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Revision notes on Group 7/17 Halogens: Titrations - volumetric analysis using halide ions

Advanced A level inorganic chemistry exam revision notes for

Part 9: The chemistry of the Group 17 HALOGENS

9.9 Volumetric analysis involving the halogens or halide ions

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 All my Group 7/17 halogens advanced chemistry level revision notes

 All my advanced A level inorganic chemistry revision notes

 GCSE Level Group 7 Halogens revision notes

PLEASE NOTE KS4 Science GCSE/IGCSE/O Level GROUP 7 HALOGENS NOTES

are on a separate webpage

Pd s block d blocks and f blocks of metallic elements p block elements
Gp1 Gp2 Gp3/13 Gp4/14 Gp5/15 Gp6/16 Group7/17 Gp0/18
1

1H

 2He
2 3Li 4Be ZSymbol, z = atomic or proton number

highlighting position of Group 7/17 Halogens

outer electrons ns2np5

 5B 6C 7N 8O 9F

fluorine

 10Ne
3 11Na 12Mg 13Al 14Si 15P 16S 17Cl

chlorine

 18Ar
4 19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br

bromine

 36Kr
5 37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I

iodine

 54Xe
6 55Cs 56Ba 57-71 72Hf 73Ta 74W 75Re 76Os 77Ir 78Pt 79Au 80Hg 81Tl 82Pb 83Bi 84Po 85At

astatine

 86Rn

9.9 Volumetric Analysis – titrations involving halogens or halide ions

Volumetric titration analysis questions involving halogens or halide ions e.g. iodine titrated with sodium thiosulfate, chloride ion titrated with silver nitrate solution.

(1) Titrating chloride with silver nitrate

(2) Titrating iodine with sodium thiosulfate

(1) Chloride ions can be titrated with standardised silver nitrate solution

The questions below are from the Volumetric (non–redox) Titration Calculations Page with ANSWERS!

  • Theory of reaction and using potassium chromate (VI) indicator.

    • As the silver nitrate is run into the chloride solution, white silver chloride is precipitated.

      • Ag+(aq) + Cl–(aq) ==> AgCl(s)

    • When all the chloride has been precipitated, the first drop of excess silver nitrate solution causes a dark brick red precipitate of silver chromate to form, thus indicating the end–point.

      • 2Ag+(aq) + CrO42–(aq) ==> Ag2CrO4(s)

    • The indicator is chosen on the basis that the solubility product of silver chloride is exceeded before that of silver chromate(VI).

Q10 25 cm3 of seawater was diluted to 250 cm3 in a graduated volumetric flask. A 25 cm3 aliquot of the diluted seawater was pipetted into a conical flask and a few drops of potassium chromate(VI) indicator solution was added.

 On titration with 0.1 mol dm–3 silver nitrate solution, 13.8 cm3 was required to precipitate all the chloride ion. [Atomic masses: Na = 23, Cl = 35.5]

(a) Give the ionic equation for the reaction of silver nitrate and chloride ion.

(b) Calculate the moles of chloride ion in the titrated 25cm3 aliquot.

(c) Calculate the molarity of chloride ion in the diluted seawater.

(d) Calculate the molarity of chloride ion in the original seawater.

(e) Assuming that for every chloride ion there is a sodium ion, what is the theoretical concentration of sodium chloride salt in g dm–3 in seawater?

 

Q11 0.12 g of rock salt was dissolved in water and titrated with 0.1 mol dm–3 silver nitrate until the first permanent brown precipitate of silver chromate was seen.

19.7 cm3 was required to titrate all the chloride ion. [Atomic masses: Na = 23, Cl = 35.5]

(a) How many moles of chloride ion was titrated?

(b) What mass of sodium chloride was titrated?

(b) What was the % purity of the rock salt in terms of sodium chloride?

 

Q12 5.0 g of a solid mixture of anhydrous calcium chloride(CaCl2) and sodium nitrate (NaNO3) was dissolved in 250 cm3 of deionised water in a graduated volumetric flask. A 25 cm3 aliquot of the solution was pipetted into a conical flask and a few drops of potassium chromate(VI) indicator solution was added.

On titration with 0.1 mol dm–3 silver nitrate solution, 21.2 cm3 was required to precipitate all the chloride ion. [Atomic masses: Ca = 40, Cl = 35.5]

(a) Calculate the moles of chloride ion titrated.

(b) Calculate the equivalent moles of calcium chloride titrated.

(c) Calculate the equivalent mass of calcium chloride titrated.

(d) Calculate the total mass of calcium chloride in the original 5.0 g of the mixture.

(e) The % of calcium chloride and sodium nitrate in the original mixture.

 

The questions above are from the

Volumetric (non–redox) Titration Calculations Page with ANSWERS!

(2) Iodine can be titrated with standardised sodium thiosulfate solution.

  • The redox theory

  • Half–cell reaction data:

    • (i) 1/2I2(aq) + e– ==> I–(aq)

    • (EØ = +0.54V, reduction of the oxidising agent, iodide gets oxidised)

    • (ii) 1/2S4O62–(aq) + e– ==> 2S2O32–(aq)

    • (EØ = +0.09V, S4O62– will act as reducing agent, EØ less positive)

  • The iodine is reduced by the thiosulfate ion to form iodide, ox. state of I (0) to (–1).

