Redox - Yr13

Question 1

What is an oxidising agent?

Answer 1

Takes electrons

Question 2

What is a reducing agent?

Answer 2

Loses electrons

Question 3

Is Cu2+ + 2e- —-> Cu reduction or oxidation?

Answer 3

Reduction

Question 4

Is Al —-> Al3+ + 3e- reduction or oxidation?

Answer 4

Oxidation

Question 5

What is the overall equation for Cu2+ + 2e- —-> Cu and Al —-> Al3+ + 3e- ?

Answer 5

3Cu2+ + 2Al —-> 3Cu + 2Al3+

Question 6

How do you write half equations

Answer 6

Balance the atoms - can use H2O or H+

Balance the charges - can use e-

Question 7

What does the no. of electrons tell you in a half equation?

Answer 7

The change in the oxidation number

Question 8

What is the half equation for the reduction of VO2+ to V3+?

Answer 8

VO2+ +2H+ + e- —-> V3+ + H2O

Question 9

What is the half equation for the reduction of Cr2O7 2- to Cr3+?

Answer 9

Cr2O7 2- + 14H+ 6e- —-> 2Cr3+ + 7H2O

Question 10

What is the half equation for the reduction of MnO4 - to Mn2+?

Answer 10

MnO4 - + 8H+ +5e- —-> Mn2+ + 4H2O

Question 11

What is the half equation for the reduction of H2S to S?

Answer 11

H2S —-> S + 2H+ + 2e-

Question 12

What is the half equation for the reduction of H2SO4 to H2S?

Answer 12

H2SO4 + 8H+ +8e- —-> H2S + 4H2O

Question 13

What should the value of a hydrogen electrode always be?

Answer 13

0.00V

Question 14

On which electrode does oxidation happen?

Answer 14

The half cell with the most negative standard electrode potential

Question 15

What does it mean if the electrode has the more negative electrode potential?

Answer 15

More reactive the metal

Question 16

What is the strongest reducing agent?

Answer 16

Li (s)

Question 17

What is the strongest oxidising agent?

Answer 17

F2 (g)

Question 18

How do you calculate electrode cell potential?

Answer 18

(Positive electrode) - (negative electrode)

Question 19

What does it mean if there is a bigger positive voltage?

Answer 19

Reaction is feasible

Question 20

Why may it not be feasible even if it is positive?

Answer 20

Conditions (not STD) may affect feasibility

Ea may be too high/rate too slow

Question 21

What is the anti-clockwise rule?

Answer 21

Arrange redox equation so that…
Electrons on the left
Largest negative electrode on top
Reaction takes place anticlockwise

Question 22

What is the overall equation for Cu2+ + 2e- —-> Cu (+0.34) and Zn2+ + 2e- —-> Zn (-0.76)?

Answer 22

Zn + Cu2+ —-> Zn2+ + Cu

Question 23

Why is the Zn and Cu reaction feasible?

Answer 23

Since Zn reacted with Cu

And lost electrons

Question 24

What would happen if you increased Zn2+ conc?

Answer 24

Equilibrium =shift left

Reduces ease of electron loss

Question 25

What would happen if you increased Cu2+ conc?

Answer 25

Equilibrium = shift right | Increases ease of electron gain

Question 26

When does it go wrong?

Answer 26

Changing conc

Question 27

What does it mean if there is less electrons in the system?

Answer 27

More positive the system

Question 28

What does it mean if it shifts left?

Answer 28

Negative

Question 29

What does it mean if it shifts right?

Answer 29

Positive

Question 30

When can an element oxidise another element?

Answer 30

When it is more positive

Question 31

When can an element reduce another element?

Answer 31

When it is less positive

Question 32

How do you make a standard solution for a redox titration (part 1)?

Answer 32

Make up 250cm3 of standard solution using 7.0g of iron sulphate By weighing difference in volumetric flask Record mass in table

Question 33

How do you carry out a redox titration (part 2)?

Answer 33

Pipette 25cm3 of standard solution into conical flask Add 10cm3 of 1.0M of H2SO4 to conical flask Fill burette with 0.02M potassium manganate (purple) Carry out titration until end point reached End point = first permanent pink colour Record results in table Repeat until have 2 concordant results

Question 34

What is the overall equation for the redox titration of potassium manganate?

