1. Topic 15 Energetics (HL) 15.1 Standard enthalpy changes of reaction
15.2 Born-Haber cycle
15.3 Entropy
15.4 Spontaneity

2. 15.1 Standard Enthalpy Change of reaction
Standard state: 101kPa, 298K

3. Standard enthalpy of formation DHfq
DHfq : The energy absorbed or evolved when 1 mol of the substance is formed from its elements in their standard states. The enthalpy of formation of any element is zero. ½ H2(g) + O2(g)  H2O(l) DHfq = -285 kJ/mol DH = SDHf(products) - SDHf(reactants)

4. Decomposition of ammonium nitrate
NH4NO3(s)  N2O(g) + 2 H2O(l)
NH4NO3(s) DHfq= -366 kJ/mol
N2O(g) DHfq= kJ/mol
H2O(l) DHfq= -285 kJ/mol
DH = [DHf (N2O(g)) + DHf (H2O(l))] – [DHf(NH4NO3(g)] =
=[82 + 2(-285)] - [-366] = -122 kJ/mol

5. Standard enthalpy of combustion,DHcq
When a substance is fully combusted in oxygen
 CH4 + 2O2  CO2 + 2H2O DHcq= ?

6. 15.2 Born-Haber cycle
To determine the lattice enthalpy and the degree of ionic character of a salt
To find an unknown value (Hess’ law)

7. Lattice enthalpy, DHlattice
Relates to the endothermic process
MX(s) M+(g) + X-(g)
in which the gaseous ions of a crystal are separated to an infinitive distance from each other.
NaCl(s)  Na+(g) + Cl-(g) DHlattice= 769 kJ/mol Endothermic reactions

8. Factors affecting the lattice enthalpy
The greater the charge of the ions, the stronger the electrostatic attraction

9. Factors affecting the lattice enthalpy (2)
The smaller the ionic radius, the shorter the distance, the stronger the electrostatic attraction

10. Electron affinity (electron gain enthalpy)
The enthalpy change when an atom gain one electron in gas phase e.g.
Cl(g) + e-(g)  Cl-(g) DHe.a. = -349 kJ/mol.
Electron affinity can be both exothermic and endothermic depending on element.

11. Born-Haber cycle for the formation of NaCl (s)
Enthalpy of atomisation of Na
Na (s) →Na (g) DHat= +108 kJ/mol
Enthalpy of atomisation of Cl
½ Cl2 (s) →Cl (g) DHat = +121 kJ/mol
(½ energy of Cl-Cl bond)

12. Born-Haber cycle for the formation of NaCl (s)(2)
Electron affinity of Cl
Cl (g) + e- → Cl- (g) DHea= -349 kJ/mol
Ionisation energy of Na
Na (g) → Na+ (g) + e DHie= kJ/mol

13. Born-Haber cycle for the formation of NaCl (s)(3)
Lattice enthalpy of NaCl
Cl- (g) + Na+ (g) → NaCl (s) DHlatt = -769 kJ/mol

14. Theoretical value of Enthalpy of formation of NaCl = -411kJ/mol
Using Hess Law:
Enthalpy of formation of NaCl
DHf (NaCl)= DHat (Na) + DHie(Na) + DHat (Cl) + DHea(Cl) + DHlatt(NaCl)
(-364) + (-771)= -393 kJ/mol

15. Use of Born-Haber cycles
In the Chemistry Data Booklet the lattice enthalpies is given booth as:
Experimental values (obtained by Born-Haber cycle)
Theoretical values (calculated from electrostatic principles)
If the value differ in a significant way => indicate more covalent character of the salt

16. 15.3 Entropy- disorder Entropy, S Unit: J/K*mol
DS = change in disorder
DS = Sproducts - Sreactants
It’s possible to measure absolute values of S

18. Increasing entropy- DS positive
Solid  Liquid  Gas
increase in S
Ice  Water  Steam
JK-1mol-1
Mixing different types of particles-
dissolving NaCl in water
Increasing no of particles-
N2O4 (g) →2 NO2 (g)

19. Decreasing entropy- DS negative
System becomes more ordered
Formation of solid ammonium chloride from hydrogen chloride and ammonia gas
NH3(g) + HCl(g)  NH4Cl(s) DS = J/K*mol

20. In any conversion there is both a change in DH and DS.
DS is probably positive if number of mol of gas increases and number of mol of solid/liquid decrease.
NH4Cl(s)  NH3(g) + HCl(g) DS = + 285J/K*mol
Pb2+(aq) + 2 I-  PbI2(s) DS = - 70 J/K*mol

21. 15.4 Spontaneity of a reaction- DG
Nature likes low internal energy (DH to decrease) and high disorder (DS to increase)
Spontaneity: is a reaction going to occur
A reaction will occur if the final state is more probable than the initial state.
=> Decrease in DH
=> Increase in DS

22. DGq = DHq - TDSq Standard free energy, DGq (or Gibbs free energy)
Temperature is important for spontaneity
A reaction will be spontaneous if DG has a negative value (DGq < 0)
A positive DGq is a non-spontaneous reaction.
If DGq = 0 then its a equilibrium

24. DGq = DHq - TDSq DH DS DG Spontaneity Negative (Exothermic) Positive
(More random)
DG < 0
Always negative
Always
spontaneous
Positive (Endothermic)
(More order)
DG > 0
Always positive
Never
Depends on T
Spontaneous at low Temperature
Spontaneous at high Temperature

25. Just the fact that a reaction is spontaneous doesn’t mean that it will occur at once- it might need activation energy (topic 6)