redox 2 p1 Flashcards
what is oxidation?
loss of electrons
what is reduction?
the gain of electrons
what is oxidation in terms of oxidation number?
oxidation is an increase of oxidation number
what is reduction in terms of oxidation number?
reduction is a decrease in oxidation number
what happens when a rod of a metal is dipped into a solution of its own ions?
an equilibrium is set up between the solid metal and the aqueous metal ions
what are the two methods to measure standard electrode potential (E^o)?
- metals or non-metals in contact with their ions in aqueous solution
- ions of the same element with different oxidation numbers
write a half-equation for zinc (s) to zinc (II)
Zn(s) ⇌ Zn2+(aq) + 2e-
what is the simplest salt bridge made of?
filter paper soaked in a saturated solution of KNO3 (potassium nitrate)
why are salt bridges necessary?
they complete the circuit allowing a flow of charge through ion movement. they also contain inert ions so they don’t react with the electrodes
what symbol is used to represent a salt bridge in standard notation?
||
what type of species goes on the outside (furthest from the salt bridge) in standard cell notation?
the most reduced species
what does | indicate in standard cell notation?
phase boundary (solid/liquid/gas)
how would an aluminium/copper cell be represented using standard cell representation?
Al(s)|Al3+(aq)||Cu2+(aq)|Cu(s)
what happens at the left hand electrode?
oxidation occurs
the left hand electrode is the half cell with the most negative Eo value
what happens at the right hand electrode?
reduction occurs
the right hand electrode is the half cell with the most positive E^o value
which side of the cell has the most negative Eo value and what happens to the metal with the most negative Eo value?
the left hand electrode is most negative
the metal will be oxidised
what conditions are required to use the standard hydrogen electrode?
temp - 298K
pressure - 100kPa
[H+] = 1.00 mol dm-3
what is the standard hydrogen electrode used for?
used as a standard for comparing other cells against
Eo of SHE is defined as 0 so all other Eo values are compared to it
why might you occasionally use other standard electrodes?
they are cheaper, easier and quicker to use and can provide just as good a reference
platinum electrodes of the SHE are expensive
if an Eo value is more negative, what does it mean in terms of oxidising/reducing power?
it means it is a better reducing agent (i.e. easier to oxidise)
if an Eo value is more positive, what does it mean in terms of oxidising/reducing power?
it means it is a better oxidising agent (i.e. easier to reduce)
what factors change Eo values/
concentration of ions
temperature
what happens if you reduce the concentration of the ions in the left hand half cell?
equilibrium moves to the left to oppose the change of removing ions; this releases more electrons, the Eo of the left hand cell becomes more negative so the e.m.f of the cell increases
how do you calculate the emf of a cell from Eo values?
Ecell = Ered(right) - Eox(left)
when would you use a Platinum electrode?
when both the oxidised and reduced forms of the metal are in aqueous solution
why is Platinum used?
it is inert so does not take part in the electrochemistry
it is a good conductor that completes the circuit
how would you predict if a reaction would occur?
- take the 2 half equations
- identify the species that is being reduced (this effectively the right hand electrode)
- calculate its Eo value minus the Eo value of the species that is being oxidised (effectively the left hand cell)
- if Eo cell > 0 a reaction will occur
what was the first commercial cell made from (Daniel cell)?
zinc/copper (II)
what are zinc/carbon cells more commonly known as?
disposable batteries
what are the two reactions that take place in zinc/carbon cells?
Zn is oxidised to Zn2+
NH4 + is reduced to NH3 at carbon electrode
IGNORE what are the reactions that occur in a lead/acid battery (car batteries)?
Pb + SO4 2- –> PbSO4 + 2e-
PbO2 + 4H+ + SO4 2- –> PbSO4 + 2H2O
how are some cells able to be recharged?
if the cell reactions are reversible, they can be reversed by running a higher voltage through the cell than the cell’s Eo
IGNORE Nickel/cadmium cells are rechargeable AA batteries. What reactions occur at the electrodes?
Cd(OH)2(s) + 2e- –> Cd(s) + 2OH-
NiO(OH)(s) + H2O + e- –> Ni(OH)2(s) + OH-
where are lithium ion cells commonly used?
mobile phones and laptops
IGNORE what reactions occur on discharge in lithium ion cells?
Li+ + CoO2 + e- –> Li[CoO2]-
Li –> Li+ + e-
what is a fuel cell?
a cell that is used to generate electric current; it does not require electrical recharging
what are the reactions that take place at the two electrons in an alkaline hydrogen fuel cell?
2H2 + 4OH- —> 4H2O + 4e-
O2 + 2H2O + 4e- —> 4OH
why is it better to use a fuel cell than to burn H2 in air, even though the same overall reaction occurs?
in combustion, sulfur and nitrogen containing compounds are produced due to high temperatures and the S and N in air. these are bad for the environment
this does not occur in a fuel cell; the only product is water. also more efficient
disadvantages of fuel cells?
hydrogen is a flammable gas with a low b.p. meaning it is hard and dangerous to store and transport - also expensive to buy
fuel cells have a limited lifetime and use toxic chemicals in their manufacture
how do you find the weakest reducing agent from a table of electrode potential data?
the weakest reducing agent will have the most positive E value
the species is the product of the reduction equation i.e. when the equation goes from right to left
why can some cells not be recharged?
when the reaction of the cell is not reversible
the product produced either dissipates or cannot be converted back into the reactants
why might the e.m.f. of a cell change after a period of time?
the concentrations of the ions change as the reagents are used up
how can the e.m.f. of a cell be kept constant?
if the reagents are supplied constantly, the concentrations of the ions will be constant meaning E remains constant
