Topic 9: Redox Processes Flashcards
LEO GER
Loss of electrons is oxidation (loss of hydrogen, gain of oxygen).
Gain of electrons is reduction (gain of hydrogen, loss of oxygen).
Oxidation-reduction Reactions
The transfer of electrons from one species to another - one species loses electrons and the other species gains electrons.
Define the oxidation number
The charge that an atom would have if all the covalent bonds in a compound were broken.
How is the oxidation number written?
Number after the charge! eg. +2
(Opposite to when we write ion charges).
What happens if the oxidation number reduces in size?
Reduction has occurred.
What happens if the oxidation number increases in size?
Oxidation has occurred.
What is the oxidation number for elements?
Zero. Eg. Cl2 is 0
What is the oxidation number for simple ions?
The same as the charge. Eg. Cl- is -1
What is the oxidation number for compounds?
Oxygen is -2 (except in peroxides when it is -1).
Hydrogen is +1 (except in metal hydrides when it is -1).
Chlorine is -1 (except with O and F).
Potassium is +1.
Work backwards. Compounds are neutral so = 0.
eg. MgO
x - 2 = 0
+2 - 2 = 0
So must be Mg+2O-2
What is the oxidation number for polyatomic ions?
Adds up to the net charge on the ion.
What ions do we leave out of redox equations?
Spectator ions. Eg. Na+, K+, SO4 2-
What ions are present in redox equations?
Reactive ions (these always have a transition metal). Eg. Cu, MnO4 -, Cr2O7 2-, Fe2+
How does reactivity affect redox reactions?
More reactive metals are stronger reducing agents as they lose their electrons more readily.
More reactive non-metals are stronger oxidising agents as they gain electrons more readily.
Define the biological oxygen demand (BOD)
The amount of oxygen used to decompose the organic matter in a sample of water over a specified time period (usually five days) at a specified temperature.
What are the two types of electrochemical cells?
Voltaic (galvanic) and electrolytic cells.
Explain voltaic (galvanic) cells
In a voltaic cell, electricity is generated by the spontaneous redox reaction taking place.
Explain electrolytic cells
In an electrolytic cell, electricity is used to drive non-spontaneous reactions.
What is the standard hydrogen electrode used for?
The SHE is used as a reference to measure the electrode potential of other half cells.
What are the standard conditions for the hydrogen electrode reaction?
Temperature = 298K
Pressure = 100kPa
Concentration = 1mol dm-3
Define the standard electrode potential of a half-cell
The electrode potential relative to the standard hydrogen electrode (SHE), measured under standard conditions (100kPa, 298K, all solutions 1 mol dm-3).
What way is the electron flow if the half cell contains a metal above Hydrogen in the Activity Series?
Electrons flow from the half-cell to the hydrogen electrode (electrode has a negative value).
What way is the electron flow if the half cell contains a metal below Hydrogen in the Activity Series?
Electrons flow from the hydrogen electrode to the half cell (electrode has a positive value).
(Spontaneity)
If the cell EMF is positive…
reaction is spontaneous.
(Spontaneity)
If the cell EMF is negative…
reaction is non-spontaneous and the reverse reaction is spontaneous.
Metals with low Eo values (most negative) will be . . .
the strongest reducing agents (themselves oxidised).
Non-metals with high Eo values (most positive) will be . . .
the strongest oxidising agents (themselves reduced).
Electrolysis of aqueous solutions: equations for the reduction of water at the cathode and oxidation of water at the anode
At the cathode:
2H20 + 2e —> H2 + 2OH- (-0.83V)
At the anode:
2H20 —> O2 + 4H+ + 4e
Overall equation:
2H20 —> 2H2 + O2
What are the observed changes at the electrodes for the electrolysis of water?
- Colourless gas is produced at both electrodes. This is seen through bubbling (H2 at the cathode and O2 at the anode)
- Ratio of the volume of gases is 2H2 : O2 (i.e. two times the amount of H2 compared to O2)
- The pH at the cathode will decrease as H+ is produced. The pH at the anode will increase as OH- is produced.
What is selective discharge determined by?
- At the cathode, it is determined by the E0 value of the ions
- At the anode, it is determined by the concentration of the ions in the electrolyte
- The materials from which the electrodes are made
What species are unable to be oxidised further in electrolysis?
Sulfates and nitrates.
Define electroplating
The process of using electrolysis to deposit a layer of a metal on top of another metal or other conductive object.
Define Faraday
The charge carried by one mole of electrons (96500 Colulomb)
Steps to work out the amount of product in electrolysis
Q=It
F=Q/96500
Mole ratios to find moles of product
m=nxM
What factors affect the amount of product formed in electrolysis?
The charge on the ion. Cu+ produces twice as much than Cu2+.
i.e. the greater the charge the more electrons per reduction so less product formed.
The current (I).
Increase in current = more product
The duration of the electrolysis (t).
Increase in time = more product
