DM - Redox titrations Flashcards
What are acid-base titrations used for?
To see how much acid is needed to neutralise a base.
What do titrations allow us to do?
Find out exactly how much acid is needed to neutralise a quantity of alkali.
How do you carry out an acid-base titration?
- Measure out some alkali using a pipette and put it in a flask, along with some indicator (e.g. phenolphthalein).
- Do a rough titration to get an idea of where the end point is. Add the acid to the alkali using a burette - giving the flask a swirl.
- Now do an accurate titration. Run the acid in to within 2cm3 of the end point, then add the acid dropwise.
- Record the volume of acid used to neutralise the alkali. Repeat until the volume of the acid recorded is the same (or within 0.1cm3) for 2-3 titres.
What do redox titrations let us find out?
How much oxidising agent is needed to exactly react with a quantity of reducing agent.
What do we need to know to carry out a redox titration?
The concentration of either the oxidising or reducing agent.
Which redox titration do we need to know about?
The one where manganate(VII) ions (MnO4^-) are the oxidising agent.
Describe the redox titration where manganate(VII) ions are the oxidising agent.
1) Measure out a quantity of reducing agent (e.g. Fe2+ ions aq), using a pipette, and put it in a conical flask.
2) Add some dilute sulfuric acid to the flask - this is in excess, so you don’t have to be too exact.
3) Now add the aqueous manganate(VII) ions (the oxidising agent) to the reducing agent using a burette, swirling the conical flask as you do so.
4) Stop when the mixture in the flask just becomes tainted with the colour of MnO4^- (the end point) and record the volume of oxidising agent added. This is the rough titration.
5) Now do some accurate titrations until two or more of the readings are within 0.1cm3 of each other.
How do you know when the reaction in a redox titration is completed?
There’s a sharp colour change.
What colour are manganate(VII) ions (MnO4^-) in aqueous potassium manganate (KMnO4)?
Purple
What happens when manganate(VII) ions (MnO4^-) are added to the reducing agent?
They start reacting with the reducing agent and are reduced to colourless Mn2+ ions.
When does the reaction between manganate(VII) ions (MnO4^-) and the reducing agent end and how can this be spotted?
Continues until all of the reducing agent has reacted. The very next drop into the flask will give the mixture the pink colour of the oxidising agent.
What can you use to find the concentration of a reagent?
Titration results.
What can titration results be used for?
Finding the concentration of a reagent.
Why don’t you need to add an indicator when doing a redox titration with manganate(VII) ions?
They are already coloured purple (and turn colourless when they react with the reducing agent).
