Redox, Electrochem And Mole Concept

Question 1

[definition] What is a redox reaction?

Answer 1

A redox reaction is a reaction in which electrons are transferred between the two reactants participating in it

A redox reaction involves both oxidation and reduction

Question 2

Define a Comproportionation and disproportionation reaction

Answer 2

Disproportionation: a reaction in which an element is simultaneously oxidised and reduced to form two products

Comproportionation: a reaction in which 2 reagents containing same element but w diff OS, forms a product in which element involved reaches the same OS.

Question 3

How to construct half equations and full equations? Are half equations affected by medium?

Answer 3

No, half equations remain the same regardless of medium. General rule of thumb is this:

Elephants On High Chair

Elements, Oxygen (via H2O), Hydrogen (via H+), Charges (via electrons).

For acidic medium, if there are still H+ remaining after combining half equations, leave it
For alkaline mediums, add OH- on both sides of the equation and cancel away H2O that appear on both sides.

Eg, 8H2O + A —> 6H2O + B
Reduce to
2H2O + A —> B

Question 4

What is the difference between electrolytic cells and electrochemical cells? What can you say about these types of cells with regards to the spontaneity of a reaction

Answer 4

Electrochem cells converts chemical energy into electrical energy (voltmeter in setup)

Electrolytic cells convert electrical energy into chemical energy (battery) —> for non spontaneous reactions

Question 5

What is oxidation state of C in CH4 molecule? Explain why this is so, using electronegativity concepts

Answer 5

-4 as in the C-H bond, C is more electronegative than H and is thus assigned the negative oxidation number

Question 6

Draw an electrochemical cell setup. What are its defining characteristics?

Answer 6

• Electrons flow from anode to cathode (more electron dense to less electron dense)
• Salt bridge
• voltmeter, no battery

Question 7

In an electrochemical cell, which are the negative and positive electrodes? Why?

Answer 7

The anode is the negative electrode because electrons are constantly being released at the anode via oxidation reaction. High concentration of e-

The cathode is the positive electrode because electrons are constantly being consumed at the cathode via reduction. Low concentration of e-

Question 8

As time passes, why does the anode become less negative (more tve) and the cathode become less positive (more -ve)? Hint: its about effect of reactions on the concentration of IONS in the solution

Answer 8

At the anode:
As oxidation continues to occur, concentration of cations increases, thus anode becomes less negative.

At cathode:
As reduction continues to occur, concentration of cations decrease. Cathode becomes less positive.

Question 9

State standard conditions

Answer 9

Temperature: 298K (ie 25 degree celcius), 1 bar (10000 Pa), concentration of all solutions 1 mol dm^-3

Question 10

What is the purpose of a salt bridge? What does a salt bridge contain?

Answer 10

The purpose of a salt bridge is to

-maintain electrical neutrality in each half cell (eg in the cell where oxidation takes place and there is excess of cations, the anion in the salt bridge will move there to neutralise it
- complete the circuit
-prevent mixing of the two electrolytes

A salt bridge will contain an aqueous solution whose ions do not participate in the reaction, eg, K+, NO3-

Question 11

Define standard electrode potential (both school and crucible)

Answer 11

(School):
Standard electrode potential is the electromotive force between the standard hydrogen electrode and the half cell measured at standard conditions

(Crucible):
Standard electrode potential of an electrode is the relative potential of this electrode under standard conditions as compared with the standard hydrogen electrode, whose electrode potential is assigned as 0V at 298K.

Question 12

Do E cell values change depending on the number of electrons being transferred?
Eg, Cl2 + 2e- —> 2Cl-
2Cl2 + 4e- —> 4Cl-

Will they have different E cell values?

Answer 12

No. Unlike Kc, where the quantity of reactants will affect the amount of heat produced, E cell is not affected by the number of electrons being transferred.

Question 13

Draw the setup used to determine standard electrode potential of a reactive metal electrode. What are some things to take note of/include in the setup?

Answer 13

Label your standard conditions ([ion in solution] = 1 mol dm-3, temp at 298K, for any gases, pressure at 1 bar)

Include all species in the reduction/oxidation equation of the half cell in the drawing. Includes the electrode, its oxidised or reduced form, and electrons.

Salt bridge!!

Question 14

What is EROSORE? How do you write the full and half cell notations for galvanic cells?

