REDOX CORE THEORY Flashcards
What qualifies a redox reaction?
A redox (REduction OXidation) reaction is a reaction in which there is a transfer of electrons
Which way do electrons move in a redox reaction?
- There is a net movement of electrons from the anode to the cathode:
- Electrons are lost at the anode (oxidation site) and gained at the cathode (reduction site)
What are the processes of oxidation and reduction? What are oxidizers and reducers?
Oxidation and reduction are simultaneous processes which occur in a redox reaction:
- Oxidation is the loss of e- (signified by an increase in oxidation number/state).
- A reactant that undergoes oxidation: a reducer. A stronger reducer has a higher oxidation potential: loses e- more readily.
- Reduction is the gain of e- (signified by a reduction in oxidation number/state.
- A reactant that undergoes reduction: an oxidizer. A stronger oxidiser has a higher reduction potential: gains e- more readily.
Relative reducing/oxidising strength is ultimately largely tied to periodic table trends: electronegativity, reactivity, etc.
How are oxidation numbers (or oxidation states) used in discussing redox reactions?
Oxidation numbers are used to track the flow of electrons: a change in an oxidation number/state signals a gain or loss of electrons. The size of the oxidation number helps determine relative reducing/oxidising strength.
- Oxidation is shown as an increase in ox number
- An increase in ox no of 1 implies loss of 1e-
- The lower the ox no. the higher the oxidation potential and hence the stronger the reducer
(bc they have a greater ability to lose/donate electrons and so result in another reactant gaining electrons/being reduced. - Reduction is shown as a reduction in ox number.
- Decrease in ox no of 1 implies gain of 1e-
- The higher the ox no. the higher the reduction potential and hence the stronger the oxidiser
(bc they have a greater ability to gain/accept electrons and so result in another reactant losing electrons/being oxidised.
What is the relationship between reducing and oxidising strengths of a redox pair?
- A strong reducer reacts to form a weak oxidiser
- A strong oxidiser reactions to form a weak reducer
- When the two sides are of relative strengths it may go back and forth quite readily.
In this way, the conjugate pairs of lot of strong reducers or oxidiers may be found all around us as once the strong reducer or oxidiser reacts leaving its weak and opposing conjugate pair, it will not readily enounter a strong enough oxidiser/reducer to change it back: e.g. sodium ions are very weak oxidisers around us everywhere often as salts, as sodium metal is a very strong reducer so is capable of reacting with even fairly weak oxidizers, so if it were to exist in the metal form it would quickly react away leaving ions which have too low a reduction potential to react with very much and so the reverse reaction is less likely.
How is a redox reaction identified?
By the transfer of electrons.
What is the electrochemical series?
The electrochemical series is a method of organising relative reducing/oxidsing strength of redox half cells by voltages generated against a given reference cell.
Allows us to predict whether reactions will occur spontaneously, what cell voltages may be derived, etc.
How does standard reduction potential (E^0 value) correspond to reducing/oxidising strength?
- Higher the E^0 value, the stronger the oxidiser.
- Lower the E^0 value, the stronger the reducer
Aka if reduction leftwards oxidation rightwards, upwards more readily gains electrons, downwards more readily loses electrons
What is the reference cell?
A reference cell is an arbitrary reference point for determining red/ox potential of a given half cell, where E^0 net values are usually calculated from the voltage generated by a given half cell paired with the reference cell.
Advice on building an ECS?
Remember, a spontaneous reaction requires…
A cathode cell with a higher reduction potential (more +ve E^0 in volts) than the anode cell, (so that mathematically when you subtract the value of the reducer/anode cell the overall cell voltage is still positive).
AKA for a spontaneous redox reaction, pair your oxidiser with a reactant of a lower reduction potential. Pair your reducer with a reactant with a higher reduction potential.
AKA an oxidiser paired with the conjugate pair of a weaker oxidiser, OR a reducer paired with the conjugate pair of a stronger reducer.
AKA NOT an oxidiser paired with the conjugate pair of a stronger oxidiser, OR a reducer paired with the conjugate pair of a weaker reducer.
