Cells and redox

• Oxidation is loss of electrons and reduction is gain of electrons.
• Oxidation and reduction occur simultaneously in a reaction because one species loses electrons which are then gained by other species.
• The oxidation state of an element is zero.
• Oxidation states in a neutral compound add up to to zero.
• Oxidation states in a charged compound add up to the charge.
• Hydrogen has an oxidation state of + 1.
• Oxygen has an oxidation state of - 2.
• An oxidising agent accepts electrons from the species being oxidised and is therefore reduced itself, and visa versa for a reducing agent.
• Electrochemical cells use redox reactions as the electron transfer between products creates a flow of electrons, and thus a current and potential difference between the two electrodes.
• A salt bridge connects the two half cells and carries the flow of charge, but is made of unreactive ions so will not interfere with the reaction.
• Conventional cell diagrams have the positive half cell on the right and the most oxidised species next to the salt bridge.
• A change of phase is represented by a line and a salt bridge a double line.
• The standard hydrogen electrode has a cell potential of zero at standard conditions, and all other potentials are measured against it, directly or indirectly.
• SHE consists of hydrochloric acid, hydrogen gas and platinum electrodes.
• Negative potentials mean the substances are easily oxidised and will lose electrons, and positive potentials mean the substances are easily reduced and will gain electrons.
• Cell EMF is the potential of the right cell minus the potential of the left cell (positive minus negative).
• Increasing the concentration of the solutions used makes the cell EMF more positive as fewer electrons are produced.
• Increasing the pressure of the cell makes the cell EMF more negative as more electrons are produced.
• Lithium ion cells are used as rechargeable batteries and consist of a lithium cobalt oxide electrode and a graphite electrode, with an electrolyte of lithium salt. Applying a current reverses the reaction.
• Fuel cells can generate an electrical current without needing to be charged, and just requires a continuous supply of hydrogen and oxygen from air.
• Fuel cells only produce water as a waste product and so are environmentally friendly, however hydrogen is flammable and they are very expensive to produce.
• An alkaline hydrogen fuel cell has the half equation H2 + 2 OH - -> 2 H2O + 2 e - at the negative electrode and the half equation O2 + 2 H2O + 4 e - -> 4 OH - at the positive electrode.
• An acidic hydrogen fuel cell has the half equation H2 -> 2 H + + 2 e - at the negative electrode and the half equation O2 + 4 H + + 4 e - -> 2 H2O at the positive electrode.
• A hydrogen fuel cell has the overall equation 2 H2 + O2 -> 2 H2O and EMF + 1.23 V.