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chemistry a level > Electrode potential and cells > Flashcards

Electrode potential and cells Flashcards

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1
Q

What happens when 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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2
Q

Write half equations for zinc (s) and zinc (II)

A

Zn (s) (equilibrium sign) Zn2+ (aq)+ 2e-

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3
Q

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

A

Cu 2+(s) (equilibrium sign) Cu3+(aq)+ e-

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4
Q

Whats the simplest salt bridge made of ?

A

Filter paper soaked in saturated solution of KNO3 (potassium nitrate)

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5
Q

Why r salt bridges necessary

A

Its purpose is to not move electrons from the electrolyte
But To maintain charge balance
Cos electrons r moving from one half cell to the other
Salt bridge prevents diffusion of solution from one half cell to another

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6
Q

What symbol is used to represent a salt bridge in standard notation

A

||

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7
Q

What type of species goes on the outside (furthest from the salt bridge) in standard notation

A

The most reduced species

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8
Q

What does | indicate?

A

Phase boundary (s/l/g)

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9
Q

How would an aluminium/copper cell b represented

A

Al (s) | Al3+ (aq) || Cu2+ (aq) | Cu (s)

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10
Q

What happens at the left hand electrode ?

A

Oxidation occurs

Left hand electrode is the half cell with the most neg E° value

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11
Q

What happens at the right hand electrode

A

Reduction

Half cell w the most positive E° value

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12
Q

Which side of the cell has the most negative E° value?

What happens to the metal w the most neg E° value

A

Oxidation - left hand electrode

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13
Q

Whats E°

A

Standard electrode potential of a cell

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14
Q

Draw the standard hydrogen electrode

A

What conditions is the standard hydrogen electrode used in ?

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15
Q

What conditions is the standard hydrogen electrode used in ?

A

Temperature 298k
Pressure 100 kPa
[H+] = 1.00 mol dm-3

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16
Q

Whats the standard hydrogen electrode used for?

A

Comparing other cells against

E° of standard hydrogen electrode is defined as 0

So All other E° values are compared against it

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17
Q

Y might u use other standard electrodes occasionally?

A

They r cheaper and quicker to use and can provide just as a good reference

Pt is expensive

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18
Q

If an E° value is more positive

What does it mean in terms of oxidising/reducing power

A

Better oxidising agent (easier to reduce

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19
Q

What factors will change e° values

A

Concentration of ions

Temperature

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20
Q

Why happens if u reduce the concentration of the ions in the left hand half cell

A

Equilibrium moves to the left to oppose the change of removing ions
This releases more electrons
The e° of the left hand cell becomes more negative so the e.m.f of the cell increases

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21
Q

How do u calc the emf of a cell from e° values

A

e° (right) -e° (left) = e° cell

22
Q

When would you use a pt electrode

A

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

23
Q

Whys pt used

A

Inert

Good conducter

24
Q

How would u predict if a reaction would occur

A

Take the two half equations

Find the species thats being reduced ( this is effectively the right hand electrode)

Calculate its e° value minus the e° value of the species thats being oxidised ( effectively the left hand side)

If E° overall > 0, reaction will occur

25
Q

What was the first commercial cell made from (daniell cell)

A

Zinc / copper (||)

26
Q

What r zinc carbon cells more commonly known as?

A

Disposable batteries

27
Q

What r the two reactions that take place in zinc carbon cells

A

Zn is oxidised to Zn2+

NH4+ reduced to NH3 at carbon electrode

28
Q

What r the reactions that occur in a lead/acid battery (car batteries)

A

Pb + SO42- —-> PbSO4 (s) + 2e-

PbO2 + 4H+ + SO4 2- + 2e- —> PbSO4 + 2H2O

29
Q

How r cells recharged (if they r rechargeable)

A

Reactions r reversible

Reversed by running a higher voltage thru the cell than the cell’s E°

30
Q

Nickel / cadmium cells r rechargeable AA batteries etc.

What reactions occur at the electrodes

A

Cd(OH)2 (s) + 2e- —> Cd (s) + 2OH-

NiO(OH) (s) + H2O + e- —> Ni(OH)2 (s) + OH-

31
Q

Where r lithium ion cells used?

