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Enthalpy 1

Enthalpy 1. H(reactants). Δ H(reaction). H(products). H(products). Δ H(reaction). H(reactants). notes.  H = H(products) - H(reactants) Enthalpy change same as “heat evolved or absorbed” (except for combustion!) Exothermic – negative enthalpy change

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Enthalpy 1

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  1. Enthalpy 1

  2. H(reactants) ΔH(reaction) H(products)

  3. H(products) ΔH(reaction) H(reactants)

  4. notes • H = H(products) - H(reactants) • Enthalpy change same as “heat evolved or absorbed” (except for combustion!) • Exothermic – negative enthalpy change • Endothermic – positive enthalpy change • Remember signs, negative and positive • Standard conditions: • 1atm, 298K, solutions at 1M dm-3, substances in standard states

  5. HӨr for a reaction = enthalpy change when the reaction happens, as given in the equation • even if some things are 2 moles or more! • E.g. making ammonia: • N2 +3H2 2NH3

  6. Enthalpy of combustion: HӨc • enthalpy change when 1 mole of substance is burned completely in O2 under standard conditions. • E.g. H2 (g) + ½O2 (g) H2O (l) exothermic negative • HӨc = -286 kJ mol-1 • Note: ½ may need to be used because it’s per mole of fuel, not oxygen.

  7. Enthalpy of neutralisation: HӨneut • Enthalpy change when 1 mole of water is created by neutralisation of acid and alkali under standard conditions. • E.g. NaOH(aq) + HCl(aq) H2O(l) + NaCl(aq) exothermic negative • HӨneut = -58 kJ mol-1 • note: ½ may need to be used because it’s per mole of water.

  8. Enthalpy of formation: HӨf • Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions. • E.g. C(s) + O2(g) CO2(g) • exothermic negative • HӨf = -394 kJ mol-1 • again, you may need fractions: • Fe + ½O2 FeO • note: this enthalpy of formation is also an enthalpy of combustion

  9. Note • For most elements, the enthalpy of combustion is the same as the enthalpy of formation of the main oxide: • E.g. • HӨf(SO2) = HӨc(S) • From S + O2  SO2

  10. Hess’s Law: • The enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.

  11. homework • Read & digest pp 92 – (mid)99

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