The concentration of all solutions is l mol.dm The temperature is 25°C (298 K) Platinum is the electrode used when the half cell is a gas. - Example of a galvanic cell: Cu - Zn galvanic cell operating at standard conditions Anode and cathode a spontaneous, In a galvanic In order for a spontaneous react with a stronger Using table Table cell, exothermic exothermic reducing REDOX reaction takes place. reaction to take place, a stronger oxidising 4A and table 4B to determinewhether Cu or Zn is Table 4A a stronger reducing Zn table 4A. the right is than a stronger Cu, as Zn is on reducing Zn is the reducing C Suitable electrolytes for the half - cells half- cell: half -cell: ; Zn(NO) Cu(NO), Miss Angler 188 is table 4B. agent the oxidising agent higher up, to Conclusion: Zn (zinc) is the anode electrode. Cu (copper) is the cathodeelectrode. agent 4B Zn is a stronger reducing agentthan Cu, as Zn is lower down to the right on Zn Cu agentmust agent. agent Cu - Zn galvanic Structure of the Cu Cathode cell Zn Anode (+) Reduction (-) Oxidation Zn Cu NOS NO Cu(NOJ{aq) ( mol.dm3) Zn(NO:),(aq) (l NaCl (aq) Copper half -cell [anode(-)] • Zn (reducing agent) • Electrode decreases ions (oxidising agent)undergo reduction Electrode increases undergoesoxidation in mass mass in - TIPS PRO Reductionhalf reaction: Cuz* When + 2eCu (from table 4A or table 4B) Balanced net reaction: electrons ZnZn + half reaction: writing the overall, balanced net reaction, the (from table 4A or table 4B) Oxidation ) Zinc half cell [cathode(+)] Cu 2+ mol.dm Zn + lost number of must equal the numberof electrons gained. A reaction/s can be multiplied 2e half throughout by a factor to ensure that the number of electrons lost Cu-Zn + Cu equals the number of electrons gained. NOTE: • A stronger reducing agentmust react with a stronger oxidising agentin a galvanic cell. When writing reactions, only a half are only either undergoing • No electrons are shown in singlearrow must be used, oxidation OR the balanced as the substances reduction. net reaction, as the must equal the number of electrons gained (i.e., it number of electrons lost "cancels out") Process • When the Zn electrode (anions) from the The mass of the • The Cu* ions in undergoesoxidation, Zn ions are added to the solution, increasing solution. To maintain electrical neutrality, Cl' ions Zn" (cations)in the concentration of bridge are expelled salt Zn electrode (anode) into solution. decreases. solution undergo reduction, forming Cu (s)which is deposited onto the Cu electrode. The Cu electrode (cathode) increases Cu The concentration of (cations)from the salt 2* ions in in mass. To maintain Cu half - cell. solution decreases. bridge are expelled into the Miss Angler 189 electrical neutrality, Na Cell notation The cell notation electrochemical is hand way of representing the reactions that occur short- a • The electrodes are written outside of the cell notation. • A single vertical line () separates substances that are in A comma • The ()separates bridge salt in substances that by two represented is different phases. are in the same phase. lines (|) vertical The cell notation for a standard electrochemical cell with be represented as: | Reducing agent after loses electrons solid electrodes can Oxidising agent Oxidised species Substance produced Substance that I Reduced species Substance produced after the oxidising Substance that gains Salt the reducing bridge electrons agent loses electrons agent gains electrons Anode Example: Cathode Cu- Zn Cell notation for the Z |Z(aq, of mol.dm') ||Cu*(aq,I mol.dm I Write down the cell notation • Mn 4A or table )|Cu () at 25°C C+A for an Mn and Cl, (9) galvanic cell 4B, the following can be concluded: a stronger reducing agent than is cell: matter and standard conditions (if requested): Worked example Using table galvanic Z I Cu" Cu Cell notationwith states Zn (5) an cell. Cl Cl, is a stronger oxidising agentthan Mn". For a spontaneous exothermic REDOX reaction to take place: A stronger reducing agentmust react with a stronger oxidising agent. Mnis *Clz is will undergo oxidation. agentand will undergo reduction. the reducing agent and the oxidising Reductionhalf reaction: CL(a) + 2e2CI' PRO (from table 4A or table 4B) Oxidation Mn Mn half reaction: Write out the oxidation '+2e Mn | Mn I| CL,|Cl half reaction