Reaction won’t occur if you have a weak reducer with a weak oxidiser, or usually not with one that’s mild and one that’s weak.
Which half of the cells to use?
- Try several combinations to deduce strength, but ultimately trial and error based off the following rules:
test cathode cell + reference anode cell has a positive voltage: test cathode cell stronger oxidiser then ref cell.
test anode cell + reference cathode cell has a positive voltage: test anode cell stronger reducer than ref cell.
test cathode cell (oxidiser) + reference anode cell (reducer) generates no voltage:
test oxidiser paired with the flipside of a stronger oxidiser/test oxidiser weaker than flipside of reference oxidiser AKA test reducer stronger than flipside of reference reducer.
test anode cell + reference cathode cell generates no voltage, test oxidiser paired with the flipside of a stronger oxidiser/test
On the ECS on the info sheet which orders half cells against a reference cell by standard reduction potentials, how do we tell which are strong oxiders and which are strong reducers?
The key is standard REDUCTION potentials: while it is true that we have both oxidation and reduction potentials, it’s perfectly valid to measure both oxidisers or reducers in either terms… While we often measure oxidizers by reduction potentials and reducers by oxidation potentials it’s just as correct and even simpler to measure both sides of the half cell in the one format, (even though one half is a reduction and the other is an oxidation) it saves writing out the series twice for no point.
high reduction potential means a greater ability to gain electrons; its a stronger oxidiser; while lower reduction potential means a lesser ability to gain electrons: or, most crucially, better at LOSING electrons aka its a stronger reducer.
both sides of a test cell have the same reduction potential, yet one is reduced and one is oxidised. Say there’s a high reduction potential, this means the side which is reduced (the oxidiser) is a strong oxidiser,
REMEMBER THAT:
A stronger oxidiser has a greater capacity to gain electrons (be itself reduced) and so is marked by a high reduction potential (and a low oxidation potential).
A stronger reducer has a greater ability to lose electrons (be itself oxidised) and so is marked by a high oxidation potential (and a low red. pot).
In this way, if one potential is high the other is low e.g. if something is a strong reducer its conjugate pair is a weak oxidiser, so it has both a high oxidation potential and a low reduction potential.
Both sides of a half-cell will have the same red/ox potential, but only one will actually be reduced or oxidized (at least at that value).
THUS strong reducers (= weak oxidizers) have high ox. pot. has low red. pot. strong oxidiser (=weak reducers) have high red. pot. has low ox. pot.
SO. Given that both sides will have the same reduction or oxidation potential, this works because high reduction potential corresponds to a low oxidation potential aka if its better at gaining electrons its worse at losing electrons and vice versa: it can’t be both a strong reducer and a strong oxidiser. If we organise them by reduction potentials, therefore, higher up the list we have strong oxidizers and their flipsides which are reactants that tend to be oxidised, yet have a high reduction potential (because both sides have the same reduction and oxidation potential) and have low oxidation potentials AKA are weaker reducers and we show that at either end of the series when we measure with either reduction OR oxidation we can sort out strength of both reducers and oxidizers: at one end we have strong oxidizers and weak reducers, at the other end we have weak oxidizers and strong reducers. Which is at the top or bottom?
Standard red. pot.: (higher red. pot. at top to lower red. pot. at bottom, infer inverse as ox. pot.) means stronger oxidizers and weak reducers at top, weaker oxidizers and strong reducers at the bottom.
Standard ox. pot.: (infer the inverse as red. pot.) means stronger reducers and weaker oxidizers (reminder, the flipside of high ox. pot. is low reduction pot.)
e.g. if a _not half cell__ has a low reduction potential its flipside has a high oxidation potential e.g. if it has a low reduction potential it must be a weak oxidiser, and if its a weak oxidiser it must be a strong reducer.
if a ___ has a low reduction potential it has a high oxidation potential e.g. if it has a low reduction potential it must be a weak oxidiser, and if its a weak oxidiser it must be a strong reducer.
e.g. Al(s) is a strong reducer so Al^3+(aq) is a weak reducer.
