C23 Redox And Electrode Potentials Flashcards

1
Q

What’s an oxidising agent

A

Electron acceptor

A species that’s reduced in a reaction and causes another species to be oxidised

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2
Q

What’s a reducing agent

A

Electron donor

A species that’s oxidised in a reaction & causes another species to be reduced

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3
Q

Define oxidation

A

Loss of electrons

An increase in oxidation number

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4
Q

Define reduction

A

Gain of electrons

Decrease in oxidation number

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5
Q

What happens in a redox reaction

A

Electrons transferred from 1 species to another

1 element is oxidised whilst other is reduced

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6
Q

Why’s 2CrO4^2- + 2H+ —> Cr2O7^2- + H2O not a redox reaction

A

Chromium is oxidised whereas hydrogen remains the same oxidation state (no element is reduced)

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7
Q

Define standard electrode potential

A

The e.m.f of a half-cell compared with a standard hydrogen half-cell measured at 298K with solution concentration of 1 mol dm^-3 and a gas pressure of 100kPa

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8
Q

What’s happens when a rod of a metal is dipped into a solution of its own ions?

A

An equilibrium is set up between the solid metal and the aqueous metal ions

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9
Q

Write a half-equation for zinc(s) to zinc(II)

A

Zn(s) <=> Zn^2+(aq) + 2e-

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10
Q

Write a half-equation for copper(II) to copper(III)

A

Cu^2+(aq) <=> Cu^3+(aq) + e-

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11
Q

What’s a standard hydrogen half-cell made of?

A

HCl 1mol dm^-3

Hydrogen gas at 100kPa

Inert platinum electrode

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12
Q

Why’s is hydrogen half-cell used as a standard half-cell

A

Easy to control its purity and reproducibility

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13
Q

How to make a simple salt bridge

A

Soak a piece of filter paper in an aq of KNO3 or NH4NO3

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14
Q

Why are salt bridges necessary

A

To complete the circuit by connecting the 2 solutions.
This enables charge to be transferred between the half-cells. They don’t react with electrodes.

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15
Q

Why might you use other standard electrodes occasionally?

A

They’re cheaper/easier/quicker to use and can provide just as good a reference.

Platinum is expensive

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16
Q

If an E^o value is more negative, what does it mean in terms of oxidising/reducing power

A

Better reducing agent (easier to oxidise)

17
Q

If an E^o value is more positive, what does it mean in terms of oxidising/reducing power

A

Better oxidising agent (easier to reduce)

18
Q

How do you calculate the emf of a cell from E^o values

A

E^o cell = E^o positive - E^o negative

19
Q

When would you use a platinum electrode

A

When both the oxidised and reduced forms of the metal are in aq

20
Q

Why is platinum chosen

A

Inert and good conductor to complete circuit

21
Q

How would you predict if a reaction would occur

A

Take the 2 half equations

Find the species that is being reduced

Calculate its E^o value minus the E^o value of the species that is being oxidised

If E^o overall > 0.4V, reaction will occur

22
Q

What are the 3 main types of electrochemical cells

A

Non-rechargeable cells

Rechargeable cells

Fuel cells

23
Q

Describe how non rechargeable cells work

A

They provide electrical energy until all the chemicals have reacted

24
Q

Describe how rechargeable cells work

A

Chemicals in the cell provide electrical energy. When recharging the reactions of the cells can be reversed

25
Give some examples of rechargeable cells
Nickel and cadmium batteries Lithium ion batteries Lithium polymer batteries
26
Explain why lithium is used in laptop batteries
Lithium has low density so the electrode is light and its very reactive
27
What are the drawbacks of using lithium batteries
They’re toxic if ingested Rapid discharge of current can cause fore or even explosions
28
Describe how fuel cels work
The cell uses external supplies of fuel and an oxidant. These external supplies need to be continuously supplied.
29
Modern fuel cells are based on what type of fuels
Hydrogen Hydrogen rich fuels e.g. methanol
30
What are the reactions that take place at the 2 electrons in an alkaline hydrogen fuel cell
2H2 + 4OH^- —> 4H2O + 4e- O2 + 2H2O + 4e^- —> 4OH-
31
What are the disadvantages of fuel cells
Hydrogen is a flammable gas with a low b.p. —> hard and dangerous to store and transport —> expensive to buy fuel cells have a limited lifetime and use toxic chemicals in their manufacture
32
What’s the reason that some cells cannot be recharged
Reaction of the cells is not reversible - a product is produced that either dissipates or cannot be converted back into the reactants
33
Why might the e.m.f of a cell change after a period of time
Concentrations of the ions change - the reagents are used up
34
How can the e.m.f of a cell be kept constant
Reagents are supplied constantly, so the concentrations of the ions are constant; E^o remains constant