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AQA A-Level Physical Chemistry > 1.11 Electrochemical Cells > Flashcards

1.11 Electrochemical Cells Flashcards

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1
Q

What is an electrochemical cell?

A

A metal is placed into a solution containing its ions
An equilibrium is established

One of these is called a half cell

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2
Q

What are two half cells called?

A

A Daniell cell

Two half cells combined by a salt bridge
With a voltmeter in circuit

An electric current flows so the electrons pass from the more reactive metal to the less reactive metal = equilibrium’s can be different

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3
Q

What is a salt bridge?

A

A piece of filter soaked in a solution of potassium nitrate KNO3
Containing electrolyte which doesn’t react

It completes the circuit so ions can move in and out of it
It is unreactive with half cells

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4
Q

What is significant about the voltmeter?

A

A high resistance voltmeter will not draw current from the circuit

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5
Q

What is the standard hydrogen electrode?

A

It is used to compare how easily metals lose electrons

It is the standard that we measure the emf values against

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6
Q

In the standard hydrogen electrode what is the general set up?

A

H2 gas is bubbled in

H+ are in aqueous solution provided by and acid eg H2SO4 or HCl

Platinum electrode

It goes on the LEFT ALWAYS!

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7
Q

What does the standard hydrogen electrode include?

A

A platinum electrode

Which is platinum foil covered in porous platinum to increase surface area

It is unreactive and a good conductor of electricity

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8
Q

What are the conditions for the standard hydrogen electrode?

A

100kPa
298K
1.0 moldm-3

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9
Q

What is the standard electrode potential? E°

A

The potential difference between any electrode under standard conditions and the standard hydrogen electrode

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9
Q

How do we determine if something is a strong/weak reducing/oxidising agent?

A

More negative E° = stronger reducing agent

More positive E° = stronger oxidising agent

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11
Q

How do you workout the overall E° cell?

A

E° right hand side - E° left hand side

Or

E° comparatively more positive - E° comparatively more negative

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12
Q

What does an overall more positive E° value mean?

A

The reaction will happen spontaneously

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13
Q

In a Daniell Cell set up whig side does each half cell go on?

A

The more negative E° value goes on the left hand side

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14
Q

What is included in cell representation?

A

|| = salt bridge (goes between the two half cells)

, = separates different substances in the same phase

= phase boundary (only between different states of matter)

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15
Q

How do you know where the half cells go in cell representation?

A

The form of the substance with the highest oxidation state is put next to the salt bridge

Eg 0, 2+, 2+, 0

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16
Q

What can we deduce from cell representation?

A

Zn(s)| Zn2+(aq) || Cu2+(aq) | Cu(s)

Reactant, product, reactant, product

And so we can easily form half equations in the order we read them
Zn -> Zn2+ + 2e-

17
Q

In cell representation what happens if there is no solid surface?

A

No solid surface to conduct electricity

A platinum electrode is used like in the SHE (standard hydrogen electrode)

Eg
Pt(s) | Fe2+(aq), Fe3+(aq) || Cl2(g) | Cl-(aq) | Pt(s)

Only put on an end of a half cell with no (s)

18
Q

What happens to the E° if concentration is changed?

A

Increase concentration = E° increases

Decrease concentration = E° decreases

19
Q

How can temperature change an E° cell?

A

Most cells are exothermic in the spontaneous direction

So

Increase in temperature moves the equilibrium to the left hand side
= decrease E° cell

20
Q

How can activation energy affect an E° cell?

A

If the reaction in the cell has a high activation energy

There’s a chance the reaction won’t happen regardless of a positive E° cell value

21
Q

What are the types of chemical cells?

A

Primary cells - irreversible and must not be recharged

Secondary cells - reversible and can be recharged by an electrical current

Fuel cells - generated electricity by the continual oxidation of an external source of fuel

22
Q

Give an example of a primary cell?

A

Zinc-Carbon dry cell

Positive electrode: Graphite rod(carbon)
Negative electrode: Zinc container

Surrounded by a paste of manganese dioxide and ammonium chloride (MnO2 + NH4Cl)

23
Q

What is the zinc-carbon dry cell overall equation?

A

Zn2+ + 2NH4+ ⇌ 2NH3 + H2 + Zn

24
Q

Why is Zinc-carbon dry cell a primary cell?

A

You don’t recharge it as the zinc container will wear away

Therefore not safe

25
Q

What batteries are most common?

A

Alkaline batteries

They contain a paste of KOH
And use Zinc as a reducing agent

Zn + 2MnO2 + 2H2O ⇌ 2MnO(OH) + 2OH- + Zn+

26
Q

What do secondary cells do?

A

They provide current to an external circuit while discharging

They use current from mains electricity while charging

27
Q

What are some examples of secondary cells?

A

Lead-acid battery
Nickel-cadmium cell
Lithium ion ***

28
Q

What is the overall equation for a lead-acid battery?

A

Discharging:
PbO2 + Pb + 2H2SO4 ⇌ 2PbSO4 + 2H2O

Charging is the other way around

29
Q

What are nickel-cadmium cells used for? Fact?

A

Electric shavers
Drills

They can be recharged up to 500 times

30
Q

What are the discharging and charging equations of the lithium ion? What is used?

A

Discharging
Li + CoO2 ⇌ LiCoO2

Charging
LiCoO2 ⇌ Li + CoO2

Reagents are absorbed onto powdered graphite

31
Q

What is a fuel cell?

A

An electrochemical cell that maintains a constant voltage due to constant concentration of reactants (continuous flow of fuel)

32
Q

What does an alkaline (hydrogen-oxygen) fuel cell include?

A

Porous platinum electrodes with a semi-permeable membrane

NaOH or KOH electrolyte

33
Q

What is significant about the rate of fuel cells?

A

Under standard conditions the rate is too slow for a decent current
Higher temperatures are used - to increase rate

But it is exothermic so the E° cell falls
Can be counteracted with high pressure

34
Q

What are some factors of acidic hydrogen oxygen fuel cells?

A

They are more efficient than combustion of H2 - as it releases electrical rather than heat energy

H2 comes from electrolysis of water - via steam reformation of methane

Hydrogen under high pressure = explosive

35
Q

What are the benefits and risks of cells, non-rechargeable cells and rechargeable cells?

A

Cells - portable source of electrical energy
BUT there are waste issues

Non-rechargeable cells - cheap
BUT there are waste issues

Rechargeable cells - less waste & cheaper in the long run
BUT some waste issue (at end of life)

36
Q

What are some benefits and risks of using hydrogen fuel cells?

A

Benefits:
Only waste product is water
Don’t need recharging
Very efficient

Risks:
Needs a constant supply
Hydrogen = flammable & explosive 
Hydrogen is often made using fossil fuels
High cost of fuel cells

AQA A-Level Physical Chemistry (12 decks)

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