Water & Aqueous Systems (solutions)

1. Water & Aqueous Systems (solutions) Chemistry Chapter 15 & 16

2. Water in the Liquid State • Water is a simple triatomic molecule • Oxygen bonds covalently with 2 hydrogen atoms. • Oxygen has greater electronegativity, so it does what to the electron pair shared with H? • Electron pairs pulled closer to O resulting in O having a slightly“-” charge & H having a slightly “+” charge; results in a highly polar molecule. • These partial charges allow an attraction between water molecules known as hydrogen bonding.

3. The bond polarities are equal, but the two poles do not cancel each other because a water molecule is bent.

4. Important Propertiesof Water • Surface tension • Hydrogen bonding between water molecules results in water’s low vapor pressure. • the tendency of molecules to escape is low & evaporation is low • Expands when frozen

5. Solvents & Solutes • An aqueous solution is water that contains dissolved minerals & gases. • The dissolving medium is the solvent • The dissolved particles are the solute • Solute usually atoms, ions, or molecules (very small less than 1 nm) • Ionic & polar covalent compounds dissolve most readily. • Homogenous aqueous solutions – the solute is evenly dispersed, non distinguishable A solvent dissolves the solute. The solute becomes dispersed in the solvent.

6. As individual solute ions break away from the crystal, the negatively and positively charged ions become surrounded by solvent molecules and the ionic crystal dissolves. The process by which the positive and negative ions of an ionic solid become surrounded by solvent molecules is called solvation. The Solution Process

7. Polar liquids will dissolve ionic compounds & other polar compounds Non-polar liquids will dissolve only non-polar compounds “Like Dissolves Like”

8. Solution Formation • Recall, solutions are homogeneous mixtures that can be solid, liquid or gaseous. • The compositions of the solvents & solute determine whether a substance will dissolve. • Stirring (agitation), temperature, and the surface area of the dissolving particles determine how fast the substance will dissolve.

9. Solubility • A saturated solution contains the max amount of solute for a given quantity of solvent at a constant temp & pressure. • The solubility of a substance is the amount of solute that dissolves in a given quantity of solvent at a specified temp & pressure – to produce a saturated solution. • Solubility is often expressed in grams of solute per 100g of solvent (or grams of solute per liter solvent). • A solution that is not saturated at a given temp & pressure is said to be an unsaturated solution.

10. Factors Affecting Solubility • Temperature • Temp affects the solubility of solid, liquid, and gaseous solutes in a solvent. • Solubility of solids increases as temp increases • Solubility of gases decreases as temp increases • supersaturated solution contains more solute that it can theoretically hold at a given temperature • Pressure • Changes in pressure have little affect on solids & liquids, but strongly influence the solubility of gases. • Henry’s Law states that at a given temp, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) above the liquid.

11. Concentrations of Solutions • Chemist use several methods to calculate concentrations of solutions, however the most common is Molarity. • Morality (M) is the number of moles of solute dissolved in one liter of solution.

13. Making Dilutions • Diluting a solution reduces the # of moles of solute per unit volume, but the total # of moles of solute in solution does not change. • Rearranging the Molarity equation gives an expression for moles of solute. • Moles of solute = M1 x V1 = M2 x V2; • M1 and V1 are the molarity and volume of the initial solution, and M2 and V2 are the molarity and volume of the diluted solution. Volumes can be in liters or milliliters, as long as the same units are used for both V1 and V2. • The dilution equation; M1 x V1 = M2 x V2

15. Electrolytes & Nonelectrolytes • An electrolyte is a compound that conducts an electrical current when in an aqueous solution or molten state. (Strong vs Weak electrolytes) • Ions must be mobile to conduct current • All ionic compounds are electrolytes because they can be dissociated into ions. • A compound that doesn’t conduct current in solution or molten state is called a nonelectrolyte. • Most molecular compounds & compounds of carbon.

16. A) Sodium chloride, a strong electrolyte, is nearly 100% dissociated into ions in water. B) Mercury(II) chloride, a weak electrolyte, is only partically dissociated in water. C) Glucose, a nonelectrolyte, does not dissociate in water.