electrochemical cells Flashcards
Explain what happens when current is allowed to flow between two half cells.
→ reactions occur separately at each electrode.
→ voltage falls to zero as reactants are used up
→ most positive electrode undergoes reduction
→ most negative electrode undergoes oxidation
State why a platinum electrode is used.
→ unreactive
→ can conduct electricity
Explain why a wire is not used as a salt bridge.
the metal wire would set up its own electrode system with the solutions
Give a suitable example of salt solution used to make the salt bridge & state why.
- potassium nitrate
→ unreactive with both the electrode & solution
State the conditions when measuring the standard electrode potential.
→ 1 mol dm⁻³ solution of ions
→ 298K
→ 100kPa
State what is meant by the SHE.
→ standard used for comparison of potentials
→ assigned a potential value of 0 volts
State why standard conditions are needed when measuring electrode potentials.
→ position of redox equilibrium changes with condition
→ this affects the electrode potential of the half-cell
In terms of equilibrium, explain how changes in temperature, pressure or concentration affect the value of the electrode potential.
→ if equilibrium shifts to the left, oxidation reaction is favoured so electrode potential becomes more negative
→ if equilibrium shifts to the right, reduction reaction is favoured so electrode potential becomes more positive
State what the value of an electrode potential tells us about the electrode’s reducing/oxidising ability.
→ more negative = more easily oxidised = great reducing agent
→ more positive = more easily reduced = great oxidising agent
State two ways of predicting weather a reaction is feasible or not.
→ combine half-equations to obtain feasible reaction & compare with the one given in question
→ all feasible reactions have a positive Ecell value
Give examples of non-rechargeable batteries.
- zinc/carbon batteries
- alkaline batteries
State the advantages of non-rechargeable batteries.
- cheap
- portable + easy to replace
- wide range of applications
State the disadvantages of non-rechargeable batteries.
→ must be disposed of once reactants are used up (can be toxic)
→ battery casing may be corroded by electrolyte
→ non-sustainable
State what lithium ion cells are used for.
used to power phones/cameras
State the components of a lithium ion cell.
- lithium cobalt oxide electrode
- graphite electrode
- lithium salt dissolved in solvent (electrolyte)
State the two half equations for a lithium ion cell.
Li⁺ + e⁻ ⇌ Li (-3.04V)
Li⁺ + CoO₂ + e⁻ ⇌ Li⁺[CoO₂]⁻ (+0.56V)
State the overall equation when a lithium ion cell is discharging.
Li+ + CoO₂ + e- → Li⁺[CoO₂]⁻
Briefly explain how rechargeable batteries are charged.
→ current is supplied, forcing e- to flow in the opposite direction
State :) and :( for rechargeable batteries.
:)
- longer lifespan
- more efficient
:(
- expensive
- need to be charged regularly
Describe how an alkaline hydrogen-oxygen fuel cell works.
- hydrogen is oxidised by hydroxide ions to water at the negative electrode
- e- flow to positive electrode, react with oxygen to release hydroxide ions
- hydroxide ions travel to negative electrode (through semi-permeable membrane) and react with hydrogen (cycle repeats)
Give the two half equations and the overall equation for the reactions in an alkaline hydrogen-oxygen fuel cell.
oxidation: 2H₂ +4OH⁻ → 4e⁻ + 4H₂O
reduction: 4e⁻ + 2H₂O +O₂ → 4OH⁻
Overall reaction:
2H₂ + O₂ → 2H₂O
State the advantages of alkaline hydrogen-oxygen fuel cells.
→ more efficient than internal combustion engines + batteries
→ does not produce pollutants
State the limitations of alkaline hydrogen-oxygen fuel cells.
→ expensive
→ hydrogen is explosive + needs to be stored under pressure
→ use of fossil fuels in the production of hydrogen through electrolysis
Explain how a battery works.
→ flow of electrons from one electrode to another
→ through an external circuit where they can do work & transfer energy to appliances
Write the conventional representation for an alkaline hydrogen– oxygen fuel cell.
Pt|H₂ (g)|OH⁻ (aq), H₂O (l) || O₂ (g)|H₂O (l), OH⁻ (aq)|Pt