and the (from table 4A or table 4B) Cell notation: -TIPS reductional | reaction to half make it easier to write the cell notation. Nocoefficients Pt included in are the cell notation. NOTE: One of must the electrodes be indicated at is a gas [CL,(9)]. Therefore a platinum the end of the cell notation Miss Angler 190 solid with the gaseous (Pt)electrode is used, this half -cell. • The standard reduction potential, table • (E) emf Cell 4B as measured in volts, is The standard reduction potential gives an indication of how reduced 4A and present on table E°(V). or oxidised, and can a substance likely is able to be interpreted as follows: OA large positive standard reduction potential indicates that the substance is more to be reduced (it is a stronger oxidising agent) large negative standard reduction potential indicates that a substance is more likely to be oxidised (it is a stronger reducing agent). likely 9A A REDOX reaction has (electrodes) must the emf cell TWO half- potential difference across the or cell potential (E®cel)at standard conditions, reduction potentials and is The two half cells two half cells, known as combination of the two standard reactions taking place simultaneously. be connected and the measured a voltmeter or usingg a is can it be calculated using the one of the following formulae: What do these variables mean and what are the Slunits? cell cell =Ecattodeanode Ecel emf in volts (V) At the cathode reduction OR OR cathode OR EPeduction Een -Eoatcn =Educhon E anode Eosdisinggent E oxidation Eeducingagen reduction is the agent =emf (standard E undergoing oxidising agent Eoxidising OR E reducing () reduction potential) in volts At the anodeoxidation OR OR takes place and the substance undergoing oxidation is the reducing agent agent zemf (standard reduction potential) in volts (M) NOTE: E represents emf (E) at standard conditionsmeasured in volts (V) When the REDOX reaction a spontaneoUs exothermic reaction, Eeell (cell emf) is positive.For Example: Ecell a galvanic cell, the cell Calculate the cell emf for the (0,34)-(-0,76) Elell= +1,10 is Cu -Zn galvanic cell A reaction is taking place. V emf and chemical equilibrium • All galvanic cells can reach equilibrium. • When a galvanic cell reaches equilibrium, the rate of oxidation reduction OR the cell (battery) is alwayspositive. positive cell emf indicatesthat a spontaneous exothermic = Eathode -Eanode Feell Cell initial emf takes place and the substance rate of the forward goes "flat" and the cell equals the rate of reaction equals the rate of the reverse reaction. emf is The zero (0). changingthe standard conditions - chemicalequiibrium Favouring the forward reactionincreasesthe cell emf. The cell emf is Effects of • greater than that calculated under standard conditions. Favouring the reverse reactiondecreases the cell emf. The cell emf is lower calculated under standard conditions. Miss Angler 191 than that GALVANIC CELLS: STANDARD HYDROGEN ELECTRODE • The standard hydrogenhalf cell operating at standard conditions potential (standard reduction potential) of • The standard reduction potential (E value) of relative to the standard hydrogenhalf Figure 1 below represents a was assigned a half cell 0,00V. all other half cells in measured cell. hydrogenhalf - cell connected to another - cell X. half NOTE: In the standard electrode: • Hydrogen gas The hydrogen glass tube at l atm and is hydrogen pumped through a a standard pressureof is bubbled over a platinum electrode which is in contact with a acid solution witha concentration of l mol.dm Platinum 1 -x*(aq) mol-dmH(aq) Figure 1 Standard conditionsunder which the standard hydrogen electrode operates: V The pressure of H,gas is atmosphere(l atm) OR 101, 3 kPa. The concentration of the strong acid solution (containing H* The temperature is 25°C (298 K). l Solid platinum is the electrode A positive 1 mol. dm used. Conclusions regarding the voltmeter reading when to a standard hydrogen ions) is a half- cell is connected half - cell voltmeter reading indicates that the other electrode is the cathode A negative voltmeter reading indicates that the (+) other electrode is theanode (-) Miss Angler 192 Worked example A 1. A+P galvanic cell operates according ()+ Zn Ag'(aq)+Zn (3)Ag NAME Write down the 1.1 1.2 Write The Ag 1.3 1.4 1.5 down electrode