1.11 - Electrode Potentials and Electrochemical Cells

What happens when a rod

of a metal is dipped into a

solution of its own ions?

An equilibrium is set up between the solid metal

and the aqueous metal ions

Write a half-equation for

zinc (s) to zinc (II).

Zn (s) ⇌ Zn 2+ (aq) + 2e

Write a half-equation for

copper (II) to copper (III).

Cu 2+ (aq) ⇌ Cu 3+ (aq) + e

What is the simplest salt

bridge made of?

Filter paper soaked in saturated solution of KNO 3

(potassium nitrate)

Why are salt bridges

necessary?

Complete the circuit, but avoid further metal/ion

potentials as does not perform electrochemistry.

Allows ion movement to balance the charge. Do

not react with electrodes

What symbol is used to

represent a salt bridge in

standard notation?

||

What type of species goes

on the outside (furthest from

the salt bridge) in standard

cell notation?

The most reduced species

What does | indicate?

Phase boundary (solid/liquid/gas)

How would an

Aluminium/Copper cell be

represented?

Al(s) | Al 3+ (aq) || Cu 2+ (aq) | Cu(s)

What happens at the

left-hand electrode?

Left hand electrode is where oxidation occurs.

Left hand electrode is the half cell with the most

negative E o value

What happens at the

right-hand electrode?

right hand electrode is where reduction occurs.

Right hand electrode is the half cell with the most

positive E o value

Which side of the cell has

the most negative E° value?

what happens to the metal

with the most negative E o

value?

Oxidation - left hand electrode

Draw the standard hydrogen

electrode

What conditions is the

standard hydrogen

electrode used in?

Temperature = 298 K

Pressure = 100 kPa

[H + ] = 1.00 mol dm -

What is the standard

hydrogen electrode used

for?

Comparing other cells against. E O of SHE is

defined as 0, so all other E o values are compared

against it.

Why might you use other

standard electrodes

occasionally?

They are cheaper/easier/quicker to use and can

provide just as good a reference.

Platinum is expensive

If an E o value is more

negative, what does it mean

in terms of

oxidising/reducing power?

Better reducing agent (easier to oxidise)

If an E o value is more

positive, what does it mean

in terms of

oxidising/reducing power?

Better oxidising agent (easier to reduce)

What factors will change E o

values?

Concentration of ions

Temperature

What happens if you reduce

the concentration of the ions

in the left hand half cell?

Equilibrium moves to the left to oppose the

change of removing ions; this releases more

electrons, the E o of the left hand cell becomes

more negative, so the e.m.f. Of the cell

increases.

How do you calculate the

emf of a cell from E o

values?

E o

cell = E o

right - E o

left

When would you use a

Platinum electrode?

When both the oxidised and reduced

forms of the metal are in aqueous

solution

Why is Platinum chosen?

Inert so does not take part in the electrochemistry

Good conductor to complete circuit

How would you predict if a

reaction would occur?

Take the 2 half equations.

Find the species that is being reduced (this is effectively the

right hand electrode)

Calculate its E o value minus the E o value of the species that

is being oxidised (effectively the left hand cell).

If E o overall > 0, reaction will occur.

What was the first

commercial cell made from

(Daniell cell)?

Zinc/copper (II)

What are zinc/carbon cells
more commonly known as?

Disposable batteries

What are the two reactions

that take place in

zinc/carbon cells?

Zn oxidised to Zn 2+

NH 4

+ reduced to NH 3 at carbon electrode

What are the reactions that

occur in a lead/acid battery

(car batteries)?

Pb + SO 4

2- → PbSO 4 (s) + 2e -

PbO 2 + 4H + + SO 4

2- + 2e - → PbSO 4 + 2H 2

O

How are cells recharged (if

they are rechargeable)?

Reactions are reversible and are reversed by

running a higher voltage through the cell than the

cell’s E o

Nickel/cadmium cells are

rechargeable AA batteries

etc. What reactions occur at

the electrodes?

Cd(OH) 2 (s) + 2e - → Cd(s) + 2OH -

NiO(OH) (s) + H 2

O + e - → Ni(OH) 2 (s) + OH

Where are lithium-ion cells

used?

Mobile phones

Laptops

What reactions occur on

discharge in lithium-ion

cells?

Li + + CoO 2 + e - → Li + [CoO 2 ] -

Li → Li + + e -

What is a fuel cell?

A cell that is used to generate electric current;

does not require electrical recharging

What are the reactions that

take place at the two

electrons in an alkaline

hydrogen fuel cell?

2H 2 + 4OH - → 4H 2 O + 4e -

O 2 + 2H 2 O + 4e - → 4OH

Draw a diagram of a

hydrogen fuel cell.

Why is it better to use a fuel

cell than to burn H 2 in air,

even though the same

overall reaction occurs?

In combustion, sulfur containing compounds (SO 2 , SO 3 ) and

nitrogen containing compounds (NO 2 , NO x

) are produced due

to the high temperatures and the S and N in air. These are

bad for the environment.

This does not occur in a fuel cell; the only product is water.

More efficient

Disadvantages of fuel cells?

Hydrogen is a flammable gas with 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

How do you find the

weakest reducing agent

from a table of electrode

potential data?

Most positive E θ value. Then it is the PRODUCT

of the reduction equation i.e. imagine equation

going from right to left

What is the reason that

some cells cannot be

recharged?

Reaction of the cell is not reversible - a product is

produced that either dissipates or cannot be

converted back into the reactants

Why might the e.m.f. Of a

cell change after a period of

time?

Concentrations of the ions change - the reagents

are used up

How can the e.m.f. Of a cell

be kept constant?

Reagents are supplied constantly, so the

concentrations of the ions are constant; E o

remains constant