Redox and Standard Electrode Potential
- 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
- Problems
- Placed in burette
- Deep purple --> colourless
- Potassium dichromate (VI)
- Orange to green
- Indicator require as colour change not as obvious
- Sulphuric acid as Hal chloride ions would interfere
- Orange to green
- Iodine
- Brown to colourless
- Potassium manganate (VII)
- An oxidising agent is a substance that has been oxidised in a reaction
- 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
- 3. Add H+ to balance H
- 2. Add waters to balance oxygens
- 1. Work out the balanced formal of the species before and after
- 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
- 3. Add the two equations and canal out the electrons on either side
- 2. Multiply the equations so that the number of electrons in each is the same
- 1. Write out the two half equations
- 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)
- Equilibrium lies to the left
- 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
- Equilibrium lies to the right
- 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
- Platinum electrode used when there is no solid metal involved in the half equation
- 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
- A standard electrode that has been calibrated against the primary standard
- Secondary Standards
- 0 volts
- The potential to which all others are compared
- 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
- SHE
- emf= potential= Right - Left
- A measure of the maximum amount of energy which can be given by the cell
- Oxidation-anode-negative
- When a piece of metal is dipped into a solution of its meal ions an equilibrium is set up
- 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
- An equation with a more positive value will reverse a less positive one
- Electrode potentials can be used to predict feasibility
- 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
- Safety
- 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
- Reduction is gain of electrons
- 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
- Oxidation is loss of electrons
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