Redox- equations and titrations

Question 1

what is reduction in terms of electrons and oxidation number?

Answer 1

-gain of electrons
-decrease in oxidation number

Question 2

what is oxidation in terms of electrons and oxidation number?

Answer 2

-loss of electrons
-increase in oxidation number

Question 3

what is the oxidation number of hydrogen?

Answer 3

+1

Question 4

what is the oxidation number of oxygen?

Answer 4

-2

Question 5

what is an oxidising agent?

Answer 5

takes electrons from the species being oxidised, contains the species that is reduced

Question 6

what is the reducing agent?

Answer 6

adds electrons to the species being reduced, contains the species that is oxidised

Question 7

what two common redox titrations will be studied?

Answer 7

-potassium promanganate (VII) under acidic conditions
-sodium thiosulfate for determination of iodine

Question 8

How do the manganate (VII) (MnO4-) act as an oxidising agent?

Answer 8

MnO4-(aq) ions are reduced to Mn2+ so the other chemical used must be a reducing agent that is oxidised

Question 9

how does the potassium manganate (KMnO4) titration lay out?

Answer 9

-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

Question 10

how do you read the meniscus?

Answer 10

read top of the meniscus

Question 11

Manganate (VII) titrations can be used for the analysis of what reducing agents?

Answer 11

-iron (II) ions, Fe2+
-ethanoedioic acid (COOH)2

Question 12

Describe non-familiar redox titrations?

Answer 12

-MnO4- reduced to Mn2+
-KMnO4 can be replaced with other oxidising agents, the commonest used being acidified dichromate (VI)

Question 13

how do you write redox reactions from half equations?

Answer 13

-make sure equation is balanced
-balance electrons
-combine equations
-cancel electrons
-cancel any other species (e.g H+ ions or OH- ions)

Question 14

how do you write a redox equation from oxidation numbers?

Answer 14

-make sure equation. is balanced
-assign oxidation numbers
-balance the electrons
-balance the charges
-balance remaining atoms using water

Question 15

how do you write half equations?

Answer 15

-make sure equation is balanced
-assign oxidation numbers
-balance the electrons
-balance remaining atoms with water

Question 16

Equation for the oxidation of manganate (VII) ions?

Answer 16

8H+ + MnO4- + 5e- -> Mn2+ +4H2O

Question 17

What do we use as a source of manganate (VII) ions?

Answer 17

potassium permanganate

Question 18

Why are manganate ions (VII) a good oxidising agent?

Answer 18

be MnO4- ions (purple) are a different colour to the Mn2+ ions (colourless)

Question 19

what is a manganese ion?

Answer 19

Mn2+

Question 20

what is the equation for the oxidation of iron?

Answer 20

Fe2+ -> Fe3+ + e-

Question 21

what are the steps needed to write redox titrations?

Answer 21

-balance electrons
-balance charges using H+ ions
-balance atoms using water

Question 22

what is the overall redox equation for the iron permanganate titration?

Answer 22

8H+ + MnO4- + 5Fe2+ -> Mn2+ + 4H2O + 5Fe3+

Question 23

what occurs at the endpoint of the iron permanganate titration?

Answer 23

all the Fe2+ ions are used up and there is a build up of MnO4- ions

Question 24

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

Answer 24

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

Question 25

manganate (VII) titrations are used to analyse what? and potassium permanganate (KMnO4) can be replaced with what other oxidising agent?

Answer 25

-manganate titrations are used to analyse reducing agents that reduce MnO4- to Mn2+
-KMnO4 can be replaced with acidified dichromate (VI), H+/Cr2O7 2-

Question 26

In the iodine-thiosulfate titrations, present the oxidation, reduction and overall equation?

Answer 26

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)

Question 27

iodine/thiosulfate titrations can be used to determine?

Answer 27

-the ClO- content in household bleach
-the Cu2+ content in copper(II) compounds
-the Cu content in copper alloys

Question 28

what is the key aim of the iodine/thiosulfate titration?

Answer 28

to work out the concentration of the oxidising agent

Question 29

outline the procedure for the iodine/thiosulfate titration

Answer 29

-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.

Question 30

when the titrated solution becomes pale, how do we overcome this problem to view the end point?

Answer 30

-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-)

Question 31

how do chlorate ions (ClO-) produce iodine to react with thiosulfate ions(S2O3 2-) ? (equations)

Answer 31

• 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)

Question 32

outline the procedure for the analysis of household bleach

Answer 32

-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

Question 33

what can iodine/thiosulfate titrations also be used to determine?

Answer 33

the copper content of copper(II) salts or alloys

Question 34

how are copper ions (Cu2+) produced in copper (II) salts and insoluble copper compounds?

Answer 34

-in copper (II) salts, they are dissolved in water
-in insoluble compounds, they react with ions
-Cu(s) -> Cu2+(aq)

Question 35

for copper alloys, such as brass or bronze, how are copper ions produced?

Answer 35

alloy is dissolved in concentrated nitric acid followed by neutralisation

Question 36

Outline the analysis of copper using iodine/thiosulfate titration?

Answer 36

-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)