Electrode potentials and electrochemical cells

• What is the standard electrode potential?

  The emf of a half cell compared to the standard hydrogen half cell at a concentration of 1moldm^-3, a pressure of 100KPa and a temperature of 298K
• Draw the Daniel cell

  
• What is the purpose of the salt bridge and what is it?

  It is filter paper soaked in a salt solution(usually potassium nitrate KNO^3)

  The salt should be unreactive with the electrodes and electrode solutions. (otherwise the emf will be altered)

  It has 2 purposes:

  1) To complete the circuit

  2) It allows ions to flow
• What conditions do electrochemical cells have to be under?


  Standard conditions:

  100 KPa pressure

  Solutions 1 moldm^-3 (to get 1M of H^+ ions we need 1M HCl or 0.5M H^2SO^4 etc)

  298K temperature
• What is the EMF of a cell?


  The voltage
• What is EMF measured in?


  Volts (V)
• What is every electrochemical cell made up of? Use the Daniel cell as an example.


  2 half cells

  1) Zn^2+ + 2e^- ---> Zn

  2) Cu^2+ + 2e^- ---> Cu
• What does each half cell have?


  An associated EMF number
• What do we get when adding 2 half cells?


  A full redox cell
• Which of these 2 equations is oxidised and which one is reduced?

  1) Zn^2+ + 2e^- ---> Zn EMF = - 0.76V

  2) Cu^2+ + 2e^- ---> Cu EMF = + 0.34V


  NO = negative one is oxidised

  PR = positive one is reduced

  
  

  Cu is reduced and Zn is oxidised

  
  

  (If both are negative values, the one that is oxidised is the more negative one)

  (If both are positive values, the one that is oxidised is the less positive one)
• Calculate the redox equation for the 2 equations below:

  1) Zn^2+ + 2e^- ---> Zn EMF = - 0.76V

  2) Cu^2+ + 2e^- ---> Cu EMF = + 0.34V


  The equation with the more negative EMF value flips:

  
  

  Zn + Cu^2+ ---> Zn^2+ + Cu
• When finding the redox equation, why don't we include the electrons?


  They cancel out so you need to ensure both equations have the same number of electrons, if not you need to balance it.

  E.g. The 2 equations below both have 2 electrons

  1) Zn^2+ + 2e^- ---> Zn

  2) Cu^2+ + 2e^- ---> Cu
• What is the equation for the EMF of an electrochemical cell?

  Calculate the EMF for the equations below:

  1) Zn^2+ + 2e^- ---> Zn EMF = - 0.76V

  2) Cu^2+ + 2e^- ---> Cu EMF = + 0.34V


  E^cell = E^red - E^ox

  e.g. E^cell = 0.34 - (-0.76)

  = 1.10V

  
• In the equation below, what happens if you increase the concentration of Cu^2+:

  Zn (s) + Cu^2+ (aq) ---> Zn^2+ (aq) + Cu (s)

  EMF = 1.10V


  The reaction will favour the forward reaction, so the voltage will increase.
• What are the observations of the equation below if Cu^2+ ions are blue:

  Zn (s) + Cu^2+ (aq) ---> Zn^2+ (aq) + Cu (s)


  1) Cu^2+ ions are being used up so the colour of the solution begins to go lighter

  2) Copper is being produced so the raw copper begins to stick to and coat the copper electrode
• Draw the cell representation of:

  Zn (s) + Cu^2+ (aq) ---> Zn^2+ (aq) + Cu (s)


  You write the more oxidised form of the metal (higher oxidation number) next to the salt bridge

  The more reactive one/ the metal which is oxidised (the beaker where the electrons are coming from) is the one written on the left

  
• How do you know which of the metals is more reactive?

  You write out the half equations so you can see the loss of electrons:

  Zn ----> Zn^2+ + 2e^- EMF = + 0.76V

  Cu ----> Cu^2+ + 2e^- EMF = - 0.34V

  The EMF values have had their sign flipped from before because we have written the equation the other way around.

  The higher EMF value (when losing electrons) is the more reactive one so Zn is more reactive than Cu
• What do you do if one of the half cells is missing its electrode?