  • The thiosulfate ion is oxidised to the tetrathionate ion. In doing so the sulphur atom changes ox. state from an average of four at (+2) in the two S2O32– ions to an average of four at (+2.5) in the single S4O62– ion.

    • A bit of an awkward one in analysing sulphur and it is best to reason in terms of an average oxidation state of sulphur.

  • 2 x reduction half–cell, (i)  I2(aq) + 2e– ==> 2I–(aq)
    2 x oxidation half–cell, (ii) rev. 2S2O32–(aq) ==> S4O62–(aq) + 2e–
    added gives full equation 2S2O32–(aq) + I2(aq) ==> S4O62–(aq) + 2I–(aq)

  • This is used to quantitatively estimate iodine in aqueous solution.

  • Indicator theory

  • The indicator is a few drops of starch solution which forms a blue–black complex with iodine.

  • The end–point is when the solution first becomes colourless with no remaining iodine to form the coloured complex.

The questions below are from the Volumetric Redox Titration Calculations Page with ANSWERS!

Question 2: Given the following two half–reactions

(a) Given (i) S4O62–(aq) + 2e– ==> 2S2O32–(aq)

and (ii) I2(aq) + 2e– ==> 2I–(aq)

construct the full ionic redox equation for the reaction of the thiosulfate ion S2O32–,and iodine.

(b) what mass of iodine reacts with 23.5 cm3 of 0.012 mol dm–3 sodium thiosulfate solution.

(c) 25cm3 of a solution of iodine in potassium iodide solution required 26.5 cm3 of 0.095 mol dm–3 sodium thiosulfate solution to titrate the iodine.

What is the molarity of the iodine solution and the mass of iodine per dm3?

 

Question 17: 25.0 cm3 of an iodine solution was titrated with 0.1 mol dm–3 sodium thiosulfate solution and the iodine reacted with 17.6 cm3 of the thiosulfate solution.

(a) give the reaction equation.

(b) what indicator is used? and describe the end–point in the titration.

(c) calculate the concentration of the iodine solution in mol dm–3 and g dm–3.

 

Question 14: Given the half–cell reaction IO3–(aq) + 6H+(aq) + 5e– ==> 1/2I2(aq) + 3H2O(l)(see also Q2)

(a) Deduce the redox equation for iodate(V) ions oxidising iodide ions.

(b) What volume of 0.012 mol dm–3 iodate(V) solution reacts with 20.0 cm3 of 0.100 mol dm–3 iodide solution?

(c) 25.0 cm3 of the potassium iodate solution were added to about 15 cm3 of a 15% solution of potassium iodide (ensures excess iodide ion). On acidification, the liberated iodine needed 24.1 cm3 of 0.05 mol dm–3 sodium thiosulfate solution to titrate it.

(i) Calculate the concentration of potassium iodate(V) in g dm–3

(ii) What indicator is used for this titration and what is the colour change at the end–point?

 

Question 18: 1.01g of an impure sample of potassium dichromate(VI), K2Cr2O7, was dissolved in dil. sulphuric acid and made up to 250 cm3 in a calibrated volumetric flask. A 25 cm3 aliquot of this solution pipetted into a conical flask and excess potassium iodide solution and starch indicator were added. The liberated iodine was titrated with 0.1 mol dm–3 sodium thiosulfate and the starch turned colourless after 20.0 cm3 was added.

(a) Using the half–equations from Q3(a)(ii) and Q2(a)(ii), construct the full balanced equation for the reaction between the dichromate(VI) ion and the iodide ion.

(b) Using the half–equations from Q2(a) construct the balanced redox equation for the reaction between the thiosulfate ion and iodine.

(c) Calculate the moles of sodium thiosulfate used in the titration and hence the number of moles of iodine titrated.

(d) Calculate the moles of dichromate(VI) ion that reacted to give the iodine titrated in the titration.

(e) Calculate the formula mass of potassium dichromate(VI) and the mass of it in the 25 cm3 aliquot titrated.

(f) Calculate the total mass of potassium dichromate(VI) in the original sample and hence its % purity.

The questions above are from the Volumetric Redox Titration Calculations Page with ANSWERS!

WHAT NEXT?

PLEASE NOTE GCSE Level GROUP 7 HALOGENS NOTES are on a separate webpage

INORGANIC Part 9 Group 7/17 Halogens sub–index:

9.1 Introduction, trends & Group 7/17 data

9.2 Halogen displacement reaction and reactivity trend

9.3 Reactions of halogens with other elements - halides

9.4 Reaction between halide salts and conc. sulfuric acid

9.5 Tests for halogens and halide ions

9.6 Extraction of halogens from natural sources

9.7 Uses of halogens & compounds

9.8 Oxidation & Reduction – more on redox reactions of halogens & halide ions

9.9 Volumetric analysis – titrations involving halogens or halide ions

9.10 Ozone, CFC's and halogen organic chemistry links

9.11 Chemical bonding in halogen compounds

9.12 Miscellaneous aspects of halogen chemistry

Advanced Level Inorganic Chemistry Periodic Table Index: Part 1 Periodic Table history Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr and important trends down a group * Part 7 s–block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p–block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots

Group numbering and the modern periodic table

The original group numbers of the periodic table ran from group 1 alkali metals to group 0 noble gases. To account for the d block elements and their 'vertical' similarities, in the modern periodic table, group 3 to group 0 are numbered 13 to 18. So, the halogen elements are referred to as group 17 at a higher academic level, though group 7 is still often used at a lower academic level.

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