Transcript Lecture 37 (Slides) November 9

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).
Energy Changes in the Formation of Ionic
Crystals
• Born Haber Cycle
• Enthalpy diagram for the formation of an ionic
crystal
General Chemistry: Chapter 12
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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
∆H1
∆H2
∆H3
∆H4
∆H5
Description of
Physical/Chemical Change
Enthalpy of sublimation of Na(s)
½ x (Bond energy of Cl2)
1st ionization energy of Na(g)
Electron affinity of Cl(g)
Lattice 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.
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General Chemistry: Chapter 13
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General Chemistry: Chapter 13
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Solution Concentration.
• Mass Percent
• Volume Percent
• Mass/Volume percent
(m/m)
(v/v)
(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)
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General Chemistry: Chapter 13
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Familiar Glassware
for Handling
Solutions
Popular Solutes
and Solutions
Molarity and Molality
Amount of solute (in moles)
Molarity (M) =
Volume of solution (in liters)
Amount of solute (in moles)
Molality (m) =
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Mass of solvent (in kilograms)
General Chemistry: Chapter 13
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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.
Molarity and Molality
• 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%
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Molarity and Molality
• 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 and Molality
Amount of solute (in moles)
Molarity (M) =
Volume of solution (in liters)
Amount of solute (in moles)
Molality (m) =
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Mass of solvent (in kilograms)
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Intermolecular Forces and the Solution
Process
FIGURE 13-2
•Enthalpy diagram for solution formation
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Intermolecular Forces in Mixtures
Magnitude of ΔHa, ΔHb, and ΔHc
depend on intermolecular forces.
Ideal solution
Forces are similar between all
combinations of components.
ΔHsoln = 0
FIGURE 13-3
•Intermolecular forces in a solution
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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?
FIGURE 13-4
Two components of a nearly ideal solution
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General Chemistry: Chapter 13
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Formation of Ionic Solutions
FIGURE 13-6
•An ionic crystal dissolving in water
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General Chemistry: Chapter 13
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Solution Formation and Equilibrium
FIGURE 13-7
•Formation of a saturated solution
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General Chemistry: Chapter 13
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