23 - redox and electrode potentials Flashcards
what do oxidising agents do in terms of electrons
takes electrons from species being oxidised
what do reducing agents do in terms of electrons
adds electrons to the species being reduced
how to create an overall redox electrons from 2 half equations
- balance electrons
- add and cancel electrons
- cancel any species on both sides of equation
how to form an overall redox equations from oxidation numbers
- write initial equation
- assign oxidation numbers
- match the oxidation changes by balancing only that species
- balance remaining atoms
what to take into account when predicting products
- In aqueous redox reactions – H2O often formed
- Balance O first then H+
what is reduced in the manganate (VII) redox titrations
- MnO4 (-) reduced to Mn(2+)
write out the method for the manganate titration in full against a solution being analysed
why does manganate decolourise as it is being added
because it is reduced
colour change in mangante titration
colourless to permenant pink
what occurs if insufficient amount of acid is added
solution will not be acidic enough and a brown solid, MnO₂, will be produced instead of Mn²⁺ ions.
why is sulfuric acid used instead of ethanoic acid
weak acid
Why can concentrated hydrochloric acid not be used instead of sulfuric acid
Concentrated hydrochloric acid cannot be used as the manganate (VII) ions will oxidise the chloride ions to chlorine.
determine percentage purity
determine value of x
in iodine-thiosulfate titrations what is oxidised and what is reduced
thiosulfate ions - oxidised
iodine - reduced
for an iodine thiosulfate titration - write out the half equations for oxidation and reduction and work out the overall equation
what can iodine-thiosulfate titrations be used for
how to find copper content in copper alloys - initial step
Alloy reacted + dissolved in nitric acid and neutralised to form Cu2+
method of iodine-thiosulfate titration against an oxidising agent
in an iodine-thiosulfate titration how to determine on an end point and method
- iodine is being reduced back into iodide ions and brown colour faded to pale straw colour
- add small amount of starch indicator
- deep blue black colour formed due to presence of iodine
- once all iodine has been reduced - blue black colour fades quickly
how does the iodine thiosulfate actually measure concentration of oxidising agent
measures how much thiosulfate needed to turn the solution from brown to colourless
oxidising agent reacts with iodide ions to form iodine (brown)
iodine (brown) is then reduced back into iodide by thiosulfate ions
when brown colour fades - it means the oxidising agent isnt producing any more iodine - because it is used up
so you know how much thiosufate exactly uses up the oxidising agent
how to analyse conc of household bleach experimentally
- form solution of bleach
- add potassium iodide
- titrate this against sodium thiosulfate in burette - using previous method
what is bleach
oxidising agent
copper alloys
brass or bronze
how to find copper content of copper salts - initial step
Cu2+ made by dissolving copper salt in water OR insoluble copper compounds reacted with acids
write equations for the analysis of copper through an iodine-thiosulfate titration
oxising agent react with iodide
how to find stoichiometry between two different equations
follow round like a circle - double then half etc
what is a voltaic cell
converts chemical energy into electrical energy
what do half cells contain
chemical species in redox half-equation
why do in a full cell - the chemicals in the 2 half cells must be kept apart
Or electrons flow in uncontrollable way + heat energy released instead of electrical energy
draw zinc and copper half cells
how to set up a Metal / metal ion half cell
- Metal rod dipped into solution of its aq metal ion
when you write a half cell in short form - you use a vertical line between the ion and the element - what is that
phase boundary between aqueous solution + metal
when writing equation for half cell - what is the convention
ALWAYS WRITTEN THAT REDUCTION IS FORWARD REACTION
what is a Ion / ion half-cells
- Contains ions of same element in different oxidation states
E.g. aq iron 2+ and aq iron 3+
what electrode do we use for ion / ion half cell
platinum
why do we use platinum
- inert
- no metal to transport electrons in / out of cell - need metal that is electrically conductive
draw a half cell for Fe 3+ / Fe 2+ and write the half equation
what is the standard electrode potential
Tendency to be reduced and gain electrons
OR
the e.m.f of half-cell connected to standard hydrogen half-cell under standard conditions
what is THE standard half cell
half cell with hydrogen gas + solution with H+ ions
what is the electrode potential on THE standard half cell
0V
draw THE standard half cell
standard conditions for standard electrode potential
Solution conc – 1moldm-3
Temp 298K
Pressure 100kPa / 1 bar
how do we determine standard electrode potentials of half cells
e.m.f of half-cell connected to standard hydrogen half-cell under standard conditions
if don’t know standard electrode potential values how do we know which species is oxidised + reduced
- Electrode with more reactive metal loses electrons + is oxidised
= Negative cathode - Electrode with less reactive metal gains electrons + is reduced
= Positive anode
how to measure standard electrode potentials - set up
Half-cell connected to standard hydrogen electrode
electrode connected by wire
solutions connected by salt bridge (strip of filter paper soaked in KNO3)
purpose of wire
flow of electrons
purpose of salt bridge
flow of ions
what does a salt bridge contain and give example
contains concentrated solution of electrolyte that doesn’t react with any solution
E.g. strip of filter paper soaked in KNO3
draw a set up to measure standard electrode potential of copper 2+
the more negative standard electrode potential value…
Greater tendency to lose electrons + undergo oxidation
the more positive standard electrode potential value…
lesser tendency to lose electrons + undergo oxidation
(more likely to be reduced)
what standard electrode potentials do metals tend to have
– tend to have negative values + lose electrons
what standard electrode potentials do non-metals tend to have
positive values + gain electrons
write out method to measure standard cell potentials between two half cells
from that method if the value is 1.1V what is that actually
E_cell
formula for E_cell
figure out what is feasible
C has a more positive electrode cell potential value - will be reduced
so that OXIDISING AGENT should react with the other REDUCING AGENTS
how to write an overall redox equation for 2 half equations using electrode cell potential values
- identify what’s being oxidised + what’s being reduced
- rewrite half equations in that way for froward reaction = standard is reduction
- combine two half equations
limitations of these feasibility predictions
- reaction rate
- concentration
how is reaction rate a limitation
some reactions have large activation energy – very slow rate
how is concentration a limitation
standard electrode potentials – 1moldm-3
if conc not that – value of electrode potential will be different from standard
if conc is bigger than 1 – shift to right + remove electrons + electrode potential is less negative
whats another limitation
- standard electrode potentials only apply to aqueous equilibria – not all of them are
colour change in manganate titration
colourless to pale pink
what is formed when copper ion reacts with iodide
copper iodide and iodine
what is copper iodide
white precipitate
what colour is iodine
brown
manganate colour change
how do you know which is anode + which is cathode