Write NAME the down is now of the (aq) ANODE that the reaction: is made of. referringto the oxidation numbers. metal that the CATHODE solid replaced Mn|Mn with a (2) made of. is () electrode. the: 1.3.1 Oxidation half 1.3.2 Reduction half 1.3.3 Balanced Write down Which of the substance answerby Give a reason for the UNBALANCED to the following reaction (2) reaction (2) net reaction (2) the cell notation for the galvanic cell (Ag - Zn or (4) cell. Zn - Mn) produced a a calculation. period the Zn Mn greater cell emf? Prove the answerwith After a certain time 1.6 why Explain this (5) numbers to was reduced. determine which substance was oxidised and which 0 +1 +2 Ag(aq)+Zn (s) Ag ()+ The oxidation number of Ag *decreased from underwentreduction oxidising agent The oxidation number of Zn increased from underwentoxidation reducing agent Zinc. a cell emf of 0V. occurs. 1.1Assign oxidation substance (2) galvanic cell has - +1 to 0. 0 to (aq) Zn Ag* was reduced/ +2. Zn was The oxidation number of Zn increased from and is therefore the anode. oxidised/ 0to +2, therefore Zn underwent Oxidation C'12 Ag'(insolution) were reduced. The electrode is a solid, therefore it is silver. 1.3 Analysis: Compare the strength of Table Mn is is Mn and Zn as agentusing 4A right on table Mn is Conclusion: agent: Oxidising agent: Undergoesoxidation Mn Zn 2+ 4A or table 4B: 4B Mn is a stronger reducing agentthan Zn, as Mn higher up, to the right 4A. on Reducing table Table a stronger reducing agent than Zn, as lower down to the a reducing Undergoes reduction Miss Angler 193 table 4B. C1.3.1 Oxidation reaction: half Remember: Mn- Mn+ 2e electrons. table Reduction 1.3.2 reaction: half Zn+ 2e Oxidation The reverse 4A Zn 4B and table Oxidation Balanced Mn net reaction: reaction, make the is writing the overall balanced + Zn?Mn+Zn on reaction. half Remember: When 1.3.3 a loss of is reaction net sure of the following: The number of electrons equals the lost number of electrons gained. G1.4 Remember Reducing Oxidised agent species Substancethat loses Substance electrons : Mn after produced the reducing reaction Salt bridge Substancethat agent | Mn The cell emf emf Ag Cell gains Substance electrons produced agent gains electrons for each galvanic cell needs to be calculated and compared: Cell emf Zn - Mn galvanic cell - Zn galvanic cell =Ecathode- E°anode Ecel = (-0,76)-(-1,18) E®cell = Ecell (+0,80)- (-0,76) The 1.6 e Ag - Zn All after the oxidising E®cell =E®cathode - Eanode E®cell=1,56 balanced. species Zn|Zn || is Reduced Oxidising agent loses electrons C1.5 * The net the order of the cell notation. Ecell =0,42 V V galvanic cell produces a greater cell emf. galvanic cells can reach equilibrium. • Whena galvanic reduction reaction. cell reaches equilibrium, the rate of oxidation equals the rate of OR the rate of the forward reaction equals the rate of the reverse The cell (battery) goes "lat" and the Miss Angler 194 cell emf is zero (0V). ELECTROLYTIC CELLS • An electrolytic cell • different is energy from a galvanic cell in that in an electrolytic cell, energy. An electrolytic cell therefore requires additional energy from a DC (direct current) power source,such as a battery becausethe REDOX reaction that takes place in a electrolytic cell is a non - spontaneous endothermic reaction, where a weaker reducing agent reacts with a weaker oxidising agent. electrical Definition: Electrolytic into chemical NOTE: In into chemical converted is cell: a cell in which electrical energy converted is energy an electrolytic cell: • The anode is electrode (+) the positive fOpposite of a galvaniccell The cathode is the negative electrode (-) The anode is connected to the positive terminal of the battery and the cathodeis connected to the negative terminal of the battery. Electrolytic • • processes covered in this - TIPS PRO appliesto an electrolytic cell: AN OX (at the anode oxidation takes place) RED CAT (at the cathode reduction takes place) Still The cell emf in an electrolytic cell is negative because a non- spontaneous REDOX reaction takes chapter: place. The decomposition of copper(l) chloride Electroplating, e.g. the electroplating of Refining of metals, e.g. The electrolysis of solution of sodium chloride copper (l)chloride (electrolysis) • A solution of copper (1)chloride (CuCl,)of concentration • The