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Reactivity 1. What drives chemical reactions?

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  1. 1.1 Measuring enthalpy changes

    1. • Chemical reactions transfer energy between system and surroundings • Total energy conserved • Heat vs. temperature

    2. • Endothermic: energy absorbed (T decreases) • Exothermic: energy released (T increases)

    3. • Relative stability determines endo/exothermic nature • Energy profiles • Sketch and interpret labelled energy profiles for endothermic and exothermic reactions

    4. • Heat transferred at constant pressure, standard conditions • Q = mcΔT • ΔH = −Q/n • Calculate enthalpy change from calorimetry data

  2. 1.2 Energy cycles

    1. • Bond-breaking absorbs energy • Bond-forming releases energy • Calculate enthalpy change from average bond enthalpies

    2. • Enthalpy change independent of pathway • Apply Hess's law to multistep reaction cycles

    3. • Combustion: ΔHc⦵ • Formation: ΔHf⦵ • Deduce equations and solve problems using standard enthalpy data

    4. • ΔH⦵ = ΣΔHf⦵(products) − ΣΔHf⦵(reactants) • ΔH⦵ = ΣΔHc⦵(reactants) − ΣΔHc⦵(products)

    5. • Energy changes in ionic compound formation • Univalent and divalent ions • Interpret and determine cycle values; include lattice enthalpy and electron affinity terms

  3. 1.3 Energy from fuels

    1. 1.3.1Combustion reactions

      • Reactive metals, non-metals, organic compounds + O₂ • Deduce complete combustion equations for hydrocarbons and alcohols

    2. 1.3.2Incomplete combustion

      • Produces CO and C • Deduce incomplete combustion equations for hydrocarbons and alcohols

    3. 1.3.3Fossil fuels

      • Coal, crude oil, natural gas • CO₂ emissions and greenhouse effect • Compare fuels by carbon dioxide released and energy output

    4. 1.3.4Biofuels

      • From biological carbon fixation (photosynthesis) • Renewable vs. non-renewable • Consider advantages and disadvantages of biofuels compared with fossil fuels

    5. 1.3.5Fuel cells

      • Convert chemical energy directly to electrical energy • Half-equations for electrode reactions • Include hydrogen and methanol fuel cells; PEM details are not assessed

    6. 1.4.1(HL)—Entropy (S)

      • Measure of dispersal of matter/energy • More energy distributions = higher entropy • Order: Sgas > Sliquid > Ssolid • Calculate ΔS⦵ from S⦵ values

    7. 1.4.2(HL)—Gibbs energy (ΔG)

      • ΔG⦵ = ΔH⦵ − TΔS⦵ • Relates energy from reaction to ΔH, ΔS, T

    8. 1.4.3(HL)—Spontaneity

      • Spontaneous when ΔG < 0 • Temperature at which reaction becomes spontaneous • Interpret signs of ΔG from thermodynamic data

    9. 1.4.4(HL)—Equilibrium and Gibbs energy

      • At equilibrium: ΔG = 0 • ΔG = ΔG⦵ + RT ln Q • ΔG⦵ = −RT ln K • Link ΔG, Q, and K to reaction direction and equilibrium position