Topic 9: Redox Processes

loss of electrons

gain of electrons

• the sum of all oxidation states in a compound always corresponds to overall charge • in a compound, oxidation state usually corresponds to group no (except for transition metals) • H is +1 (except when bonded to a metal, in which case it’s -1) • O is -2 (except in H2O2, in which case it’s -1) • F is always -1 • pure elements are always 0

AKA oxidant • readily accept electrons • thus gets reduced in a rxn

AKA reductant • readily donates electrons • thus gets oxidized in a rxn

• oxidation state increases when oxidized • vice versa for reduction • if there’s no change in oxidation state in a rxn, the rxn is not a redox rxn

• when oxygen is deficient, add H2O (l) - when hydrogen is deficient, add H+ (aq)

down the series: • ↓ reactivity • ↑ reducing ability • ↓ oxidizing ability • thus for metals, ↑ reactivity = ↑ reducing ability • as they are reductants, they undergo oxidation • more reactive metals can reduce less reactive metals • in redox rxns, this explains why H in acids can only be displaced by metals above it in the activity series

2M (s) + 2H2O (l) → 2M+ (aq) + 2OH- (aq) + H2 (g) M can only be alkali metals

M (s) + 2H2O (l) → MOH* (aq) + H2 (g) *dependent on M’s charge

down the series: • ↓ reactivity • ↓ reducing ability • ↑ oxidizing ability • thus for non-metals, ↑ reactivity = ↑ oxidizing ability • as they are oxidants, they undergo reduction

• application of redox process - used to measure BOD (biological oxygen demand)

Biological Oxygen Demand - measure of the dissolved oxygen (in ppm) required to decompose organic matter in water over a set time period (usually 5 days)

↑ pollution and ↑ BOD = cannot sustain aquatic life

• • a measured vol of the sample is incubated for 5 days - microorganisms in the water oxidize the organic material - after 5 days, excess Mn (II) salt is added • In alkaline conditions, Mn (II) is oxidized to Mn (IV) oxide by remaining oxygen: 2Mn2+ (aq) + 4OH- (aq) + O2 (aq) → 2MnO2 (s) + 2H2O (l) • KI is added, and in acidic conditions is oxidized by Mn (IV) oxide to form iodine: MnO2 (s) + 2I- (aq) + 4H+ (aq) → Mn2+ (aq) + I2 (aq) + 2H2O (l) • The released iodine is titrated with sodium thiosulfate: I2 (aq) + 2S2O3 2- (aq) → S4O6 2- + 2I- (Aq) • The oxygen concentration in the sample can be deduced by determining the number of moles of iodine released.

mass (in mg) dissolved in 1 dm^3 of water

metal in contact with an aq soln of its own ions

• 2 diff half cells • connected to enable e- transfer during redox rxns • in order to produce energy (in the form of electricity) • cells are connected by a salt bridge and external wire to enable free movement of ions • ions flow through the salt bridge to complete the circuit and keep half-cells electrically neutral • in voltaic cells, anode is negative (where oxidation occurs)

• half-cell undergoing oxidation is always placed on the left • 2 vertical lines ( || ) are used to separate them • presented as electrode | before , after || before , after | electrode

• substance that doesn’t conduct electricity when solid - but can conduct when liquid

• used to make non-spontaneous redox rxns occur • by supplying energy from an external power source • the electricity is passed through the electrolyte to allow electrical energy to convert into chemical energy • it’s industrially used to obtain reactive metals from their ores • in electrolytic cells, anode is positive e. g. electrolysis of molten NaCl

• used to determine the unknown concentration of a known analyte • finds equivalence point where the redox reaction has been stoichiometrically completed via e- transfer • doesn’t necessarily require an indicator (as some redox rxns change color at equivalence point)

• food industry • pharmaceutical industry • water and environment analysis

• analysis of iron content using KMnO4 (potassium permanganate) • as part of the winkler method: iodine-thiosulfate redox titration with starch as an indicator