Redox

Question 1

What is a redox reactions?

Answer 1

A reaction where both oxidation and reduction takes place.

Question 2

What is oxidation?

Answer 2

The loss of electrons or an increase in oxidation number.

Question 3

What is reduction?

Answer 3

A gain in electrons or a decrease in oxidation number.

Question 4

What is an oxidising agent?

Answer 4

A species which oxidises another species and is itself reduced in the process.

Question 5

What is a reducing agent?

Answer 5

A species which reduces another species and is itself oxidised in the process.

Question 6

What are redox titrations generally used for?

Answer 6

To work out the concentration of transition metal ions present in a solution (since most transition metals have variable oxidation states so can be easily oxidised/reduced by a suitable agent).

Question 7

What are redox titrations involving MnO₄^- often used for?

Answer 7

To work out the concentration of reducing agents, mainly Fe²⁺ ions.

Question 8

What are the reagents required for a MnO₄^- titration?

Answer 8

• KMnO₄.
• H₂SO₄ to acidify titration solution.

Question 9

What is the half equation for the reduction of MnO₄^- ions to Mn²⁺ ions?

Answer 9

MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O

Question 10

What is the end-point of the MnO₄^- titration?

Answer 10

When the solution stays a consistant purple colour. This is because when Fe²⁺ ions are still present in solution, purple MnO₄^- ions are oxidised to pale pink Mn²⁺ ions. The solution is usually colourless due to the low Mn²⁺ concentrations in titration mixture. When all Fe²⁺ ions are depleted, purple MnO₄^- ions remain in mixture, turning it purple and signalling the end-point of titration.

Question 11

What are redox titrations involving I₂/S₂O₃^- generally used for?

Answer 11

Determining the concentration of an oxidising agent capable of oxidising I^- to I₂ in a solution.

Question 12

What is the half equation for the reduction of I₂?

Answer 12

I₂ + 2e^- → 2I^-

Question 13

What is the equation for the reaction of I₂ with S₂O₃^2-?

Answer 13

I₂ + 2S₂O₃^2- → 2I^- + S₄O₆^2-

Question 14

How is the I₂/S₂O₃^2- titration carried out?

Answer 14

1. Excess I^- ions are added to a solution of the oxidising agent in question, along with starch indicator. The indicator should turn solution blue/black due to presence of I₂.
2. Mixture is titrated against known concentration of Na₂S₂O₃.

Question 15

What is the end-point of the I_2/S₂O₃^2- titration?

Answer 15

When the blue-black colour caused by starch indicator disappears.

Question 16

What oxidising agents can the I₂/S₂O₃^2- titration be used for?

Answer 16

Cu²⁺, Cr₂O_7^2-, ClO^-

Question 17

What does a half-cell consist of?

Answer 17

An element in an equilibrium between 2 different oxidation states. One direction is an oxidation reaction whilst the other is a reduction reaction. E.g. A strip of copper metal is placed in solution of CuSO₄:

Cu²⁺ (aq) + 2e^- ⇔ Cu (s)

Question 18

What is the sturcture of a simple electrochemical cell?

Answer 18

2 half cells with different electrode potentials are connected together with a conducting wire (usually copper) and a salt bridge across the 2 solutions. A voltmeter is connected between the 2 half cells to measure the potential difference.

Question 19

What are the principles behind an electrochemical cell?

Answer 19

In the instance of a Cu/Cu²⁺ and Zn/Zn²⁺ cell, the following 2 equilibria are involved:

Cu²⁺ (aq) +2e^- ⇔ Cu (s)

Zn²⁺ (aq) + 2e^-⇔ Zn (s)

The Zn equilibrium lies to the left and the Cu equilibrium lies to the right. This means that the Zn half cell loses electrons more readily than Cu half cell. The electrons produced by Zn half cell flows through wire to the Cu half cell where they are used to reduce Cu²⁺ ions to Cu. This makes Zn half cell the negative electrode and the Cu half cell the positive electrode.

Question 20

What is the purpose of the salt bridge?

Answer 20

To prevent the build-up of charge in the solutions of the 2 electrodes which would hinder the transfer of electrons.

Question 21

What is a non-metal/non-metal ions half cell?

Answer 21

A half cell consisting of the elementary non-metal and its associated aqueous ion in an equilibrium. E.g.:

2H⁺ (aq) + 2e^- ⇔ H₂(g)

Question 22

Why is a platinum electrode used in a non-metal/non-metal ion half cell?

Answer 22

In order to enable the transfer of electrons into and out of the half cell via the electrode which is in contact with both the element and the ion, but is completely inert so will not take part in any reactions with either.

Question 23

What is a metal/metal ion half cell?

Answer 23

A half cell consisting of a mixture of 2 metallic ions of the same element in equilibrium between 2 different oxidation states. E.g.:

Fe³⁺ (aq) + e^-⇔ Fe²⁺(aq)

Question 24

What is a standard metal/metal ions half cell?

Answer 24

Half cell consisting of a equimolar mixture of both metallic ions with an inert platinum electrode to enable transfer of electrons into and out of half cell.

Question 25

What is the standard electrode potential, Eº, of a half cell?

Answer 25

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

Question 26

How is the standard hydrogen half cell set up?

Answer 26

H₂ at 100 kPa pressure is bubbled through HCl solution (which provides H⁺ ions) at 1 mol dm^-3, under temperatures of 298 K. A platinum electrode is used in order to allow the transfer of electrons into and out of the half cell. The platinum electrode is coated with spongy black substance called platinum black, which allows for the efficient exchange of electrons between H₂ and H⁺ ions.

