YEAR 12 HOMEWORK

SHEET 6 - Redox Reactions

Read pp 39 - 48 of Chemistry 2

For these questions you must need to remember n=m/M, n=cV and pV=nRT and the table of solubilities.

Q1. Calculate the oxidation number of the underlined atom for each of the following:

Q2. Which of the following reactions are redox? For those that are redox, state which sp[ecies is the oxidant?

(a) Cl_{2 (g)} + Zn _(s) ---> Zn²⁺ _(aq) + 2Cl^-_(aq)

(b) NaCl _(aq) + AgNO_{3 (aq)} ---> AgCl _(s) + NaNO_{3 (aq)}

(c) Fe₂O₃ (s) + 3H₂ (g) ---> 2Fe (s) + 3H₂O

(d) S₈ (s) ---> 4S₂ (g)

(e) ICl₅ (s) + Cl₂ (g) ---> ICl₇ (s)

(f) 2KMnO₄ (aq) + 5H₂S (g) + 6HNO₃ ---> 2Mn(NO₃)₂ (aq) + 5S (s) + 2KNO₃ (aq) + 8H₂O (l)

(g) 6CO₂ (g) + 6 H₂O (l) ---> C₆H₁₂O₆ + 6O₂ (g)

Q3. Write balanced half equations for the following processes

(a) NO₃^- (aq) to NO₂ (g)

(b) S₂O₃^- (aq) to SO₂ (g)

(c) V₂O₅ (s) to VO₂^2-

(d) MnO₂ (s) to Mn₂O₃^- (aq)

Q4. Balance the following skeleton redox reactions

(a) Cr₂O₇^2- + H⁺ + I₂ (s) ---> IO₃^- + Cr³⁺ + H₂O

(b) NO₃ + H⁺ + S + H₂O ---> NO (g) + H₂SO₃

Q5 What mass of potassium permanganate is required to oxidise 2.40 g of iron (II) sulfate acidified with sulfuric acid?

Q6. Dilute hydrogen peroxide (H₂O₂) is sold as a general purpose bleach. One brand ("WundaWhite) is tested by volumetric analysis. a 25.00 ml aliquot of the peroxide is titrated with 0.0525 M potassium permanganate solution. The average titre is 31.12 ml.

(a) Write a flow chart for the procedure of the volumetric analysis

(b) Write the equation for the reaction

(c) Determine the concentration of peroxide in WundaWhite in both mol/L and g/L.

(d) What volume of oxygen at 24 ^oC and 750 mmHg would be produced during the titration?

(e) This titration did not use an indicator. Could you suggest a reason why?

Q7. 4.00 g of steel wool (impure iron) was dissolved in excess sulfuric acid (which converted the Fe atoms into Fe²⁺) and the solution made up to 500.0 ml in a standard flask. 25.0 ml of this solution required 20.00 ml of 0.0300 M potassium permanganate solution to be completely converted into Fe³⁺. What percentage of the wool is iron?

Q8. Household beach contains the active ingredient NaOCl which is usually listed as "available chlorine" on the product label. A sample of "Wally's Whitener" claims to have 40g/L a available chlorine.

A 20.0 ml sample of the bleach is made up to 100ml in a standard flask and 20.0 ml aliquots of the diluted solution are pipetted into conical flasks and excess acidified KI solution added. The iodide is oxidised into iodine:

OCl^- (aq) + 2I^- (aq) + 2H⁺ (aq) ---> I₂ (aq) + Cl^- (aq) + H₂O (l)

The free iodine is then titrated with a standard solution of 0.500M sodium thiosulfate solution with an average titre of 18.52 ml according to the reaction.

I₂ (aq) + 2S₂O₃^2- (aq) ---> S₄O₆^2- (aq) + 2I^- (aq)

(a) Calculate the number of moles of S₂O₃^2- added

(b) from (a), calculate the number of moles of I₂ added

(c) Calculate the number of moles of OCl^- in each aliquot of diluted solution

(d) Calculate the number of moles of OCl^- in the original sample of bleach

(e) Calculate the mass of "free" chlorine in the original sample

(f) Calculate the concentration of free chlorine as g/L