LECTURE

Standard Enthalpies

Born-Haber Cycle

IB Chemistry Power Points

Topic 15

Energetics

www.pedagogics.ca

Great thanks to

JONATHAN HOPTON & KNOCKHARDY PUBLISHING

www.knockhardy.org.uk/sci.htm

Much taken from

ENTHALPY

CHANGES

Step 1 Energy is required as reactants

are broken into their elements.

Step 2 Energy is released as products

are formed from their elements.

H = Step 2 - Step 1

In Step 1 the route involves going in the OPPOSITE DIRECTION to the defined enthalpy change, it’s value is subtracted (or alternatively the sign of the enthalpies is reversed)

Enthalpy of reaction from enthalpies of formation

Consider a reaction as reactants elements products

NEGATIVE SUM OF THE

ENTHALPIES OF FORMATION OF

THE REACTANTS

REACTANTS

PRODUCTS

ELEMENTS

H

SUM OF THE ENTHALPIES OF FORMATION OF THE PRODUCTS

ΔH = Σ ΔHf products

- Σ ΔHf reactants

Sample calculationCalculate the standard enthalpy change for the following reaction, given that the standard enthalpies of formation of water, nitrogen dioxide and nitric acid are -286, +33 and -173 kJ mol-1 respectively;

2H2O(l) + 4NO

2(g) + O

2(g) ———> 4HNO

3(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

Enthalpy of reaction from enthalpies of formation

the value for the enthalpy of formation for oxygen is ZERO as it is already in elemental form

Enthalpy of reaction from enthalpies of combustion

Enthalpy of reaction from enthalpies of combustion

Enthalpy of reaction from enthalpies of combustion

Enthalpy of reaction from enthalpies of combustion

The Born–Haber cycle

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 enthalpies[1]) 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.”

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 form Mg2+ 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.”

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

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

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

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.

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.

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