  You add in an electrode that is both inert but can still conduct e.g. platinum or graphite
• If there's no state change between species, what do you write in the cell representation?


  You write a comma instead of a line
• Where do we get our half cell EMF values from?


  The standard hydrogen half cell
• Draw the standard hydrogen half cell


  You have to name the acid, you can't just write H^+ ions

  The acid is most likely nitric acid (HNO^3)

  
• How does the standard hydrogen half cell work?


  It is connected to the half cell whose EMF value you want to find. The value on the voltmeter is the EMF value for the other half cell because the EMF value of the standard hydrogen half cell is 0.00V
• Write the half equation for the standard hydrogen half cell.


  2H^+ (aq) + 2e^- ----> H^2 (g)
• Consider the equations below, which is the most powerful oxidising and reducing agent? How can we remember this?:

  Li^+ + e ^- ----> Li EMF = -3.03

  Mn^2+ + 2e^- ----> Mn EMF = -1.19

  2H^+ + 2e^- ---------> H^2 EMF = 0

  Ag^+ + e^- -----> Ag EMF = +0.8

  F^2 +2e^- ------> 2F^- EMF = +2.87


  BLOXTR - "Bottom Left Oxidises Top Right"

  TRRA - "Top Right Reducing Agent"

  Most powerful oxidising agent = F^2

  Least powerful oxidising agent = Li^+

  Most powerful reducing agent = Li

  Least powerful reducing agent = F^-

  
  

  Mn^2+ can oxidise Li but not H^2

  Ag reduces F^2 but not H^+

  
  

  (HAS TO BE ORDERED -ve ---> +ve TO WORK SO YOU MAY NEED TO REORDER THE QUESTION)
• What can electrochemical cells be useful for?


  A useful source of energy for commercial use.
• What are the types of commercial electrochemical cells?


  Rechargeable cells

  Non-rechargeable cells

  Fuel cells
• What kind of a reaction must take place within a cell for it to be rechargeable?


  Reversible
• What is a rechargeable cell?


  The reaction takes place within a rechargeable cell is called a reversible reaction, meaning the reactants can reform. Therefore, the cell can be 'reformed' meaning it's a rechargeable cell.
• What is a commonly used rechargeable cell and what are they used in?

  Lithium ion cells are commonly used as rechargeable batteries in phones, laptops and cars.
• What does a lithium ion cell consist of?


  A lithium cobalt oxide electrode and a graphite electrode.

  An electrolyte of a lithium salt in an organic solvent is used to carry the flow of the charge
• Draw a lithium ion cell


  Check notes
• What are the half cell equations for the lithium ion cell?


  Negative electrode: Li^+ + e^- ⇌ Li

  Positive electrode: e^- + Li^+ + CoO^2 ⇌ Li^+[CoO^2]^-
• How can a rechargeable cell be charged?


  A current has to be applied over the cell which forces electrons to move in the opposite direction. This causes the reaction to reverse, recharging the cell
• What are some advantages of rechargeable cells?


  1) Have a longer life than non-rechargeable batteries

  2) Can be recharged hundreds of times

  3) Some rechargeable batteries last longer on a single charge than non-rechargeable batteries

  4) More energy efficient as charging takes less energy than making new batteries
• What are some disadvantages of rechargeable cells?


  1) Need to be regularly recharged

  2) Cost more than non-rechargeable batteries

  3) Not all appliances are suitable to used with rechargeable batteries
• Why can't non-rechargeable cells be charged?


  The reactions are irreversible
• What is an example of a non-rechargeable cell?


  The Daniel Cell
• What are some advantages of non-rechargeable cells?


  1) Cheaper than rechargeable cells

  2) Portable and easy to replace

  3) Easy to use and has a wide range of applications
• What are some disadvantages of non-rechargeable cells?


  1) Non-sustainable (materials to make the cells are finite)

  2) The casing of the battery can be corroded by the electrolyte, causing leakages

  3) Once the reactants have been used up, the battery must be disposed of. Toxic chemicals can leach from landfill sites into water sources
• What are fuel cells?


  This type of electrochemical cell is used to generate an electrical current without needing to be recharged.