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Electrochemical Cells - IB HL

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Published in: Chemistry
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Discussing about electrochemical cells in electrochemistry Cell formation Representation Calculation of potential Previous year questions of them same including option C paper

Salman F / Doha

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  2. 8. 31 -S 0-0 00 2- I C 500 e 13. A voltaic cell is made up of nickel and magnesium half-cells. (a) (b) Mg (s) I Mg2' (aq) I I Ni2W(aq) I Ni (s) Write the balanced equation for the reaction in this voltaic cell. Calculate the cell potential for 0.0100 mol drW3 Mg2+(aq) and 0.800moldrn• Ni2• (aq) at 298K. Use sections 1 , 2 and 24 of the data booklet.
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  5. 20. A proton-exchange membrane (PEM) fuel cell uses pure hydrogen gas as the fuel and a proton exchange membrane as the electrolyte. Excess fuel Fuel in Anode Water and heat out 02 Air in Cathode Electrolyte (Source: https:/,'en_wikipedia.org/wiki/FiIe:Proton_Exehange_FueI_CeII_Diagrarn_svgl (a) Deduce the half-equations for the reactions occurring at the electrodes. [21 Anode (negative electrode): Cathode (positive electrode): t..u.D (b) anode: cathode: overall: anode: cathode: overall: Calculate the cell potential, e, in V, using section 24 of the data booklet. f-t.ceJv cat CH30H(aq) + H20(l) -+ C02(g) + +6e- + 6e- —9 3H20(l) CHAOH(aq) C02(g) + 2H20(1) A methanol fuel cell can also be constructed using and the reactions are: CH.OH(aq) + 60iV(aq) C02(g) + 5H20(1) + 6e- 3H20(1) CHAOH(aq) +202(g) C02(g) +2H20(1) oxidation reduction oxida tion reduction
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  8. Lit Positive plate Separator Negative plate external circuit Li• ions LiPF6 electrolyte C002 electrode containing Li+ ions, forms Li.C002 Li,C6 Figure C.37 A lithium-ion battery — lithium ions are exchanged between complexed lithium and graphite electrodes. T he a.vw-cl-e Us of ca.rbo-n a-do-vws aed cd-ho-du a. maevgawUZ(lV) 0-xa.e m- ccboZt(lV) 0-xa.e LÜ+ io-vvs I-aft-ice. The Us o. L•t4a-f-tA0-ro phosphate (LüPF6) o-rgao-üz . The each-o-{gte to 0 0 L L i CO O
  9. * Go Similarities and differences between fuel cells and rechargeable batteries Similarities: • They both generate electrical energy from chemical energy. • They are both composed of an anode, a cathode and an electrolyte. • They both generate a current based on the separation of reduction and oxidation reactions and the flow Of electrons through an external circuit. Differences: • Fuel cells require an external source of chemical energy (fuel), but rechargeable batteries have their chemical energy source within them. • Fuel cells newer run out so long as there is a constant supply of fuel from an external source; rechargeable batteries run out and then have to be recharged by connecting to an electricity supply which reverses the reactions. Rechargeable batteries are far cheaper than fuel cells. • Fuel cells are capable of generating a far greater quantity Of electricity than rechargeable batteries because the fuel is supplied constantly. • Fuel cells are — hydrogen fuel cells produce only product; rechargeable batteries may contain toxic metals so are difficult to dispose of. • Fuel cells can produce drinkable water as a byproduct; there is no byproduct of rechargeable batteries. • Rechargeable batteries can only be recharged so many times — they have a finite life and must be replaced eventually; fuel cells have much better longevity. Cell type Fuel cells Lead—acid batteries Nickel—cadmium batteries Lithium-ion batteries Size/mass Can be made in a variety of sizes. Fuel cell stacks used to power buses are quite large and heavy, but small fuel cells have been developed for laptops etc. Some fuel cells have a very high power-to-mass ratio and high power-to-volume ratio. Large and heavy — not suitable for portable devices. Low power-to-mass ratio and power-to-volume ratio. Small and light — used in a variety of portable devices. Intermediate power-to-mass ratio and power-to-volume ratio. Small and light - used in a variety of portable devices. Produce the greatest amount of power per unit mass or unit volume of the rechargeable batteries. Voltage Typically about 0.6—0.8 V but depends on the type of fuel cell. Used in a stack with multiple fuel cells connected in series. 2V per cell — usually used as a battery with six cells in series 1.2V About 3.7 V — the highest voltage per cell. Table C. Il A comparison Of different tynes Of cell