solution is initially blue in colour due to the presence • Two conductive graphite electrodes are used. Figure 2 below Used silver/nickel copper of a concentrated The decomposition an iron spoon with I mol.dm- prepared. is of Cu'ions in solution. -TIPS PRO represents the structure of electrolytic cell for the decomposition of CuCl, (ag) A DC power source Battery ) (DC power source) -Cathode Reduction Anode (+) Oxidation must be Used electrolytic ensure that an in to cell the electrons flow in one only i.e., from the anode to the direction cathode. Anions move towards the anode Cations move towardsthe cathode -CuCl,(aq) Figure 2 Through a process of electrolysis,the decomposition Definition: converted chemical Electrolysis: The chemical to chemical energy. OR of process the use of change. Miss Angler 195 copper ()chloride in which electrical electrical takes place. energy is energy to produce a Analysis of the electrolysis Reduction half (from table of copper (I)chloride + reaction: Cu 2e 4A or table 4B) Oxidation half 2C1Cl,+ 2e reaction: (from table 4A or table 4B) Balanced net reaction: + C, Cu 2+ 2CI» Cu Observations Solid copper (reddish - brown in A green chlorine gas bubblesat • Due to the decreases concentration in of Cu colour) the deposited is at the cathode. anode. ions in solution decreasing, the initial blue solution colour. Electroplating What is electroplating? Electroplating The process is the process of depositing a layer of one metal onto another metal. of electrolysis takes place. What is the purpose of electroplating? Electroplating is used to improve the metals appearance,protect the metal or to give it other Useful properties. Process • The object being electroplated cathode (it is connected to • The object (metal oxidation and is (coated) must undergo reduction and is therefore the the negative terminal of the battery). X) doing the electroplating (i.e., the coating) must undergo therefore the anode (it is connected to the positive terminal of the battery). The electrolyte is a solution of a compound containing ions of the metal (metal X) doing the electroplating. 9Example: A plastic ring (coatedin graphite to ensure that it is conductive), is coated with silver(Ag). Silver (Ag) electrode Anode (+) Oxidation NO3 Plastic AgNO ,(aq) Ag ring coated Cathode ) Reduction MissAngler 196 in graphite Analysis of the electroplating of a Oxidation half reaction: (from table 4A or table 48) Reduction half reaction: (from table 4A or table 48) Ag+ Ag Ag+ Balanced net reaction: e Ag+ Ag coated plastic ring in graphite with silver e Ag + Ag Ag Refining of metals e.g., copper (electrorefining of copper) • Electrorefining is Used to The impure copper platinum (P) and purify copper. This is a very similar process to electroplating. [which contains other metal impuritiessuch as zinc (Zn),silver (Ag), gold (Au)] is the anode and undergoesoxidation. (+), which is connected to the positive terminal impuremetals do not undergooxidation, and form a solid precipitate at the bottom of the container. The anode decreases in mass. • The pure copper, the cathode (-), which is connectedto the negative terminal of the battery and undergoes reduction. The cathodeincreases in mass due to the pure copper of the battery being deposited • The electrolyte These onto the cathode. must be a solution containing Cu PRO -TIPS ions. Example: Impurecopper undergoeselectrorefining to produce pure Cu is a stronger copper. reducing e Pure copper Impure copper Anode Cathode (+) -CuCl,(aq) Cut (impurities) Analysis of the electrorefining Oxidation half reaction: (from table 4A or table 4B) Reductionhalf reaction: (from table 4A or table 4B) Balanced net reaction: (-) Reduction Oxidation Precipitate agent than Cu Cu Cu+ of copper Cu+ 2e + 2eCu Cu Cu + C MissAngler 197 Pt, Ag and Au, therefore Cu will be reduced. Electrolysis of a concentrated alkali industry) solution • a concentrated hydrogengas [H,(9)] and NaOH In the Chlor - alkali industry,through the electrolysisof chloride (NaCl), chlorine gas (Cl, (9)), • • In (NaCl)is solution of sodium chloride A concentrated of sodium chloride (Chlor the Chlor-alkaliindustry, a membrane cell is solution of (aq) sodium produced. is called brine. used which consists of an - exchange ion membrane. • The ion exchangemembrane - is selectively