9. Redox Flashcards
Define elctrolysis
Electrolysis - a process which uses an electric current to drive oxidation and reduction
Define oxdiation and reduction
Oxidation - loss of electrons
Reduction - gain of electrons
Define oxidation state
Oxidation state - the apparent charge that an atom has in a molecule or ion
Oxidation state vs ion charge notation
Oxidation state: +2
Ion charge: 2+
Oxidation state - reduction / oxidation / - why
The higher (more positive) is the oxiidation state - the more e the atom has gained - the better it is oxidising agent - the more it experienced reduction (the more reduced it is) -> because it has a high EN - attracts e due to nuclear charge / octet
Oxidation state tells which redox reaction the atom has undergone
Oxidation state vs oxidation number
Oxidation state: Fe+2
Oxidation number: iron (II)
Common oxidising and reducing agents’ examples
Oxidising agent = oxidant
Reducing agent = reductant
Which substance can act both as oxidiing and reducing agent?
H2O
Relationship between strength of metal and its strength of reducing agent
The more reactive is the metal (the more easily loses e) - the stronger reducing agent it is
Strongest reducing: Li, Mg
What is an activity series?
Relative reducing agent strength of metals can be compared by displacement reactions - combinations of metals and their ions - line up according to strength - activity series +> predict whether redox recation would occur or not
Relationship between the reactivity of non-metals and their strentgh as oxidising agents
reactivity (ability to accept e) decreases down group - oxidising agent strength also decreases
Reactivity can be compared by displacement reactions
For what are redox titrations used
Like acid-base titrations, redox titrations are used to determine the unknown concentration of a substance in solution
Usually apparent by change of colour - d elements involved
Explain Winkler method
Winkler Method used to measure dissolved oxygen in water - BOD is calculated
Explain BOD
As pollution increases - O2 conc decreases - biological oxyge demand (BOD) used to measure pollution
BOD - the amount of oxygen used to decompose organic matter in a sample of water over a specified period fo time (usually 5 days at specific temp)
High BOD - low leevl of dissolved O2 - greater pollution
What are the types of electrochemical cells?
Components of a voltaic cell (galvanic cell)
Between the half cells - charge seprataion - electrode potential - reason for e flow
Voltmeter records the generated voltage
Salt bridge (glass tube or absorbtive paper strip with ions) completes the circuit - neutralises the charge and maintains the potential difference - ion movement in salt bridge opposes the movement of e (anions migrate from cathode to anode and vice versa)
What is a cell diagram convention
Cell diagram convention is a simple representationn of a voltaic cell
What is cell potential?
- The difference in potentials (tendencies to undergo reduction) of the two half-cells in a voltaic cell - notation E
- Half cell potential values - determined by comparing to SHE
- Cell potential values refeer to reduction reaction - the more positive Ecell value for half cell - the more readily it is reduced
Explain SHE
Standrad hydrogen electrode - baseline for measuring and comparing half-cell electrode potentials
Assigned value - 0 V - other values created on this baseline
Explain standard electrode potential
Cell potential generated using SHE
Required conditions:
- all solutions 1M
- gasses as 100kPa
- all substances pure
- temp at 298K
- platinum used as electrode of no solid metal
Cu half-cell has a greater tendency to be reduced than H+ => its standrad potential is 0.34V
How to determine spontaneity of a redox reaction from cell potential?
What are the two measures of spontaneity of redox recations?
H - cell potential
G - Gibbs free energy
Direct relationship between H and G
Explain the structure of electrolytic cell
Electrolytic cell does the reverse: it uses an external source of electrical energy to bring about a redox reaction that would otherwise be nonspontaneous.
- Electricity is used to chemcial breakdown
- Reactant - electrolyte
- Redox occurs at electrodes
- Ions are discharged
Electrolysis is the only means of extracting these metals from their ores
Power sources pushes e towards negative electrode
Comaparison of oxidation and reduction sites in voltaic vs electrolytic cell
Which conditions affect the yield of products in electrolysis?
- the current
- duration of electrolysis
- charge of ion
What are the Ecell and G values for voltaic and electrolytic cells to work?
Electrolysis of molten Cu salt with Cu electrode
At cathode: Cu2+ +2e- -> Cu
At anode: Cu -> Cu2+ + 2e-
- no change in pH
- pinky brown colour is deposited on cathode
- no change in the intensity of blue colour
