Ionic Bonding – the Born Haber Cycle

Ionic Bonding – the Born Haber Cycle

• Insight into the stability of ionic compounds can be obtained if we imagine breaking a reaction forming a binary ionic compound (from a metal and a nonmetal) into several steps. We’ll look at this for the formation of NaCl(s). In the next slide we will identify ΔH’s for familiar processes and introduce a new ΔH – the enthalpy of crystallization (lattice energy).

Slide 2 of 61

Energy Changes in the Formation of Ionic Crystals

Copyright © 2011 Pearson Canada Inc.General Chemistry: Chapter 12

• Born Haber Cycle

• Enthalpy diagram for the formation of an ionic crystal

Born Haber Cycle - Comments• We consider a binary ionic substance being

formed from its constituent elements in their standard states. Along the way we first form neutral gaseous atoms of each element (a metal and a nonmetal) in the gas phase. We next form a metal ion (Na+(g), Mg2+(g)……) and a non-metal ion (Cl-(g), O2-(g)…..). Finally we combine the two metal ions to form an ionic crystal.

Born Haber Cycle

• For the case of NaCl(s) formation you should be able to identify the signs of ∆H1, ∆H2, ∆H3

and ∆H5. (∆H4 is “trickier”?). You also should be able to see what physical or chemical process is occurring at each step. If ∆H5 were not a highly exothermic step would ionic compounds be as stable?

Born Haber Cycle for NaCl(s) Step or ∆H Value

Description of Physical/Chemical Change

∆H1Enthalpy of sublimation of Na(s)

∆H2½ x (Bond energy of Cl2)

∆H31st ionization energy of Na(g)

∆H4Electron affinity of Cl(g)

∆H5Lattice energy of NaCl(s)

Class Examples

• 1. How would the Born Haber cycle for the formation of NaBr(s) differ from the Born Haber cycle already considered for NaCl(s) formation?

• 2. How would the Born Haber cycle for the formation of MgO(s) and MgCl2(s) differ from the Born Haber cycle already considered for NaCl(s) formation?

Physical Properties of Mixtures

• At a specified T and P a pure substance has well-defined (unique) values for a range of physical properties. These include density, colour, electrical conductivity, vapor pressure and so on. For example, at -5.0 OC ice (H2O(s)) has a vapor pressure of 0.402 kPa and a density of 0.917 g∙cm-3. (As the ice is cooled below this T the vapor pressure drops quickly).

Physical Properties/Mixtures – cont’d:

• Changing the chemical composition of a mixture will affect physical properties. Many food items and biologically important fluids are mixtures. In St. John’s the city council is planning to make “mixtures” this winter by adding rock salt, NaCl(s), to ice. The objective here will be to melt ice - lower the melting point of ice.

Types of Solution: Some Terminology

• Solutions are homogeneous mixtures and are uniform throughout.

• Solvent.– Determines the state of matter in which the solution exists.– Is the largest component.

• Solutes– Other solution components said to be dissolved in the

solution.

Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 13 Slide 9 of 46


Solution Concentration.• Mass Percent (m/m)• Volume Percent (v/v)• Mass/Volume percent (m/v)

• Isotonic saline is prepared by dissolving 0.9 g of NaCl in 100 mL of water and is said to be:

0.9% NaCl (mass/volume)


Familiar Glassware for Handling Solutions

Popular Solutes and Solutions

Molarity and Molality

Copyright © 2011 Pearson Canada Inc. Slide 15 of 46General Chemistry: Chapter 13

Molarity (M) = Amount of solute (in moles)

Volume of solution (in liters)

Molality (m) = Amount of solute (in moles)

Mass of solvent (in kilograms)

Class Examples:• 1. A popular consumer product is 5.21%

ethanol (C2H5OH) by volume. Assuming that the remaining 94.8% by volume of this product is water (and that ethanol has a density of 0.789 g/mL) calculate:

• (a) the % by mass of ethanol in this solution.• (b) the molar concentration of ethanol in this

solution.• (c) the molality of ethanol in this solution.


• For dilute aqueous solutions the molality and molality of a solution are usually very similar.

• Why is this the case?

Class Examples

• 2. A solution is prepared by dissolving 44.6g of Cu(NO3)2

.6H2O(s) in enough water to make 825 mL of solution. What is the molar concentration of Cu2+(aq) ions and NO3

-(aq) ions in this solution?• 3. 2.25 L of 0.400 mol.L-1 Al(NO3)3 (aq) and 2.00L

of 0.350 mol.L-1 Ba(NO3)2 (aq) are mixed. What is the molar concentration of nitrate ions in the resulting solution?

Physical Properties – Concentrations: :

• The most useful concentration units for physical properties studies show the relative numbers of molecules (or ions) of each substance. The relative number of molecules (of each substance) is the same as the relative number of moles (of each substance). Often we employ mole fractions – especially for vapor pressure calculations.

Mole Fraction and Mole Percent


i = Amount of component i (in moles)

Total amount of all components (in moles)

1 + 2 + 3 + …n = 1

Mole % i = i 100%


• Molarity (mol∙L-1), does not indicate the relative amounts of solute(s) and solvent. The next slide helps demonstrate why. An alternate concentration unit, molality, does give an indication of the relative amounts of solute(s) and solvent. We can convert from molarity to molality given the solution density.



Molarity (M) = Amount of solute (in moles)

Volume of solution (in liters)

Molality (m) = Amount of solute (in moles)

Mass of solvent (in kilograms)

Intermolecular Forces and the Solution Process


FIGURE 13-2

•Enthalpy diagram for solution formation

Intermolecular Forces in Mixtures


FIGURE 13-3•Intermolecular forces in a solution

ΔHsoln = 0

Magnitude of ΔHa, ΔHb, and ΔHc depend on intermolecular forces.

Ideal solutionForces are similar between all combinations of components.

Similar Intermolecular Forces

• Molecules with similar structures often have intermolecular forces of the same type and of similar strength. The next slide shows the structures of benzene and the slightly more complex toluene molecule. What intermolecular forces are important for these two molecules?

Two components of a nearly ideal solutionFIGURE 13-4


Formation of Ionic Solutions


FIGURE 13-6

•An ionic crystal dissolving in water

Solution Formation and Equilibrium


FIGURE 13-7

•Formation of a saturated solution

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Ionic Bonding – the Born Haber Cycle