Making solutions In order to dissolve - the solvent molecules must come in contact with the solute. Stirring moves fresh solvent next to the solute. The solvent touches the surface of the solute. Smaller pieces increase the amount of surface of the solute. When a solute dissolves, its individual atoms, molecules, or ions interact with the solvent, become solvated, and are able to diffuse independently throughout the solution. • If the molecule or ion collides with the surface of a particle of the undissolved solute, it may adhere to the particle in a process called crystallization (formation of a solid with a well-defined crystalline structure). • Dissolution and crystallization continue as long as excess solid is present, resulting in a dynamic equilibrium. dissolution solute + solvent crystallization solution

Water Molecule H2O d+ d+ d- d- Water is a POLAR molecule O2- H+ H+ Draw the Lewis dot structure of a water molecule. Recall, oxygen has 6 valence electrons and hydrogen has one valence electron. The ‘electron cloud’ is more dense around the oxygen than the hydrogen. Therefore, the oxygen side of the molecule is more negatively charged and has a partial negative charge. The hydrogen have a lower electron density around them and receive a partial positive charge. This molecule does not have a formal (+) and (-) charge – as in an ionic compound. http://images.google.com/imgres?imgurl=http://www.rpc.msoe.edu/cbm/lib/images/nc2.jpg&imgrefurl=http://www.rpc.msoe.edu/cbm/lib/nc.php&h=183&w=289&sz=17&hl=en&start=161&tbnid=_Ead2aBRZUSliM:&tbnh=73&tbnw=115&prev=/images%3Fq%3Dchemical%2Bbonds%26start%3D160%26gbv%3D2%26ndsp%3D20%26hl%3Den%26sa%3DN

create surface tension Water molecules “stick” together to create surface tension to support light weight objects. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

Water Molecule What is a polar molecule? d- Hydrogen bond d+ O H How does the polarity of water effect this molecule? An individual water molecule consists of two hydrogen atoms bonded to an oxygen atom in a bent (V-shaped) structure. The oxygen atom in each O–H covalent bond attracts the electrons more strongly than the hydrogen atom. O and H nuclei do not share the electrons equally. – Hydrogen atoms are electron-poor compared with a neutral hydrogen atom and have a partial positive charge, indicated by the symbol δ+. – The oxygen atom is more electron-rich than a neutral oxygen atom and has a partial negative charge, indicated by the symbol 2δ-. Unequal distribution of charge creates a polar bond.

Hydrogen bonds occur between two polar molecules, or between different polar regions of one large macro-molecule. One “relatively” negative region is attracted to a second “relatively” positive region. O H Electronegative atoms Hydrogen bond H N

Interstitial Spaces Oil Non-polar "immiscible" Polar dissolved solid Layer Water Water Water Water Water Water Water Water Polar This is an overly simplified model of spaces between water molecules. However, it is in these interstitial spaces that gases (e.g. oxygen and carbon dioxide) dissolve and solids also dissolve. red food coloring

Interstitial Spaces Oil Non-polar "immiscible" Polar dissolved solid Layer Water Water Polar This is an overly simplified model of spaces between water molecules. However, it is in these interstitial spaces that gases (e.g. oxygen and carbon dioxide) dissolve and solids also dissolve. red food coloring

Dissolving of solid NaCl Due to the arrangement of polar bonds in a water molecule, water is called a polar substance. Due to the asymmetric charge distribution in the water molecule, adjacent water molecules are held together by attractive electrostatic interactions. Energy is needed to overcome these electrostatic interactions. Unequal charge distribution in polar liquids, like water, makes them good solvents for ionic compounds. When an ionic solid dissolves in water, the partially negatively charged oxygen atoms of water surround the cations, and the partially positively charged hydrogen atoms in water surround the anions. Individual cations and anions are called hydrated ions.

