1. Chemistry- F325 Fuel cells and electrode potentials By Otrivine

2. Cells and half cells  Firstly, the basics of equilibrium for example if we have copper,  Cu2+ + 2e- equilibrium sign Cu(s), we know that the forward reaction involves the gaining of electrons (reduction) and the backwards reaction is the loss of electrons( oxidation), REMEMBER= OILRIG, the patten is always if electron is in left side then its reduction but if the electron is on the right hand side of the equation, it is oxidation.  In the exam we may be asked to draw half cells Cells from metal/metal ion half cell. This consists of two beakers each of the beakers are filled with a particular solution and the concentration always has to be 1 mol dm-3 (standard states), between the two beakers( we call it half cell) you would use a salt bridge, the purpose of this bridge is to allow the ions to transfer from one half cell to the next. Also, we place a voltmeter and this is used to measure the electrode potential of the cell and has a high resistance and minimises the current that flows. In addition, the electrons move in the wire and always remember, electrons flow from a high electrode potential to the more negative electrode potential. Remember that this is a common exam question. Salt bridge is made of a strip of filter paper soaked in aqueous solution such as ammonium nitrate and potassium nitrate. This is a good example of a potential question that could crop up as they never asked this, so memorize it. A simple diagram on next slide representing the metal/metal ion half cells.

3.  This diagram gives an excellent representation of the metal and non metal ion half cell.  Learn the diagrams, as many candidates did not draw good half cells in the Jan 2013 paper and memorise the standard conditions which are 298K(25 degrees celcius),  100 Kpa( 1 atmospheric pressure), concentration of 1 mol dm-3 for each solution.

4.  This diagram is another one you should now which is between non metal/ non metal ion half cells.  The difference between this and the metal/metal ion half cell is that this has got a platinum electrode ( to allow transfer of electrons) and its inert so it does interfere with the solutions. Also, a hydrogen gas enters at 298K and 1 atm pressure.  Sadly, could not get a diagram of the non metal/non metal ions but in the OCR book on page 185, it does show a nice diagram so learn that.  Also, the platinum electrode is coated with pt black, a spongy coating in which allows the transfer of electrons between the non metal and its ions. Learn this again as this could potential come up.

5. COMMON EXAM QUESTION!!  In exams they could ask (about 80% of the time) to draw a half cell between metal and non metal ions half cell.  In this cause you have to draw this:-  http://chemed.chem.purdue.edu/gen chem/topicreview/bp/ch20/electro.ph p http://chemed.chem.purdue.edu/gen chem/topicreview/bp/ch20/electro.ph p  Its on this website,very good indeed!

6. Cell potentials  A likely chance of this question coming up is very high so learn this:-  The definition of standard electrode potential has not come up for the past 2 or 3 years.  Standard electrode potential = Is the electro motive force of a half cell compared with a standard hydrogen half cell under standard conditions which are ____ Kpa, ___ K and __ mol dm-3. (YOU SHOULD KNOW WHAT THE CONDITIONS ARE!)  You could be asked to work out the electrode potential of a half cell which is simply:-  The more positive terminal – negative terminal, always remember the electrode potential values give you a positive value not negative!!  Sometimes you may be given two equations with electrode potential values and you have to make an overall equation and to do this you use the electrode potential values. The more positive electrode potential is reduced and you leave the equation as it is, but the one which is more negative/less positive you oxidised and you change the equation around. A good example is found on my next slide !

7. Examples!  Using this example we can see that the electrode potential of Zn is more negative and hence, is oxidised and so swap everything around, Zn(s)-  Zn2+(aq) + 2e- and you leave the Cu untouched, and now you did this you can just cancel the electrons and will result in the overall redox equation forming. And remember, The two equilibria essentially turn into two one- way reactions:  The equilibrium with the more negative (or less positive) E° value will move to the left.  The equilibrium with the more positive (or less negative) E° value will move to the right.  Now try yourself, to give overall, and post it on TSR so I can check your answer, but I am sure all of you got it correct as its pretty simple.

