Redox and Electrode Potential

Question 1

What does the oxidation agents do

Answer 1

They accept electrons (they are reduced in reaction )

Question 2

What does the reducing agent do in the reaction

Answer 2

It donates electrons (they are oxidised)

Question 3

True or false
Different substances are more or less efficient at reducing/ oxidising?

Answer 3

True different substances are better or worse at being oxidising or reducing agent

Question 4

Separate this redox equation into its two half equations
CuSO4 + Zn —> ZnSO4 + Cu
Blue. + grey —> colourless + brown

Answer 4

Remove spectator ions
Cu2+ + Zn —> Cu + Zn2+

Create the two half equations
Cu2+ + 2e- —> Cu
Zn —> Zn2+ + 2e-

Question 5

When can the reducing be the oxidising agent and vice Versa

Answer 5

1) the reducing agent has a form which could oxidise something
2) every oxidising agent has a form which could reduce something
They just wouldn’t be as good at this

Question 6

What does the electrode potential allow us to do

Answer 6

Electrode potential allows us to quantify how good an oxidising or reducing agent a species is

(The scale is based on the tendency for the oxidising form of a redox pair to enquire electrons thus becoming reduced)

Question 7

If there is a more positive electrode potential what does that mean

Answer 7

The more positive the value the more likely they are to acquire electrons
(Better oxidising agents)

Question 8

If the potential differences is negatives then…

Answer 8

Electrons tend to be produced and the reaction is moving from the RHS to the LHS
(Negative voltage means more electrons being thrown out )

Question 9

If the potential difference is positive then electrons tend to….

Answer 9

Electrons tend to be absorbed and the reaction is moving from the LHS to the RHS

Question 10

What is the standard electrode potential a measure of

Answer 10

Standard electrode potential - a measurement of the potential difference between an electrode and the solution in a half cell relative to the standard hydrogen electrode

Question 11

What conditions are considered standard for standard electrode potential

Answer 11

1 mol dm-3 solution
298 K (25oc)
10^5 Pa (for gaseous)

Question 12

How do we measure electrode potential

Answer 12

It cannot be measured directly so is measured next to another half cell.

Question 13

Def of half cell

Answer 13

An electrode in contact with a solution of its own ions forming part of a cell
E.g Zn metal dipped in ZnSO4 solution

Question 14

When measure a porous partition is needed
+ what is this called
+ what is it for

Answer 14

Porous partition can be as simple as a salt bridge - solution of KCl soaked into filtered paper
It allows for electrical connection of 2 half cells by virtue of free ions provided by KCl

Question 15

Which way is the electromotive force taken

Answer 15

The electromotive force of the cell is taken by convention to act from L —> R through the cell
This means
E cell = E RSH electrode — E LHS electrode

Question 16

What is the equation and value of hydrogen electrode

Answer 16

H+ (aq) + e- —> 1/2H2 (g)
Electrode cell value = 0.00 v
(Needs to specifically by to 2DP)

Question 17

Sketch a diagram of measuring the hydrogen electrode potential value

Answer 17

See notes
Upside down …

Question 18

In a standard cell when E cell is positive electrons are moving from…

Answer 18

Electrons are moving from the LHS—> RHS.
This means the RHS half cells is being reduced
.ie. A+ + e- —> A
This means that A+ is a good oxidising agent ( as it is being reduced)

Question 19

If the electrode potential is more positive what does that mean for A+

Answer 19

More positive the value the better the oxidising agent A+ is

Question 20

The more negative the electrode potential value what is true for A

Answer 20

More negative value the better the reducing agent A is
(I.e the reduced version)

Question 21

What does the potential difference tell us

Answer 21

Potential difference between the two sides allows us to know the direction that electrons are moving

Question 22

What type of electrode use in hydrogen electrode

Answer 22

Platinum electrodes are used for half cells when the electrode is not a metal

Question 23

When predicting reaction that will occur using electrode potentials
How would we know which way they react

Answer 23

The more negative electrode potential of the pair if they are both positive will go from RHS —> LHS

Question 24

Why might the reaction be feasible with E cell but why might the reaction not take place

Answer 24

• reaction takes a more favourable route
• activation energy is too high
• value of E are too similar
• equilibrium is disrupted

Question 25

What is a cell

Answer 25

Two half cells with different potentials electrically connected is a cell (Connected by a wire rather than salt bridge)

Question 26

Which terminal on the battery would the more negative half cell be?

