Download Salt Solutions Ionic Bonding

Salt Solutions
Salts have a non-directional type of bonding. The interaction
between anions and cations is mainly electrostatic. Anions
pack into regular lattices. The cations fit into the holes
within the lattice. Minerals are crystalline salts on or in the
Earth. These dissolve in rainwater to the extent of their
solubility and their ions become components of seawater.
Outline
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Ionic Bonding
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Solubility of Minerals
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Common Ion Effect
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Homework
Ionic Bonding
Sodium Chloride
To understand salt solutions, we must first explore the
nature of the solid, crystalline salts. Sodium chloride NaCl,
ordinary table salt, is the most common salt in the ocean
and is part of the solution inside cells of many organisms.
It is composed of sodium cations and chloride anions
packed in a crystal with 6 anions around every cation and 6
cations around every anion.
Instead of covalent bonds, ionic materials like NaCl are
held together by the electrostatic forces between anions and
cations in the salt.
Electrostatic forces are attractive between particles of opposite sign (for example, cations for anions) and
repulsive between particles of the same sign (for example, cations for cations). Electrostatic forces are
governed by Coulomb's Law. You don't need to memorize this. The important thing is that the forces are
inversely proportional to 1/r2 so they decrease rapidly as the distance between the the two particles
increases.
Chemistry 102
Prof. Shapley
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Cubic Close Packing
There are several types of packing of the large anions in salts. A common one is cubic close packing. A
close-packed structure is the most efficient and has the least amount of empty space. In each layer, there
are six anions packed around each each one.
The next layer of anions fits into the depressions of the first layer and the third layer depressions in the
second. The fourth layer is directly above the first to give a repeat pattern A-B-C.
There are "holes" to accommodate cations between every two layers.Three atoms in one layer covered by
another anion in the second layer make a tetrahedral hole.
Chemistry 102
Prof. Shapley
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Three atoms in one layer and three atoms in the next form an octahedral hole.
Chemistry 102
Prof. Shapley
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In a close-packed structure, tetrahedral holes are smaller than octahedral holes so a salt with small cations
relative to the anions will have some or all of the tetrahedral holes filled. If the cation is too big to fit into
a tetrahedral hole, it will go into an octahedral hole.
In sodium chloride, the sodium cations fill all the octahedral holes in the chloride lattice.
Lattice Energy
Each Na+ is surrounded by 6 Cl- anions. The electrostatic attraction between cations and anions reduces
the energy of the system. Farther away are 12 Na+ cations.
The Na+-Na+ repulsion increases the energy of the system. Even farther are more Cl- anions, and so on
through the crystal.
Chemistry 102
Prof. Shapley
page 4
The sum of the energy changes from the electrostatic attractions and repulsions of all the anions and
cations in a mole of a salt is the energy of crystallization. The inverse of the energy of crystallization, the
energy required to separate all the ions in a crystal to gas phase ions, is called the lattice energy. The
lattice energy of NaCl is 786 kJ/mol.
Solubility of Minerals
Minerals in the Crust
The relative abundance of elements in the Earth's crust is quite different than that of the atmosphere
(nitrogen,78 %; oxygen, 21 %) and of the oceans (mostly oxygen and hydrogen). The top 10 elements that
make up the crust are listed below:
Other important but less abundant elements include sulfur and chlorine, each at 500 parts per million.
The minerals are all crystalline salts. They are typically classified by the nature of their anions.
Chemistry 102
Prof. Shapley
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Water from oceans, lakes, rivers continually evaporates. Wind moves the water vapor and this vapor
continually condenses as rain. Water flows across the surface and dissolves minerals. The anions and
cations from minerals flow into the ocean.
Solubility Product Constant
Minerals have widely different solubilities in water. Sodium chloride is very soluble with 359 g or 6.1
mol dissolving in each 1.0 L of water at equilibrium. Concentration units of mol/L are in molar
concentration, abbreviated as M.
Chemistry 102
Prof. Shapley
page 6
There is an equilibrium between solid sodium chloride and the solvated ions Na+(aq) and Cl-(aq) when
NaCl is added to water. Like all equilibria, an equilibrium constant is equal to the ratio of the
concentrations of products to the concentrations of reactants.
The concentration of any solid is defined as 1. The concentration of pure water is a constant, 55.556 M.
By multiplying the equilibrium constant by the concentration of water, we define a more useful constant
Ksp or the solubility product constant.
Calcium hydroxide has a solubility product constant of 5.0 x 10-6.
How can we use this to determine the concentration of Ca2+(aq) when excess Ca(OH)2 is added
to water?
You can find a chart of solubility product constants of other minerals in the tables section.
Common Ion Effect
Equilibrium
Consider a reversible reaction: A + B
Chemistry 102
C + D.
Prof. Shapley
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There is a certain initial concentration of A, B, C, D. The ratio of the concentrations of products and
reactants is called Q. Q varies as the forward and reverse reactions proceed and, at each value of Q, the
system has a specific energy content. The system will always seek the lowest possible energy. At the
lowest energy point, the rate of the forward reaction will equal the rate of the reverse reaction. The value
of Q at this point is the equilibrium constant, Keq.
The concentration values of A, B, C, and D are the total concentrations from all sources. If one of these is
added from some other source in the reaction, the ration of the others will adjust until the ratio Q again
equals Keq.
Ions from Other Sources
The type of equilibrium we are considering in this unit is the equilibrium between a mineral (crystalline
salt) in water with the aquated anions.
Let's consider our saturated solution of Ca(OH)2 from the last page. We calculated that the concentration
of Ca+2 would be 1 x 10-2 M and the concentration of HO- would be 2 x 10-2 M.
Calcium chloride, CaCl2, is much more soluble than Ca(OH)2. What would happen if 1.1 g (0.010 mol) of
CaCl2 were added to 1.0 L of our saturated solution of Ca(OH)2?
Chemistry 102
Prof. Shapley
page 8
Some of the less soluble Ca(OH)2 would precipitate from solution. The resulting solution would have
0.010 mol/L of Ca+2 and 0.020 mol/L Cl- from the CaCl2 that dissolved along with 0.008 mol/L Ca+2 and
0.016 mol/L HO- from the remaining Ca(OH)2.
Removing Ions
Strong acids, such as HCl, dissociate completely in water and the H+ ions react with HO- ions to make
H2O.
If we add an acid to our saturated solution of Ca(OH)2, the acid protons would react with some of the HOions and drive the equilibrium to the right. More of the solid Ca(OH)2 should dissolve. Let's see what
happens if we add 0.01 mol/L of HCl to the solution.
Chemistry 102
Prof. Shapley
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We see that an additional 0.0045 mol/L of the salt would dissolve.
Chemistry 102
Prof. Shapley
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