Electrode potential and cells

Question 1

What happens when a metal is dipped into a solution of its own ions

Answer 1

An equilibrium is set up between the solid metal and the aqueous metal ions

Question 2

Write half equations for zinc (s) and zinc (II)

Answer 2

Zn (s) (equilibrium sign) Zn2+ (aq)+ 2e-

Question 3

Write a half equation for copper (II) to copper (III)

Answer 3

Cu 2+(s) (equilibrium sign) Cu3+(aq)+ e-

Question 4

Whats the simplest salt bridge made of ?

Answer 4

Filter paper soaked in saturated solution of KNO3 (potassium nitrate)

Question 5

Why r salt bridges necessary

Answer 5

Its purpose is to not move electrons from the electrolyte
But To maintain charge balance
Cos electrons r moving from one half cell to the other
Salt bridge prevents diffusion of solution from one half cell to another

Question 6

What symbol is used to represent a salt bridge in standard notation

Answer 6

||

Question 7

What type of species goes on the outside (furthest from the salt bridge) in standard notation

Answer 7

The most reduced species

Question 8

What does | indicate?

Answer 8

Phase boundary (s/l/g)

Question 9

How would an aluminium/copper cell b represented

Answer 9

Al (s) | Al3+ (aq) || Cu2+ (aq) | Cu (s)

Question 10

What happens at the left hand electrode ?

Answer 10

Oxidation occurs

Left hand electrode is the half cell with the most neg E° value

Question 11

What happens at the right hand electrode

Answer 11

Reduction

Half cell w the most positive E° value

Question 12

Which side of the cell has the most negative E° value?

What happens to the metal w the most neg E° value

Answer 12

Oxidation - left hand electrode

Question 13

Whats E°

Answer 13

Standard electrode potential of a cell

Question 14

Draw the standard hydrogen electrode

Answer 14

What conditions is the standard hydrogen electrode used in ?

Question 15

What conditions is the standard hydrogen electrode used in ?

Answer 15

Temperature 298k
Pressure 100 kPa
[H+] = 1.00 mol dm-3

Question 16

Whats the standard hydrogen electrode used for?

Answer 16

Comparing other cells against

E° of standard hydrogen electrode is defined as 0

So All other E° values are compared against it

Question 17

Y might u use other standard electrodes occasionally?

Answer 17

They r cheaper and quicker to use and can provide just as a good reference

Pt is expensive

Question 18

If an E° value is more positive

What does it mean in terms of oxidising/reducing power

Answer 18

Better oxidising agent (easier to reduce

Question 19

What factors will change e° values

Answer 19

Concentration of ions

Temperature

Question 20

Why happens if u reduce the concentration of the ions in the left hand half cell

Answer 20

Equilibrium moves to the left to oppose the change of removing ions
This releases more electrons
The e° of the left hand cell becomes more negative so the e.m.f of the cell increases

Question 21

How do u calc the emf of a cell from e° values

Answer 21

e° (right) -e° (left) = e° cell

Question 22

When would you use a pt electrode

Answer 22

When both the oxidised and reduced forms of the metal are in aqueous solutions

Question 23

Whys pt used

Answer 23

Inert

Good conducter

Question 24

How would u predict if a reaction would occur

Answer 24

Take the two half equations

Find the species thats being reduced ( this is effectively the right hand electrode)

Calculate its e° value minus the e° value of the species thats being oxidised ( effectively the left hand side)

If E° overall > 0, reaction will occur

Question 25

What was the first commercial cell made from (daniell cell)

Answer 25

Zinc / copper (||)

Question 26

What r zinc carbon cells more commonly known as?

Answer 26

Disposable batteries

Question 27

What r the two reactions that take place in zinc carbon cells

Answer 27

Zn is oxidised to Zn2+ NH4+ reduced to NH3 at carbon electrode

Question 28

What r the reactions that occur in a lead/acid battery (car batteries)

Answer 28

Pb + SO42- —-> PbSO4 (s) + 2e- PbO2 + 4H+ + SO4 2- + 2e- —> PbSO4 + 2H2O

Question 29

How r cells recharged (if they r rechargeable)

Answer 29

Reactions r reversible | Reversed by running a higher voltage thru the cell than the cell’s E°

Question 30

Nickel / cadmium cells r rechargeable AA batteries etc. What reactions occur at the electrodes

Answer 30

Cd(OH)2 (s) + 2e- —> Cd (s) + 2OH- NiO(OH) (s) + H2O + e- —> Ni(OH)2 (s) + OH-

Question 31

Where r lithium ion cells used?

