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Chemistry A2 > Electrochemical Cells > Flashcards

Electrochemical Cells Flashcards

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1
Q

Half Cell

A

Rod of metal in a solution of its own ions where an equilibrium is set up

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2
Q

Suggest how current flows between electrodes in an electrochemical cell

A
  • electrons flow from negative electrode where oxidation occurs
  • to positive electrode where reduction occurs
  • (flow from most reactive to less reactive metal)
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3
Q

Suggest why a salt bridge is used in preference to a wire

A

Avoid further metal/ion potentials in circuit

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4
Q

Suggest purpose of salt bridge

A
  • allows ions to move in salt bridge

- charge balance / completes circuit

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5
Q

Describe a salt bridge

A

Inverted U tube stoppered with cotton filled with potassium nitrate solution / soaked filter paper

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6
Q

Standard Electrode Potential

A
  • potential of a cell consisting of an electrode to release electrons when attached to hydrogen electrode
  • provides a measure for reducing power of metal electrode
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7
Q

Describe hydrogen electrode

A
  • inverted test tube supplied with hydrogen gas
  • platinum foil attached to platinum wire (unreactive)
  • electrode potential of hydrogen electrode is 0V by definition
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8
Q

Suggest standard conditions for electrode potential

A

298K, 100kPa, 1 moldm-3 (only mention relevant)

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9
Q

Suggest purpose of platinum in hydrogen electrode

A
  • H2 cannot conduct electricity
  • platinum provides surface for transfer of electrons
  • increasing surface area of platinum increases rate of reaction
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10
Q

Ecell or EMF

A

Ecell(R)- Ecell(L)

NOT positive minus negative!

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11
Q

How to tell if reaction in electrochemical cell is feasible

A

Ecell is positive

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12
Q

Describe lithium ion battery

A
  • negative electrode is carbon (graphite)
  • positive electrode is lithium cobalt oxide
  • solid polymer electrolyte
    -ve : Li -> Li+ + e-
    +ve: Li+ + CoO2 + e- -> LiCoO2
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13
Q

Suggest how lithium ion battery is recharged

A
  • external voltage applied greater than voltage of cell
  • drives electrons in opposite direction
  • reactions reversed to reform reactants
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14
Q

Describe alkaline hydrogen-oxygen fuel cell

A
  • platinum catalyst electrodes
  • NaOH electrolyte
  • ve : H2 + 2OH- -> 2H2O + 2e-
    +ve : O2 + 2H2O + 4e- -> 4OH-
    (remember H2>H2O and O2>OH- for electrolyte then write ionic equations and replace H+ with OH-)
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15
Q

Suggest why emf of hydrogen cell is constant and why it does not need recharging

A
  • continuous supply of reagents

- concentrations of H2/O2 constant

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16
Q

Suggest how hydrogen is stored

A
  • as liquid under pressure

- absorbed onto solid metal hydride

17
Q

Evaluate hydrogen cell

A

Advantages
- no pollution (only H2O)
- higher efficiency
- renewable
- hydrogen readily available from electrolysis of water
Disadvantages
- difficulty storing and transporting due to safety (H2 is highly flammable so explosive)
- H2 from crude oil which is finite
- electrolysis of water is expensive
- energy required comes from combustion of fossil fuels

18
Q

Suggest why aluminium electrode is rubbed with sandpaper prior to use

A

To remove oxide layer

19
Q

Suggest why certain solutions are not suitable for use in salt bridges

A
  • react with ions in solution
  • change concentrations of ions
  • hence emf changes
20
Q

Conventional Cell Representation

A
  • vertical solid line for phase boundary
  • double vertical line for salt bridge
  • species with HIGHEST OXIDATION states closer to salt bridges
  • positive electrode on RHS
  • do NOT balance 2H+ for H2 electrode just simply H+
21
Q

Suggest why emf of cell changes when electrodes are connected and current flows

A
  • concentrations of ions change

- so no longer standard conditions

22
Q

Suggest conditions a salt bridge must satisfy

A
  • must allow ions to flow between half cells
  • must not react with chemicals used in cell
  • must not be oxidised or reduced in process
23
Q

Suggest why salt bridges are plugged with cotton at both ends of the U tube

A
  • prevent two-half cell solutions from mixing

- only allow passage of ions

24
Q

Explain how salt bridges work and describe what would happen to galvanic cell if salt bridge was removed

A
  • at negative half cell, oxidation causes solution to become positively charged so negative ions from salt bridge flow into solution
  • at positive half cell, reduction causes solution to become negatively charged so positive ions from salt bridge move into solution
  • main charge neutrality in cell
  • without salt bridge charges in half cells become imbalanced and cell voltage will drop to zero
25
Q

Describe how to draw conventional cell representation for measuring standard electrode potential for a given metal

A
  • standard hydrogen electrode on LHS by convention

- Pt electrode for standard hydrogen electrode

26
Q

Oxidising Agent

A

Electron acceptor

27
Q

Reducing Agent

A

Electron donor

28
Q

How to quote a half cell in an explanation

A

Species with highest oxidation state in half cell quoted first, e.g. O2 | H2O

29
Q

Explain why a cell cannot be recharged

A
  • reactions cannot be reversed

- so reactants are not regenerated

30
Q

For Pt | Fe2+, Fe3+ || BrO, Br2 | Pt deduce the change in concentration of Fe3+ that results in an increase in emf of cell

A
  • decrease
  • shifts position of equilibrium for Fe2+ -> Fe3+ + e- to right
  • electrode potential is less positive
    ALWAYS MENTION CHANGE IN ELECTRODE POTENTIAL
31
Q

Suggest two properties of platinum that make it suitable for use as an external electrical contact in the cell

A
  • conductor of electricity

- unreactive / inert

32
Q

Explain why water is not used as a solvent in lithium ion cell

A

Reacts with Li to form LiOH

33
Q

Why is electrode potential of hydrogen half cell 0.00V

A

Hydrogen electrode potential is 0.00V by definition

34
Q

Suggest why hydrogen-oxygen fuel cell has same emf when operating in acidic and alkaline conditions

A

Overall reaction is the same

35
Q

Suggest the main advantage of hydrogen cells in cars over rechargeable electric cells

A

H2 and O2 supplied continuously without stopping to recharge
(NOT higher efficiency - only more efficient than internal combustion engine since no moving parts)

36
Q

Explain why a high resistance voltmeter is used when measuring standard electrode potential of cell

A
  • infinitely high resistance so avoids flow of current
  • since emf changes when current flows
  • concentrations of ions change so no longer under standard conditions
37
Q

Conventional cell representation of lithium ion cell

A

Li | Li+ || Li+, CoO2 | LiCoO2 | Pt

MUST include Pt electrode

38
Q

For Fe | Fe2+ || Ag+ | Ag

Suggest why KCl not suitable to use in salt bridge?

A
  • reacts with Ag+ to form AgCl is a precipitate so removes the ions from the solution
  • penalise reacts with Fe2+ since FeCl2 is soluble so ions will remain in solution

Chemistry A2 (12 decks)

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