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Chemistry > Redox and electrode potentials > Flashcards

Redox and electrode potentials Flashcards

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1
Q

Define oxidising agent

A

A species that is reduced in a reaction and causes another species to be oxidised.

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2
Q

Define reducing agent

A

A species that is oxidised in a reaction and causes another species to be reduced.

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3
Q

Define oxidation

A

Loss of electrons
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 are transferred from one species to another.
One element is oxidised whilst the other is reduced.

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6
Q

What are the half equations and the ionic equation for :
SnO + Zn ——> ZnO + Sn

A

Half equations :
Sn2+ + 2e- —–> Sn
Zn ——> Zn2+ + 2e-
Ionic equation :
Sn2+ + Zn ——-> Sn + Zn2+

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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 of concentration of 1.00moldm-3 and gas pressure of 100Kpa.

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8
Q

What 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 it’s aqueous ions.

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9
Q

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

A

Zn (s) <—–> Zn2+ + 2e-

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10
Q

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

A

Cu2+ (aq) <——> Cu3+(aq) + e-

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11
Q

What is a standard hydrogen half cell made of (3)

A

Hydrochloric acid - 1moldm-3
Hydrogen gas - 100KPa
Inert platinum electrode

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12
Q

Why is a 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 aqueous solution of KNO3 or NH4NO3.

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14
Q

Why are salt bridges necessary

A

To complete the circuit by connecting the two solutions. This enables charge to be transferred between the half cells. They do not react with the electrodes

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15
Q

Why might you use other standard electrodes occasionally

A

Cheaper as platinum is expensive

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16
Q

If the E0 value is more negative, what does it mean in terms of oxidising/reducing power

A

Better reducing agent (easier to oxidise)

17
Q

If the E0 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 E0 value

A

Positive - negative

19
Q

When would you use a platinum electrode

A

When both the oxidised and reduced forms of the metal are in aqueous solutions

20
Q

Why is platinum chosen

A

Inert and a good conductor to complete circuit

21
Q

What are the three kinds of electrochemical cells

A

Non-rechargeable cells
Rechargeable cells
Fuel cells

22
Q

Describe how non-rechargeable cells work

A

They provide electrical energy until all the chemicals have reacted

23
Q

Describe how rechargeable cells work

A

Chemical in the cell provides electrical energy. When recharging the reactions of the cells can be reversed.

24
Q

Give an example of rechargeable cells

A

Lithium ion batteries

25
Q

Explain why Lithium is used in computer batteries

A

Low density so the electrode is light and it is very reactive,

26
Q

What are the drawbacks of using Lithium batteries

A

Toxic if ingested
Rapid discharge of current can cause fire

27
Q

Describe how fuel cells work

A

The cell uses external supplies of fuel and an oxidant. These external supplies need to be continuously supplied.

28
Q

Modern fuel cells are based on what type of fuels

A

Hydrogen
Hydrogen rich fuels eg.methanol

29
Q

What are the two reactions that take place at the two electrons in alkaline hydrogen fuel cell

A

2H2 + 4OH- ——> 4H2O + 4e-
O2 + 2H2O +4e- ——-> 4OH-

30
Q

What are the disadvantages of fuel cells

A

Hydrogen is a flammable gas with a low b.p so dangerous to store
Fuel cells have a limited lifetime and use toxic chemicals in their manufacture

31
Q

What is the reason some cells cant be recharged

A

Reaction of the cell is not reversible

32
Q

Why might the emf of a cell change after a period of time

A

Concentrations of the ions change- the ions are used up

33
Q

How can the emf of a cell be kept constant

A

Reagents are supplied constantly, so the concentrations of the ions remain constant, E0 remains constant

34
Q
A

Chemistry (7 decks)

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