Answer 31

Mobile phones | Laptops

Question 32

What reactions occur on discharge in lithium ion cells

Answer 32

Li+ + CoO2 + e- —-> Li+[CoO2]- Li —-> Li+ + e-

Question 33

What is a fuel cell?

Answer 33

A cell thas used to generate electric current Does not require electrical recharging

Question 34

What r the reactions that take place at the two electrons in an alkaline hydrogen fuel cell?

Answer 34

2H2 + 4OH- —-> 4H20 + 4e- O2 + 2H2O + 4e- —-> 4OH-

Question 35

Draw a diagram of a hydrogen fuel cell

Answer 35

Ugh

Question 36

Y is it better to use a fuel cell than to burn H2 in air | Even tho the same overall reaction occurs

Answer 36

In combustions sulphur containing compounds and nitrogen containing compounds r produces die to high temperatures and the s and n in air These r bad for the environment This doesn’t occur in a fuel cell, the only product is water More efficient

Question 37

Disadvantages of fuel cells

Answer 37

h is a flammable gas w a low b.p.—> hard and dangerous to store and transport Expensive to buy Fuel cells have a Limited lifetime and use toxic chemicals in their manufacture

Question 38

How do u find the weakest reducing agent from a table of electrode potential data?

Answer 38

Most pos e° value | Then it is the product of the reduction equation i.e. imagine equation going from left to right

Question 39

Whats the reason that some cells cannot b recharged

Answer 39

Reaction of the cell is not reversible - a product is produced that either dissipated or cannot b converted back into the reactants

Question 40

Why might the emf of a cell change after a period of time?

Answer 40

Concentrations of the ions change - the reagents are used up

Question 41

How can the emf of a value b kept constant

Answer 41

Reagents r supplied constantlt so tye concentrations of the ions are constant E° remains constant

Question 42

Give two important conventions that you have to follow when your drawing electrochemical cells using the conventional representation

Answer 42

The half cell with the more negative potential goes on the left the oxidised forms go in the centre of the cell diagram the reduced forms go at the edge of the cell diagram

Question 43

If three factors that can influence electrode potentials

Answer 43

Temperature Concentration of reactants Pressure

Question 44

What is the electrode potential of the standard hydrogen electrode? why is it this value

Answer 44

0.00 v By definition

Question 45

Describe how a standard hydrogen electrode is set up and give the standard conditions used when measuring electrode potentials

Answer 45

Hydrogen gas is bubbles into a solution of aqueous H+ ions electrode is made of Pt standard conditions temperature 298k pressure 100 kPa all solutions of ions have a concentration of 1.00 mol dm-3

Question 46

Describe the term standard electrode potential

Answer 46

The voltage measured under standard conditions when a half cell is connected to a standard hydrogen electrode

Question 47

Aluminium has a standard electrode potential of -1.66 whilst Nickel has a standard electrode potential of -0.25 states which species is more reactive and explain how you know in terms of electrode potential

Answer 47

Al - they r both metals and Al has a more negative electrode potential

Question 48

Electrode potential of bromine is +1.07 whilst chlorine has an electrode potential of +1.36 state which species is more reactive and explain how you know in terms of electrode potential

Answer 48

Chlorine - they are both nonmetals and chlorine has the more positive standard electrode potential

Question 49

Electrode potential of silver is +0.80 whilst copper has an electrode potential of +0.34 state which species is more reactive and explain how you know in terms of electrode potential

Answer 49

Copper - They r both metals and copper has the more negative electrode potential

Question 50

Calculate the EMF for the following reactions Al(s) + 3Ag+(aq) —> Al3+(aq) + 3Ag(s) Cu (s) + Cl2(aq) —> Cu2+(aq) + 2Cl-(aq) Al3+ (aq) E° = -1.66 Ag+ (aq) E° = +0.80 Cu2+ (aq) E° = +0.34 Cl2(aq) E° = +1.36

Answer 50

0. 80-(-1.66)=2.46v | 1. 36-0.34=+1.02v

Question 51

The direction of redox reactions of the following half-cells Mg2+(aq)/Mg(s) and Ni2+(aq)/Ni(s)

Answer 51

Mg2+(aq) +2e—-> Mg(s) E° = -2.38v Ni2+(aq) + 2e- —> Ni (s) E°