Answer 14

ER: electrode (solid reactant in the half equation) , O: oxidised form of the electrode, S: salt bridge (represented by ||), O: oxidised form of the other electrode, RE: electrode (solid reactant)

Note: Reduction on the Right, oxidation on the left.

Half cell notation: E(Zn^2+/Zn)
Oxidised form first always.

Question 15

State the significance of standard electrode potentials (found in Data Booklet)

Answer 15

Value of standard electrode potentials is indicative of the position of equilibrium of the equation
More positive value —> forward reduction reaction is favoured.

More negative value —> backward oxidation reaction is favoured.

Question 16

Qn: Comparing two E cell values, E(Zn2+/Zn) = +1.36V and E(Ca2+/Ca) = +1.24V, explain which will be reduced and why.

What does this indicate about its strength as a oxidising agent?

Answer 16

Since E(Zn2+/Zn) = +1.36V is more positive than E(Ca2+/Ca), Zn2+ is preferentially reduced.

If you have a greater tendency to undergo reduction, you are stronger as an oxidising agent. Hence, Zn2+ is also a stronger oxidising agent

Question 17

In your own words, how do you find the Ecell value in these two scenarios:

1. You are only given the names of the solutions which react together. Eg, reaction between aqueous copper sulfate and potassium iodide, find Ecell.
2. Given the full equation depicting the redox reaction, find Ecell value

Answer 17

1. first list down all the ions present in the solution and categories them into can be oxidised and can be reduced. Using standard electrode potential values, you can figure out what is preferentially oxidised/reduced and form your full chemical equation
2. Take note of the REACTANTS in the equation, and locate them in the Data Booklet (whether it’s on the left or right side of the half equation matters). In instances where species can undergo more than 1 type of reaction in the data booklet, eg Fe2+ which can oxidise to Fe3+ or be reduced to Fe, choose the one that will enable you to still form a redox reaction

Question 18

What is one precaution to take whenever selecting half equations for species where there exists >1 half eqn in the data booklet?

Answer 18

The medium of the solution is extremely important. Acidic or Alkaline?
Acidic then reactions with H+ as reactant will be taken into consideration

Question 19

What is the formula for calculating Gibbs free energy from E cell potential? How do you determine number of moles of electrons transferred if they are different for the two half equations?

Answer 19

G=-nF(E cell)
n being the number of moles of electrons transferred and F being faraday’s constant (96500C per mol)

If number of moles of electrons diff, balance the eqns and take the BALANCED number of electrons

Question 20

How does POE affect standard electrode potential ?

Answer 20

POE shifts left : potential becomes more negative
POE shifts right: becomes more positive

Note: the standard electrode potential measures the electromotive force between the electrode and the standard hydrogen electrode when it is a reduction reaction

Question 21

Explain how concentration of metal ions, temp and pressure could change the standard electrode potential of a half cell reaction

Answer 21

conc of metal ions increase, Poe shift to decrease it, shift left. Potential becomes more negative

Temperature increase, forward reaction (ionisation of metal atoms) is favoured as its endothermic, potential becomes more positive

Increased Pressure. Poe shifts to the side with less number of gaseous particles (usually the reduced form) hence reduction is favoured, and potential increases

Question 22

Explain why some reactions involving the formation of precipitates (s) will still occur despite a negative E cell value, using the example of 2Cu2+ + 4I- —>/< 2CuI(s) + I2. Use calculations of Ecell value where necessary

Answer 22

1. Write out half equations for both.
   2Cu2+ + 2e- —> 2Cu+ (red)
   2I- —> I2 + 2e (ox)

As ppt is formed,
(1) [Cu+] decreases, Poe of reduction half eqn shifts right to increase it, becomes more positive
(2) [I-] also decreases , Poe of oxidation half equation shifts left to increase it, becomes more negative

Choose either or both explanations depending on marks alloc, Ecell will definitely become more positive

Question 23

Explain what is electrolysis in your own words. What type of reactants are present at the beginning in the electrolyte?

Answer 23

Electrolysis is a process which involves the splitting of a substance into its component ELEMENTS via supplying electrical energy

Ions are present. Ions may be oxidised or reduced to form ions or component elements

Question 24

In electrolysis, what happens at the cathode and anode? What are the signs of the cathode and anode and why?

