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R3.2Reactivity 3 — What are the mechanisms of chemical change?

Electron transfer reactions — IB Chemistry HL Revision Notes & Topical Questions

50 min read 72 parts Lesson 2 of 13 1 quiz

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

R3.2 Electron transfer reactions — IB CHEMISTRY HL notes: Identify redox agents, balance half-equations, describe voltaic and electrolytic cells, organic redox, electrode potentials, Gibbs energy and electrolysis.Oxidising agent — oxidises another species by taking its electrons;…

  • Reducing agent
  • Stock notation
  • Half-equations in acid
  • Manganate(VII)
  • Iodine–thiosulfate
  • Metals in groups

Part 1 of 10Oxidising and reducing agents

Oxidising agent
  • Oxidising agent oxidises another species by taking its electrons;
    • it is itself reduced and its oxidation number decreases.
Reducing agent
  • Reducing agent reduces another species by giving electrons;
    • it is itself oxidised and its oxidation number increases.
Redox
  • Redox needs both an oxidising and a reducing agent.
Dual role
  • Dual role some species, such as H2O2, can act as either depending on what they react with.
Identify
  • Identify assign oxidation numbers on both sides of the equation.
Stock notation
  • Stock notation the Roman numeral in a name shows the metal's oxidation state, for example iron(II) sulfate contains Fe2+.

Part 2 of 10Balancing redox equations and redox titrations

1
Write the unbalanced equation and identify the atoms that change oxidation number.
2
Deduce the oxidation number changes.
3
Balance the oxidation number changes.
4
Balance the charges.
5
Balance the atoms.
Half-equations in acid
  • Half-equations in acid balance atoms other than H and O, add H2O for O, add H+ for H, add e− for charge, then combine.
Manganate(VII)
  • MnO4− + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O; self-indicating as the purple colour disappears at the end point.
Iodine–thiosulfate
  • 2S2O32− + I2 → 2I− + S4O62−; starch is added near the end point and the blue-black colour disappears.
Uses
  • Uses iron tablets with manganate(VII) and chlorine in bleach with thiosulfate.

Part 3 of 10Relative ease of oxidation and reduction

Relative ease of oxidation and reduction
Metals in groups
  • Metals in groups ease of oxidation increases down Group 1 and Group 2.
Halogens
  • Halogens oxidising power decreases down Group 17, with F2 the strongest and I2 the weakest.
Halogen displacement
  • Halogen displacement a more reactive halogen displaces a less reactive one from its halide solution.
Reactivity series
  • Reactivity series a more reactive metal displaces a less reactive metal ion, acting as a reducing agent.
Metals with acids
  • Metals with acids acid + metal → salt + hydrogen, as 2H+ + Zn → Zn2+ + H2 with H+ reduced and Zn oxidised.
Rate
  • Rate more reactive metals and stronger acids react more vigorously; copper does not react with dilute acids.
Reducing power order
  • Mg, Al, Zn, Fe, Pb, H, Cu, Ag, from strongest to weakest.

Part 4 of 10Voltaic cells

1
Voltaic cell
  • Voltaic cell generates electricity from a spontaneous redox reaction by separating the two half-cells.
2
Half-cell
  • Half-cell a metal dipped in a solution of its own ions, with an equilibrium setting up an electrode potential.
3
EMF
  • EMF the potential difference between two half-cells; only differences can be measured.
4
Anode
  • Anode oxidation, negative; cathode — reduction, positive.
5
Electron flow
  • Electron flow through the external circuit from anode to cathode.
6
Salt bridge
  • Salt bridge an inert electrolyte such as KNO3 that completes the circuit, with anions moving to the anode and cations to the cathode.
7
Cell notation
  • Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s), with a single line for a phase boundary and a double line for the salt bridge.

Part 5 of 10Primary, secondary and fuel cells

Primary cell
  • Primary cell the reaction is not reversible and the cell is discarded when used up.
Secondary cell
  • Secondary cell the reaction is reversed by applying a voltage greater than the cell voltage.
Lead–acid
  • Pb and PbO2 in H2SO4, about 2 V per cell and 12 V from six cells; heavy, with toxic lead and corrosive acid.
NiCad
  • Cd and NiO(OH), about 1.2 V; cadmium is toxic and the cell has a memory effect.
Lithium-ion
  • Lithium-ion lithium cobalt oxide and carbon, 3.5–4.0 V;
    • light with no memory effect, but cells degrade, lithium is limited and there is a fire risk.
Fuel cell
  • Fuel cell fuel and oxygen are supplied continuously;
    • the hydrogen–oxygen cell gives E = +1.23 V with only water as product.

Part 6 of 10Electrolytic cells

1Electrolysis
Electrolysis splitting a compound using electrical energy, driving a non-spontaneous reaction.
2Electrolyte
  • Electrolyte molten or dissolved ionic compound.
3Anode
  • Anode positive, where anions are oxidised.
4Cathode
  • Cathode negative, where cations are reduced ("RED CAT").
5Ion movement
  • Ion movement cations move to the cathode and anions to the anode.
6Molten binary compounds
  • Molten binary compounds metals form at the cathode and non-metals at the anode, as for PbBr2.
7Comparison
  • Comparison in a voltaic cell the anode is negative and the cathode positive;
    • in an electrolytic cell the anode is positive and the cathode negative.
8Uses
  • Uses purifying copper, plating, extracting aluminium and making chlorine and sodium hydroxide.

