CHEM UNIT 5: ELECTROCHEMISTRY

  • REDOX REACTIONS
  • ELECTRODE POTENTIALS
  • THE ELECTROCHEMICAL SERIES
  • REDOX TITRATIONS
  • MORE REDPOX TITRATIONS
  • USES OF FUEL CELLS
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  • Created by: Chynna
  • Created on: 26-03-13 12:00

REDOX REACTIONS

If electrons are transferred - it's a redox reaction

  • OILRIG
  • Reduction and oxidation happen simultaneously
  • an oxidising agent accepts electrons and gets reduced
  • a reducing agent donates electrons and gets oxidised

Oxidation numbers

  • all atoms treated as ions for this even if they're covalently bonded
  • uncombined elements have an oxidation no. of 0
  • elements just bonded to identical atoms, like O2 have an oxidation no. of 0
  • oxidation no. of a simple monatomic ion (Na+), is the same as it's charge
  • in compounds - overall oxidation no. is just the ion charge
  • sum of oxidation nos. for a neutral compound is 0
  • combined oxygen is nearly always -2, except in peroxides, where it's -1
  • combined hydrogen is +1, except in metal hydrides where it is -1 (H2 = 0)
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REDOX REACTIONS

Roman numerals give oxidation numbers - e.g. copper has oxidation no. +2 in copper(II) sulfate and manganese has oxidation no. +7 in a manganate(VII) ion(MnO4-)

oxidation states go up or down as electrons are lost or gained

  • oxidation state for an atome will increase by 1 for each electron lost
  • oxidation state will decrease by 1 for each electron gained
  • an element can be reduced and oxidised at the same time - disproportionation

you can separate redox reactions into half-equations

  • ionic half equations show oxidation or reduction
  • can both be combined to make a full equation
  • the no. of electrons lost and gained in the reaction must balance - so oxidation numbers must also balance.
  • e.g. zinc metal displaces silver ions from silver nitrate solution to form zinc nitrate and a deposit of silver metal
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ELECTRODE POTENTIALS

Electrochemical cells make electricity

  • can be made from 2 different metals dipped in salt solutions of their own ions and connected by a wire (the external circuit). Always 2 reactions within an electrochemical cell - one is oxidation and the other reduction - so it's a redox process

e.g zinc/copper electrochemical cell

  • zinc loses electrons more easily than copper - so in half cell on left, zinc (from zinc electrode) is oxidised to form Zn2+ ions - releases electrons into the external circuit
  • in the other half cell, the same number of electrons are taken from the external circuit, reducing the Cu2+ ions to copper atoms
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ELECTRODE POTENTIALS

  • Electrons flow through the wire from the most reactive metal to the least. 
  • a volmeter in the external circuit shows the voltage between the 2 half cells = cell potential or emf
  • you can also have half cells involving solutions of 2 aqueous ions of the same element, such as Fe2+/Fe3+ - the conversion between both happens on surface of electrode

electrode potentials are measured against a standard hydrogen electrode

  • standard electrode potential of a half cell - voltage measured under standard conditions when the half cell is connected to a standard hydrogen electrode
  • standard hydrogen electrode contains a piece of platinum foil submerged in a 1mol/dm3 solution of H+ ions. H2 gas is bubbled through at a pressure of 101kPa - platinum electrode is platinized - surface of foil is covered in finely powdered platinum - increasing its surface area. The platinized surface absorbs hydrogen gas and an equilibrium is set up.
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ELECTRODE POTENTIALS

The electrode potential of the half cell is defined as 0 - known as a reference cell. All other standard electrode potentials are measured against it in a set up like this one:

  • the standard hydrogen electrode is always shown on the left
  • E  represents the standard electrode potential of a half cell
  • the whole cell potential = E  right hand side - E  left hand side
  • E  left hand side = 0.00V - this means that the measured cell voltage will be the standard electrode potential for the right hand half cell. It could be negative or positive depending on which way the electrons flow.
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ELECTRODE POTENTIALS

to find the standard cell potential - combine the 2 half cells

E  cell = E  right hand side - E  left hand side

  • we make the half cell with the more negative standard electrode potential the left hand one
  • the cell potential will always be a positive voltage because the more negative E  value is being subtracted from the more positive E  value. If the positions of the half-cells were swapped over then the size of the voltage would have the same value but would be negative.

to write overall equation - combine the half-equations

  • always write the oxidised substance first
  • so the top reaction goes backwards and bottom reaction goes forwards
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THE ELECTROCHEMICAL SERIES

- shows you what's reactive and what's not

  • the more reactive a metal is, the more it wants to lose electrons to form a positive ion - have more negative standard electrode potentials
  • the more reactive a non-metal is, the more it wants to gain electrons to form a negative ion - have more positive standard electrode potentials

the anticlockwise rule predicts whether a reaction is likely to happen

For example - will zinc react with copper ions?