Question 27

What is the e.m.f of a cell?

Answer 27

The voltage produced by a cell when there is no current and thus no loss of energy as a result of internal resistance.

Question 28

How is e.m.f of a cell determined?

Answer 28

The e.m.f of a cell is the difference between the standard electrode potentials of the two half cells it’s made from.

Question 29

How is the feasibility of a redox reaction determined using standard electrode potentials?

Answer 29

1. Identify the 2 half equations of the 2 species involved in the reaction.
2. Find the difference between the standard electrode potential of the species being reduced compared to the one being oxidised (reduced - oxidised).
3. If the cell potential is +ve, the reaction is feasible. If it’s -ve, the reaction isn’t.

Question 30

What is the significance of the value of a cell potential?

Answer 30

1. The more positive the cell potential, the more likely the reaction will take place.
2. If the cell potential is less than 0.4V, the reaction is unlikely to take place

Question 31

How does shifting the position of equilibrium in a half cell reaction affect the electrode potential?

Answer 31

1. If the position of equilibrium is shifted to the right in a standard reduction half cell equation, the electrode potential becomes less negative/ more positive.
2. If the position of equilibrium is shifted to the left, the electrode potential becomes more negative/ less positive.

Question 32

What are the limitations of using standard electrode potentials to predict reactions?

Answer 32

1. Reaction may be feasible, but rate of reaction may be very slow due to high activation energy.
2. Conditions of a reaction may not be standard and so predictions made by standard electode potentials may not be accurate.
3. The states of the reactants involved may differ from those the standard electrode potentials were taken.

Question 33

What are the 3 main types of cells?

Answer 33

1. Non-rechargeable cells.
2. Rechargeable cells.
3. Fuel cells.

Question 34

What are non-rechargeable cells?

Answer 34

A cell that provides energy until the chemical reactions that drive the voltage reach a point where they can no longer generate the voltage. The cell is flat and needs to be discarded.

Question 35

What are rechargeable cells?

Answer 35

A cell that provides energy via chemical reactions that can be reversed with the input of electrical energy duing recharging so that they are able to generate more voltage. Examples include nickel-cadmium (Ni-cad), lithium ion and lithium polymer batteries.

Question 36

What are fuel cells?

Answer 36

Cells that generate electrical energy via chemical reactions between a fuel and an oxidant, whcih are depleted quickly and so need to be continuously added to the fuel cell. The energy provided is continuous as long as the fuel and oxidants are continuously replenished.

Question 37

What is the fuel, oxidant and electrolyte used in an alkaline fuel cell?

Answer 37

Fuel: H₂.

Oxidant: O₂ (from air).

Electrolyte: Aqueous OH^-ions.

Question 38

What is the anode reactions in an alkaline fuel cell?

Answer 38

2H₂O + 2e^- ⇔ H₂ + 2OH^-

Question 39

What is the cathode reaction in an alkaline fuel cell?

Answer 39

O₂ + 2H₂O + 4e^- ⇔ 4OH^-

Question 40

What is the overall reaction that occurs in an alkaline fuel cell?

Answer 40

O₂ + 2H₂ ⇔ 2H₂O

Question 41

What are the possible ways of encorporating fuel cells in cars?

Answer 41

1. Hydrogen or hydrogen rich fuels (such as methanol, natural gas and petrol, which are converted to H₂ via a reformer operating at 250-350°C via reactions like CH₃OH + H₂O → 3H₂ + CO₂) and are used in a fuel cell to generate electricity which drives an electric motor.
2. Fuel cells have been designed that used methanol directly as a fuel instead.

Question 42

What is the advantage of using methanol as fuel over hydrogen gas?

Answer 42

1. Methanol is a liquid so is much easier to store compared to H₂ which is a gas.
2. Methanol can be generated from biomass so is renewable.

Question 43

What are the advantages of using fuel cell powered cars over combustion engine cars?

Answer 43

• Hydrogen-rich fuels used for the fuel cell produces much less CO₂ compared to petrol.
• There isn’t the problem of incomplete combustion which produces toxic gases like CO.
• Fuel cells convert around 40-60% of chemical energy into kinetic energy whereas internal combustion engines are only around 20% efficient, with the rest of the energy being wasted as heat.

Question 44

Why is storing H₂ such a big problem?

Answer 44

H₂has a very low boiling point and is thus a gas at room temperature. This makes storage on cars a big problem. The only way H₂gas can be efficiently stored on cars is as a liquid under pressure, but even then, very low temperatures are required to keep it a liquid.

Question 45

What are possible methods for storing hydrogen?

Answer 45

1. H₂ gas can be adsorbed onto a solid surface via intermolecular forces.
2. H₂ can be dissociated into H atoms and incorporated into a solid lattice structure as hydrides.

Question 46

What are the limitations of hydrogen fuel cells?

Answer 46

1. Storage and transportation of H₂poses a big logistical problem (mainly due to the difficulties of transporting a gas) which is currently very costly and inefficinent. Better infrastructure would need to be developed in order to reduce cost and improve efficiency.
2. Adsorbers and abosrbers used to store H₂ have limited life-times.
3. Fuel cells are expensive and also have a limited life-time and need to be replaced regularly.
4. Fuel cells require the use of toxic chemicals in their production.