Qualitative analysis Flashcards
(23 cards)
What happens at a thiosulfate titration?
2S2O3(2-) + I2 -> 2I(-1) + S4O6(2-)
Goes from a yellow/brown solution to a colourless solution.
A start indicator is added near the end point when the iodine fades a pale yellow to emphasise it. With starch added the colour change is from blue/black to colourless.
What happens in a Manganate redox titration?
The redox titration between Fe(2+) with MnO4(1-) (purple) is a very common exercise. This titration is self indicating becuase of the significant colour change from reactant to product.
MnO4(1-) + 8H(1+) + 5Fe(2+) -> Mn(2+) + 4H2O + 5Fe(3+)
Purple to colourless.
The purple colour of manganate can make it difficult to see the bottom of the meniscus in the burette.
If the manganate is in the burette then the end point of the titration will be the first permanent pink colour.
What type of acids do you use in manganate reactions?
Dilute sulfuric acid
Why can’t Conc HCl be used in manganate titrations?
Because the conc HCl will form Cl- ions which will be then oxidised by the manganate ion.
Why can’t nitric acid be used for mantanate titration?
Because the nitric acid is an oxidising agent. It oxidises Fe(2+) to Fe(3+).
What does an electrochemical cell set up look like?
- A cell has two half-cells.
*The two half cells have to be connected with a salt bridge.
*Simple half cells will consist of a metal(acts as electrode) and a solution of a compound containing that metal. (eg. Cu and CuSO4).
*These two half cells will produce a small voltage if connected into a circuit.
*There is also a high resistant voltmeter connecting the two electrodes (this stops the current from flowing in the circuit. so we can measure the max possible potential difference.
What is a salt bridge made out of?
A salt bridge is usually made from a piece of filter paper (or material) soaked in a salt solution, usually
Potassium Nitrate. The salt should be unreactive with the electrodes and the electrode solutions.
*A wire is not used because the metal wire would set up its own electrode system with the solutions.
Components of a standard hydrogen electrode.
- Hydrogen gas at pressure of 100kPa
- Solution containing the hydrogen ion at 1 mol dm-3
(solution is usually 1 mol dm-3 HCl) - Temperature at 298K
What are the two most stable oxidation states of iron?
Fe2+ and Fe3+
Compounds containing Fe2+ ions are in what colour?
Pale green
What are the most stable oxidation states of chromium?
Cr3+
Cr6+
Cr2+
What can CrO7 (2-) be reduced to and what conditions are required?
It is reduced to Cr3+ and Cr2+ ions.
Conditions = Zinc in acidic conditions.
What is used to oxidise Cr3+ to Cr2O7(2-)?
Hydrogen peroxide in alkaline conditions.
What are the most stable oxidation states of copper?
Cu+
Cu2+
What can be used to reduce Cu2+ to Cu?
Zinc
what does Cu+ readily disproportionates to?
Cu and Cu2+
What is the test for Carbonate ions (CO3(2-)) ? Also give an equation.
When HCl or another type of acid is added, CO2 is given off, this gas is then bubbled through limewater which turns cloudy.
XCO3 + 2HCl —-> CO2 + H2O + XCl2
what is the test for sulfate ions (SO4(2-))?
BaCl2 + XSO4 —> BaSO4 + XCl2
Barium Chloride OR
Barium Nitrate
What is the test for an ammonium ion (NH4+)
Adding NaOH and gently heating will form ammonia gas, which is basic. So use can use damp red litmus paper and it should turn blue.
What happens when Mg2+ is added to these solutions?
Ammonium solution
Excess sodium hydroxide (NaOH)
Excess sulfuric acid (H2SO4)
Ammonium solution = white precipitate (Mg(OH)2)
Excess sodium hydoxide = white precipitate (Mg(OH)2)
Excess sulfuric acid = colourless solution
What happens when Ca2+ is added to these solutions?
Ammonium solution
Excess sodium hydroxide (NaOH)
Excess sulfuric acid (H2SO4)
Ammonium solution = No change
Excess sodium hydoxide = white precipitate (Ca(OH)2)
Excess sulfuric acid = slight white precipitate (CaSO4)
What happens when Sr2+ is added to these solutions?
Ammonium solution
Excess sodium hydroxide (NaOH)
Excess sulfuric acid (H2SO4)
Ammonium solution = No change
Excess sodium hydoxide = slight white precipitate (Sr(OH)2)
Excess sulfuric acid = white precipitate
(SrSO4)
What happens when Ba2+ is added to these solutions?
Ammonium solution
Excess sodium hydroxide (NaOH)
Excess sulfuric acid (H2SO4)
Ammonium solution = No change
Excess sodium hydoxide = No change
Excess sulfuric acid = White precipitate (BaSO4)
