redox

Redox reaction

rx where one substance oxidised, another reduced

Reductant

Substance oxidised and causes another to be reduced

Oxidant

Substance reduced and causes another to be oxidised

Metal displacement reaction

more reactive metal on bottom rhs displace less reactive metal above left, more reactive metals are stronger reductants and oxidise more readily, lower in ptable - lower ionisation energy - more reactive

Halogen displacement reaction

more reactive halogen displace less reactive halogen from solution, more reactive halogens stronger oxidants and reduced easily, higher in group - higher electron affinity - more reactive, top left of srp, less reactive bottom right, reactive halogen become halide

Disproportionation

Some substances can be both oxidised and reduced

Displacement

more reactive substance displaces less reactive substance from solution

galvanic cell

convert chemical potential energy into electrical, spontaneously occurring redox is used to drive electrical current

why is salt bridge/porous barrier used in connection of half cells

prevent direct contact between oxidant and reductant while permitting ions to flow thru half cells, maintain electrical neutrality. without salt bridge, pos charge build up at anode as electrons are lost, neg charge build up at cathode as electrons are gained, preventing flow of electrons thru external circuit

purpose of electrolyte solution

electrodes immersed in soln - allow charge to flow as ions

flow of ions in galvanism

anion to anode to replace lost electrons, cation to cathode to balance gained electrons thru salt bridge

emf (electromotive force)

one half cell has higher tendency to move electrons thru circuit than other. electrons are pushed by reductant, pulled by oxidant - potential diff between half cells

standard conditions

25 deg, 1atm, 1mol/l

limitation of srp

measured under standard conditions. under non standard conditions, values not reliable

dry/leclanche/primary cell

e.g. alkaline batteries. graphite cathode, zinc anode, ammonium chloride electrolyte paste. non rechargeable, redox only in fwd dx

lead acid accumulator/secondary cell

used in car batteries. lead sulfate coats electrodes which enables battery to be recharged when alternator provides 14v to reverse rx

fuel cell

fuel not stored, need to be continuously supplied. electricity produced as long as fuel supplied. h2 anode, o2 cathode. electrodes must be conducting and porous. catalysts can be added to increase rx. adv - cleaner and more efficient bc direct conversion of chem to electr w less heat waste. disadv - expensive, fuel need to be constantly supplied.

corrosion

result of redox rx, react w o2. strongest reductants react most vigorously

surface coating

prevent water/oxygen from contacting surface by coating surface in non metal or metal coating e.g. paint, tin

galvanising

coat metal w more reactive metal. if surface scratched, more reactive metal oxidise more readily. more reactive metal create protective oxidised layer stopping oxygen reaching metal

sacrificial anode

connect iron/steel to more reactive metal using wire. the more reactive metal is oxidised in preference to iron/steel

cathodic protection

iron cathode by connecting it to neg terminal of power source while connecting scrap metal anode to pos terminal, make iron neg charged and preventing oxidation bc electrons pushed towards it, not lost from it

structure of electrolysis

power source to provide flow of electrons, two inert electrodes, molten or salt soln electrolyte, no salt bridge

electrolytic cathode

attached to neg terminus of battery, electrons travel thru circuit from battery to cathode

electrolytic anode

attached to pos terminus of battery, electrons from oxidation move thru circuit to power supply

electrolysis

mvt of electrical energy from power supply thru conductive liquid. electrical energy converted into chemical. electrical current from battery causes non-spontaneous redox at anode and cathode. electrodes not separated but products are to prevent reverse rx

electroplating

make thin surface coating of one metal over another

electrorefining

smelting is used to produce impure metal. electrons drawn out of impure metal to pos terminal, producing ions (ox). pure metal is present and atoms are deposited as power source accepts electrons from ion (red). copper is strongest oxidising agent present, so only metal formed