electrochemical cells

Question 1

what is a half cell

Answer 1

If a rod of metal is dipped into a solution of its own ions, an equilibrium is set up
Each part of the cell is called a half cell

Question 2

what is an electrode potential

Answer 2

the electrode (reduction) potential (E) is a value which shows how easily a substance is reduced

if the electrode potential is more negative than it is less likely to undergo reduction ,equilibrium moves to the left

more postive electrode potentail ,equilibrium will move to the right

Question 3

how is a cell represented

Answer 3

the half cell with the most negative potential goes on the left

the most oxidised species from each half cell ,is next to the salt bridge

Question 4

what is the standard hydrogen electrode

Answer 4

0V measured under standard conditions

Question 5

what are the standard conditions of a standard hydrogen electrode

Answer 5

solution of 1 mole conc
temperature -298k
100kpa -pressure

consists of hydrochloric acid, platinum electrode and hydrogen gas

-they are inert so they wont interfere with the reaction.

Question 6

what does the cell potential mean

Answer 6

negative potential means cell will be likely to be oxidised
positive potential cell likely to be reduced

Question 7

how do you find emf

Answer 7

right potential -left potential / most positive minus most negative

/

Question 8

how to find the most weakest/powerful oxidising agent

Answer 8

its always on the left hand side of the equation
its always the most positive

the weakest is the most negative

Question 9

how do you find if reaction is feasible

Answer 9

take half equation of the most negative potential
flip it
combine half equation
find emf
if it is postive then it is feasibel

Question 10

describe non-rechargeable batteries

Answer 10

cheaper
non reversible

Question 11

describe components in a rechargeable battery

Answer 11

lithium ion batteries ,for example,
electrode A- lithium cobalt oxide
electrode B-graphite
electrolyte -lithium salt dissolved in organic solvent

Question 12

what effect does increasing the pressure and concentration have on the EMF

Answer 12

increasing the concentration makes the cell more positive as fewer electrons are produced in the

increasing the pressure makes the EMF more negative

Question 13

describe a fuel cell

Answer 13

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

Question 14

Describe the components of an electrochemical cell

Answer 14

•A cell has two half–cells.
•The two half cells have to be connected with a salt bridge.
•Simple half cells will consist of a metal which acts an electrode
and a solution of a compound containing that metal (eg Cu and CuSO4).
•These two half cells will produce a small voltage if connected into a circuit. (i.e. become a Battery or cell).

Question 15

How does voltage form

Answer 15

When one electrode has a higher tendency to oxidise and release electrons
More electrons build up on electrode creating potential difference
One electrode acts as positive terminal (one that doesn’t release electrons) and the other acts as negative terminal

Question 16

Why is it important to use a voltmeter with high resistance

Answer 16

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 possible potential difference (E).
The reactions will not be occurring because the very high resistance voltmeter stops the current from flowing.

Question 17

Describe a salt bridge

Answer 17

The salt bridge is used to connect up the circuit.

The free moving ions conduct the charge.
A salt bridge is usually made from a piece of filter paper (or material) soaked in a salt solution, usually potassium nitrate.

The salt should be unreactive with the potassium chloride would not be suitable for copper systems because chloride ions can form complexes with copper ions.electrodes and electr

Question 18

Why is potassium chloride not a suitable salt bridge

Answer 18

Chloride ions can form copper complexes with copper systems

Question 19

What type of electrode undergoes oxidation

Answer 19

Most negative electrode undergoes oxidation
Most positive electrode undergoes reduction

The most positive electrode will always undergo reduction.
Cu2+ (aq) + 2e—->Cu(s) (positive as electrons are used up)
The most negative electrode will always undergo oxidation. Zn(s) —>Zn2+ (aq) + 2e- (negative as electrons are given off)

Question 20

How are cell diagrams presented

Answer 20

The solid vertical line represents the boundary between phases e.g. solid (electrode) and solution (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 right if possible (but this is not essential)

Most oxidised is next to the double line

Question 21

What is used if a system does not have a metal

Answer 21

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 22

Why is a platinum electrode good

Answer 22

It is unreactive and conducts electricity

Question 23

How is the potential of an electrode measured

Answer 23

It is only possible to measure potential difference using two electrodes
It has to be connected to half cell with known potential

Question 24

What is the potential of a standard hydrogen electrode

Answer 24

It’s 0V

Question 25

How is a hydrogen electrode represented

Answer 25

Pt|H2(g)|H+

Question 26

How is a hydrogen electrode represented

Answer 26

Pt|H2(g)|H+

Question 27

What are the components of a standard hydrogen electrode

Answer 27

1.hydrogen gas pressure 100KPa
2.1.0 mol dm-3 solution containing hydrogen
3.temperature 298K
4. Platinum electrode

Question 28

What is standard electrode potential

Answer 28

When an electrode is connected to hydrogen electrode under standard condition ,potential difference measured is standard electrode potential.

Question 29

To calculate Emf

Answer 29

Right hand side - left hand side

Question 30

How do you know which half cell oxidises

Answer 30

The half cell which is more negative EMF value will oxidise (go backwards)
The half cell which is more positive will reduce (goes forward)

Question 31

How do you find the most powerful reducing agent

Answer 31

Will be the most negative end of series
On the right hand side

Question 32

How do you find the most powerful oxidising agent

Answer 32

Will be the most positive end of series
On the left hand side

Question 33

How do you find the weakest reducing agents

Answer 33

The most positive end of series
On the the right hand side

Question 34

How do you find the weakest oxidising agent

Answer 34

The most negative end of series
On the left hand side

Question 35

What is the effect of concentration on cell voltage

Answer 35

Increasing concentration of reactants increases the emf

Question 36

Effect on temperature on e cell

Answer 36

E cell equilibrium shifts backwards
Decreases in E cell

Question 37

What are three types of cells

Answer 37

Non rechargeable
Re chargeable
Fuel cells

Question 38

How do you know if a cell is non re chargeable

Answer 38

When the reaction is reversible

Question 39

How do fuel cells work

Answer 39

They use energy from the reaction of oxygen and a fuel to create voltage

Question 40

Why is the voltage of a fuel cell constant

Answer 40

Because they continuously fed H2 and 02
Maintaining constant concentration of reactants.

Question 41

Advantages of a fuel cell

Answer 41

Less pollution
Greater efficiency

Question 42

Limitations of fuel cells

Answer 42

Expensive
Limited lifetime
Transporting hydrogen can be limited
Uses toxic chemicals in production

Question 43

Describe ethanol fuel cells

Answer 43

Can be made from renewable sources in a carbon neutral way
Raw materials to produce ethanol by fermentation is abundant
Ethanol is explosive