AS Level Chemistry - Energetics Summary

  • Chemical Reactions and Energy

    • Chemical reactions are associated with energy changes, mainly heat energy.
    • Energy changes can be classified as:
    • Exothermic (ΔH < 0) - releases heat
    • Endothermic (ΔH > 0) - absorbs heat

  • Key Terms

    • Enthalpy Change: Change in heat content at constant pressure.
    • Standard Conditions: 1 atm pressure, 298K (25°C).
    • Bond Energy: Energy required to break bonds.

  • Calculating Enthalpy Changes

    • Use the formula: extΔH=−mcextΔText{ΔH} = -mc ext{ΔT}
    • qq = heat energy
    • mm = mass of substance
    • cc = specific heat capacity
    • extΔText{ΔT} = change in temperature

  • Hess's Law

    • Enthalpy change is independent of the reaction path.
    • Formula: extΔH<em>r=extΔH</em>1+extΔH<em>2+extΔH</em>3ext{ΔH}<em>r = ext{ΔH}</em>1 + ext{ΔH}<em>2 + ext{ΔH}</em>3

  • Exothermic Reactions

    • Release heat, example:
    • ext{CH}2(g) + 2 ext{O}2(g)
      ightarrow ext{CO}2(g) + 2 ext{H}2 ext{O}(g), ext{ ΔH} = -802 ext{ kJ mol}^{-1}

  • Endothermic Reactions

    • Absorb heat, example:
    • ext{N}2(g) + 2 ext{O}2(g)
      ightarrow ext{NO}_2(g), ext{ ΔH} = +181 ext{ kJ}

  • Standard Enthalpy Definitions

    • Formation: extΔH°fext{ΔH}°_f - formation of 1 mole of compound from elements.
    • Combustion: extΔH°cext{ΔH}°_c - energy change when 1 mole of substance combusts completely.
    • Neutralization: extΔHextneutext{ΔH}_{ ext{neut}} - enthalpy change when an acid neutralizes a base, example:
    • ext{H}^+(aq) + ext{OH}^-(aq)
      ightarrow ext{H}_2 ext{O}(l), ext{ ΔH} = -57.3 ext{ kJ mol}^{-1}
    • Atomization: extΔHatext{ΔH}_{at} - enthalpy for forming gaseous atoms from elements.

  • Calculating Enthalpy Changes

    • Find ΔH through various methods including calorimetry and using standard enthalpies.
    • Example calculations for specific reactions and enthalpy changes provided throughout the notes.