Electrodes

Question 1

Deduce how the emf of the cell Mg(s)lMg2+(aq)llFe2+(aq)lFe(s) changes when the conc of Mg2+ is decreased

Answer 1

Increases
Equilibrium displaced to Mg2+
Electrode is more negative

Question 2

Explain why iron corrodes when in contact with water which contains dissolved oxygen

Answer 2

Fe is giving electrons

Question 3

What is the function of the Pt electode

Answer 3

To allow transfer of electrons

Question 4

What are the standard conditions which apply to Fe3+/Fe2+ when measuring potential

Answer 4

Fe3+ and Fe2+ have a conc of 1moldm-3

Question 5

Pt l H2O2 , O2 ll IO3- , I2 l Pt
Write an equation for the spontaneous cell reaction

Answer 5

2IO3- + 2H+ + 5H2O2 -> 5O2 + I2 + 6H2O

Question 6

Give one reason why the emf of this cell changes when the electrodes are connected and a current flows

Answer 6

Emf is determined when no current flows

Question 7

State why the electrode potential for the standard hydrogen electrode is equal to 0V

Answer 7

Set to this value

Question 8

Write the conventional representation for an alkaline hydrogen-oxygen fuel cell

Answer 8

Pt l H2(g) l OH- (aq) ll O2 (g) l OH- (aq) l Pt

Question 9

Describe a standard hydrogen electrode

Answer 9

H2 bubbles
1moldm-3 HCl
298K
100kPA
Pt

Question 10

In other lithium cells, lithium cobalt oxiide electrodes and lithium electrodes are used
Give equations for the reaction that occurs at the pos and neg electrode

Answer 10

Pos: Li+ + CoO2 + e- -> Li+CoO2-
Neg: Li -> Li+ + e-

Question 11

State the meaning of the term electrochemical series

Answer 11

List of electrode potentials in numerical order

Question 12

Use the table to explain why fluorine reacts with water

Answer 12

Fluorine oxidises water
(Fluorine E0 is higher than O2 E0)

Question 13

Suggest one reason why the cell cannot be electrically recharged

Answer 13

Reaction not reversible

Question 14

Give one reason why the emf of the lead acid cell changes after several hours

Answer 14

Reagents get used up in the reaction

Question 15

Identify type of cell that behaves like cell X (Straight horizontal line)

Answer 15

Hydrogen-oxygen fuel cell

Question 16

Explain why the voltage remains constant in cell X

Answer 16

Reagents are supplied continuously
Atmosphetic hydrogen

Question 17

What in the SHE solution??

Answer 17

HCl !!!
1moldm-3

Question 18

Give two reasons why Pt is suitable for its purpose

Answer 18

Inert
COnducts electricity

Question 19

Identify the ‘salt bridge’ and why its used

Answer 19

KNO3
Allow movement of ions
Inert

Question 20

Explain why the ammeter reading would fall to zero after a time

Answer 20

Fe3+ used up
(Reactants used up)

Question 21

Suggest one reason why the redox reaction between chlorine and water does not normally occur in the absence of light

Answer 21

Ea is high

Question 22

‘Discharges’ meaning? How do you write the equation

Answer 22

More positive E0 is the reduction so write that first

Question 23

Deduce one essential property of the non reactive porous separate labelled in the diagram

Answer 23

Allow ions to pass

Question 24

Suggest why a cell often leaks after being used for a long time

Answer 24

Zn used up

Question 25

Suggest why ethanol can be considered to be a carbon neutral fuel

Answer 25

CO2 released by combustion
CO2 taken up by photosynthesis

Question 26

Hydrogen can also be produced by the electrolysis of acidified water using electricity produced using solar cells. Give one reason why this method is not used on a large scale

Answer 26

Cost of manufacture of solar cells