How do you balance a redox reaction

how do you balance a redox reaction

Doc Brown's Advanced Level Chemistry Quantitative redox reaction analysis

GCE Advanced Level REDOX Volumetric Analysis Titration Revision Questions

Quantitative volumetric analysis – exam practice redox titration q uestions based potassium manganate(VII)–iron(II)/ethanedioate–ethanedioc acid (oxalate, oxalic acid)/hydrogen peroxide/sodium nitrite titrations, sodium thiosulphate/thiosulfate–iodine titrations and potassium dichromate(VI)–iron(II) titration. Any suggestions?

Volumetric analysis worksheet of structured questions of REDOX VOLUMETRIC TITRATION CALCULATIONS

Titrations and calculations based on oxidation–reduction techniques–reactions – s olved problems

Relative atomic masses that may be needed, in alphabetical order of symbol.

C=12.0, Cr = 52.0, Fe=55.9, H=1.0, I=126.9, K=39.1, Mn=54.9, N=14.0, Na=23.0, O=16.0, S=32.1,

Equations needed * FULL ANSWERS and WORKING *


Qualitative Analysis

and also

acid–base and other non–redox titrations Questions * EMAIL query?comment?error

NOTE: Half reactions are usually quoted as the half–cell reduction equation. Reuse half–cell or full equations in later questions from earlier questions. It is assumed you will work through them in numerical question order. If you cannot work out the redox equations, you can just download the equations so that you can at least practice the 'pure' volumetric calculation aspects of the questions.

Questions 1/4/6/7/8/9/10/11/12/13/15/16 based potassium manganate(VII)–iron(II)/ethanedioate–ethanedioc acid (oxalate, oxalic acid)/hydrogen peroxide/sodium nitrite titrations, Q2/14/17 on sodium thiosulphate–iodine titration, Q3/5 on potassium dichromate(VI)–iron(II) titration, further Q's will be added – suggestions?


U se of the correct 'species' (e.g. usually two given/chosen as/from half–cell equation data)
  • T he 'species' direction change – which is oxidised or reduced? In other words get the half–cell equations the right way round! (If not indicated, might have to decide from E Ш data supplied, the more +ve half–cell is the reduction (oxidising agent). More in Equilibria Part 7 (being written)
  • The correct ratio of half–cell equations – the 'balance' must be based on oxidation number analysis or number of electrons transferred. The total increase in oxidation states = the total decrease in oxidation states or total electrons gained = total electrons lost by the species involved.
  • A dd up the ion charges, the totals should be the same on both sides of the equation (I find this a handy extra check especially with stray H2 O's or H + 's!).
  • 'traditional' atom count – placed last because its not completely reliable with redox equations!
  • In some Q's the full equation may be given or the two half–cell equations to be put together the right way round and in the right ratio (see Redox Chemistry Part 2 (being written) )

  • I've tried to quote the data to the appropriate significant figures and associated 'trailing zeros'.

    Note that it is standard convention to show half-cell reactions as reductions, i.e. atom/ion/molecule + electron(s) to give the reduction product. This means you have to judge whether the half-reaction needs to be reversed to derive the full ionic redox equation, and any multiples of it are needed, - so take care, and don't get confused by conventions! they are there help!

    Question 1. Given the following two half–reactions: ( Q1 can be done as an experimental 'word–fill' version )

    Question 1 has many parts covering the titration of iron(II) ions with a standard solution of potassium manganate(VII) and the problems are solved.

    The relative atomic mass of iron = 55.9

    Q1(e) was a late addition and the worked out answers are better presented than maybe others on this page?

    (a) Construct the fully balanced redox ionic equation for the manganate(VII) ion oxidising the iron(II) ion

    (b) 24.3 cm 3 of 0.0200 mol dm –3 KMnO4 reacted with 20.0 cm 3 of an iron(II) solution acidified with dilute sulfuric acid.

    (i) Calculate the molarity of the iron(II) ion.

    (ii) How do recognise the end–point in the titration?

    (c) Calculate the percentage of iron in a sample of steel wire if 1.51 g of the wire was dissolved in excess of dilute sulphuric acid and the solution made up to 250 cm 3 in a standard graduated flask.

    A 25.0 cm 3 aliquot of this solution was pipetted into a conical flask and needed 25.45 cm 3 of O.0200 mol dm –3 KMnO4 for complete oxidation.

    (d) Suggest reasons why the presence of dil. sulfuric acid is essential for an accurate titration and why dil. hydrochloric and nitric acids are unsuitable to be used in this context.

    (e) The analysis of a soluble iron(II) salt to obtain the percentage of iron in it.

    8.25g of an iron(II) salt was dissolved in 250 cm 3 of pure water. 25.0 cm 3 aliquots were pipetted from this stock solution and titrated with 0.0200 mol dm –3 potassium manganate(VII) solution.

    The titration values obtained were 23.95 cm 3. 23.80 cm 3 and 23.85 cm 3.

    (i) What titration value should be used in the calculation and why?

    (ii) Calculate the moles of manganate(VII) used in the titration.

    (iii) calculate the moles of iron(II) ion titrated

    (iv) Calculate the mass of iron(II) titrated

    (v) Calculate the total mass of iron in the original sample of the iron(II) salt.

    (vi) calculate the % iron in the salt.


    Category: Forex

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