1. IB Chemistry Power Points
Topic 15
Energetics
www.pedagogics.ca
LECTURE
Standard Enthalpies
Born-Haber Cycle

2. Much taken from
ENTHALPY
CHANGES
Great thanks to
JONATHAN HOPTON & KNOCKHARDY PUBLISHING
www.knockhardy.org.uk/sci.htm

3. Enthalpy of reaction from enthalpies of formation
Consider a reaction as reactants  elements  products
Step 1 Energy is required as reactants
ELEMENTS
are broken into their elements.
NEGATIVE SUM OF
Step 2 Energy is released as products THE ENTHALPIES OF
are formed from their elements. FORMATION OF THE SUM OF THE
REACTANTS ENTHALPIES OF
FORMATION OF
H = Step 2 - Step 1 THE PRODUCTS
REACTANTS
In Step 1 the route involves going in the
OPPOSITE DIRECTION to the defined H
enthalpy change, it’s value is subtracted (or
alternatively the sign of the enthalpies is
reversed) PRODUCTS
ΔH = Σ ΔHf products - Σ ΔHf reactants

4. Enthalpy of reaction from enthalpies of formation
Sample calculation
Calculate the standard enthalpy change for the following reaction, given that the
standard enthalpies of formation of water, nitrogen dioxide and nitric acid are -286,
-1
+33 and -173 kJ mol respectively;
2H2O(l) + 4NO2(g) + O2(g) ———> 4HNO3(l)

5. Enthalpy of reaction from enthalpies of formation
Sample calculation
Calculate the standard enthalpy change for the following reaction, given that the
standard enthalpies of formation of water, nitrogen dioxide and nitric acid are -286,
-1
+33 and -173 kJ mol respectively;
2H2O(l) + 4NO2(g) + O2(g) ———> 4HNO3(l)
By applying Hess’s Law ... The Standard Enthalpy of Reaction will be...
PRODUCTS REACTANTS
ΔH° = 4 x (-173) MINUS 2 x (-286) + 4 x (+33) + 0
ANSWER = - 252 kJ
the value for the enthalpy of formation for oxygen is ZERO as it is already in elemental form

6. Enthalpy of reaction from enthalpies of combustion

7. Enthalpy of reaction from enthalpies of combustion

8. Enthalpy of reaction from enthalpies of combustion

9. Enthalpy of reaction from enthalpies of combustion

10. The Born–Haber cycle

11. Background Information (from Wikipedia)
“The Born–Haber cycle is an approach to analyzing
reaction energies. It was named after and developed by the
two German scientists Max Born and Fritz Haber.
The Born–Haber cycle involves the formation of an ionic
compound from the reaction of a metal (often a Group I or Group
II element) with a non-metal. Born–Haber cycles are used primarily
as a means of calculating lattice energies (or more precisely
[1]
enthalpies ) which cannot otherwise be measured directly.
The lattice enthalpy is the enthalpy change involved in formation
of the ionic compound from gaseous ions. Some chemists define it
as the energy to break the ionic compound into gaseous ions. The
former definition is invariably exothermic and the latter is
endothermic.”

12. Background Information (from Wikipedia) continued
“A Born–Haber cycle calculates the lattice enthalpy by comparing
the standard enthalpy change of formation of the ionic compound
(from the elements) to the enthalpy required to make gaseous ions
from the elements. This is an application of Hess's Law.
This latter calculation is complex. To make gaseous ions from
elements it is necessary to atomise the elements (turn each into
gaseous atoms) and then to ionise the atoms. If the element is
normally a molecule then we have to consider its bond dissociation
enthalpy (see also bond energy). The energy required to remove
one or more electrons to make a cation is a sum of
successive ionization energies; for example the energy needed to
2+
form Mg is the first plus the second ionization energies of Mg.
The energy released when one electron is added to an atom to
make it an anion is called the electron affinity.”

13. A schematic of a Born-Haber cycle
The sum of the enthalpies on any two sides must equal
the enthalpy change of the remaining side

14. There are many different
standard enthalpy values
depending on what is being
measured

15. Here is a cycle for NaCl. Make sure you understand
the changes occurring and which enthalpy values are
used for each reaction.

17. 774 kJ/mol

18. Factors affecting lattice enthalpy
Observation: A decrease in the size of any ion increases
the lattice enthalpy. (more positive/endothermic).
Explanation: This is because small ions can be close
together and the smaller distance of separation the
larger the attractive force between the ions.

19. Factors affecting lattice enthalpy
Observation: An increase in charge also increases lattice
enthalpy. (more positive/endothermic).
Explanation: This is because the force of attraction
between ions increases as the charge on the ion
increases.

20. Differences between experimental and theoretical values
in lattice energy.
In general, the larger the difference between empirical
and theoretical value, the more covalent character the
bond has.
You should be able to relate this to electronegativity