electrochemistry

Question 1

what is a cell

Answer 1

a cell has two half cells
the two half cells have to connected with a salt bridge

these two half cells will produce a SMALL voltage if connected into a circuit ( become a battery or cell)

Question 2

what do simple half cells consist of

Answer 2

a simple half cell will consist of a metal (that acts as the electrode) and a solution of a compound containing that metal (e.g. Cu and CuSO4)

Question 3

why does a voltage form

Answer 3

take half cells with a zinc metal and a copper metal

when connected together the zinc half cell has more of a tendency to oxidise to the Zn2+ ion and release electrons than the copper half cell
Zn → Zn2+ + 2e-

more electrons will therefore build up on the zinc electrode compared to the copper electrode
A potential difference is therefore created with the zinc electrode being more negative and the copper electrode being more positive

Question 4

what is the potential difference measured with

Answer 4

the potential difference is measured with a high resistance voltmeter and is given the symbol E.

e.g. the E for the zinc and copper cell is +1.1V

Question 5

why do we use a high resistance voltmeter

Answer 5

The voltmeter needs to be of very high resistance to stop the current from flowing in the circuit

In this state it is possible to measure the maximum potential difference (E)
The reactions will not be occurring because the very high resistance stops the current from flowing

Question 6

what is a salt bridge used for

Answer 6

The salt bridge is used to connect up the circuit.

The free moving ions ( in the salt bridge) conduct the charge

Question 7

what is a salt bridge made of

Answer 7

a salt bridge is made of filter paper (or material) soaked in a slat solution, usually POTASSIUM NITRATE

Question 8

what are some necessary properties of a salt bridge

Answer 8

the salt bridge should be unreactive with the electrodes and electrode solution

e.g. potassium chloride would not be suitable for copper system because the chloride ion can form complexes with copper ions

Question 9

why is a wire not used (instead of a salt bridge)

Answer 9

a wire is not used because it would set up its own electrode system with the solutions

Question 10

what happens if the current is allowed to move (if we replace the high resistance voltmeter with something like a bulb)

Answer 10

if the current is allowed to flow meaning the voltmeter is replaced with something else like a bulb or if the circuit is short circuited, the reactions will the occur separately at each electrode.

The voltage will therefore fall to zero as the reactants are used up.

Question 11

what reactions occur at each electrode

Answer 11

The most positive electrode will always undergo a reduction reaction:

Cu2+ 2e- (aq) → Cu(s) ( it is positive because electrons are being used up)

The most negative electrode will always undergo reduction:

Zn → Zn2+(aq) + 2e- (negative as electrons are given off)

Question 12

how can electrochemical cells be represented

Answer 12

they can be represented by cell diagrams:

Cd (s) | Cd2+ (aq) || Ag+ (aq) | Ag (s) E = ?V

• the solid vertical line represents the boundary between phases e.g. solid (electrode) and solutions (electrolyte)

The double line represents the salt bridge between the two half cells
The voltage produced is indicated
The more positive half cell is written on the left if possible ( but the is not necessary )

Question 13

What happens when a system does not include metals

Answer 13

If a system does not include a metal but can act as a electrode,the a platinum electrode must be used and included in the cell diagram.
It provides a conducting surface for electron transfer

A platinum electrode is used because it is unreactive and can conduct electricity

Question 14

How do we write the cell diagram if the half equation have several physical states

Answer 14

If a half equation has several physical states then the solid vertical line should be used between different states boundaries

Question 15

How do we measure the electrode potential of a cell

Answer 15

It is not possible to measure the absolute potential tial of a half electrode on its own

It is only possible to measure the potential difference between two electrodes
To measure it, it has to be connected to another half-cell of known potential, and the potential difference between the two half -cells measured

By xinbentuin we can assign a relative potential to each electrode vy linking utto a reference electrode (hydrogen electrode), which is give a potential of zero Volts

Question 16

How is the potential of all electrodes measured

Answer 16

By comparing their potential ta to that of the standard hydrogen electrode

The standard hydrogen electrode (SHE) is assigned with a potential 0 volts

Question 17

What is the hydrogen electrode equilibrium

Answer 17

H2 (g) ⇄ 2H+ (aq) + 2e-

Question 18

How is the hydrogen electrode represented in a cell diagram

Answer 18

Pt|H2 (g)|H+ (aq)

Because the equilibrium does not include a conducting metal surface a platinum wire is used whig is coated in finely divided platinum.

