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Energy changes

Energy changes. The precise value of an enthalpy change depends on; 1) The number of moles. 2) Temperature 3) Pressure 4) The physical states of reactants. In order to compare reactions standard enthalpy changes are used, represented by the symbols;

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Energy changes

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  1. Energy changes

  2. The precise value of an enthalpy change depends on; • 1) The number of moles. • 2) Temperature • 3) Pressure • 4) The physical states of reactants. • In order to compare reactions standard enthalpy changes are used, represented by the symbols; • ΔHΘ(Δ= differenceΘ=standard)

  3. Standard conditions • ΔHΘis defined as the enthalpy change when molar quantities react under standard conditions. • Standard conditions are defined as; • A temperature of 298 K (25oC) • A pressure of 101 kPa (1atm) • All substances in their natural physicalstates at 298 K and 101kPa. • Eg; Water must be liquid, not steam.

  4. ΔHΘfStandard enthalpy of formation • The standard enthalpy of formation is the enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions. • Eg;C (s)+ 2H2(g)→ CH4(g) • As the elements are in their standard states, and one mole is being produced ΔH for this reaction is; • ΔHr = ΔHΘfCH4

  5. ΔHΘc Standard enthalpy of combustion • The enthalpy change when one mole of a substance is completely burnt in excessoxygen under standard conditions. • Eg; CH4+ 2O2→ CO2+ 2H2O • One mole is being completely burnt so; ΔHr = ΔHΘcCH4 • NB; If combustion was incomplete, due to a lack of oxygen, carbon monoxide or carbon would be produced instead.

  6. ΔHΘnStandard enthalpy of neutralisation • The enthalpy change when one mole of water is formed when an acid is neutralised by a base to form an infinitely dilute solution. • Eg; HCl + NaOH→ NaCl + HOH • As one mole of water is produced this represents; • ΔHr = ΔHΘn

  7. NB H2SO4 + 2NaOH→ Na2SO4 + 2HOH • Two molesof water are produced. • ΔHr = 2ΔHΘn • ½H2SO4 + NaOH→ ½Na2SO4 + HOH • Now one mole of water is produced, so the enthalpy change is; • ΔHr = ΔHΘn

  8. Write equations to represent the standard neutralisation enthalpies of; • Hydrochloric acid and calcium hydroxide. • HCl + ½Ca(OH)2→ ½CaCl2 + H2O • Phosphoric acid and sodium hydroxide • ⅓H3PO4 + NaOH→ ⅓Na3PO4 + H2O • Aqueous ammonia and sulphuric acid. • ½ H2SO4 + NH4OH→ ½ (NH4)2SO4 + H2O

  9. ΔHΘa the enthalpy of atomisation • This is the enthalpy change when one mole of gaseous atoms are formed from the element at stp. • Eg; C(s)→ C(g) • One mole of gaseous atoms are being formed so this represents • ΔHr = ΔHΘaC

  10. NB For H2(g)→ 2H(g) • ΔHris not ΔHΘaH • In this case 2 moles of gaseous atoms are produced so; • ΔHr= 2ΔHΘaH • But for ½H2(g)→ H(g)onemole of gaseous atoms are produced so; • ΔHr = ΔHΘaH

  11. NB; The state of the element does not matter. • ½Cl2(g)→ Cl(g) • ½Br2(l)→ Br(g) • ½I2 (s) → I(g) • In each caseΔHΘr=ΔHΘa

  12. Mercury. Aluminium Fluorine Phosphorous Hg (l)→ Hg (g) Al (s)→ Al (g) ½F2(g)→ F (g) ¼P4(s)→ P(g) Write an equations to represent ΔHΘafor;

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