electrode potentials Flashcards
Explain, in terms of electrode potentials and equilibrium, why the pH of the solution in the hydrogen half-cell decreases as this cell delivers current.
H redox system is more negative
(e.g. has a more –ve E OR less +ve E OR is –ve electrode)
OR H redox system releases electrons
(May be in equation, e.g. H2 → 2H+ + 2e–)
Equilibrium shifts to increase [H+] OR H+
OR standard hydrogen equation shifts to increase [H+] OR H+
Explain, in terms of equilibrium, why acidic conditions should not be used with cells containing CN–(aq) ions.
H+ reacts with CN– OR HCN forms
OR equation: H+ + CN– → HCN (ALLOW ⇌)
OR CN– accepts a proton/H+
OR equilibrium shifts right AND CN– is removed
State one important difference between a fuel cell and a modern storage cell.
Fuel reacts with oxygen/oxidant to give electrical energy/voltage
Suggest one advantage of using ethanol, rather than hydrogen, in a fuel cell for vehicles.
ethanol is a liquid OR is less volatile
OR ethanol is easier to store/transport/stored more safely
OR hydrogen is explosive/more flammable
OR ethanol has more public/political acceptance
The overall reaction in a DEFC is the same as for the complete combustion of ethanol.Write the equation for the overall reaction in a DEFC.
C2H5OH + 3O2 → 2CO2 + 3H2O
Deduce the half-equation for the reaction at the oxygen electrode in a DEFC.
O2 + 4H+ + 4e– → 2H2O
State two other large-scale uses of the hydrogen.
- fuel cells
- manufacture of margarine
OR hydrogenation of alkenes/unsaturated fats/unsaturated
oils/unsaturated molecules - making of ammonia OR Haber process
- making of HCl/hydrochloric acid
- making of methanol
Define the term standard electrode potential.
Include all standard conditions in your answer.
Definition
The e.m.f. (of a half-cell) compared with/connected to a
(standard) hydrogen half-cell/(standard) hydrogen electrode
Standard conditions Units essential
Temperature of 298 K / 25ºC
AND (solution) concentrations of 1 mol dm–3
AND pressure of 100 kPa OR 105 Pa OR 1 bar
Explain, in terms of equilibrium, why the cell potential increases.
Assume Cu2+|Cu OR Cu half cell unless otherwise stated.
[Cu2+] decreases OR < 1 mol dm–3
AND
Equilibrium (shown in table) shifts to left
more electrons are released by Cu
The cell has a bigger difference in E
State one advantage of using hydrogen as a fuel compared with conventional fuels.
no/less CO2 OR H2O is only product OR greater efficiency
State one other way that hydrogen can be stored as a fuel for vehicles.
liquefied/as a liquid AND under pressure/pressurised
Give two reasons why it is uncertain whether reactions predicted from Eovalues may actually take place.
High activation energy OR slow rate
Conditions not standard OR concentrations not 1 mol dm–3
Draw a labelled diagram to show how the student could have set up this cell to measure its standard cell potential.
complete circuit with voltmeter and salt bridge linking
two half-cells
Pt electrode in Fe3+/Fe2+ half-cell with same
concentrations
Cr electrode in 1 mol dm–3 Cr3+ half-cell
Explain, in terms of their electrode potentials, why ‘chromium is more reactive than copper’ in this reaction.
E o for chromium (redox system) is more negative/lower/less (than copper redox system) ORA chromium system shifts to the left / Cr(s) Cr3+(aq) + 3e– AND copper system shifts to the right / Cu2+(aq) + 2e– Cu(s)
When this experiment was carried out, the student observed some bubbles of a gas.Suggest an explanation for this observation.
Cr reacts with H+ ions/acid to form H2 gas
Suggest two advantages of using methanoic acid as the fuel in a fuel cell rather than hydrogen.
- Methanoic acid is a liquid AND easier to
store/transport
OR hydrogen is a gas AND harder to store/transport
OR hydrogen as a liquid is stored under pressure - Hydrogen is explosive/more flammable
- HCOOH gives a greater cell potential/voltage
- HCOOH has more public/political acceptance than
hydrogen as a fuel
The student made the following observations.
• In Cell A, the nickel electrode lost mass.
• In Cell B, the nickel electrode gained mass.
• In both cells, the measured cell potential slowly changed.
Explain these observations. Include equations in your answer.
Cell A (based on 1 and 2) Ni + 2Fe3+ Ni2+ + 2Fe2+ Cell B (based on 1 and 3) 2Cr + 3Ni2+ 2Cr3+ + 3Ni concentrations (of the ions in each cell) change OR concentrations are not standard
State a method, other than absorption, that is being developed to store hydrogen for possible use as a fuel in cars.
adsorbed (on a solid) OR on the surface (of a solid)
OR as a liquid under pressure
State two large-scale uses for the hydrogen produced.
Two from points below:
1. fuel OR fuel cells
2. manufacture of margarine
OR hydrogenation of alkenes/unsaturated fats
3. manufacture of ammonia OR ‘Haber process’
4. manufacture of HCl/hydrochloric acid
5. reduction of metal ores/metal oxides
As the cell approaches full charge, the aqueous KOH electrolyte starts to decompose, forming hydrogen gas at one electrode and oxygen gas at the other electrode.
Predict half-equations that might take place at each electrode for the decomposition of the electrolyte to form hydrogen and oxygen.
4OH– ⎯→ O2 + 2H2O + 4e–
2H2O + 2e– ⎯→ H2 + 2OH–
Write an equation for the complete combustion of methanol.
CH3OH + 1½O2 CO2 + 2H2O
State two advantages of vehicles using fuel cells compared with the combustion of conventional fossil fuels.
less CO2 OR less greenhouse gases
greater efficiency
Suggest one advantage of using methanol, rather than hydrogen, in a fuel cell for vehicles. Justify your answer.
methanol is a liquid
AND
methanol is easier to store/transport
State the charge carriers that transfer current through the wire, ………………………………………………………………………………………………….
through the solution. ………………………………………………………………………………………..
electrons AND ions
Fuel cells are a type of electrochemical cell being developed as a potential source of energy in the future.
• State one important difference between a fuel cell and a conventional electrochemical cell.
• Write the equation for the overall reaction that takes place in a hydrogen fuel cell. • State two ways that hydrogen might be stored as a fuel for cars.
• Suggest why some people consider that the use of hydrogen as a fuel for cars consumes more energy than using fossil fuels such as petrol and diesel.
A fuel cell converts energy from reaction of a fuel with oxygen into a voltage/electrical energy 2H2 + O2 2H2O ALLOW multiples, e.g. H2 + ½O2 H2O IGNORE state symbols Two from: under pressure OR at low temperature OR as a liquid adsorbed on solid absorbed within solid Energy is needed to make the hydrogen OR energy is needed to make fuel cell