permeableand only allows cations (e.g., Na') to migrate through. Figure 3below represents the structure of the membrane cell used in the electrolysis of sodium chloride. concentrated Cl, (9) 1 + NaCl (aq) Dilute H,(9) NaOH (aq) Anode Cathodg H,O H,0 () CL Concentrated NaCl (aq) Process Figure 3 At theanode (+) • Concentrated sodium chloride (brine) is pumped into this NaCl dissociates into Na* (aq) and Cl' (aq). concentrated • The positive electrode (anode) attracts the negatively • The Cl' ions become oxidised and form Cl, (g) • Oxidation reaction:2Cl C+ 2e cell charged compartment.The CI' ions in the solution. half- (Fromtable 4A At the cathode 4B) or table (-) • Na' ions migrate through • Water (H20)is pumped There are H'ions, Na' the ion - exchange membrane. this component and partially ionises. and H,O molecules in this compartment. into ions • Due to Na" being a very weak being very low, Reduction half (from table 4A H, O is - reaction: 2H,O or table Oxidation half reaction: (from table 4A or table Reductionhalf (from table reducing the concentration of + 2eH,+ 20H NOTE: The Na' ions 2ClCl,+2e lons 2H+ 2e-, +2OH 4B) Balanced net reaction: H* 48) 48) reaction: 4A or table agentand reduced. 2H,O + 2CE-Cl,+ H,+20H Miss Angler 198 in this reaction OH to form NaOH are spectator and bond to (aq) Worked example A+P The simplified diagram below purification of copper represents an electrochemical DC cell used for the source Electrode A copper) (impure Give a reason why a direct current (DC) source 1.2 Write down the half 1.3 Due is used in this (2) A. reaction which takes place at electrode (2) amountsof zinc impuritiesin the impurecopper,the becomes contaminated with Zn ions. Refer to the table of standard reduction potentials to explain ions will not affect the After the purification 2,85 x 10moles The initial that was initially of the purity C1.2 • • In copper obtained the Zn experiment. impurecopper was completed,it was formed. of the mass of electrode present in B was 2,0 g.Calculate electrode (3) found that Cu half reaction: in impurities (4) that the electrons flow the electrolyticcell, electrode cathode where the the percentage B. B (impure undergoes oxidation. Electrode A is the pure copper and Reduction in this why of copper were ADC power source ensures anode to the cathode. 1.1 experiment. to small electrolyte 1,4 B Electrode Electrolyte is in one direction only i.e., from the copper)is theanode,which the cathode. Reduction takes place at the solution are reduced. CU" + 2eCu C'1.3 To explain this answer, the strength of the oxidising agents must be compared and a conclusion drawn: Zn is weaker oxidising agentthan Cu therefore the Cu Cu. ; in solution ions will (Zn"andCu²) be reduced to form OR Cu is a stronger oxidising agent than Zn, therefore the form Cu. Miss Angler 199 Cu ions will be reduced to C14 • OPTION 1 . The number of moles of pure copper formed in the reaction is known, using the formula n ,themass of pure copper formed can be determined This mass ot pure copper formspart of the total mass (2,0g) of the impurecopper (electrode B). • To determinethe of electrode mass of impurities, subtract the mass of pure copper from the total mass B. • The percentageimpurities can be determined the total mass of electrode B and then by converting taking the to a this mass of impurities percentageby divided by x 100. Cu) = n(pure (2,85 x 102) (63.5) m(pure Cu) = 1,809.g Mass Mass Mass of impurities= Total mass of electrode B -mass of pure Cu =2-1,809. of impurities of impurities =0,19...g 0.19. = Percentage impurities Percentage impurities =9,51 OPTION x 100 OR % 2 • The number of moles of pure copper formed formula n= in the reaction the mass of pure copper formed can M • This mass ot pure copper formspart of the total be is known, using the determined. mass (2,0 g)of the impurecopper (electrode B). The percentagepurity can first be be determined by by the total mass of electrode B and then converting taking the To determine percentagepure Cu from percentageimpurities, subtract the Cu) =" n(pure (2,85 the x 102) =m (63.5) m(pure Cu) =1,809.g Percentage pure Cu Percentage pure Cu = 90,487... =x Percentage impurities Percentage impurities 100 % = 100- 90,487... =9,51% Cu 63.55 Copper Miss Angler 200 this to mass of pure Cu divided a percentageby x 100. 100.