A

Mobile phones

Laptops

32
Q

What reactions occur on discharge in lithium ion cells

A

Li+ + CoO2 + e- —-> Li+[CoO2]-

Li —-> Li+ + e-

33
Q

What is a fuel cell?

A

A cell thas used to generate electric current

Does not require electrical recharging

34
Q

What r the reactions that take place at the two electrons in an alkaline hydrogen fuel cell?

A

2H2 + 4OH- —-> 4H20 + 4e-

O2 + 2H2O + 4e- —-> 4OH-

35
Q

Draw a diagram of a hydrogen fuel cell

A

Ugh

36
Q

Y is it better to use a fuel cell than to burn H2 in air

Even tho the same overall reaction occurs

A

In combustions sulphur containing compounds and nitrogen containing compounds r produces die to high temperatures and the s and n in air

These r bad for the environment

This doesn’t occur in a fuel cell, the only product is water
More efficient

37
Q

Disadvantages of fuel cells

A

h is a flammable gas w 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

38
Q

How do u find the weakest reducing agent from a table of electrode potential data?

A

Most pos e° value

Then it is the product of the reduction equation i.e. imagine equation going from left to right

39
Q

Whats the reason that some cells cannot b recharged

A

Reaction of the cell is not reversible - a product is produced that either dissipated or cannot b converted back into the reactants

40
Q

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

A

Concentrations of the ions change - the reagents are used up

41
Q

How can the emf of a value b kept constant

A

Reagents r supplied constantlt so tye concentrations of the ions are constant
E° remains constant

42
Q

Give two important conventions that you have to follow when your drawing electrochemical cells using the conventional representation

A

The half cell with the more negative potential goes on the left the oxidised forms go in the centre of the cell diagram the reduced forms go at the edge of the cell diagram

43
Q

If three factors that can influence electrode potentials

A

Temperature
Concentration of reactants
Pressure

44
Q

What is the electrode potential of the standard hydrogen electrode?

why is it this value

A

0.00 v

By definition

45
Q

Describe how a standard hydrogen electrode is set up and give the standard conditions used when measuring electrode potentials

A

Hydrogen gas is bubbles into a solution of aqueous H+ ions
electrode is made of Pt
standard conditions
temperature 298k
pressure 100 kPa
all solutions of ions have a concentration of 1.00 mol dm-3

46
Q

Describe the term standard electrode potential

A

The voltage measured under standard conditions when a half cell is connected to a standard hydrogen electrode

47
Q

Aluminium has a standard electrode potential of -1.66 whilst Nickel has a standard electrode potential of -0.25 states which species is more reactive and explain how you know in terms of electrode potential

A

Al - they r both metals and Al has a more negative electrode potential

48
Q

Electrode potential of bromine is +1.07 whilst chlorine has an electrode potential of +1.36
state which species is more reactive and explain how you know in terms of electrode potential

A

Chlorine - they are both nonmetals and chlorine has the more positive standard electrode potential

49
Q

Electrode potential of silver is +0.80 whilst copper has an electrode potential of +0.34
state which species is more reactive and explain how you know in terms of electrode potential

A

Copper - They r both metals and copper has the more negative electrode potential

50
Q

Calculate the EMF for the following reactions

Al(s) + 3Ag+(aq) —> Al3+(aq) + 3Ag(s)

Cu (s) + Cl2(aq) —> Cu2+(aq) + 2Cl-(aq)

Al3+ (aq) E° = -1.66
Ag+ (aq) E° = +0.80
Cu2+ (aq) E° = +0.34
Cl2(aq) E° = +1.36

A
  1. 80-(-1.66)=2.46v

1. 36-0.34=+1.02v

51
Q

The direction of redox reactions of the following half-cells Mg2+(aq)/Mg(s) and Ni2+(aq)/Ni(s)

A

Mg2+(aq) +2e—-> Mg(s)
E° = -2.38v

Ni2+(aq) + 2e- —> Ni (s)
E°

chemistry a level (12 decks)

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