Electrochemical Cells Flashcards
How do you make a half cell and therefore a electrochemical cell
When a metal is dipped into a solution of its ions, you get a half-cell
The combination of two half cells gives an electrochemical cell
What is the standard hydrogen electrode (SHE)
The reference electrode, which is assigned an electrode potential of 0.00V under standard conditions
How can the standard electrode potentials of other systems be found
By combining them with a standard hydrogen electrode and measuring the EMF of the cell formed
How is the equation for each half cell written
As a reduction process e.g.
M2+ + 2e- → M
What are the standard conditions referred to be standard electrode potential
298K
100kP
1.00 mol dm^-3 solution of ions
How do you display the conventional representation of cells
Draw the salt bridge.
The half cell with the more positive E is always reduction and so placed on RHS
The standard hydrogen electrode is ALWAYS on the left hand side
Write the ions/atoms beside the salt bridge with the higher oxidation atoms closer to salt bridge.
Separate different phases with a boundary e.g. separate solids and gases with I
If one side does not have a solid, add a Pt(s) catalyst to the end of that side
How do you calculate the EMF of a cell
E of RHS - E of LHS
How can you determine whether a reaction is thermodynamically feasible based on the E value of the cell
If the E value of the cell is positive, the reaction is feasible
What should always be used for the solution
A nitrate as all nitrates (EXCEPT CuSO4 and ZnSO4) are soluble
Why is a salt bridge used
It completes the circuit and allows the movement of ions
Keeps the two solutions electrically neutral as it provides a reservoir of positive and negative ions
Why is a high resistance voltmeter used
A high resistance voltmeter is used since it draws no current from the cell, and the so the cell has its maximum potential difference
Why are two half-cells required to make the complete cell
Both oxidation and reduction must take place separately, so that electrons are transferred via the external circuit
Why does a voltage form when a zinc electrode in zinc solution is connected to a copper electrode in copper solution
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 than the copper electrode. ELECTRONS GO TO COPPER
A potential difference is created between the two electrodes.
The zinc strip is the negative terminal and the copper strip is the positive terminal.
This potential difference is measured with a high resistance voltmeter
Identify the substances and conditions used in the standard chlorine half cell
A platinum electrode
1.00 mol dm^-3 solution of Cl- ions
Chlorine gas at 100kPa pressure
Temperature at 298K
How do you know which cell is being reduced and oxidised
The cell on the right is the cathode unless told otherwise
CRAO
Cathode is reduction
Anode is oxidation
How is a salt bridge made
From a piece of filter paper soaked in a salt solution, usually potassium nitrate
What property should the salt used to make the salt bridge have
Unreactive with the electrodes and electrode solutions/electrolyte.
Why is a wire not used as a salt bridge
The metal wire would set up its own electrode system with the solutions
What happens if current is allowed to flow
If the voltmeter is removed and replaced with a bulb or if the circuit short circuited, a current flows.
The reactions occur separately at each electrode.
Voltage falls to 0 as the reactants are used up.
What are the components of the standard hydrogen electrode
A platinum electrode immersed in 1.00 mol dm^-3 dilute HCl
Hydrogen gas at 100kPa bubbled over the platinum electrode
298K temperature
What would a diagram of the standard hydrogen electrode look like
A platinum electrode immersed in HCl and a test tube upside down over the electrode with a gap to let in hydrogen gas
What is the purpose of a platinum electrode
It is an inert/unreactive electrode
Allows electron transfer
Conducts electricity
Allows hydrogen gas to be adsorbed onto its surface
Why are platinum electrodes made by coating porous material with platinum rather than using platinum rods
To increase the surface area and speed up the rate of reaction
What problems are encountered with the standard hydrogen electrode
It is difficult to maintain the pressure of hydrogen at 100kPa
It is a very slow electrode reaction
Why is a reference electrode necessary to measure the standard electrode potential
A reference electrode is needed to measure the potential difference.
You cannot measure the electrode potential of a single electrode as electron transfer to another electrode is necessary to complete the circuit
Why is a saturated solution used to make the salt bridge
So no ions enter the salt bridge but ions can come out of the salt bridge.
Why is a salt bridge used and not a metal piece of wire
A salt bridge is used to allow the movement of ions
The metal wire would set up its own potential difference and interfere with cell reaction
What are the standard electrode potentials of oxidising and reducing agents
Oxidising agents have a HIGH POSITIVE potential
Reducing agents have a HIGH NEGATIVE potential
Consider the reaction :
Ag+ + e- → Ag
E = +0.80V
What effect would decreasing the concentration of silver ion have on the value of the electrode potential
The position of equilibrium shifts to the left, producing more Ag+ ions and producing more electrons. This makes the electrode potential less positive.
Consider the reaction :
Ag+ + e- → Ag
E = +0.80V
State and explain the effect on the cell emf on using a smaller silver rod
There is no effect on the cell EMF since the equilibrium is unaffected by the amount of solid. It is only affected by the conc of Ag+ ions
Why can the Zn2+/Zn half cell reduce the Co2+ ions to cobalt metal
The E of the Zn2+/Zn half cell is more negative than that of the Co2+/Co half cell. Therefore Zn releases electrons to Co2+ and reduces it to Co.