  10. Calculate the cell potential of the cell at 25 oc when the concentration of Zn 21. 22. + (aq) is 0.200 mol din-3 and that ofAg+(aq) is 0.100 moldm- Coal is Often converted to liquid hydrocarbon fuels through initial conversion to carbon monoxide and hydrogen. (a) (b) State how these gases are produced, giving the appropriate equation(s). Outline how the carbon monoxide is then converted to a hydrocarbon fuel. [21 As well as being burnt, methanol can also be used to provide electricity through a fuel cell. A schematic diagram Of such a fuel cell, that depends on the transfer Of hydrogen ions between the electrodes, is shown below. Anode negative electrode Cathode positive electrode Carbon dioxide Unused methanol / water Fuel Methanol / water Water, air (unused 02) Oxidant Air (02) (a) Proton-Exchange Membrane (PEM) [Source: adapted from http:/,'greenbigtruck.coml Deduce half-equations for the reactions at the two electrodes and hence the equation for the overall reaction. Anode (negative electrode): Cathode (positive electrode): Overall:
  11. (b) (c) Even though fuel cells, primary cells and rechargeable cells have similar fundamental characteristics, there are important differences between them. Suggest a way in which they are similar. Outline the difference between primary and rechargeable cells. Identify one factor that affects the voltage of a cell and a different factor that affects the current it can deliver. Voltage: Current: (Option C, question 17 continued) (c) Methanol fuel cells provide a portable energy source. The process can be represented by the overall equation CHyOH(aq) + — (g) C02 (g) + 2H20 (g). (i) Deduce the half-cell equations occurring at each electrode during discharge. Anode (negative electrode): Cathode (positive electrode): (ii) (iii) Outline the function of the proton-exchange membrane (PEM) in the fuel cell. Explain how the flow of ions allows for the operation of the fuel cell. [21 [21 [21
  12. 14. Modem electric cars store their energy in lithium ion batteries. (a) The diagram represents a cell in such a battery delivering a current (i) Complete the half-equations on the diagram and identify the species moving between the electrodes. . -+2LiC002 Species moving: Cathode (LiC002) Anode (graphite lattice) Load [3] (2) (ii) State the factor that limits the maximum current that can be drawn from this cell and how electrodes are designed to maximize the current. Limiting factor: Electrodes design: (Option C, question 13 continued) (c) Fuel cells have a higher thermodynamic efficiency than octane. The following table gives some information on a direct methanol fuel cell. Anode reaction CH30H(aq) + H20(l) —0 6H+ (aq) + 6e- + C02 (g) Cathode reaction —02 (g) + 6Hf (aq) + 6e- 3H20(l) Net equation AH = -726kJmoV CHJOH(aq) + —o.(g) -9 coz(g) + 2H20ß) Determine the thermodynamic efficiency of a methanol fuel cell operating at 0.576 V. use sections 1 and 2 of the data booklet. [31
  13. (c) A voltaic cell consists of a nickel electrode in 1.0 moldm-3 solution and a cadmium electrode in a Cd2- (aq) solution of unknown concentration. Cd(s) + + Ni (s) Determine the concentration of the Cd2+(aq) solution if the cell voltage, E, is O. 19V at 298K. Use section 1 of the data booklet. (Option C continued) 21. A fuel cell is an energy conversion device that generates electricity from a spontaneous redox reaction. (a) The Geobacter species of bacteria can be used in microbial fuel cells to oxidise aqueous ethanoate ions, CH3COO-(aq), to carbon dioxide gas. State the half-equations for the reactions at both electrodes. Negative electrode (anode): Positive electrode (cathode): (b) A concentration cell is an example of an electrochemical cell. (i) State the difference between a co ntration cell and a standard voltaic cell. (2] [2] [1

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