Answer 24

At the anode, anions are oxidised. At the cathode, cations are reduced.

Anode is positive
Cathode is negative
Electrons flow out from the negative terminal of the battery which is connected to the cathode, ie, supplying electrons to the cathode. It is thus negatively charged.

Electrons are withdrawn from the anode by the positive terminal of the battery, making anode positively charged.

Question 25

What’s the preliminary step before you find standard electrode potentials of possible reactants? What are some considerations in selective discharge?

Answer 25

1. Need to identify which species are present at each electrode (anion —> anode, cation —> cathode)

Considerations:
1. E value
2. Concentration of electrolyte and thus ion (high conc, will be preferentially reacted)
3. Reactivity of electrodes

Question 26

What ions are present in an aqueous electrolyte? Eg, NaCl (aq)

Answer 26

Na+, Cl-, H2O. NO H+ or OH-!!!

Question 27

How does concentration of electrolyte affect the products formed in an electrolysis experiment?

Answer 27

Increased concentration of ions will shift POE and thus standard electrode potential accordingly. When you write the half equation, keep it in reversible arrow, reduction form first as that is how standard electrode potentials are derived

Question 28

State the formula for calculating Q, the quantity of charge passed during electrolysis (measured in C).
How does n(e) in this formula differ from the n used in the calculation of Gibbs free energy?

Answer 28

Q=It (I = current, units A, t = time in seconds)
Q= n(e) F (number of moles of electrons)x Faraday’s constant (quantity of charge of one mole of electrons)

N(e) is the actual number of moles of electrons used (to be calculated)
N in Gibbs free energy is the relative ratio of the number of moles of electrons in the chemical equation (ie its stoichio coefficient)

Question 29

Define Faraday’s constant and state the formula to derive it

Answer 29

Faraday’s constant is the total charge of one mole of electrons.
F= Le, where L is 6.02x10^-23 and e is the elementary charge, ie the charge carried by a single electron

Question 30

[applications of electrolysis]
Describe in your own words how the Purification of copper works. What is the electrolyte used?

Answer 30

Electrolyte used must definitely contain Cu2+ ion

The pure metal must have an E cell value that is IN BETWEEN the two impurities. Let’s call the two impurities A and B, and the metal be Cu

Cu+ + e- —>/< Cu (s)
A+ + e- —>/< A (s)
B+ + e- —>/< B (s)

Assuming A has the most negative E value, at the anode, A will be readily oxidised first. Next, Cu oxidises causing anode to dissolve. B does not undergo oxidation and will drop off as anode sludge.

At the cathode, you now have Cu2+ ions and A+ ions to choose from. Since Cu2+ E cell value is more positive than A+ (rmb how i said the E cell value must be in between the two impurities??), Cu2+ will be preferentially reduced to Cu which will coat the cathode. A+ remains in the solution

Question 31

[applications of electrolysis] What is anodising of aluminium?

Answer 31

Anodisation of aluminium is the process of coating aluminium objects with a layer of corrosion resistant aluminium oxide via electrolysis

Question 32

What are the pros and cons of using a hydrogen fuel cell?

Answer 32

Cons:
being gaseous, the fuel cell can be rather bulky as compared to liquid fuel cells which can be transported easily in a more compact form
- more explosive than liquid fuel cells too

H2 does not produce pollutive gas and is thus a cleaner energy source. Methanol when used as a fuel will produce co2, a pollutive gas,.

Question 33

What are the equations at the anode and cathode for anodising of aluminium? What is the electrolyte used

Answer 33

Electrolyte: aqueous H2SO4
Anode:
H2O —> O2 + 4H+ + 4e-
3/2 O2 + 2Al —> Al2O3

At cathode: H+ reduced to H2

Question 34

What are the factors affecting selective discharge of ions

Answer 34

1. E value (standard electrode Potential)
2. Reactive electrode
3. Concentration of ions in the electrolyte

Question 35

When do you bring in concentration of electrolyte as a factor affecting selective discharge? How does high concentration of electrolyte affect the discharge of ions?

Answer 35

When there is an unexpected GIVEN observation, which indicates that it has smth to do with POE

High [] of ions will shift POE so as to decrease conc of the ion in the solution, affecting electrode potential value and affecting selective discharge.m