Dissolves in excess of water under standard

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dissolves in excess of water under standard conditions - May be exothermic or endothermic Examples: NaCl (s) + water Na + (aq) + Cl - (aq); H soln Θ = +4.2 kJ/mol CaCl 2 (s) + water Ca 2+ (aq) + 2Cl (aq) H soln Θ = – 120 kJ/mol
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18 vap
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19 fus
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20 subl
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21 Extra First ionisation energy of an element It is the energy needed to remove 1 electron from 1 mol of gaseous atoms to form 1 mol gaseous cations Mg (g) Mg + (g) + e Δ H = + 736 kJ/mol First electron affinity of an atom It is the energy change when one electron is added to each atom in 1 mol of neutral gaseous atoms O (g) + e O (g) Δ H = – 142 kJ/mol Lattice energy of an ionic compound It is the enthalpy change when 1 mol of the ionic solid is formed from its constituent gaseous ions Na + (g) + Cl (g) NaCl(s) Δ H = – 787 kJ/mol
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22 8.6 Hess Law If a chemical reaction can take place through different pathways, the overall enthalpy change of the chemical reaction is the same regardless of the pathway taken H overall =  H individual steps Useful in the calculation of enthalpy change which cannot be determined directly from experiments
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23
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24 2 N 2 + 6 H 2 + N 2 + 3/2 F 2 4(– 46) – 114 3(– 467) H r Θ = – 4 (– 46) + (– 114) + 3 (– 467) = – 1331 kJ/mol OR 8.6 Hess Law Q1
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25 CO 2 – 394 H r Θ – 283 H r Θ = – 394 – (– 283) = – 111 kJ mol – 1 8.6 Hess Law Determine the enthalpy change for the reaction below: C (s) + ½ O 2 (g) CO (g) The following information were determined, H c Θ of C = – 394 kJ mol – 1 ; H c Θ of CO = – 283 kJ mol – 1 Q2 C (s) + ½ O 2 (g) CO (g)
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26 + C (s) + 3/2 O 2 (g) H r Θ – 394 H r Θ = (– 411) + (– 286) + (– 394) – (– 25) = – 1066 kJ mol – 1 8.6 Hess Law Using Hess’ Law to calculate the enthalpy of formation of calcium carbonate. Given H f Θ of CO 2 = – 394 kJ mol – 1 ; H f Θ of H 2 O = – 286 kJ mol – 1 Q3 – from lab work Ca (s) + C (s) + 3/2 O 2 (g) CaCO3 (s) 2 HCl CaCl 2 (aq) + H 2 (g) – 286 CaCl 2 + H 2 O + CO 2 2 HCl – 411 – 25
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27 8.7 Born-Haber Cycle A series of hypothetical steps and their standard enthalpy changes used to calculate lattice energy E.g.
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