1. ENERGETICS /THERMOCHEMISTRY (AS)

2. 1.Often chemical changes are accompanied by changes in heat content / enthalpy of the materials reacting (H)
2. This change is shown by a change in temperature.

3. a. heat is lost to the surroundings
Temperature of reaction mixture rises / increases
H is negative
Exothermic reaction

4. Enthalpy diagram / energy level diagram
OR Reaction pathway/reaction coordinate/extent of reaction

6. b. heat is absorbed from the surroundings , temperature of reaction mixture decreases / falls
H is positive
Endothermic reaction

9. 3. a. The value of H depends on temp , pressure and concentrations of reactants.
b. H are measured under standard conditions :
Temperature = 298K ( 250 C )
Pressure = 1 atm / 1.01 x 105 Pa
Concentrations = 1 mol dm-3

10. c. Any H measured under standard conditions is described as standard enthalpy change.
Symbol : H

11. State symbols must be included in equations
STANDARD ENTHALPIES
Definitions
State symbols must be included in equations

12. STANDARD ENTHALPHY OF FORMATION ( Hf )
Enthalpy change when 1 mole of a compound is formed from its elements in their standard states , under standard conditions temperature 298 K and pressure of gases at 1 atm

13. Steps in writing equations:
1. write formula of compound formed
2. identify elements required to form the compound
3. balance equation to form 1 mole of the compound

14. Examples Mg (s) + ½O2 (g) → 1MgO (s) -602 kJ mol-1
602 kJ evolved for every 1 mole of MgO formed
½ H2 (g) + ½ Cl2 (g) → 1HCl (g)
kJ mol-1

15. Examples Notes : 1) Hf of elements is zero
Eg : Cu(s)  Cu(s) , Hf = 0
2) Hf are often theoretical only

16. Exercise : Write the equation for enthalpy of formation for : KMnO4 (s) and H2O (l)
K(s) + Mn(s) + 2O2(g) 1KMnO4(s)
H2(g) + ½O2(g)  1H2O (l)

17. STANDARD ENTHALPY CHANGE OF COMBUSTION ( Hc )
Enthalpy evolved when 1 mole of the element or compound is completely burned in excess oxygen , under standard conditions

18. Steps in writing equations :
1. identify products formed from burning of compound in excess oxygen
2. balance equation for 1 mole of compound burnt

19. Examples 1 C (s) + O2 (g) → CO2 (g) Hc = - 394 kJ mol-1
394 kJ evolved for every 1 mole of carbon burnt
1 H2 (g) + ½O2 (g) → H2O (l)

20. Exercise : Write an equation for the enthalpy of combustion for C3H6 and
CH3OH

21. 1C3H6 (g) + 9/2 O2 (g)  3CO2 (g)
+ 3H2O (l)
1CH3OH (l) + 3/2 O2 (g)  CO2 (g)
+ 2H2O (l)

22. STANDARD ENTHALPY OF ATOMISATION ( Hat )
Enthalpy required/absorbed
1 mole of gaseous atoms formed
From the element in its standard state under standard conditions
Examples :
Fe (s)  1 Fe (g)

23. Example :
½Cl2(g)  1 Cl (g) kJ mol-1
molecules atoms
Note : from Data Booklet
Bond energy Cl-Cl = kJ mol-1
Cl-Cl  2 Cl (g) 2 moles atoms
Hat of Cl = ½ x bond energy Cl-Cl

24. STANDARD BOND DISSOCIATION ENTHALPY
Also called bond energy
Energy absorbed
Separate the 2 atoms in a covalent bond in gaseous state under standard conditions , per mole of bond.

25. Examples 1) HCl (g) → H (g) + Cl (g) , +431 kJmol-1
2) CH4 contains 4 x C-H bond
Total energy required to break all bonds in CH4 = 1640 kJ
¼ CH4 = ¼ ( 4 C-H bond ) = one C-H bond
Average bond energy of one C-H bond
= ¼ ( 1640 ) = +410 kJ
¼ CH4 (g)→¼ C (g) + H (g),+ 410 kJmol-1

26. STANDARD ENTHALPY OF NEUTRALIZATION
Energy evolved
Acid reacts with base to form 1 mole of water , under standard conditions
Examples :
NaOH + HCl → NaCl H2O
KOH + HCl → KCl H2O
Ionic equation(strong acid + strong base:
H+(aq) + OH-(aq)  1 H2O (l),
H neutralisation = -57 kJ mol-1

27. Generally,
1) Strong acid / strong alkali :
H = - 57 kJ mol-1
2) Weak acid / base :
H = -(<57) kJ mol-1 (eg. -54 kJmol-1)
Reason : Certain amount of energy required to ionise the weak acid or base first

28. STANDARD ENTHALPY OF HYDRATION ( Hhyd )
Energy evolved
1 mole of separate gaseous ions dissolved in water under standard conditions
Exothermic as attraction/bond forms between the ions and polar water molecules
Called ion dipole attraction

29. Examples :
1 Na+ (g) → Na+ (aq) kJmol-1
1 Cl- (g) → Cl- (aq) kJmol-1

30. Hhyd for compounds =
sum of Hhyd of constituent ions
Eg : Hhyd MgCl2 = Hhyd Mg2+ +
2 x Hhyd Cl-
= (-381)
= kJ

31. H (hyd)  charge density of ions
Charge density = charge/size
Higher charge density , stronger ion dipole attraction , more exothermic H(hyd)
Eg :
H(hyd) Cl- > H(hyd) Br –

32. STANDARD ENTHALPY OF SOLUTION ( Hsolution )
Enthalpy change when 1 mole of a substance dissolved in a stated amount of solvent under standard conditions
Example :
1 KOH (s)  K+ (aq) + OH- (aq)
or KOH (aq)
- 57 kJ mol-1

33. STANDARD ENTHALPY CHANGE OF REACTION ( Hr )
Enthalpy change in a chemical reaction , for the number of moles of reactants as shown in a balanced chemical equation under standard conditions
Example :
4 H2O Fe  Fe3O H2
 Hr = x kJ when 4 moles H2O reacts with 3 moles Fe