module 5 (chapter 23) - electrode potentials and redox

transfer chemical energy into electrical energy -electrical energy is produced by the movement of electrons and so redox reactions are needed as these transfer electrons

• contains a chemical species present in a redox half-equation • the chemicals of the two cells are kept apart as if allowed to mix electrons flow uncontrollably releasing heat energy not electrical

metal rod dipped into a solution of its aqueous metal ion • the phase boundary is where the metal is in contact with its ions and this sets up an equilibrium • phase boundary is shown by a vertical line in the equation • in an isolate half cell, there is no net transfer of electrons either into or out of the cell. • when there are two, the direction of flow depends upon the relative tendency of each electrode to release electrons

• Contains ions of the same element in different oxidation states. • An inert electrode made out of platinum is used. These help to transport electrons either into or out of the half-cell

• The electrode with more reactive metal loses electrons and is oxidised. This is the negative electrode • The electrode with the less reactive metal gains electrons and is reduced. This is the positive electrode.

• The e.m.f of a half-cell connected to a standard hydrogen half-cell under standard conditions. • A standard Half-cell contains hydrogen gas, a solution containing H+ ions and an Inert platinum electrode

• Concentration of exactly 1moldm-3 • 298K • 100Kpa

• The more negative the electrode potential value, The greater the tendency to lose electrons and undergo oxidation. • The more positive the electrode potential value, The greater the tendency to gain electrons and undergo reduction. • Metals tend to have more negative electrode potential values than non-metals.

E(positive electrode) – E(negative electrode)

• They do not give an indication of rate of reaction of activation energy • If the concentration is different to 1moldm-3, it won’t work. • The actual conditions may be different to standard. • standard electrode potentials apply to aqueous equilibria, many reactions take place that are not aqueous • f the potential difference is less than 0.4V, the reaction is unlikely to happen

• allows ions to flow and contains a solution that doesn’t react with the half cell solutions • e.g. filter paper socked in KNO3

-reduction as the forwards reaction

• the concentration of the electrolyte chances - this means that the conditions are standard

• electrical energy is produced by redox reactions at the electrodes. • however, the reactions cannot be reversed. this means they are non-renewable and therefore when chemicals are used up they will go flat

• Low current, long-storage devices such as wall clocks and smoke detectors • Most modern primary cells are alkaline based on Zinc and Magnesium dioxide, and a potassium hydroxide electrolyte solution

• They have high specific energy • Light weight so can be carried around on aircrafts • Cheap to make and buy • Long storage times

• non-renewable and therefore can only be used once • Made from toxic materials (heavy metals) • Difficult to dispose of and often sent to landfill.

• electrical energy produced by redox (simultaneous oxidation and reduction) • They can be recharged as the cell reaction producing the electrical energy can be reversed • The chemicals in the cell are regenerated and can be used again • The recharging process is just the opposite of the discharging reaction so is essentially the redox reaction in reverse for recharging.

• Lead-acid batteries used in cars • Nickel-cadmium cells form the batteries used in radios and torches • Lithium ion cells are used in laptops, camera and mobile phones. They are also being developed for use in cars.

• They use less toxic materials and therefore toxic waste. • Sometimes generates hydrogen gas at the cathode when charging • Produce less waste because they can be recharged • Motion can be used to recharge batteries

• more expensive • Don’t have a huge lifespan • Extremely sensitive to high temperatures and therefore are prone to exploding and burning

• Anode is where oxidation takes place (electrically negative), cathode is where reduction takes place (electrically positive) • Electrodes are porous and made of platinum materials reinforced carbon – acts as catalysts • can have either an acid or alkali electrolyte

• use the energy from a reaction of a fuel with oxygen to create voltage • the fuel and oxygen flow into the fuel cell and the products flow out. the electrolyte remains in the cell • they don’t need to be recharged and can operate continuously along as they are supplied.

• Produced in an environmentally manner • Water and heat are the only by-product • More efficient than combustion engines • Less noise • Never run flat if you have a constant supply of fuel and therefore are more reliable. Secondary cells eventually become less efficient, primary cells are non-renewable as you have to replace the cell when it runs out.