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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 • Ionic Bonding • Solubility of Minerals • Common Ion Effect • 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 page 1 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 page 2 Three atoms in one layer and three atoms in the next form an octahedral hole. Chemistry 102 Prof. Shapley page 3 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 page 5 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 page 7 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 page 9 We see that an additional 0.0045 mol/L of the salt would dissolve. Chemistry 102 Prof. Shapley page 10