Electrode Potentials and Fuel Cells

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  • Created by: ava.scott
  • Created on: 03-02-15 22:07

REDOX

Redox: when both oxidation and reduction take place

oxidation number: allows you to see the oxidation state of an atom/molecule e.. if they have lost or gained lectrons

half-reaction: Showing the loss or gain of electrons from a species

Oxidising agent: oxidises species, by taking there electrons, making them positive 

Reducing agent: reduces species by giving electrons,  making them more  negative. 

REDOX equation: combines two half equations

MAKE PREDICTIONS FOR REACTIONS????

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Electrode Potentials

Standard electrode potential

The electron potential of a cell compared to a hydorgen half cell under all standard conditions e.g. 298k, 1 atm, and all solutions 1moldm-3.

Measuring electrode potential of a metal which is in contact ith its metal ions

Hydrogen gas cahmber, with platnum electron and c.HCl

Metal solid dipped in solution with ions

Measuring electrode potential of ions of the same element in different oxidation states

Hydrogen half cell withj gas chamber, platinum electrode and HCl solution

Platinum electrode dipped in solution with ions of both oxidation states.

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Feasability of reactions

  • difference of 0.4V????

not just o.4 v-- other limitations

  • concentration of the ions/acid may be wrong, forcing equlibirum in the opposite way as intended
  • too high an activation energy
  • circuit incomplete

PREDICTIONS ARENT ALWAYS USEFUL BECAUSE

  • predictions cannot be made about rate, which could be slow because of an extremely high activation energy
  • different conditions may be used in real life, so the elcytrode potential will be different
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Storage and fuel cells

Modern cells and batteries: three types

  • Non-rechargeable- provide electrical energy until the chemicals have reacted to such an extent that voltage fails.
  • Rechargeable cells- the cell recation can be reversed during recharging. e.g. lithium ions
  • fuel cells- the cell reaction uses a constant supplie of fuel and a oxidant, which are consumed, and must be provided constantly.
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Benefits of methanoic acid

compared to H2

  • HCOOH is a liquid so more easily stored
  • Less explosive/flammable than H2
  • Great voltage/e.m.f difference
  • HCOOH is more sociallya ccepted or recognsied.
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CHECK

when asked to find one of the half equation from an overall equation + one other half equation:

  • remove reactants and products provided by the otehr half equation.
  • Balance oxygens with water/ oh- IS ALKALINE
  • balance hydrogens with protons IF ACIDIC

When asked to write overall reaction using EMF

  • reverse one with negative emf- are the right reactants still provided??
  • if not there must be another equation.
  • Multiply equatiosn to balance electrons.
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