electrochemistry [P2] PAPER 1 Flashcards
A salt bridge was used in a cell to measure electrode potential.
Explain the function of the salt bridge.
the ions in the ionic substance in the salt bridge move through the salt bridge to maintain the charge balance and complete the circuit
What does standard electrode potential, E°, refer to?
• temperature of 298K
• pressure of 100kPa (if gases involved in the half equations)
• 1.00 mol/dm3 solution (of ions involved in the half equations)
• zero current
what happens at the cathode?
reduction
what happens at the anode?
oxidation
which species is at the cathode?
one with the more positive E°
the more positive electrode ___ electrons
gains
what happens to the reducing and oxidising power as you go down the electrochemical series?
reducing power increases
oxidising power decreases
How to determine which half equation is at what electrode?
The higher E° = reduction = cathode
(more positive = GAINS electrons)
how to calculate the EMF of a cell?
right E°- left E°
cell representation?
INCLUDE STATE SYMBOLS
RO||OR
left hand cell || right hand cell
R = reduction
O = oxidation
|| = salt bridge
| = in between RO / OR if there is a change in state
if conductor needed - Pt (s) - goes on the ends
There are two ways to use hydrogen as a fuel for cars. One way is in a fuel cell to power an electric motor, the other is as a fuel in an internal combustion engine.
Suggest the major advantage of using the fuel cell.
a fuel cell converts more of the available energy from combustion of hydrogen into kinetic energy of the car
Aluminium is used as an electrode in an experiment to measure the emf of a cell. Suggest why the aluminium is rubbed with sandpaper prior to use.
to remove the oxide layer on the aluminium
Explain how a salt bridge provides an electrical connection between two electrodes in an electrochemical cell.
has mobile ions / ions can move through it
Suggest why the recharging of a lithium cell may lead to release of carbon dioxide into the atmosphere.
Electricity for recharging the cell may come from power stations burning fossil fuels
Rechargeable lithium cell electrode reactions?
The electrode reactions in an alkaline hydrogen–oxygen fuel cell?
negative electrode:
H2 + 2OH– → 2H2O + 2e–
positive electrode:
O2 + H2O + 4e– → 4OH–
overall equation:
2H2 + O2 → 2H2O
Explain how the electrode reactions in an alkaline hydrogen–oxygen fuel cell can be used to generate an electric current.
• Hydrogen releases electrons
• Oxygen accepts electrons
Explain why a hydrogen fuel cell doesn’t need to be recharged.
Hydrogen is supplied continuously
To provide energy for a vehicle, hydrogen can be used either in a fuel cell or in an internal combustion engine.
Suggest the main advantage of using hydrogen in a fuel cell rather than in an internal combustion engine.
In the fuel cell, a greater proportion of the energy available from the hydrogen-oxygen reaction is converted into useful energy
Identify one major hazard associated with the use of a hydrogen-oxygen fuel cell in a vehicle.
• hydrogen is flammable
•hydrogen is explosive
• H+ is corrosive
• OH– is corrosive
Solar cells generate an electric current from sunlight. These cells are often used to provide electrical energy for illuminated road signs.
Explain why rechargeable cells are connected to these solar cells.
• solar cells don’t provide electrical energy all the time
• rechargeable cells can store electrical energy for use when the solar cells aren’t working
Suggest one reason why many waste disposal centres contain a separate section for cells and batteries.
prevent pollution of the environment by toxic substances
State the meaning of the term electrochemical series.
list of electrode potentials in numerical order
• • E° (Li+ / Li) < E° (H2O / H2 ,OH-)
• Lithium would react with the electrolyte/water
An EMF value for a commercial lithium-iodine cell is 2.80 V.
Suggest why this value is different from a value calculated.
non-standard conditions used
what material would you use as an electrode in a cell involving only aqueous solutions and why?
• platinum
• conducts electricity
• inert
State one essential requirement of the soluble ionic compound used to make the salt bridge in a cell.
must not react with the electrolyte
Standard electrode potentials are measured by comparison with the standard hydrogen electrode.
State the substances and conditions needed in a standard hydrogen electrode.
• H2 (g) at 100kPa
• 1.0 mol/dm3 HCl
• platinum electrode
• temperature of 298K
• E° (H+ / H2) < E° (Cu2+ / Cu), therefore H+ cannot oxidise Cu to Cu2+
• E° (Cu2+ / Cu) < E° (NO3– / NO), therefore NO3– can oxidise Cu to Cu2+
State the change that needs to be made to the apparatus in Figure 1 to allow the cell reaction to go to completion.
replace voltmeter with lamp/wire/ammeter
Describe a standard hydrogen electrode.
• hydrogen gas
• 1.0m moldm-3 HCl
• at 298K and 100kPa
• Pt electrode
State why the electrode potential for the standard hydrogen electrode is equal to 0.00V.
by definition
Give one reason, other than cost, why the platinum electrodes are made by coating a porous ceramic material with platinum rather than by using platinum rods.
Increases the surface area, so the reaction is faster
Suggest why the e.m.f. of a hydrogen–oxygen fuel cell, operating in acidic conditions, is exactly the same as that of an alkaline fuel cell.
overall reaction is the same
Other than its lack of pollution, state briefly the main advantage of a fuel cell over a re-chargeable cell such as the nickel–cadmium cell when used to provide power for an electric motor that propels a vehicle.
hydrogen and oxygen are supplied continuously
Hydrogen–oxygen fuel cells are sometimes regarded as a source of energy that is carbon neutral. Give one reason why this may not be true.
Hydrogen may need to be made using an energy source that is not ‘carbon neutral’
Name the standard reference electrode against which all other electrode potentials are measured.
standard hydrogen electrode
What is the function of the platinum electrode in an electrochemical cell?
to allow the transfer of electrons
why may a cell not be recharged?
the cell reaction cannot be reversed
In the external circuit of this cell, the electrons flow through the ammeter from right to left.
Suggest why the electrons move in this direction.
• Cu2+ ions in the left-hand electrode are more concentrated
• So, Cu → Cu2+ + 2e– happens at right-hand electrode
Write the conventional representation for an alkaline hydrogen–oxygen fuel cell.
State what must be done to maintain the EMF of a fuel cell when in use.
constantly add reactant
