Chapter 9: Redox Processes

Question 1

Define redox reaction

Answer 1

When oxidation and reduction occur simultaneously

Question 2

Define oxidation:
- ___ of oxygen
- ___ of hydrogen
- ___ of electrons
- ___ in oxidation number

Answer 2

• Gain of oxygen
• Loss of hydrogen
• Loss of electrons
• Increase in oxidation number

Question 3

Define reduction:
- ___ of oxygen
- ___ of hydrogen
- ___ of electrons
- ___ in oxidation number

Answer 3

• Loss of oxygen
• Gain of hydrogen
• Gain in electrons
• Decrease in oxidation number

Question 4

What is a disproportionation reaction?

Answer 4

When the same reactant is both oxidised and reduced

Question 5

How do you assign oxidation numbers to:
- Elements
- Elements in an ionic salt
- Elements in a covalent compound
- Elements in an ionic compound

Answer 5

• Elements: O.N always 0
• Elements in an ionic salt: O.N = Ionic charge
• Elements in a covalent compound: Assign O.N as if ionic (most electronegative element is assigned negative charge)
• Elements in an ionic compound: Sum of O.N of elements = Total charge of compound

Question 6

What are the exceptions to rules for O.N?

Answer 6

• Transition metals: Can have multiple O.N, and are named using oxidation state (roman numerals)
• O.N of H in metal hydrides (Eg. NaH): -1
• O.N of O in OF2: +2 (because F is more electronegative)
• O.N of O in hydrogen peroxide (H2O2): -1

Question 7

Define oxidising and reducing agents

Answer 7

• Oxidising agent: A species that oxidises another species (takes electrons), and is themselves reduced in the process (gain electrons)
• Reducing agent: A species that reduces another species (donates electrons), and is themselves oxidised in the process (lose electrons)

Question 8

What are half equations?

Answer 8

Equations that show oxidation and reduction separately

Question 9

How are half-equations balanced and re-combined to form an overall redox equation?

Answer 9

• Balance number of (non H and O) atoms
• Balance O by adding H2O to the O-deficient side
• Balance H by adding H+ to the H-deficient side
• Balance charges by adding electrons to the electron-deficient side (more positive side)
• Recombine half-equations by multiplying entire equation to balance number of electrons (no. of e- lost during oxidation = no. of e- gained during reduction)
• Subtract any duplications

Question 10

Define the activity series

Answer 10

Arranging metals according to how easily they are oxidised (how readily they lose electrons)

More active = more easily oxidised = loses electrons more readily = stronger reducing agent

*Link to chemical bonding: first ionisation energy

Question 11

How can the activity series be used to predict the feasibility of redox reactions?

Answer 11

More reactive metals will displace less reactive ones to create displacement redox reactions, as they are stronger reducing agents and will reduce the less reactive metal while losing electrons more readily to be oxidised

When reacting with an acid (H+), metals more reactive than H will reduce H+ to H2 gas (*Link to Acids and Bases: acid/reactive metal reaction)

Question 12

Give an example of a redox titration

Answer 12

Winkler method

S₂O₃ : O₂
4 : 1

Question 13

Define Biochemical Oxygen Demand (BOD)

Answer 13

Amount of oxygen used by aerobic microorganisms to decompose the organic matter in water over 5 days at 20°C

BOD (mg dm⁻³/ppm) = Initial concentration of dissolved oxygen - Concentration of dissolved oxygen after 5 days
(mol dm⁻³ x mR = g dm⁻³)
(g dm⁻³ x 1000 = mg dm⁻³ = ppm)

Question 14

Example question (Winkler method):
MnSO₄ , KI and H₂SO₄ were added to 100cm³ of water. Iodine formed was titrated against 16.00 cm³ of 5.00x10⁻³ mol dm⁻³ of Na₂S₂O₃. What is the BOD of the water sample?

Answer 14

• Find ₙ[Na₂S₂O₃]
• Find ₙ[O₂]
• Find [O₂]
• Find mass of O₂
• Convert from g dm⁻³ to ppm

Question 15

Describe the energy conversion in a voltaic cell

Answer 15

Conversion of chemical energy (redox reactions) to electrical energy (voltage) through displacement reactions (electrons lost are transferred through the external circuit)

Question 16

Describe/draw the features and functions of the features in a voltaic cell, and write the cell notation for a voltaic cell

Answer 16

• Half-cells
• Electrode (negative anode): Where oxidation of the more reactive metal takes place (electrons lost are transferred through the external circuit to the less reactive electrode, causing the anode to shrink over time)
• Electrode (positive cathode): Where reduction of the less reactive metal takes place (gain electrons from the more reactive metal, causing the cathode to expand over time)
• Electron flow: from negative to positive (anode to cathode)
• Salt bridge (filter paper soaked in ionic salt): To provide an electrical connection between the two half-cells by allowing ions to flow into and out of the half-cells to balance the charges in the half-cells (build-up of positive charges in the anode causes anions flow into anode; build-up of negative charges in the cathode causes cations flow into cathode) and achieve electrical neutrality of the voltaic cell
  *Link to equilibrium
• Voltmeter: measures the voltage of the cell (the bigger the difference in reactivity between the metals, the higher the voltage)
• Cell notation
  Eg. Zn(s)lZn²⁺(aq)llCu²⁺(aq)lCu(s)
  Eg. Pt(s)lH₂(g),H⁺(aq)llFe³⁺(aq),Fe²⁺(aq)lPt(s)

Question 17

Describe the features of an electrolytic cell

Answer 17

Electrolysis: Breaks down a molten electrolyte (an ionic salt) into its constituent ions by passing electricity through it (electrical energy is converted into chemical energy)

Electrolyte: Molten ionic salt that conducts electricity and contains ions that move towards the electrodes
(*Link to Chemical Bonding to explain why ionic salt has to be molten)

• Anode (positive electrode): Anions move towards the positive electrode, and are oxidised (lose electrons). The electrons are transferred through the external circuit to the positive terminal of the battery.
• Cathode (negative electrode): Electrons flow from the negative terminal of the battery to the negative cathode. Cations move towards the negative electrode, and are reduced (gain electrons) by gaining electrons from the cathode.

Electron flow: Electrons taken from the external circuit at the cathode are returned at the anode, completing the circuit

Products: Constituent elements of the salt (metal is formed at the cathode, non-metal formed at the anode)

Question 18

Contrast a voltaic cell and an electrolytic cell

Answer 18

• A voltaic cell relies on spontaneous redox reactions, whereas an electrolytic cell relies on electricity to induce non-spontaneous redox reactions
• A voltaic cell converts chemical energy to electrical energy, whereas an electrolytic cell converts electrical energy to chemical energy
• In a voltaic cell, the anode is the negative electrode and the cathode is the positive electrode. In an electrolytic cell, the anode is the positive electrode and the cathode is the negative electrode