electrode potentials and electrochemical cells

• redox reactions involve the transfer of electrons, which is associated with electrical energy. So, redox reactions can be used in electrochemical cells to generate electricity
• electrochemical cell: a device capable of generating a potential difference from redox reactions
• electrochemical cells consist of two half cells. In one half cell the oxidation reaction occurs, and in the other the reduction. electrons flow between the two cells driving the redox reaction. Half cells always contain an element in two different oxidation states. In each cell the metal is being oxidised and the ions reduced, forming an equilibrium.
• When writing half cell equations, the forward reaction should shoe the reduction (gain of electrons) and equilibrium signs must always be used as it is not a one way reaction
• If one of the oxidation states of a species is present as a gas, it can be bubbled through the solution at 100KPa, with a platinum electrode providing a surface for the reaction to take place on
• platinum electrode: inert so doesn't react with anything in the cell, conducts electricity so electrons can be lost or gained, coated in porous platinum black giving it a large surface area
• an electrode is a solid surface which allows the transfer of electrons to and from it
• If both species in a half cell are in aqueous solution, the solution must be eqimolar. With a platinum electrode present so electrons can be transferred between the two species.
• standard electrode potential: the voltage measured under standard conditions, when the half cell is connected to a standard hydrogen electrode
• standard conditions: temperature - 298K pressure - 100KPa concentration of solutions: 1 mol dm-3
• The standard electrode potential tells us a half cells tendency to accept or release electrons. If the equilibrium lies further to the left is better at donating electrons, itself being oxidised. While if the equilibrium lies further to the right it is better at accepting electrons, itself being reduced. The position of equilibrium can be measured using standard electrode potential
• the standard electrode potential of a cell is measured against a standard hydrogen electrode. Under standard conditions a hydrogen electrode has a standard electrode potential of 0V, so whatever the voltage displayed is the value for the half cell you are interested in. A voltometre and salt bridge should connect the two cells
• cell's are typically represented using shorthand. The half cell with the most negative potential goes on the left, while the half cell with the most positive potential goes on the right. A double line in the centre represents a salt bridge, and the species with highest oxidation states go closest to the salt bridge. A solid line represents a phase boundary between species if they are of different states, if they are of the same state a comma is used.
• the electrochemical series is a list of standard electrode potentials of all the possible half cells. Spieces on the left hand side of the half cell equation are more readily reduced and the right hand side are more readily oxidised. SO couples near the top of the electrochemical series favour the backwards (oxidation) reaction and couples near the bottom of the series favour the forwards (reduction) reaction
• The cell reaction with the more positive EMF proceeds in the forwards (reduction) direction and accepts electrons from the other half cell. It becomes the positive terminal of the electrochemical cell. The cell with the less positive EMF proceeds in the backwards (oxidation) direction and donates electrons to the other half cell. It becomes the enegative terminal of the electrochemical cell
• non-rechargeable batteries undergo a reaction that generates electrical energy until the chemicals within the battery have fully reacted. Then the battery must be disposed of and cannot go on to produce more energy
• In rechargeable batteries once chemicals have reacted fully a potential difference can be applied to the cell in the opposite direction. This will regenerate the original chemicals allowing us to reuse the battery
• Fuel cells differ from typical electrochemical cells as they don't have a stock of chemicals in the electrodes or electrolyte, instead chemicals are stored externally and fed into the cell when electricity is required
• in alkaline fuel cells oxygen and hydrogen gas are fed into two seperate tubes at the top of the cell, the gases cannot pass through the anion exchange membranes, but the OH- ions can. Electrons move from the negative to positive electrode, while OH- ions move from the positive to the negative electrode
• in alkaline hydrogen fuel cells the electrolyte allows transport of OH- ions, in acidic hydrogen fuel cells the electrolyte allows transport of H+ ions
• hydrogen fuel cell positive electrode: O2 + 2H20 + 4e- = 4OH- (reversible)
• hydrogen fuel cell negative electrode: 2H2 + 4OH- = 4H2O + 4e- (reversible)
• hydrogen fuel cell equation : 2H2 + O2 = 2H2O
• advantages of hydrogen fuel cells: more efficient than burning fossil fuels, release non-toxic and non-polluting water, don't need to be recharged as long as they have fuel they keep producing electricity
• disadvantages of hydrogen fuel cells: energy is needed to build the cell and produce the hydrogen. This energy is typically derived from burning harful fossil fuels. Hydrogen is also very flammable, making it difficult to safely store.
• non-rechargeable batteries are convinient and cost less per battery than the rechargeable alternative. However, are disposed of after use and build up in landfill, and over time may be more expensive as you must keep repurchasing them
• rechargeable batteries like lithium ion batteries are more environmentally friendly, as they are replaced less often so contribute less to landfill and conserve natural resources. However, they are toxic if ingested and can malfunction and explode