  • write down the 2 half equations - with the more negative standard electrode potential on top - then draw anticlockwise arrows - gives you direction of each half reaction
  • to find the cell potential you always for E  bottom - E  top
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THE ELECTROCHEMICAL SERIES

sometimes the prediction is wrong

  • anticlockwise rule tells you if a reaction is feasible under standard conditions
  • if the cell potential is positive and greater than about 0.4V, the reaction should go to completion
  • if the value is between 0.0 - 0.4V then the reaction will still happen, but it'll be reversible

could be wrong if:

  • the conditions are not standard - changing conc or temp of the solution
  • the reaction kinetics are not favourable - rate of reaction may be so slow that the reaction might not appear to happen or if a reaction has a high activation energy, this may stop it happening

cell potential is related to entropy and the equilibrium constant

- bigger the cell potential, the bigger the total entropy change taking place during the reaction in the cell.

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USES OF FUEL CELLS

-generate electricity by reacting a fuel with an oxidant

  • Fuel usually hydrogen and oxidant usually oxygen
  • At the anode the platinum catalyst splits the H2 into protons and electrons
  • The polymer electrolyte membrane (PEM) only allows the H+ ions across and this forces the e- to travel around the circuit to get to the cathode
  •  Electric current is created in the circuit – used to power something like a car, a bike – voltage produced is usually 0.6V
  • At the cathode, O2 combines with the H+ ions from the anode and the e- from the circuit to make H2O – the only waste product
  • The cell goes on producing current as long as it’s supplied with hydrogen and oxygen 

Hydrogen seems like the perfect fuel – only waste product is water, no greenhouse gases and supply is limitless BUT it has to be made first.

  •  Most hydrogen is produced by reacting natural gas w/ steam – gives CO2 as a waste product and needs fossil fuels to heat the process.
  • Can also be made from electrolysis of water but this needs lots of electricity which is usually generated by fossil fuels 
  • Only way to sustainably produce hydrogen without causing pollution is to generate electricity using a renewable source – hydroelectric, solar, wind
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USES OF FUEL CELLS

Hydrogen is only really as clean as the way you produce it

Fuel cells don’t just use hydrogen

Scientists developing fuel cells that are hydrogen-rich fuels – have a high percentage of hydrogen in their molecules and can be converted into H2 in the car by a reformer – these fuels include methanol and ethanol.

There are also new generation of fuel cells that can use alcohols directly without having to reform them to produce hydrogen 

The alcohol is oxidised at the anode in the presence of water  

The H+ ions pass through the electrolyte, and are oxidised themselves to water

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USES OF FUEL CELLS

Advantages of using alcohol instead of hydrogen in fuel cells

  •  Higher hydrogen density than liquefied hydrogen – more hydrogen atoms per dm3
  • Already an infrastructure in place for making both methanol and ethanol on a large scale and can both be produced from renewable biomass
  • Alcohols are liquids at room temp so they don’t need special refrigerated storage
  • Methanol can be made from CO2 so it offers a possible way to reduce CO2 levels in the atmosphere

Fuel cell vehicles have some important advantages over normal cars 

  • Produce far less pollution than a regular car when in use – only waste product is water and even those that run on hydrogen-rich fuels produce a lot less CO2 than petrol or diesel engines
  •  A fuel cell is at least twice as efficient as at converting fuel to power as a petrol engine 

Making fuel cells is expensive and have to be made using toxic chemicals which then need to be disposed of. Also have a limited life span – have to keep making new ones and getting rid of old ones

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USES OF FUEL CELLS

Fuel cells have other uses too

  •  Used in space – space shuttle uses 3 fuel cell power plants, each made up of many fuel cells, that can produce up to 12kW of power – water is used as drinking water by astronauts. Conventional batteries weigh too much to be used.
  • Used in breathalysers – amount of alcohol in someone’s breath is directly related to the amount in their blood stream. Alcohol vapour is breathed out with CO2 as the blood passes through the lungs. 2100cm3 of breath contains the same amount of alcohol as 1cm3 of blood. The amount can be measured using a breathalyser 

Old way of detecting alcohol in the breath – use reaction between ethanol and potassium dichromate(VII), orange dichromate is reduced to green chromium as ethanol is oxidised to ethanoic acid. How much the colour changes is measured using a photocell system

Breathalysers used in police stations – use IR spectrometry to detect the presence and quantity of ethanol – machines are very accurate but not usually easily portable

Recently – use an ethanol fuel cell – amount of alcohol is proportional to the current produced when the suspect’s breath is fed to the anode of the cell. These devices are less susceptible to giving false readings due to other substances in the suspect’s breath – also easily portable and accurate  

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