Why might a reaction not take place even though the E of the cell is positive
- Reaction has a high activation energy
- Actual conditions different to the standard conditions used to measure E values and this will affect value of the electrode potential.
V3+ + e- = V2+ and E = -0.26V
(SO4)2- + 4H+ + 2e- = SO2 + 2H2O and E = +0.17V
Explain why SO2 will not reduce V3+ ions to V2+ ions
SO2 will not reduce V3+ ions to V2+ ions because E of SO4 2-/ SO2 half cell is more positive than that of V3+/V2+ half cell. Therefore SO2 will not release electrons to the V3+ ions and wont’t reduce them.
Why might a battery be used instead of a cell
A battery is 2 or more cells linked together in series. A cell may not have a high enough voltage.
What are electrochemical cells used as
A commercial source of electrical energy
What are the different types of cells
Non-rechargeable (irreversible)
Rechargeable
Fuel cells
What are the electrode reactions in a lithium cell
Positive electrode:
Li+ + CoO2 + e- = Li+[CoO2]-
Negative electrode:
Li = Li+ + e-
How does a fuel cell generate a voltage
It uses the energy released from the reaction of a fuel with oxygen to produce a voltage
Uses of Lithium ion cells
To power cameras and mobile phones
Why would water not be a good solvent in lithium ion cells
It would react with the lithium metal
Why do fuel cells not need to be charged
They maintain a constant voltage over time because they are continuously fed with fresh O2 and H2 so maintaining constant conc of reactants.
What are the reactions in a hydrogen fuel cell under alkaline conditions
4e- + 4H2O = 2H2 + 4OH-
4e- + 2H2O + O2 = 4OH-
What are the limitations of hydrogen fuel cells
Expensive
Difficult to store and transport hydrogen
Hydrogen is explosive
Limited lifetime
The hydrogen made from water by electrolysis uses electricity generated in power plants that burn fossil fuels
Use of toxic chemicals in their production
Advantages of fuel cells
less pollution and CO2 produced as the only by product is water
Greater efficiency
Give one reason why the EMF of the cell changes when the electrodes are connected and a current flows
Concentration of ions changes
No longer standard
EMF is determined when no current flows
How does the EMF of a cell change when the SA of each platinum electrode is doubled
No change
Direction of electron movement in an external circuit
Anode to cathode as the anode is where oxidation occurs therefore electrons are released.
Explain why the current in the external circuit of the cells falls to zero after the cell has operated for some time
The ions in each electrode will eventually be at the same concentration
Suggest why the recharging of a lithium cell may lead to the release of CO2 into the atmosphere
Electricity for recharging cell may come from power stations burning fossil fuel
Function of a porous separator
Allows ions to move.
Suggest why the EMF values of the acidic and alkaline hydrogen-oxygen fuel cells are the same
Same OVERALL reaction
State the substances and conditions needed in a standard hydrogen electrode
H2 (g)
1 mol dm^-3 HCl
100kPa
Platinum electrode
298K temperature
Describe an experiment the student does to show that the standard electrode potential for
the TiO2+(aq) / Ti(s) electrode is −0.88 V
The student is provided with:
* the Cu2+(aq) / Cu(s) electrode set up ready to use
* solid titanium(IV) oxysulfate (Mr = 159.9)
* 0.50 mol dm−3 sulfuric acid
* a strip of titanium
* laboratory apparatus and chemicals.
Your answer should include details of:
* how to prepare the solution of acidified TiO2+(aq)
* how to connect the electrodes
* measurements taken
* how the measurements should be used to calculate the standard electrode potential
for the TiO2+(aq) / Ti(s) electrode
Weigh 7.995 / 8.00 g TiOSO4
Dissolve in / add (allow react with) (0.50 mol dm-3) sulfuric acid
Transfer to volumetric flask and make up to the mark
Stage 2: Set up cell
Piece of Ti immersed in (1 mol dm−3 acidified) TiO2+(aq) / the solution
(connect solutions with) salt bridge or description
(connect metals through high R) voltmeter
Stage 3: Measurements and calculation
Record voltage/potential difference/emf of the cell
Ecell = ERHS – ELHS
Ecell = Ecopper – Etitanium
ELHS = ERHS – Ecell OR Ecell should be +1.22 V if Cu on RHS (or −1.22 if Cu electrode
on LHS
Why is aluminium metal rubbed with sandpaper before using
To remove the oxide layer on the aluminium
State why a fuel cell does not need to be electrically recharged
Reactants supplied continuously
Suggest one advantage of using methanol, rather than hydrogen, in a fuel cell for use in cars
Methanol is liquid so can be stored/transported easily
What is the major advantage of using the fuel cell
A fuel cell converts more of the available energy from combustion of hydrogen into KE of car
State the main environmental advantage of using a hydrogen-oxygen fuel cell to power a car
Water is the only product
Suggest why the use of a hydrogen-oxygen fuel cell might not be carbon neutral
CO2 released because fossil fuels are burned to produce electricity to generate hydrogen