(The platinum black is used because it is proof and can absorb the hydrogen gas(

Question 19

What are the component of a standard hydrogen electrode

Answer 19

Tommy the electrode a standard reference eleclectrode some conditions apply:

1. Hydrogen gas at pressure of 100kPa
2. Solution contains the hydrogen ion at 1.0 mol dm-3 (solutions is usually 1 mol dm-3 HCl)
3. temperature at 298K
4. platinum electrode

Question 20

Why are standard conditions needed for the hydrogen electrode

Answer 20

Standard consulting str needed vecause the position of thr redo equilibrium change with conditions

E.g.
M+1 + e- ⇄ M(s)

Na increase in the concentration of M+1 would move the equilibrium to the right so making the potentialmore positive

Question 21

Whatvrw secondary standards

Answer 21

The standard hydrogen electrode is difficult to use, so often a different tstqndaed is used which is easier to use

These other standards are themselves calibrated against the SHE
This is known as using a secondary standard - I .e. a standard electrode that has been xlaibrated against the primary standard

Question 22

What are the most common secondaries standards

Answer 22

The common ones are:

Silver/silver chloride E = +0.22 V

Calomel electrode E = +0.27 V

Question 23

What is the potential difference measured called when an electrode is connected to a hydrogen electrode system using standard conditions

Answer 23

The potential difference difference measured is called the standard electrode potential

Question 24

Where can we find the standard electrode potential

Answer 24

The standard electrode potentials can be found in data books

The more oxidised form is on the left

Question 25

How do we calculate te EMF of a cell

Answer 25

In order to calculate the E of a cel, we must used the standard electrode potential for the half cells

Each half cell has a standard potential value

We have to use the equation Ecell = Erhs - Elhs

Question 26

What are the uses of the electrode potentials

Answer 26

The most useful application of electrode potentials is to show the directin of spontaneous change for redox reactions

Question 27

What is the easiest way to use electrode potentials

Answer 27

For any two half equations:
The more negative half cell will always oxidise (gi backwards)

The more positive e half cells will always reduce (go forwards)

To work out the Ecell that corresponds with the spontaneous change then use Ecells = Ered - Eox

A spontaneous change will always have a positive Ecell

Question 28

What chemicals can be used for powerful reducing agents

Answer 28

The most powerful reducing agents will be found at the most negative end of the series on the right (the one with the lower oxidation number)

Question 29

what is the effect of conditions on Cell voltage E cells

Answer 29

the effects of changing conditions on Ecell can be made by applying Le Chatelier’s principle

Question 30

what is a Ecell

Answer 30

Ecell is a measure of how far from equilibrium the cell reaction lies

The more positive the Ecell the more likely the reaction is to occur

Question 31

if the current is allowed to flow (…)

Answer 31

if the current is allowed to flow, the cell reaction will occur and the Ecell will fall to zero as the reaction proceeds and the reactant concentration drop

Question 32

what is the effect of concentration on Ecell

Answer 32

Looking at cell reactions is a straightforward application of le Chatelier.

So increasing, the concentration of “reactants” would increase Ecell and decreasing them would cause Ecell to decrease

look at the example

Question 33

what is the effect of temperature on Ecell

Answer 33

Most cells are exothermic in the spontaneous direction so applying Le Chatelier to a temperature rise to these would result in a decrease in Ecell because the equilibrium reactions would shift backwards

Question 34

what does it mean when the Ecell is positive

Answer 34

If the Ecell is positive, it indicates a reaction might occur

There is still a possibility, however, that the reaction will not occur or it will occur very slowly so that it effectively will not occur