Part 7 of 10Organic redox reactions

Primary alcohol
  • Primary alcohol oxidised to an aldehyde and then to a carboxylic acid (reflux with excess oxidant).
    • (distil off as formed)
Secondary alcohol
  • Secondary alcohol oxidised to a ketone only.
Tertiary alcohol
  • Tertiary alcohol not oxidised, because the functional group carbon has no hydrogen.
Oxidant
  • Oxidant acidified potassium dichromate(VI), orange Cr2O72− turning green Cr3+.
Reduction
  • Reduction carboxylic acids to primary alcohols with LiAlH4 in dry ether;
    • aldehydes to primary alcohols and ketones to secondary alcohols with LiAlH4 or NaBH4.
Reagents
  • NaBH4 cannot reduce carboxylic acids; both give hydride H−.
Alkenes and alkynes
  • Alkenes and alkynes hydrogenation with a nickel catalyst, about 200 °C and 1000 kPa, as in making margarine.

Part 8 of 10Standard electrode potentials (HL)

Standard electrode potentials (HL)
Standard hydrogen electrode
  • H2 at 100 kPa, 1.00 mol dm−3 H+ and platinum, 2H+ + 2e− ⇌ H2, E° = 0.00 V.
Standard electrode potential
  • Standard electrode potential the EMF with the SHE on the left and the half-cell on the right at 1.0 mol dm−3, 100 kPa and 298 K.
Reduction potentials
  • Reduction potentials half-reactions are written as reductions, as in Section 19 of the data booklet.
More negative E°
  • More negative E° stronger reducing agent, equilibrium to the left.
More positive E°
  • More positive E° stronger oxidising agent, equilibrium to the right.
Cell EMF
  • E°cell = E°right − E°left = E°reduction − E°oxidation.
    What the symbols mean: Ecell = standard cell potential (V) · Eright = electrode potential of the positive (reduction) half-cell · Eleft = potential of the negative half-cell (V)
Spontaneity
  • Spontaneity a positive E°cell means the reaction is spontaneous, but thermodynamics predicts and kinetics controls, so a reaction may not be seen.

Part 9 of 10Gibbs energy and cell potential (HL)

Equation
  • ΔG° = −nFE°, where n is the moles of electrons and F = 9.65 × 104 C mol−1.
Positive E°
  • Positive E° negative ΔG°, spontaneous.
Negative E°
  • Negative E° positive ΔG°, so the forward reaction is not spontaneous and the reverse is.
Equilibrium
  • ΔG = 0 and E = 0, as when a voltaic cell is exhausted.
Data booklet
  • Data booklet the ΔG° = −nFE° equation and F are given.

Part 10 of 10Electrolysis of aqueous solutions and electroplating (HL)

Electrolysis of aqueous solutions and electroplating (HL)
Competing species
  • Competing species water can be reduced to H2 or oxidised to O2, so it competes with the ions.
Factors
  • Factors standard electrode potentials, concentration of the ions and the identity of the electrodes.
Dilute H2SO4 or NaOH
  • Dilute H2SO4 or NaOH water is electrolysed to hydrogen and oxygen.
NaCl(aq)
  • NaCl(aq) hydrogen forms at the cathode; at high chloride concentration chlorine forms at the anode, giving NaOH(aq).
    • (above about 25 %)
CuSO4(aq) with inert electrodes
  • CuSO4(aq) with inert electrodes copper forms at the cathode and oxygen at the anode.
CuSO4(aq) with copper electrodes
  • CuSO4(aq) with copper electrodes the anode dissolves, which purifies copper, with impurities falling as anode slime.
Electroplating
  • Electroplating the object is the cathode and the anode is the plating metal, which keeps the ion concentration stable.
Thickness
  • Thickness controlled by current, time and electrolyte concentration.
Plastics
  • Plastics they are etched and given embedded metal particles to conduct before plating.

Key terms

TermMeaning
Oxidising agentA species that accepts electrons and is reduced.
Reducing agentA species that donates electrons and is oxidised.
Voltaic cellA cell generating electricity from a spontaneous redox reaction.
Electrolytic cellA cell using electricity to drive a non-spontaneous reaction.
Salt bridgeA link of inert electrolyte completing the circuit.
Standard hydrogen electrodeThe reference half-cell with E° = 0.00 V.
ElectroplatingCoating an object with a metal by electrolysis.

Avoid these mistakes

1
Putting oxidation at the cathode — oxidation is always at the anode.
2
Saying the anode is always negative — it is negative in voltaic cells and positive in electrolytic cells.
3
Writing E°cell as left minus right.
4
Using ΔG° in kJ and F in C mol−1 without converting to the same energy unit.
5
Assuming a reaction with positive E° will always be observed.

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