REDOX and ELECTRODE Potentials Flashcards
limitations of predictions using E values ?
if Ecell value is positive, reaction is feasible
if conc of solution is NOT 1 moldm-3 then the then it would disrupt equllibrium in one of the cells
EG ZINC
zn (2+) + 2e- -( reversible arrow) – >zn(s). if you inrease the concentration of zn(2+) to GREATER than 1 moldm-3 , the neq shifts right to REMOVE electrons in the system to make the electrode potential LESS NEGATIVE , thus changing the whole cell potential
Other factors ? non standard conditions (Eg unaqeous reactions) and high activation energy/ slow reaction rate
explanation and use of terms reducing agent and oxidising agent
oxidising agent takes electrons from species being oxidised and contains species being reduced
reducing agents adds electrons to species being reduced and contains species that is oxidised
the techniques and procedures used when carrying out redox titrations including those Fe 2+ / mno4 - reacitons
manganate VII titrations
. add standard solution of pottasium manganate (VII) to burette
.add MEASURED VOLUME OF solution being analsysed from pipette to concical flask. also add excess dilute sulfuric acid (for reduction of Mno4- ions )
.reaction= self indicating
.manganate (VII) solution reacts and gradually decolourises. end point is seen by first permanent pink colour show excess Mno4- ions present.
.REPEAT TITRATION UNTIL YOU SEE CONCORDANT TITRES (TWO TITRES WITHIN 10 CM FROM EACH OTHER ! )
. note kmno4 = deep purple thus idfficult to see bottom of mensicus through intense colour so read burette from the top. titre= difference between readings provided that measuring taken from top of the mensicus = difference between readings taken from bottom thus they are the same
analysing percentage purity of iron compound procedure?
- prepare 250 cm3 of impure feSO4: 7h2O in volumetric flask
- use pipette to meaure 25 cm3 of solution in conical flask then add 10cm 3 of 1 moldm3 of sulfuric acid in excess
3 use bureette to titrate solution using standard 0.02 moldm3 solution of pttasium manganate and anaylse results to deteermine percentage purity
using iodine/ thiosulfate redox titrations to analyse oxidising agents ? ( titrating iodine with sodium thiosulfate )
iodine thiosulfate titrations; s203 (2-) ions are oxidised and iodine is reduced . these titrations are usually used for the analysis of oxidising agents
this practical tries to analyse concentration of aqeous iodine
- add solution of NA2S2O3 to burette
- pipette in oxidising agent to be analysed into concial flask then add excess ptoassium iodide; oxidising agent reacts w iodide ions thus producing iodine turning solution yellow brown
- titrate solution with na2s2O3 to reduce iodine back to I- ions thus colour fades gradually thus making it difficult to decide end point. solve by adding starch indicator which forms blue black colour to identify end point – more iodine thiosulfate solution added- blue black colour fades. at end point all iodine will react and blue black colour disappears as all the iodine has been reduced to I-
analysis of household bleach ( an iodine thiosulfate reaction ) ( we are analysing hte ocncentrateion of chlorate ions! )
in this analysis we calculate the concentration of CL0- ions in bleach
procedure:
1. use pipette and add 10 cm3 of bleach to 250cm3 volumetric flask and ad water to prepare 250cm3 solution
- use pipette to measure 25 cm3 of solution into conical flask then add 10cm3 of 1 moldm3 of pottasium iodide follwoed by same conc of HCL to acidify solution ( the HCL provides H+ ions for the reaction!)
- titrate solution using burette using standard 0.05 mol/dm3 of sodium thiosulfate
- repeat titration to obtain concordant results
- analyse results to determine conc of chlorate ions in bleach
measuring standard cell potentials practical
- prepare 2 standard half cells
metal/ metal ion; metal ion MUST have conc of 1 moldm3
ion/ion; both metal ions must be equimolar and inert platinum electrode must be used
gas hal cell (Eg hydrogen half cell): 100 kpa pressure must be used in contact with 1 moldm-3 ionic conc. platinum electrode must be used
- connect metal electrodes to volt meter by wires
- prepare salt bridge via soaking filter paper in pottasium nitrate solution
- connect 2 solutions of half cells via salt bridge
- record cell potential from voltmenter
Iodine /thiosulfate reaction equations ?
oxidation: (2)S2O3(2-) –> S4O6(2-) + 2e-
reduction : I2 +2e- –> 2I-
Overall: 2S2O3 (2-) + I2 –> 2I- + S4O6 (2-)
difference between primary cells , secondary cells and
primary cells -= non rechargeable; irreversible reactions , battery goes flat and cell has to be discarded/ recycled. primary cells can be used as wall clocks and smoke detectors
secondary = rechargeable ; eg car batteries, NiMH batteries in radios , lithium ion polymer cells in laptops
key phrase about fuel cells from spec?
uses energy from reaction of fuel with oxygen to create voltage !
.fuel and oxygen flow into fuel cell and prducts flow out; electrolyte remains in cell
.feul cells operate continously provided fuel and o2 are supplied to cell
.dont have to be recharged
APPLICATIONS OF SCIENCE : Benefits of electrochemical cells counteracted by risks of toxicity and fire from Li based cells
.polar
.light metal = high energy density when used in lithium ion batteries
.they are flexible and can be formed into various sizes/shapes for fitting around laptops, mobile phones etc
IMPORTANT POINT: .limitations of lithium ion batteries; unstable at high temperatures and catches fire to ignite devices. Thus care must be taken in their recyclying !
advantages and disadvantages of electrochemical cells
advantages ; more efficient at producing energy than combustion engines, less pollution, eg in hydrogen fuel cells only waste product is water
disadvantages: uses toxic chemicals needed to be disposed on and chemicals used to make cells = flammable such as LITHIUM
summary of how a hydrogen fuel cell works
h2 supplied to anode, splits into electrons and protons
.PEM allows H+ across; forces electrons to travel around circuit to get to cathode, creating current to power stuff
.at cathode, o2 combines with H+ and e- from circuit to make H2O
