Module 5 Chapter 23
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How do you balance complex half equations
Using KOHES
What is KOHES
Key elements Oxygen Hydrogen Electrons States
What occurs in alkaline conditions too do with KOHES
OH- is added instead of H+ when doing KOHES
What is potassium manganate
It is an oxidising agent under acidic conditions and is reduced to form Mn2+
What is required for MnO4- to be reduced
H+ ions
What is potassium manganate commonly used for
The analysis of reducing agents that are oxidised
What are common reducing agents that potassium manganate is used to analyse
Iron (II) Fe 2+ Ethanedioic acid (COOH)2
What are the differences in a redox titration and an acid titration
It does not need na indicator as MnO4- is purple and Mn2+ is colourless Due to the dark colour of KMnO4, readings are taken from the top of the meniscus instead of
What is the endpoint
It is the first permanent pink colour
What is the method for determining the concentration of a solution of potassium manganate (1)
• Pipette a known volume of the standard solution into a conical flask • Add an excess of an acid to the flask • Rinse the burette with a small sample of potassium manganate solution
What is the method for determining the concentration of a solution of potassium manganate (2)
• Refill the burette with the • Complete the rough titration until you achieve a persistent pink colour, record reading • Complete more accurate titrations until you obtain concordant sets of results
What can iodine thiosulphate titrations be used for
To determine the content of different oxidising agents
What are some oxidising agents that iodine thiosulphate titrations can be used to determine the content of
ClO- in bleaches Cu 2+ compounds Cu in alloys
What is reduced in the reaction and give the half equation
I2 + 2e- —> 2I-
What is oxidised in the reaction and give the half equation
S2O3 2- 2S2O3 2- —> S4O6 2- + 2e-
How is a sodium thiosulphate titration done (1)
You add thiosulphate solution dropwise to a small sample of aqueous iodine until pale yellow.
Why does the solution turn pale yellow when the thiosulphate is added
As the iodine is in excess, most of the iodine is reduced to I- however some iodine remains causing it to have a change in colour from brown to pale yellow.
How is a sodium thiosulphate titration done (2)
Add one drop of starch solution Then continue to add thiosulphate solution dropwise
Why does the solution turn blue when the starch is added
As the iodine forms a complex with the starch causing it to turn blue-black
Why does the solution turn colourless after the remaining thiosulphate is added drop by drop
It reduces the remaining I2 to I- and when there are no more I2 molecules the starch can’t form the complex causing a change in colour from blue to clear.
How do you modify the method to analyse an oxidant
• Pipette/weigh a sample of oxidant into a conical flask • Add excess of potassium iodide solution Oxidant + iodide —> iodine + reduced form of oxidant • Carry out the titration normally
What can you use this method to analyse
Copper
What is an extra step that is required if the copper salt is insoluble in water
You don’t need to react the salt with acid to form aqueous ions.
What occurs in a electrochemical cell
Two redox half equations take place separately , with electron transfer caused by by external circuits.