Redox and Standard Electrode Potential

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  • Created by: dsmeikle
  • Created on: 26-12-16 19:07
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  • Redox and Standard Electrode Potential
    • Reduction is gain of electrons
      • An oxidising agent is a substance that has been oxidised in a reaction
        • Potassium manganate (VII)
          • Deep purple --> colourless
            • MnO4- ions no longer being reduced
          • Powerful- used to determine reducing agents
            • Placed in burette
              • Problems
                • Intense colour means difficult to see graduation marks on the burette- less accurate
                • KMNO4 reacts slightly with the glass causing a slight stain if the burette is used too often
              • Easier to detect than the gradual disappearance of the pink colour if the other way around
              • Excess sulphuric acid in conical flask
        • Potassium dichromate (VI)
          • Orange to green
            • Indicator require as colour change not as obvious
          • Sulphuric acid as Hal chloride ions would interfere
        • Iodine
          • Brown to colourless
    • Balancing half equations
      • 1. Work out the balanced formal of the species before and after
        • 2. Add waters to balance oxygens
          • 3. Add H+ to balance H
            • 4. Add e- to balance charge
    • Combining Half equations
      • 1. Write out the two half equations
        • 2. Multiply the equations so that the number of electrons in each is the same
          • 3. Add the two equations and canal out the electrons on either side
            • 4. If necessary, cancel any other species which appear on both sides
    • Electrode Potentials
      • When a piece of metal is dipped into a solution of its meal ions an equilibrium is set up
        • The tendency for the metal to form positive ions and go into solution
        • The tendency for the metal ions in solution to gain electrons and form metal
        • Negative potential
          • Equilibrium lies to the left
            • Metal acquires a negative charge due to a build up of electrons on the metal (the electrode has a negative potential)
        • Positive Potential
          • Equilibrium lies to the right
            • Positive charge builds up on the metal as electrons have been used up to form metal from the metal ions
        • Position depends on the metal e.g. more reactive metals tend to form ions and so negative charge builds up on the metal
        • Half-cell or Electrode= Metal dipping into a solution of its ions
          • Platinum electrode used when there is no solid metal involved in the half equation
            • Inert (unreactive)
            • Gas electrodes
              • Gases in contact with solutions of their ions
            • Redox electrodes
      • To measure the potential the half-cell must be connected to another of known potential and the potential difference between the two measured
        • SHE
          • Standard electrode potential
          • The primary standard
            • The potential to which all others are compared
              • Cell Notation: Pt(s) l H2(g) l H+(aq)
              • The SHE is always on the left
              • Difficult to use
                • Secondary Standards
                  • A standard electrode that has been calibrated against the primary standard
                    • Silver/silver chloride
                    • Calomel electrode
            • 0 volts
          • Salt bridge
            • Joins two half-cells
            • Allows ions to move from one hal-cell to the other whilst solutions remain separate
            • Tube containing saturated solution go KCL or KNO3 in agar jelly
          • Standard conditions required- equilibrium unaffected
            • Temperature: 298K
            • H2 gas at 100kPa
            • 1M H+ (aq)
        • Electrochemical cell
      • emf= potential= Right - Left
        • A measure of the maximum amount of energy which can be given by the cell
      • Oxidation-anode-negative
    • Cell notation
      • l represents a phase boundary (i.e. between species in different states)
      • ll represents a salt bridge
      • Solid electrodes written at the two ends
        • Cell reaction goes from left to right
      • ROOR- Reduced, oxidised, oxidised, reduced
      • The half-cell with the more positive potential is set up on the right and drawn on the right
        • Except when measuring against primary or secondary standards- always the left
    • Feasibility
      • Electrode potentials can be used to predict feasibility
        • An equation with a more positive value will reverse a less positive one
          • When the equations are combined add the numerical values and if the value is positive the section is feasible
    • Fuel Cells
      • Generate electricity through electrochemical reactions which oxygen and hydrogen combine to form water
      • Advantages
        • Alternative for fossil fuels which are non-renewable and which we are currently dependent on
        • Possible solution to fuel prices rising and resources dwindling
          • Food, transport and electricity are affected by fuel prices
        • Possible solution to global warming crisis
          • Eliminates greenhouse gases if hydrogen is produced by electrolysis
          • Clean- Only by-product is water
        • Eliminates economic dependence on politically unstable countries for fossil fuels
        • Higher efficiency than diesel or gas engines
          • Convert chemical energy directly to electrical energy with no combustion process
        • Some have low heat transmission- ideal for military applications
        • Maintenance is simple since there are few moving parts in the system
        • Quiet operation
      • Disadvantages
        • Safety
          • Dangers associated with storage of hydrogen
          • Feasibility of liquified hydrogen under pressure
        • Transport
          • Low density= expensive
        • Economy
          • Limited life of absorber/ absorber
          • Limited life cycle cell
            • Driving range shorter than in traditional vehicles
          • Refuelling and starting times of fuel cell vehicles (FCV's) are longer
          • Expensive to produce- most are hand made
          • Net energy loss- energy required to produce hydrogen is more than that produced in its use
        • Environment
          • Toxic chemicals used in cell production
    • Uses
      • Metals extracted from their ore by reduction
    • Electrochemical Series
      • The most powerful reducing agents have the most negative electrode potential= top
      • The most powerful oxidising agents have the post positive electrode potentials= bottom
  • Redox Tiitrations
    • Oxidation is loss of electrons
      • A reducing agent is a substance that has been reduced in a reaction
    • Disproportionation
      • When one species is simultaneously oxidised and reduced

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