redox and electrochemistry Flashcards
rules for redox
sum of all ox no in a neutral compound is 0
sum of ox no in an ions is equal to the charge of the ion
th ox no of any uncombined element is 0
the ox no of oxygen is always -2 expect peroxides or when combined with F
the ox no of H is always +1 except hydride when -1
in a compound group 1= ox no of +1
group2=+2
group 3= +3
group 7 can have all different ox no but assume -1 unless asked to find it
what is a redox reaction
When both Ox and reduc happen
ox= loss of electron or increase of ox no
reduc= gain of electrons or decrease of ox no
metals tend to be ox and non metals tend to reduc
Oxidising agent
is a reagent that oxidises (takes electrons from) another species
reducing agent
is a reagent that reduces (adds electrons too) another species
Oxidation
is the loss of electrons/increase in oxidation number
reduction
reduction is the gain of electrons/ the decrease in oxidation number
electron transfer
oxidation is loss
reduction is gain
metals tend to lose- oxidation
non metals tend to gain- reduction
for a reaction to be redox there must be reduction and oxidation so something must lose electrons and something must gain
constructing a redox equation from half equations
First × the half equations by a factor so that the number of electrons in both are the same
next add the 2 equations together
then cancel out hydrogen or water
constructing half equations
1) balance the elements being oxidised/reduced
2)balance the oxidation numbers:use electrons
3)balance charge using H+ if acid or OH- is alkali
4) add water to balance out H and Os
constructing redox equations from oxidation numbers
identify reactants and products
balance only elements which are oxidised or reduced + identify their total ox number change
balance the change
total increase in ox no = total decrease in ox number
check if other elements need balancing
redox titration using acidified manganate
acidified K maganate is a strong ox agent
self indicating
manganate ions (Mn04 -) = purple
Mn2+ ions = very pale pink
manganate reduced to Mn2+
purple to pink
typically maganate is in burette and reducing agent is in conical flask
when complete solution in flask goes from og reducing agent colour normally colourless = very pale pink permanently
half equ =
Mn04 - +5e- +8H+ —> Mn2+ + 4H20
redox titration between iodine and thiosulfate
thiosulphate ion S2O3 2- reduces I2 to 2I- and forms S4O6 2- tetrathionate ion
2S2O3 2- + I2 —> 2I- + S4O6 2-
brown to colourless
problem- brown colour fades overtime so hard to detect when disappears
solution- add starch indicator to I2, blueblack to straw coloured (in exam say colourless)
sodium thiosulfate is in burette and iodine solution is in flask
this analysis is use to analyse
household bleach
copper compounds
copper alloys
remember between equations don’t change moles in calculations of I2 as all the i2 from 1 is used in the other
what does the electrochemical series show?
gives the standard electrode potential values for reduction half equations
relevance of standard electrode potential values
give measure of relative ease of reduction or oxidation
compare the reducing/oxidising powers of diff species
more positive greater tendency to gain electrons and be be reduced + oxidising agent (stronger the more positive)
more negative greater tendency to lose electrons and be oxidised + reducing agent (stronger the more negative)
2H+ + 2e- —> H2 = 0.00
which direction of the electrochemical series reactions is reduction and which is oxidation
forward is always REDUCTION
equ pos right
Backwards is always OXIDATION
equ pos left
what are standard half cells used for?
to measure the standard electrode potential
what conditions are required for a standard half cell
standard conditions
and must have species from half equ in equilibrium
three types of standard half cell
Metal ion/ metal half cell-
metal=electrode (where transfer of electrons occurs)
metal ion = in solution + 1moldm-3 (electrolyte)
Metal ion/ Metal ion half cell-
pt or graphite (inert) electrode (used to make electrical contact with sol + transfer of electrons occurs on surface)
both metal ions in solution + 1moldm-3
Non metal/ mom-metal ion-
pt or graphite electrode (“)
non metal ion is in electrolyte solution 1moldom-3
non metal is solid liquid or gas, usually gas + stored in container 1atm
e.g. hydrogen half cell
LEARN DRAWINGS
standard electrode potential def
KEY
of a half cell is the e.m.f of a half cell compared with a standard hydrogen half cell, under standard conditions
how to measure standard electrode potential of a half cell?
half cell is connected to a standard H half cell by a salt bridge( filter paper soaked in ionic sol) - allows movement of ions between half cells (completing the circuit)
under standard conditions
electrical current flows through- electrons flow through connecting wires
measured by high resistance voltmeter
What to remember when drawing diagram to measure standard electrode potential
Salt bridge
standard conditions
wires + voltmeter
which of the 3 types of half cell are present ( H is always non metal with cage for gas)
label what electrode made of + if pos ( larger standard electrode potential value) or neg ( smaller standard electrode potential value)
label movement of ions and electrons
equation for standard cell potential
cell = reduction - oxidation
RedOx
should always be positive
predicting feasibility of redox reaction
use standard electrode potential
species undergoing reduction MUST have a more positive standard electrode potential
or it isn’t feasible
limitations of predictions using standard cell potentials
kinetics-reaction may be too slow as Ea is too high so may not be observed despite being feasible
non standard conc of solutions-
gives value for electrode potential diff to standard electrode potential resulting in cell potential value diff to standard