Answer 26

The more negative half cell = negative terminal

Question 27

Which terminal of the battery would the less negative half cell be

Answer 27

The more positive half cell = positive terminal

Question 28

Method to work out half equations using oxidation numbers

Answer 28

1) assign oxidation numbers to all the species in equation 2) balance only the species hat contain species changing oxidation numbers 3) balance oxygens and hydrogens adding H2O

Question 29

What are the types of redox titrations?

Answer 29

1) potassium Manganate (VII) KMnO4 (aq) 2) Sodium Thiosulfate Na2S2O3 (aq)

Question 30

Explain potassium manganate (VII) titrations 1) examples of substances 2) redox occurring 3) colour change

Answer 30

Examples : - iron (II) ions - ethanedioic acid These titrations MnO4- is reduced to Mn^2+ KMnO4 is deep purple colour End point = pale permanent pink

Question 31

Explain sodium thiosulfate titrations 1) examples of substances 2) redox occurring

Answer 31

Examples - determination of I2 - chlorate (i) ions - copper (II) ions Redox occurring -S2O3^2- oxidised to S4O6^2- I2 reduced to 2I-

Question 32

Colour change that occurs in sodium thiosulfate titrations with iodine added

Answer 32

Yellow brown colour from aq iodine Yellow colour fades as Na2S2O3 added As end point approaches starch added which forms a blue-black colour which turns colourless on end point

Question 33

How is E cell calculated

Answer 33

E cell = E (positive electrode) - E (negative electrode)

Question 34

Why might cell potential be different in real life than calculations (Limitations of calculations)

Answer 34

1) RATE OF REACTION -feasibility is based on entropy, but reaction may have large activation energy, very slow rate so may not appear to react 2) CONCENTRATION - standard electrode potentials are measure using conc of 1 Mol dm-3. Many reactions are not at this conc 3) STANDARD CONDITIONS - E values are recorded under standard conditions but many conditions are not standard so values may effect results

Question 35

How does altering the concentration of solution effect electrode potential

Answer 35

If concentration increases above 1mol dm-3 Equilibrium shift to right, removing electrons from system Electrode potentials = less negative If concentration decreases below 1mol dm-3 Equilibrium shifts to left, releasing more electros from system Electrode potentials = more negative

Question 36

What is the difference between primary and secondary cells

Answer 36

Primary cells - non rechargeable Electrodes used up in redox reaction Used for low current, long storage devices Secondary cells - rechargeable which caused cell to be regenerated and used again

Question 37

What are features of fuel cell

Answer 37

Fuel cells uses energy from the reaction of fuel with oxygen to create a voltage - fuel and O2 flow into fuel cells and products flow out - fuel cells can continuously operate if fuel and oxygen supplied - fuel cells do not need to be recharged

Question 38

Draw alkali hydrogen fuel cell

Answer 38

On LHS H2(g) in H2O(l) out On RHS O2(g) in OH- ions moving from RHS to LHS

Question 39

Draw the acid hydrogen fuel cell

Answer 39

On LHS O2(g) in H2O(l) out On RHS H2(g) in H+ ions moving from LHS to RHS

Question 40

What is the overall reaction of a hydrogen fuel cell

Answer 40

H2(g) + 1/2 O2(g) —> H2O (l) E cell = 1.23 V

Question 41

What is the oxidation and reduction reaction of the hydrogen fuel cell Acid

Answer 41

Oxidation H2 (g) —> 2H+ (aq) + 2e- Reduction 1/2 O2(g) + 2H+(aq) + 2e- —> H2O (l)