Redox- equations and titrations Flashcards
what is reduction in terms of electrons and oxidation number?
-gain of electrons
-decrease in oxidation number
what is oxidation in terms of electrons and oxidation number?
-loss of electrons
-increase in oxidation number
what is the oxidation number of hydrogen?
+1
what is the oxidation number of oxygen?
-2
what is an oxidising agent?
takes electrons from the species being oxidised, contains the species that is reduced
what is the reducing agent?
adds electrons to the species being reduced, contains the species that is oxidised
what two common redox titrations will be studied?
-potassium promanganate (VII) under acidic conditions
-sodium thiosulfate for determination of iodine
How do the manganate (VII) (MnO4-) act as an oxidising agent?
MnO4-(aq) ions are reduced to Mn2+ so the other chemical used must be a reducing agent that is oxidised
how does the potassium manganate (KMnO4) titration lay out?
-KMnO4 is added to the burette
-add measured volume of unknown solution to clinical flask with excess dilute sulphuric acid added to provide H+ ions for reduction
-during titration, manganate solution reacts and decolorises
-end point is permanent pink colour, indicating excess MnO4- ions
-repeat until two concordant results obtained
how do you read the meniscus?
read top of the meniscus
Manganate (VII) titrations can be used for the analysis of what reducing agents?
-iron (II) ions, Fe2+
-ethanoedioic acid (COOH)2
Describe non-familiar redox titrations?
-MnO4- reduced to Mn2+
-KMnO4 can be replaced with other oxidising agents, the commonest used being acidified dichromate (VI)
how do you write redox reactions from half equations?
-make sure equation is balanced
-balance electrons
-combine equations
-cancel electrons
-cancel any other species (e.g H+ ions or OH- ions)
how do you write a redox equation from oxidation numbers?
-make sure equation. is balanced
-assign oxidation numbers
-balance the electrons
-balance the charges
-balance remaining atoms using water
how do you write half equations?
-make sure equation is balanced
-assign oxidation numbers
-balance the electrons
-balance remaining atoms with water
Equation for the oxidation of manganate (VII) ions?
8H+ + MnO4- + 5e- -> Mn2+ +4H2O
What do we use as a source of manganate (VII) ions?
potassium permanganate
Why are manganate ions (VII) a good oxidising agent?
be MnO4- ions (purple) are a different colour to the Mn2+ ions (colourless)
what is a manganese ion?
Mn2+
what is the equation for the oxidation of iron?
Fe2+ -> Fe3+ + e-
what are the steps needed to write redox titrations?
-balance electrons
-balance charges using H+ ions
-balance atoms using water
what is the overall redox equation for the iron permanganate titration?
8H+ + MnO4- + 5Fe2+ -> Mn2+ + 4H2O + 5Fe3+
what occurs at the endpoint of the iron permanganate titration?
all the Fe2+ ions are used up and there is a build up of MnO4- ions
how do you analyse the percentage purity of an iron (II) compound (procedure only)? e.g percentage purity of an impure sample of iron (II) sulfate, FeSO4.7H2O
1) prepare a 250cm cubed solution of impure FeSO4.7H2O in a volumetric flask
2) using a pipette, measure 25cm cubed of this solution into a clonical flask. Then add 10 cm cubed of 1moldm cubed H2SO4 (aq) (an excess).
3) Using a burette, titrate this solution using standard of 0.02moldm cubed solution of potassium manganate (VII), KMnO4 (aq)
4) analyse results to determine percentage purity
manganate (VII) titrations are used to analyse what? and potassium permanganate (KMnO4) can be replaced with what other oxidising agent?
-manganate titrations are used to analyse reducing agents that reduce MnO4- to Mn2+
-KMnO4 can be replaced with acidified dichromate (VI), H+/Cr2O7 2-
In the iodine-thiosulfate titrations, present the oxidation, reduction and overall equation?
oxidation: 2 S2O3 2-(aq) -> S4O6 2-(aq) + 2e-
reduction: I2 (aq) + 2e- -> 2I- (aq)
overall: 2 S2O3 2-(aq) +I2(aq) -> 2I-(aq) + S4O6 2-(aq)
iodine/thiosulfate titrations can be used to determine?
-the ClO- content in household bleach
-the Cu2+ content in copper(II) compounds
-the Cu content in copper alloys
what is the key aim of the iodine/thiosulfate titration?
to work out the concentration of the oxidising agent
outline the procedure for the iodine/thiosulfate titration
-add a standard solution of Na2S2O3 (sodium thiosulfate) to the burette.
-using pipette, add the solution of oxidising agent to a clonical flask. Then add excess potassium iodide. The oxidising agent reacts with iodine ions to produce iodine, which turns the solution a yellow-brown colour
-titrate this solution with sodium thiosulfate. During the titration, iodine is reduced back to I- ions and the brown colour fades, making it difficult to see the end point.
when the titrated solution becomes pale, how do we overcome this problem to view the end point?
-add starch indicator
-when starch is added the solution turns blue-black
-the more sodium thiosulfate added, the more blue-black colour disappears
-end point is when all blue-black colour disappears (all iodine is reduced to I-)
how do chlorate ions (ClO-) produce iodine to react with thiosulfate ions(S2O3 2-) ? (equations)
- ClO- ions from bleach react with I- and H+ ions to form I2: ClO-(aq) +2I-(aq) + 2H+(aq) -> Cl-(aq) + I2(aq) + H2O (l)
-in the titration, I2 reacts with S2O3 2- ions: 2 S2O3 2-(aq) + I2(aq) -> 2I-(aq) + S4O6 2-(aq)
outline the procedure for the analysis of household bleach
-using pipette, add 10cm cubed of bleach into 250 cm cubed volumetric flask and add water to prepare 250 cm cubed of solution
-using pipette, measure 25 cm cubed of this solution into a clonical flask and add 1moldm cubed potassium iodide (KI) followed by 1moldm cubed HCl(aq) to acidify solution.
-using burette, titrate solution using standard 0.05moldm cubed solution of sodium thiosulfate
-repeat till concordant results are acheived
what can iodine/thiosulfate titrations also be used to determine?
the copper content of copper(II) salts or alloys
how are copper ions (Cu2+) produced in copper (II) salts and insoluble copper compounds?
-in copper (II) salts, they are dissolved in water
-in insoluble compounds, they react with ions
-Cu(s) -> Cu2+(aq)
for copper alloys, such as brass or bronze, how are copper ions produced?
alloy is dissolved in concentrated nitric acid followed by neutralisation
Outline the analysis of copper using iodine/thiosulfate titration?
-Cu2+(aq) ions react with I-(aq) ions to form I2(aq) and a white precipitate of copper (I) iodide, CuI(s). The mixture is brown.
2Cu2+(aq) + 4I-(aq) -> 2CuI(s) + I2(aq)
-the iodine is then titrated with a standard solution of sodium thiosulfate:
2 S2O3 2-(aq) + I2(aq) -> 2I-(aq) + S4O6 2-(aq)
