Electrode Potentials

Question 1

define electrochemical cell

Answer 1

a device capable of generating a potential difference from redox reactions

Question 2

what happens in each half cells?

Answer 2

in 1 half cell, the oxidation reaction occurs, and in the other half cell, the reduction reaction occurs

Question 3

what is the conc of a solution in a standard half- cell?

Answer 3

1.00 mol dm-3

Question 4

explain the variable oxidation states in a half cell

Answer 4

• half cells always contain an element in 2 different oxidation states
• here, copper is in the 0 and +2 oxidation states
• in the cell, the ions from the solution are being reduced and the metal is being oxidised. This establishes an equilibrium between the 2 oxidation states
• its convention to have the forward reaction as the the ions gaining electrons (reduction)

Question 5

why do we use a platinum electrode?

Answer 5

to ensure the reaction has a surface to occur on- it allows electrons to be transferred.
-In this half-cell, the gas has a standard pressure of 100KPa

Question 6

what are some important properties of the platinum electrode?

Answer 6

• its inert, so it doesn’t react with anything in the cell
• it conducts electricity, allowing electrons to be lost or gained
• its coasted in platinum black, which has a large surface area

Question 7

define electrode

Answer 7

a solid surface which allows the transfer of electrons to and from it

Question 8

what is the benefit of the platinum electrode in regards to species with that are present in both oxidation states?

Answer 8

• the half- cell has an equimolar solution (equal concentrations) of Fe2+ and Fe3+ ions and a platinum electrode
• the platinum electrode is necessary as it provides a surface on which electrons can be transferred between the 2 iron species

an equilibrium is established between the different oxidation states
Fe3+ (aq) + e- ⇌ Fe2+ (aq)

Question 9

whats the difference between equilibrium that lies further left or further right?

Answer 9

• further left = better at donating electrons (the species is itself oxidised
• further right = is better at accepting electrons (he species is itself reduced

-he position of equilibrium is measured by the standard electrode potential

Question 10

define standard electrode potential

Answer 10

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

Question 11

define standard conditions

Answer 11

temp = 298K
solution concentration = 10 moldm-3
pressure = 100kPa

Question 12

whats the difference between a positive and negative EMF?

Answer 12

• more positive = forward (reduction) and accepts electrons from the other half- cell. It becomes the positive terminal of the electrochemical cell
• more negative = backwards (oxidation) and donates electrons to the other half- cell. It becomes the negative terminal of the electrochemical cell

Question 13

why are the 2 half cells connected?

Answer 13

to form a circuit

Question 14

what is the importance of a salt bridge?

Answer 14

contains free ions to complete eh circuit so that charge can flow- its is made from filter paper soaked in KNO3 (aq)

Question 15

whats the importance of the voltemeter?

Answer 15

gives the difference in standard electrode potentials of the half cells measured in volts

-the voltmeter has an effectively infinite resistance, so when its connected to measure the voltage, the redox reaction will not actually occur as no current will flow - this means the maximum possible EMF can be recorded

Question 16

whats the standard electrode potential of the hydrogen half cell?

Answer 16

0V its used as a reference

Question 17

Define electrochemical series

Answer 17

a list of standard electrode potentials of all the possible half- ells

Question 18

how do you calculate the standard cell potential?

Answer 18

E cell = E (positive terminal) - E (negative terminal)

E cell = E (reduction) - E (oxidation)

Question 19

CELL SHORTHAND

• what does a single line represent?
• what does a double line represent?
• what species is written next to the salt bridge?
• which half cell goes on the left?
• which half cell goes on the right?

Answer 19

• single line= phase boundary and if there’s no phase boundary, we use a comma. ionic cells always have a platinum electrode, which is represented by a phase boundary
• a double line= salt bridge
• the species with the high oxidation state= written next to salt bridge
• the oxidation cell (-) goes on the left
• the reduction (+) goes on the right

Question 20

what is the relationship between cell potential and feaibility?

Answer 20

the greater the cell potential, the more feasible the reaction

Question 21

what are some limitations of cell calculations?

Answer 21

• if the conditions are changed, this can change the position of equilibrium, and therefore can change the electrode potential
• although it tells us about feasibility, it doesn’t tell us anything about kinetic factors such as activation energy and rate of reaction

Question 22

whats the difference between non rechargeable cells (batteries) and rechargeable?

Answer 22

• this reaction generates electrical energy until the chemicals within the battery have fully reacted
• the battery is then disposed of

-in a rechargeable battery, when the chemicals have reacted fully, a potential difference can be applied to the cell in the opposite direction, which will regenerate the original chemicals. This allows us to re- use the battery

Question 23

what are lithium- ion batteries used for and what is the electrolyte?

Answer 23

• used in laptops and mobile phones
• one of the electrodes in a lithium- ion battery is made of graphite, and the other is made of lithium cobalt oxide, LiCoO2
• the electrolyte for a lithium-ion battery is an organic solvent

Question 24

how do fuel cells differ from normal electrochemical cells?

Answer 24

• because fuel cells do not have a stock of chemical held in the electrodes or electrolyte
• instead, the chemicals are stored externally and are fed into the cell when electricity is required

Question 25

describe alkaline hydrogen fuel cell

Answer 25

1- oxygen gas and hydrogen gas are fed into 2 sperate tubes at the top of the cell

2-the electrodes contain anion exchange membranes. These do not let oxygen or hydrogen gas trough but do let OH- ions and water to pass through

3-electrons move from the negative electrode through the wire

4-OH- ions move from the positive electrode to the negative electrode through the anion exchange membrane

5-the electricity produced by this fuel cell can power a device, such as a bulb or a car

Question 26

what are the advantages of hydrogen fuel cells?

Answer 26

• more efficient than burning fossil fuels
• they release water which isn’t harmful
• they don’t need to be recharged- they keep producing electricity for as long as they have fuel

Question 27

what are disadvantages of hydrogen fuel cells?

Answer 27

• energy is needed to build the fuel cells and produce hydrogen- this energy comes from fossil fuels
• hydrogen is highly flammable so needs to be carefully handled