If the reaction has a high activation energy the reaction will not occur

Question 35

what can electrochemical cells used in

Answer 35

Electrochemical cells can be used as a commercial source of electrical energy

Question 36

what are the different types of cells

Answer 36

Cells can be:
- non-rechargeable (irreversible)

• rechargeable
• fuel cells

Question 37

what are some examples of non-rechargeable

Answer 37

dry cell alkaline battery

cells are non - rechargeable when the reactions that occur with them are non-reversible

Question 38

what are some examples of secondary rechargeable cells

Answer 38

• lead-acid cells
• NiCad (nickel cadium)

To rechargeable batteries, a current is supplied to force electrons to flow in the opposite direction around the circuit and reverse the reactions

This is possible because none of the substances jn a rechargeable battery escape or are uses up.
The reactions that take place in non rechargeable batteries are difficult or impossible to reverse in this way

Question 39

what are fuel cells

Answer 39

a fuel cell uses the energy from the reaction of a fuel with oxygen to create voltage

Question 40

why do fuel cells maintain a constant voltage

Answer 40

fuel cells will maintain a constant voltage over time because they are continuously few fresh O2 and H2 so maintaining a constant concentration of reactants

This differs from ordinary cells where the voltage drops over time as the reactant concentration drop

Question 41

why are standard conditions not used in hydrogen fuel cells

Answer 41

using standard conditions: The rate is too slow to produce an appreciable current

Higher temperatures are therefore used to increase rate but the reaction is exothermic so applying le Chatelier would mean the E cell falls

Higher pressure can help counteract this

Question 42

what are some advantages of fuel cells over conventional petrol or diesel-powered vehicles

Answer 42

-greater efficiency

They converts more of their avaliable energy into kinetic energy to get the car moving

The only waste product is water, so there are no nasty toxic chemicals to dispose of and no CO2 emissions from the cell itself

Fuel cells do not need to be recharged like batteries- as long as hydrogen and oxygen are supplied the cell will continue to produce electricity

Question 43

what are some limitations for hydrogen fuel cells

Answer 43

• expensive
• storing and transporting hydrogen in terms of safety - because hydrogen is highly flammable , the feasibility of a pressurised liquid and a limited life cycle of solid “absorbed”
• limited lifetime (requiring regular replacement and disposal) and high production costs
• use of toxic chemicals in their production

Question 44

what are ethanol fuel cells

Answer 44

ethanol fuel cells have also been developed

Compared to hydrogen fuel cells they have a certain advantages including:

• ethanol can be made from non-renewable sources in a carbon-neutral way

Raw materials to produce ethanol by fermentation rate abundant

Ethanol is less explosive and measures to store than hydrogen. New petrol stations would not be required as ethanol is less liquid fuel

Question 45

what are the equations that occur at the oxygen and the ethanol electrodes

Answer 45

Equation that occurs at oxygen electrode:
4e- + 4H+ +O2 → 2H2O

Equation that occurs at ethanol electrode:
C2H5O5 + 3H2O → 2CO2 + 12H+ 12e-

Overall equation:
C2H5O5 +3O2 → 2CO2 + 3H2O

Question 46

how can hydrogen be stored as fuel cells

Answer 46

1. as a liquid under pressure
2. absorbed on the surface of a solid material
3. absorbed within a solid material

Question 47

How do hydrogen fuel cells work

Answer 47

Hydrogen and oxygen gases are fed into two separate platinum containing electrodes

These electrodes are usually made by coating a porous ceramic material with thin layer of platinum

The electrodes are separated by anion exchange membrane that allows anions (OH-) and water to pass through it, but stop hydrogen and oxygen from passing through. The electrolyte is an aqueous alkaline KOH solution.

Hydrogen is fed to the negative electrode

The electrons flow dorm the negative electrodes through an external circuit to the positive electrode.
The OH- ions lads rheoug the anion exchange membrane towards the negative electrode
Oxygen is fed to the positive electrode

Question 48

Why is a thin layer of platinum used

Answer 48

It is cheaper than using solid platinum rods and it provides a larger surface area so reactions go faster