Cells Flashcards

1
Q

Primary cells

A

Cells that are Not rechargeable and will eventually run out when reactants get used up

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2
Q

Secondary cells

A

Rechargeable cells that use a reversible reaction

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3
Q

What is the theory behind electrochemical cells?

A

There is a transfer of electrons in the redox reaction between half cells which acts as an electric current

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4
Q

Storage cells

A

A rechargeable cell

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5
Q

Example of a primary cell

A

Alkaline batteries: Zinc + Manganate (IV)

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6
Q

Why can Storage cells be recharged?

A

Because the overall reaction is reversible

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7
Q

Why can a current occur in an electrochemical cell?

A

Because the voltmeter was replaced with an ammeter to allow flow of electrons between half cells so overall, reactions occur

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8
Q

Why does the current in a non rechargeable cell eventually fall to 0?

A

Because the reactants get used up and form products of overall reaction
So no current will flow as no more electrons are transferred

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9
Q

What is the discharge reaction in an electrochemical cell?

A

The forward overall reaction because charge is given out when electrons from oxidised reactant is transferred to reduced reactant

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10
Q

What is the charging reaction in an electrochemical cell?

A

The backward reaction because reactants are reformed for the forward reaction to proceed again and give off more charge

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11
Q

Why do chargeable cells eventually run out of charge?

A

Because of leakage of the electrolyte solvent or side reactions occur that decrease concentration of reactants over time

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12
Q

Example of rechargeable secondary cell?

A

Lithium ion batteries

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13
Q

Why is graphite used in lithium ion batteries?

A

Because absorbs reagents so allows ions to react in absence of a solvent

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14
Q

Why can water not be used as a solvent in lithium ion batteries

A

Because it is not inert so water will react with the lithium

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15
Q

Fuel cell

A

Uses a constant supply of fuel and oxidising agent to produce a voltage via a chemical reaction (without burning the fuel)

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16
Q

General outline of how a fuel cell works

A

Fuel reacts with oxygen
Uses energy in this to create a voltage

17
Q

Examples of fuel in a fuel cell

A

Methanol
Hydrogen

18
Q

2 types of hydrogen fuel cells

A

In acidic electrolyte conditions
In alkaline electrolyte conditions

19
Q

Summary of alkaline hydrogen fuel cell: anode

A

hydrogen fed into cell and oxidised to H+ (but combines with hydroxide ions from electrolyte) to produce water
Electrons are released from oxidation

20
Q

Equation at anode of alkaline hydrogen fuel cells

A

2H₂ + 4OH⁻ —-> 2H₂O + 4e⁻
OXIDATION OF HYDROGEN GAS

21
Q

Electrolyte in an alkaline hydrogen fuel cell

A

Potassium hydroxide

22
Q

Summary of alkaline hydrogen fuel cell: cathode

A

Oxygen is fed into cell
Accepts electrons (reduced) from circuit and reacts with water to reform hydroxide ions

23
Q

Equation at cathode of an alkaline hydrogen fuel cell

A

O₂ + 2H₂O +4e⁻ —-> 4OH⁻
REDUCTION OF OXYGEN

24
Q

Why is the reaction at the anode written as reduction when it is oxidation?

A

Because the standard notation is to write electrode equations as reduction :)

25
Exchange of hydroxide ions in an alkaline hydrogen fuel cell
between a membrane from the positive electrode (produced from reduction of oxygen) to the negative electrode (gained after hydrogen was oxidised to H+)
26
Overall hydrogen fuel cell equation
2H₂ + O₂ -----> 2H₂O
27
Summary of acid hydrogen fuel cell: anode
Hydrogen is fed into the fuel cell Hydrogen is oxidised by losing electrons to form H+ and releases electron
27
Equation for the hydrogen fuel cell in acidic conditions
H₂ -----> 2H⁺ + 2e-
28
Summary of acid hydrogen fuel cell: cathode
Oxygen is fed into the fuel cell and combines with H+ (from the anode) to produce water Accepts electrons from the anode
29
Electrolyte used in acid hydrogen fuel cell
Phosphoric acid
30
How many electrons are transported in each half cell in a fuel cell?
4
31
Why do fuel cells maintain a constant voltage over time?
Continuously feeding the fuel cell with H2 and O2 to maintain concentration So concentrations of reactant remains constant over time
32
Can reactions in the hydrogen fuel occur under standard conditions?
No, rate is too slow because the activation energy is too high
33
Increasing temperature of a hydrogen fuel cell
Too high shifts position of equilibrium to prefer the backward endothermic reaction to take energy in so EMF falls
34
How can hydrogen be stored?
As a liquid under pressure Adsorbed on the surface of a solid Absorbed on solid surface material
35
Catalysts in a hydrogen fuel cell
Used to lower activation energy to provide an alternative reaction pathway so reaction can actually occur
36