Answer 34

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

Question 35

Piece of pure iron wire (0.1395g) reacts with aqueous acid to convert Fe to Fe2+. Needs 26.2cm3 of KMnO4 solution in titration. Calculate conc of KMnO4 solution

Answer 35

0.0191 moldm-3

Question 36

0.402g of impure Fe was reacted with an aqueous aid to convert Fe to Fe2+. Needs 23.5cm3 of 0.04moildm-3 kMnO4 solution in titration. Calculate percentage by mass of Fe

Answer 36

65.2%

Question 37

When reacted with CrO7 2- only Sn2+ to Sn4+. 10g of solder dissolved in acid to make 1.0dm3. 25cm3 needed 20cm3 of 0.0175 moldm-3 K2Cr2O7. Calculate percentage by mass

Answer 37

49.9%

Question 38

25cm3 of hydrogen peroxide solution poured into 250cm3 volumetric flask + made up to 250cm3 with water. 25cm3 of solution acidified + titrated against 0.02 moldm-3 KMnO4, 38cm3 required. Calculate original conc of hydrogen peroxide solution

Answer 38

0.76 moldm-3

Question 39

Why do you still divide by 25 when it was made up to 250cm3?

Answer 39

Because even though in 250cm3 we just want the "vodka" not the coke (water) as well

Question 40

What colour is iodine change from and to in the redox titration?

Answer 40

Yellow/brown solution to straw colour

Question 41

What is the colour change an indication of?

Answer 41

The completion of a redox reaction

Question 42

What is a cell?

Answer 42

A cell that has 2 half-cells connected by a salt bridge

Question 43

What does a simple half cell consist of?

Answer 43

Metal Solution of compound containing metal eg. Cu + CuSO4

Question 44

What will 2 half cells produce when connected in a circuit?

Answer 44

Small voltage

Question 45

What half cell chosen to produce "standard" potential?

Answer 45

Hydrogen

Question 46

What are the possible half equations for the standard hydrogen half cell?

Answer 46

H+ + e- ----> 1/2H2 | 1/2H2 ----> H+ + e-

Question 47

What are the conditions of the standard hydrogen half cell?

Answer 47

298K 1 moldm-3 1 atm

Question 48

What happens if you connect a hydrogen half cell to another cell (containing a different metal)?

Answer 48

It will tell you the value of the electrode potential of the metal

Question 49

Why use a salt bridge?

Answer 49

Used to connect the circuit | So free moving ions can conduct charge

Question 50

What is a salt bridge made from?

Answer 50

Made frim filter paper soaked in potassium nitrate

Question 51

Why is a salt bridge soaked in KNO3?

Answer 51

Unreactive with electrodes = suitable

Question 52

Why is a salt bridge used instead of a wire?

Answer 52

Because a wire would set up its own electrode system with the solution

Question 53

What is a two ion system?

Answer 53

When it is 2 aqueous species in a half equation

Question 54

What concentration must solutions be in a two ion system?

Answer 54

1 moldm-3

Question 55

If you aren't given a solid in one of the half equations what must be the electrode + solution of that half cell be?

Answer 55

Pt = electrode | Both ions = solution

Question 56

What is standard electrode potential?

Answer 56

The emf of a half cell compared with a standard hydrogen electrode at standard conditions - 298k, 1.0 moldm-3 + 1atm

Question 57

What is a fuel cell?

Answer 57

A device that converts energy into electrical energy, water + heat through electrochemical reactions

Question 58

What is the hydrogen fuel cell?

Answer 58

Hydrogen goes in + reacts with O2 = voltage

Question 59

Describe hydrogen fuel cell | Fuel cell Vs Battery

Answer 59

``` Open system Anode + cathode = gases React with Pt catalyst Reactants externally supplied No recharging required ```

Question 60

Describe galvanic cell (battery) | Fuel cell Vs Battery

Answer 60

Closed system Anode + cathode = metals Reactants internally consumed Needs recharging

Question 61

What are the similarities between fuel cell Vs combustible engine?

Answer 61

Both use hydrogen-rich fuel Both use compressed air as oxidant Both require cooling

Question 62

What are the differences between fuel cell Vs combustible engine? FUEL CELL

Answer 62

Output = electrochemical work Fuel + oxidant reacts electrochemically No pollution

Question 63

What are the differences between fuel cell Vs combustible engine? COMBUSTIBLE ENGINE

Answer 63

Output = mechanical work Fuel + oxidant react combustively Fossil fuels = pollution

Question 64

What are the half equations for alkali fuel cell?

Answer 64

2H2O + 2e- ----> H2 + 2OH- | 1/2O2 - 2e- ----> 2OH-

Question 65

What are bad about hydrogen fuel cells?

Answer 65

Atm methanol used to make hydrogen = fossil fuels | Hydrogen gas = explosive = dangerous