8. Equilibrium  If the electrode potentials are very high positive value, this indicates that the equation/compound is a strong oxidising agent and If value is small and negative most of the cases it’s a strong reducing agent.  I hope you are familiar with the terms oxidising and reducing if not, oxidising agent is the substance that is being reduced ( takes electron) and reducing agent is substance that is being oxidised( supplies electron)  Remember these points, very important and common exam questions

9. Limitations of electrode potentials  Predication of equilibrium can be possible but not the rate because the reaction could be slow or may have high activation energy.  The actual conditions may not be used and could affect the reaction. -Standard electrode potentials apply to aqueous equilibria = many reactions take place under different standard conditions. -The general rule for working predicting the reactions could take place under standard conditions if given electrode potential values, the larger the difference in electrode values the more likely reaction will take place usually if difference is more than 0.4V, reaction happens.

10. Storage and fuel cells  Fuel cell diagram 

11. Modern cell batteries  The three types of modern cells are  Non rechargeable cells= provides electrical energy until the chemicals have reacted to such an extent that the voltage falls.

12. Rechargeable cells  The chemicals react providing electrical energy, the cell can be reversed during recharging, Common examples are:-  Nickel and Cadmium batteries = used in rechargeable batteries  Lithium ion and lithium polymer used in laptops.

13. Fuel cells  Uses fuel and oxidant and provide electrical energy.  QUESTION FOR YOU GUYS!!  STATE THE DIFFERENCES BETWEEN THE 3 TYPES OF MODERN CELLS (4 MARKS) POST ON TSR IF YOU LIKE SO I CAN MARK.

14. How do fuel cells work?  The hydrogen and oxygen fuel cell The fuel cell uses energy from the reaction of a fuel with oxygen to create a voltage. Fuel cells do not have to be recharged. MEMORISE THE TWO HALF EQUATIONS FOR HYDROGEN AND OXYGEN FUEL CELLS ITS IN THE BOOK ON PAGE 191 REMEMBER WHEN IT IS IN ALKALI CONDITION YOU NEED TO CHANGE EVERYTHING, SO SWAP THEM AROUND FOR BOTH EQUATION, THIS WAS ASKED IN JAN 2013 AND IN JUNE 2010 PAPER (I THINK).

15. Hydrogen for future  Many scientists have now developed FCVs= hydrogen rich fuels.  Hydrogen- rich fuel includes methanol, nautral gas and petrol.  Methanol reacts with water to release co2 and hydrogen, the conditions is 250-300 degrees Celsius.  The advantage of methanol reacting with water has its advantages which is the liquid fuel can be stored and transported and is easier to store and transport than hydrogen gas and methanol can be generated from biomass. Publics and politicians accept this method of making hydrogen.  However, it does generate co2 which could contribute to global warming and does require high temperature so following high production costs.

16. Advantages of fuel cell  Less pollution and less co2 is released  Greater efficiency.  Question for you guys,  Suggest why still people are concerned of using hydrogen fuel cell ?  From your opinion, is it best to make hydrogen from methanol reacting with water or hydrogen-oxygen fuel cell?

17. Storage of hydrogen  Hydrogen can be stored as liquid under pressure  Hydrogen can be adsorbed onto solid materials  Hydrogen can be absorbed within some solid materials.  Suggest the disadvantage of storing hydrogen as liquid under pressure ?  This is a diagram showing how hydrogen is stored as liquid in big tanks.

18. Limitations of hydrogen fuel cells  Memorise them!!  1) The fuel cell uses toxic chemicals in their production  2) fuel cells have a limited life time requiring regular replacement and disposal following high production costs.  3) the current adsorbers and absorbers have a limited life time  4) The feasibility of storing a pressurized liquid  5) the large scale storage and transport all requires money!! So expensive, you can see from the slide 17, that imagine if you have 1000s of those tanks, just think each tank costs 300 pounds and if you have so many of them it would cost millions!

19. Hydrogen economy  Hydrogen can be made by reacting methane with stem Electrolysis of water! Remember common exam question!

20. Exam technique  I advise you all to the legacy questions on electrode potentials.  I will give you some good stretch and challenge questions for this topic:- ok hmmm  1) Why is the public worried about co2 being released into the air and how is co2 being made when energy is used (2 marks)  2) Give 2 substances that are needed in order for the fuel cell to be carried (2 marks)  3) Name the strongest oxidising and reducing agent (2)  4) When nitric acid and copper react, what gas is being produced and state the colour (3 marks)  These all A-A* questions, post your answers on TSR and I will be happy to help

21.  FINALLY!!  GOOD LUCK IN THE EXAM!! ANYTHING PLEASE TELL ME AND I WILL TRY MY BEST TO HELP YOU