Topic 9: Redox Processes Flashcards
oxidation
loss of electrons
reduction
gain of electrons
rules for oxidation states
- 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
oxidizing agent
AKA oxidant
- readily accept electrons
- thus gets reduced in a rxn
reducing agent
AKA reductant
- readily donates electrons
- thus gets oxidized in a rxn
oxidation and reduction in terms of oxidation states
- 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
rules for balancing redox equations
- when oxygen is deficient, add H2O (l)
- when hydrogen is deficient, add H+ (aq)
redox tendency trends in activity series
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
redox reactions: highly reactive metals + cold water
2M (s) + 2H2O (l) → 2M+ (aq) + 2OH- (aq) + H2 (g)
M can only be alkali metals
redox reactions: less reactive metals + steam
M (s) + 2H2O (l) → MOH* (aq) + H2 (g)
*dependent on M’s charge
redox tendency trends in Group VII
down the series:
- ↓ reactivity
- ↓ reducing ability
- ↑ oxidizing ability
- thus for non-metals, ↑ reactivity = ↑ oxidizing ability
- as they are oxidants, they undergo reduction
winkler method
- application of redox process
- used to measure BOD (biological oxygen demand)
BOD
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)
implications behind BOD values
↑ pollution and ↑ BOD = cannot sustain aquatic life
winkler method - process
- 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
- a measured vol of the sample is incubated for 5 days
- 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.
ppm
mass (in mg) dissolved in 1 dm^3 of water
half-cell
metal in contact with an aq soln of its own ions
voltaic cell
- 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)
rules for writing cells
- 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
electrolyte
- substance that doesn’t conduct electricity when solid
- but can conduct when liquid
electrolytic cell
- 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
redox titrations
- 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)
when redox titrations are used
- food industry
- pharmaceutical industry
- water and environment analysis
examples of common redox titrations
- analysis of iron content using KMnO4 (potassium permanganate)
- as part of the winkler method: iodine-thiosulfate redox titration with starch as an indicator
redox half-equations of the iodine-thiosulfate reaction
oxidation: 2S2O3 2- –> S4O6 2- + 2e-
reduction: I2 + 2e- –> 2I-
!! remember: halogens are very strong oxidizing agents
standard conditions to measure standard electrode potential
- conc: 1 mol dm^-3
- pressure: 100kPa
- all pure substances
- temp: 298K
- if no metals are present or can conduct, electrodes must be Pt
electrolysis: physical observations of bromine
- brown liquid
- strong smell
electrolysis: physical observations of copper
pinkish metallic solid
electrolysis: physical observations of chlorine
- greenish-yellow gas
- pungent smell
electrolysis: physical observations of lead
grey metallic solid
calculating electrolysis products
Q (charge) = lt (current x time)
no of moles of e-s = Q / F (F = faraday’s constant)
electrolysis of aqueous solutions
- ## H2O may be oxidized/reduced in place of one or both reactants
factors affecting the outcome of electrolysis of aqueous solutions
- relative cell potential values of the ions
- relative concentrations of the ions
- the nature of the electrode
special consideration for the electrolysis of NaCl (aq)
- at the cathode either OH- or Cl- may be produced
- Cl- is preferentially discharged when the concentration by mass of NaCl (aq) is > 25% of the overall solution
electrolysis of aqueous solutions
- ## H2O may be oxidized/reduced in place of one or both reactants
factors affecting the outcome of electrolysis of aqueous solutions
- relative cell potential values of the ions
- relative concentrations of the ions
- the nature of the electrode
special consideration for the electrolysis of NaCl (aq)
- at the cathode either OH- or Cl- may be produced
- Cl- is preferentially discharged when the concentration by mass of NaCl (aq) is > 25% of the overall solution
- otherwise OH- is discharged (as its E cell value is smaller)
observations when Cl- is discharged in the electrolysis of NaCl (aq)
- strong smell
- bleaching effect on damp litmus paper
- gas evolved at both anode (Cl2) and cathode (H2)
observations when copper electrodes are used in the electrolysis of CuSO4 (aq)
- pinky-brown metallic solid deposited on cathode
- no change in intensity of blue color
- anode disintegrates
- no change in pH
applications of the electrolysis of CuSO4 (aq) using copper electrodes
purification of copper:
- uses electrolysis of a solution containing Cu2+ ions, an impure copper anode, and a small pure copper cathode
- the anode disintegrates as Cu transfers to the cathode, which increases in mass
- impurities collect as sludge at the bottom of the cell
observations when inert electrodes are used in the electrolysis of CuSO4 (aq)
- gas evolved at anode (O2)
- decreased pH (due to release of H+ at anode)
- loss of intensity of blue color (due to reduction of Cu2+)
- pinky-brown metallic solid deposited on cathode
observations when copper electrodes are used in the electrolysis of CuSO4 (aq)
- pinky-brown metallic solid deposited on cathode
- no change in intensity of blue color
- anode disintegrates
- no change in pH
applications of the electrolysis of CuSO4 (aq) using copper electrodes
purification of copper:
- uses electrolysis of a solution containing Cu2+ ions, an impure copper anode, and a small pure copper cathode
- the anode disintegrates as Cu transfers to the cathode, which increases in mass
- impurities collect as sludge at the bottom of the cell
differences between voltaic and electrolytic cells
- voltaic: spontaneous rxns with chemical –> electrical energy conversion
electrolytic: non-spontaneous rxns with electrical –> chemical energy conversion - voltaic: 2 separate solns joined by salt bridge
electrolytic: 1 soln, no salt bridge required