Polar water molecules interact with positive and negative ions Animation The attraction of water dipoles for ions pulls ions out of a crystalline lattice and into aqueous solution. The ion-dipole forces exist in the solution as well, lessening the tendency for ions to return to the crystalline state. The combination of an ion in solution and the neighboring water dipoles to which it is attracted is called a hydrated ion. Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 468

Dissolving of solid NaCl salt Na+ NaCl solid NaCl (aq) = Na+ = Cl- Animation by Raymond Chang All rights reserved.

Solvation First... Then... Solvation – the process of dissolving solute particles are surrounded by solvent particles First... The interactions that determine the solubility of a substance in a liquid depend on the chemical nature of the solute whether it is ionic or molecular) rather than on its physical state (solid, liquid, or gas). • Two examples: 1. Forming a solution of a molecular species in a liquid solvent 2. Formation of a solution of an ionic compound Solutions of molecular substances in liquids – London dispersion forces, dipole-dipole interactions, and hydrogen bonds that hold molecules to other molecules are weak. – Energy is required to disrupt these interactions, and unless some of that energy is recovered in the formation of new, favorable solute-solvent interactions, the increase in entropy on solution formation is not enough for a solution to form. – For solutions of gases in liquids, the energy required to separate the solute molecules is ignored (H2 = 0) because molecules are already separated – it is necessary to only consider the energy required to separate the solvent molecules (H1) and the energy released by new solute-solvent interactions (H3). 1. Nonpolar gases are most soluble in nonpolar solvents because H1 and H3 are both small and of similar magnitude. 2. Nonpolar gases are less soluble in polar solvents than in nonpolar solvents because H1 >> H3. 3. Solubilities of nonpolar gases in water increase as the molecular mass of the gas increases. – All common organic liquids, whether polar or not, are miscible; the strengths of the intermolecular attractions are comparable, the enthalpy of solution is small, and the increase in entropy drives the formation of a solution. – If predominant intermolecular interactions in two liquids are very different from one another, they may be immiscible, and when shaken with water, they form separate phases or layers separated by an interface. – Only the three lightest alcohols are completely miscible with water; as the molecular mass of the alcohol increases, so does the proportion of hydrocarbon in the molecule, which leads to fewer favorable electrostatic interactions with water Hydrophilic and hydrophobic solutes – A solute can be classified as hydrophilic, meaning that there is an electrostatic attraction to water, or hydrophobic, meaning that it repels water. 1. Hydrophilic substance is polar and contains O–H or N–H groups that can form hydrogen bonds to water; tend to be very soluble in water and other strongly polar solvents 2. Hydrophobic substance may be polar but usually contains C–H bonds that do not interact favorably with water; essentially insoluble in water and soluble in nonpolar solvents – The difference between hydrophilic and hydrophobic substances has substantial consequences in biological systems. – Vitamins can be classified as either fat soluble or water soluble. 1. Fat-soluble vitamins (Vitamin A) are nonpolar, hydrophobic molecules and tend to be absorbed into fatty tissues and stored there. 2. Water-soluble vitamins (Vitamin C) are polar, hydrophilic molecules that circulate in the blood and intracellular fluids and are excreted from the body and must be replenished in the daily diet. solute particles are separated and pulled into solution Then...

Dissolving of NaCl - - - O + + + + Cl- Na+ hydrated ions H Timberlake, Chemistry 7th Edition, page 287

Dissolving of Salt in Water Na+ ions Water molecules Cl- ions When sodium chloride crystals are dissolved in water, the polar water molecules exert attracting forces which weaken the ionic bonds. The process of solution occurs the ions of sodium and chloride become hydrated. NaCl(s) + H2O  Na+(aq) + Cl-(aq)

Dissolving of Salt in Water Cl- ions Na+ Water molecules When sodium chloride crystals are dissolved in water, the polar water molecules exert attracting forces which weaken the ionic bonds. The process of solution occurs the ions of sodium and chloride become hydrated. NaCl(s) + H2O  Na+(aq) + Cl-(aq)

Particle Model of a Solution Before mixing Fresh water Water molecule After mixing Saltwater solution Water molecule Chloride ion, Cl- Sodium ion, Na+