Chapter 6 Notes - Rachel Lusby PDF

Preview

Summary

Chapter 6 notes...

Description

● ● ● ● ●

●

●

●

●

Properties of Solutions Solution - homogeneous mixture of two or more substances Solute – the component of a solution that is present in lesser quantity Solvent – the solution component present in the largest quantity Aqueous solution - solution where the solvent is water ○ Solutions can be liquids as well as solids and gases Examples of Solutions ○ Solutions can be liquids as well as solids and gases ○ Air - oxygen and several trace gases are dissolved in the gaseous solvent, nitrogen ○ Alloys - brass and other homogeneous metal mixtures in the solid state ○ Focus on liquid solutions as many important chemical reactions take place in liquid solutions General Properties of Liquid Solutions ○ Clear, transparent, no visible particles ○ May be colored or colorless ○ Electrolytes are formed from solutes that are soluble ionic compounds ○ Nonelectrolytes do not dissociate ○ Volumes of solute and solvent are not additive ■ 1 L ethanol + 1 L water does not give 2 L of solution Solutions and Colloids ○ Colloidal suspension - contains solute particles which are not uniformly distributed ■ Due to larger size of particles (1nm - 200 nm) ■ Appears identical to solution from the naked eye ■ Smaller than 1 nm, have solution ■ Larger than 1 nm, have a precipitate Tyndall Effect ○ Tyndall effect - the ability of a colloidal suspension to scatter light ■ See a haze when shining light through the mixture ■ Solutions: light passes right through without scattering ■ Colloidal suspension -light as haze, scatters light ● Solution - no haze Degree of Solubility ○ Solubility - how much of a particular solute can dissolve in a certain solvent at a specified temperature ○ Factors which affect solubility: ■ Polarity of solute and solvent ● The more different they are, the lower the solubility ○ Temperature ○ Increase in temperature usually increases solubility ○ Pressure ○ Usually has no effect ○ If solubility is of gas in liquid, directly proportional to applied pressure

●

●

●

● ●

●

●

●

Saturated Solution ○ A solution that contains all the solute that can be dissolved at a particular temperature Solubility and Equilibrium ○ If excess solute is added to a solvent, some dissolves ○ At first, rate of dissolution is large ○ Later, reverse reaction, precipitation, occurs more quickly ○ When equilibrium is reached the rates of dissolution and precipitation are equal, there is some dissolved and some undissolved solute ○ A saturated solution is an example of a dynamic equilibrium Henry’s Law ○ the number of moles of a gas dissolved in a liquid at a given temperature is proportional to the partial pressure of the gas above the liquid ○ Gas solubility in a liquid is directly proportional to the pressure of the gas in the atmosphere in contact with the liquid ○ Gases are most soluble at low temperatures ○ Solubility decreases significantly at higher temperatures ■ Carbonated beverages – CO2 solubility less when warm ■ Respiration – facilitates O2 and CO2 exchange in lungs Concentration Based on Mass Concentration - amount of solute dissolved in a given amount of solution Concentration of a solution has an effect on ○ Physical properties ■ Melting and boiling points ○ Chemical properties ■ Solution reactivity Mass/Volume Percent ○ Amount of solute = mass of solute in grams ○ Amount of solution = volume in milliliters ■ Concentration = amount of solute / amount of solution ○ Concentration is expressed as a percentage by multiplying ratio by 100% = weight/volume percent or % (m/V) ■ % m / V = grams of solute / mL of solution x 100% Mass/Mass Percent ○ Most useful for solutions of 2 solids whose masses are easily obtained ○ % m / m = grams of solute / grams of solutions x 100% Parts per Thousand (ppt) and Parts per Million (ppm) ○ As percentage is the number of parts of solute in 100 parts of solution, ppt and ppm change the calculation only by orders of magnitude ■ ppt = g solute / g solution ◊ 103 ppt ■ ppm = g solute / g solution ◊ 106 ppt ■ ppt and ppm are most often used for very dilute solutions Concentration Based on Moles

● ● ●

●

● ●

Chemical equations represent the relative number of moles of reactants producing products Many chemical reactions occur in solution where it is most useful to represent concentrations on a molar basis Molarity ○ The most common mole-based concentration unit is molarity ○ Molarity ■ Symbolized M ■ Defined as the number of moles of solute per liter of solution ○ M = moles of solute / L solution Dilution ○ Dilution is required to prepare a less concentrated solution from a more concentrated one ■ M1 = molarity of solution before dilution ■ M2 = molarity of solution after dilution ■ V1 = volume of solution before dilution ■ V2 = volume of solution after dilution ○ Moles of Solute = Molarity x L solution ○ Moles of solute will not change, only fewer per unit volume ■ M1V1 - M2V2 ■ Knowing any three terms permits calculation for the fourth Concentration-Dependent Solution Properties Colligative properties - properties of solutions that depend on the concentration of the solute particles, rather than the identity of the solute Four colligative properties of solutions ○ Vapor pressure lowering ■ Raoult’s law - when a nonvolatile solute is added to a solvent, vapor pressure of the solvent decreases in proportion to the concentration of the solute ■ Solute molecules (red below) serve as a barrier to the escape of solvent molecules resulting in a decrease in the vapor pressure ○ Boiling point elevation ■ Can be explained considering the definition as the temperature at which vapor pressure of the liquid equals the atmospheric pressure. ● If a solute is present, then the increase in boiling temperature is necessary to raise the vapor pressure to atmospheric temperature ■ ΔTb ■ Proportional to the number of solute particles ■ An electrolyte will affect boiling point to a greater degree than a nonelectrolyte of the same concentration ■ Each solvent has a unique boiling point elevation constant ● kb ○ Freezing point depression

