SS - Titrations and Redox Flashcards
Describe the difference between a pipette and a burette
A pipette measure only one volume of solution where as burettes are graduated so can measure different volumes
What are acid-base titrations used for?
To find how much acid is needed to neutralise a base
Describe how you would carry out an acid-base titration
1) Measure out some alkali using a pipette and put it in a flask, along with some indicator (e.g. phenolphthalein)
2) Do a rough titration to find where the end point roughly is by adding the acid slowly whilst regularly swirling the flask (neutralisation when indicator colour changes)
3) Do an accurate titration, running the acid within 2cm³ of the endpoint, and then adding the rest of the acid drop by drop.
4) Record the amount of acid used to neutralise the alkali (make sure to use repeats)
Describe the use of a redox titration
To find how much oxidising agent is needed to exactly react with a quantity of reducing agent
What must you know before starting a redox titration?
The concentration of either the oxidising agent or the reducing agent
Describe how you would carry out a redox titration with manganate (VII) ions as the oxidising agent
1) Measure out the quantity of reducing agent (e.g. Fe²⁺) and put it in a conical flask using a pipette
2) Add excess dilute sulfuric acid to the flask (about 20cm³) - so there are plenty of H⁺ ions to allow the oxidising agent to be reduced
3) Add the aqueous MnO₄⁻ (the oxidising agent) to the reducing agent using a burette and swirling the conical flask as you do so
4) Stop when the mixture in the flask just becomes tainted with the colour of the MnO₄⁻ and record the volume of oxidising agent added. This is a rough titration
5) Do more accurate titrations with repeates within 0.10cm³ of each other
What colour are Manganate (VII) ions (MnO₄⁻) in aqueous potassium manganate (VII) solution (KMnO₄)
Purple
In a titration, give the name of:
i) The reagent of known volume but unknown concentration
ii) The reagent added from the burette
i) Analyte
ii) Titrant
Describe how you would use titration results to find the concentration of the analyte
1) Work out the number of miles of the titrant added to the analyte
2) Look at the balanced equation for the reaction and find how many moles of analyte react with every mole of titrant
3) Work out the number of moles of analyte that would be in 1000cm³ (1 dm³) of solution - this is the concentration
Define redox reaction
One chemical is reduced whilst another is oxidised due to the transfer of electrons from the reducing agent to the oxidising agent (the reduced species)
What are redox reactions also referred to as?
Oxidation-reduction reactions
Give the 8 rules for working out the oxidation state of an atom
1) All atoms are treated as ions, even if covalently bonded
2) Uncombined elements have an oxidation state of 0
3) Elements just bonded to identical atoms, like O₂ and H₂, also have an oxidation state of 0
4) The oxidation state of a simple monatomic ion (e.g. Na⁺) is the same as its charge
5) In compounds or compound ions, the overal oxidation state is the ion charge (e.g. SO₄²⁻ is -2)
6) The sum of the oxidation states for a neutral compound is 0
7) Combined oxygen is nearly always -2, except in peroxides where it’s -1 (or combined with fluorine)
8) Combined Hydrogen is +1, except in metal hydrides where it is -1 (and H₂ where it’s 0)
What do the roman numerals in a chemical’s name show?
The oxidation number of the atom or group immediately before it
State the 3 ways in which a molecule can become oxidised
The loss of electrons
The loss of Hydrogen
The gain of Oxygen
State the 3 ways in which a molecule can become reduced
The gain of electrons
The gain of Hydrogen
The loss of Oxygen
