PAGS

Question 1

Iodine clock reaction

Answer 1

H2O2(aq) + 2I-(aq) –> 2H20(l) + I2(aq)
1) add small amount of sodium thiosulfate solution along with starch(indicator which turns blue-black in prescence of iodine / use of colorimetry) to an excess of hydrogen peroxide and iodide ions in acid solution
2S2032-(aq) + I2(aq) –> 2I-(aq) + S4O62-(aq)
2) the end point is when no more sodium thiosulfate, so indicator turns blue-black due to presence of iodine

varying concs of H2O2 whilst keeping others ocnstant gives different times for colour change, hence clock reaction

Question 2

measuring pH

Answer 2

1) calibrate the pH meter using distilled water whilst adjusting reading to 7.0
2) use it on standard solutions
3) rinse probe via distilled water each time

Question 3

redox + thiosulfate titrations

Answer 3

1) measure out quantity of the reducing agent(pipette into conical flask)
2) add excess dilute sulfuric acid to flask
3) gradually add oxidising agent using burette, swirling conical flask simultaneously
4) end point = colour change (use of white surface helps pin point final drop)
5) repeat titrations to calculate mean volume

1) for thiosulfate its similar but iodine fades to pale yellow so use of 2cm3 starch going blue-black helps identify end point