Topic 9: Redox Processes Flashcards
oxidation
- loss of electrons
- increase in oxidation no
- gain of oxygen
- loss of hydrogen
reduction
- gain of electrons
- decrease in oxidation no
- loss of oxygen
- gain of hydrogen
oxidation state
apparent charge of an atom in a molecule or ion.
- used to measure electron control or possession relative to the atom in pure element
relation between reducing power and reactivity in metals
more reactive metals are stronger reductants
relation between oxidizing power and reactivity in non-metals
more reactive non-metals are stronger oxidizing agents
redox titration
redox reaction between an oxidizing agent and reducing agent
chemically speaking, what happens in a redox titration?
electrons are transferred from reductant to oxidant
redox titrations: analysis of iron with manganate (VII)
5Fe2+ + MnO4- + 8H+ -> 5Fe3+ + Mn2+ + 4H2O
- uses KMnO4 in acidic soln. as oxidant
- oxidises Fe (II) ions to Fe (III)
- MnO4- is reduced to Mn2+
- colour change: deep purple to colourless
redox titrations: iodine-thiosulfate reaction
2I- (aq) + oxidant -> I2 (aq) + reduced product
- oxidant reacts with excess iodides to form iodine diatomic molecules
- oxidants can be KMnO4, KIO3, K2Cr2O7, NaOCl, etc.
2S2O3 2- (aq) + I2 (aq) -> 2I- (aq) + S4O6 2- (aq)
- the liberated iodine is then titrated with sodium thiosulfate (Na2S2O3)
- using starch as an indicator (NOT added at the start but during titration
- initially forms deep blue colour due to starch, but as I2 is reduced to I-, the blue colour disappears
redox titrations: winkler method (what it is, its function, etc)
- calculates dissolved oxygen content of water
- used to measure degree of pollution
- as oxygen is used by bacteria in decomposition reactions
biological oxygen demand (BOD)
- amount of oxygen used to decompose organic matter in a sample of water over a specified time period
- usually 5 days at a specified temp
- high BOD = high quantity of degradable organic waste = low level of dissolved oxygen
redox titrations: winkler method (process)
- Dissolved oxygen O2 (g) is fixed by the addition of a manganese (II) salt, such as MnSO4. This causes oxidation of Mn (II) to higher oxidation states:
2Mn2+ (aq) + O2 (g) + 4OH- (aq) -> 2MnO2 (s) + 2H2O (l) - Acidified iodide ions (I-) are added and are oxidised by Mn (IV) to I2:
MnO2 (s) + 2I- (aq) + 4H+ (aq) -> Mn2+ (aq) + I2 (aq) + 2H2O (l) - Iodine produced is titrated with sodium thiosulfate:
2S2O3 2- (aq) + I2 (aq) -> 2I- (aq) + S4O6 2- (aq)
redox titrations: winkler method (ratio of O2 : S2O3 2-)
1:4
voltaic cells
- generates electricity from spontaneous redox reactions
- separates two half-reactions into half-cells, allowing e-s to flow between them through an external circuit
2 connected half-cells = 1 voltaic cell
electrode potential
charge separation between the metal and its ions in solution