■

●

●

May be explained considering the equilibrium between solid and liquid states. ● Solute molecules interfere with the rate at which liquid water molecules associate to form the solid state ■ ΔTf ■ Proportional to the number of solute particles ● Solute particles - not just solute ■ An electrolyte dissociates into ions ■ An equal concentration of NaCl will affect the freezing point twice as much as glucose (a nonelectrolyte) ■ Each solvent has a unique freezing point depression constant or proportionality factor ● kf ■ ΔTf = kfM ● Osmotic pressure ○ Some types of membranes appear impervious to matter, but actually have a network of small holes called pores ○ These pores may be large enough to permit small solvent molecules to move from one side of the membrane to the other ○ Solute molecules cannot cross the membrane as they are too large ○ Semipermeable membrane - allows solvent but not solute to diffuse from one side to another ○ Osmosis: the movement of solvent from a dilute solution to a more concentrated solution through a semipermeable membrane ■ Requires pressure to stop this flow ○ Osmotic pressure: the amount of pressure required to stop the flow across a semipermeable membrane ■ (π) = MRT ○ Osmolarity: the molarity of particles in solution ■ Osmol, used for osmotic pressure calculation Vapor Pressure of a Liquid ○ Consider Raoult’s law in molecular terms ■ Vapor pressure of a solution results from escape of solvent molecules from liquid to gas phase ■ Partial pressure of gas phase solvent molecules increases until equilibrium vapor pressure is reached ■ Presence of solute molecules hinders escape of solvent molecules, lowering equilibrium vapor pressure Tonicity and the Cell ○ Living cells contain aqueous solution and these cells are also surrounded by aqueous solution ○ Cell function requires maintenance of the same osmotic pressure inside and outside the cell

○

●

● ● ● ●

●

●

●

●

Solute concentration of fluid surrounding cells higher than inside results in a hypertonic solution causing water to flow into the surroundings, causing collapse = crenation ○ Solute concentration of fluid surrounding cells too low, results in a hypotonic solution causing water to flow into the cell, causing rupture = hemolysis ○ Isotonic solutions have identical osmotic pressures and no osmotic pressure difference across the cell membrane Molality ○ Solute concentration is expressed in mole-based units ■ Number of particles is critical, not the mass of solute ○ Molality ■ M = Moles of solute / kg of solvent ■ The denominator is in kg solvent, not in kg solution Aqueous Solutions Water is often referred to as the “universal solvent” Excellent solvent for polar molecules Most abundant liquid on earth 60% of the human body is water ○ transports ions, nutrients, and waste into and out of cells ○ solvent for biochemical reactions in cells and digestive tract ○ reactant or product in some biochemical processes Two common ways of expressing concentration of ions in solution: ○ Moles per liter (molarity) ■ Molarity emphasizes the number of individual ions ○ Equivalents per liter ■ Eq / L ■ Emphasis on charge Equivalents ○ 1 M of Na3PO4 ■ What is the concentration of PO43- ions? ● 1M ■ What is the concentration of Na+ ions? ● 3M ○ Equivalent is defined by charge Equivalents/Liter ○ The eq/mol value of the ion is the number of charges on the ion ■ Regardless of whether that charge is positive or negative ○ eq/L = (eq / mol ion)(mol ion / L) Biological Effects of Electrolytes in Solution ○ Active transport: the transporting of Na+ and K+ ions across the cell membrane ■ Cellular energy must be expended to make concentration of ions different on each side of the cell membrane ■ This is accomplished via large protein molecules embedded in cell membranes

○

○

○

The two most important anions in the body fluids are Cl− and HCO3− ■ Cl− ● Acid/base balance ● Maintenance of osmotic pressure ● Oxygen transport by hemoglobin ■ HCO3− ● Most waste CO2 is removed from the body Two most important cations in the body are Na+ and K+ ■ Na+ ● Blood: 135 meq/L ● Cells: 3.5-5.0 meq/L ■ K+ ● Blood: 10 meq/L ● Cells: 125 meq/L Danger to the body occurs when Na+ and K+ both in blood and in cells becomes too high or low ■ Na+ too low: ● Decrease of urine output ● Dry mouth ● Flushed skin ● Fever ■ Na+ too high: ● Confusion, stupor, or coma ■ K+ too high: ● Death by heart failure ■ K+ too low: ● Death by heart failure

Practice Questions 1. Calculate the percent composition or % (m/V) of 2.00 x 102 mL containing 20.0 g sodium chloride a. 20.0 g NaCl, mass of solute, 2.00 ◊ 102 mL, total volume of solution b. % (m/V) = 20.0g NaCl / 2.00 ◊ 102 mL x 100% = 10.0% (m/V) sodium chloride 2. Calculate the osmolarity of 5.0 x 10-3 M Na3PO4 a. Na3PO4 is an ionic compound forming electrolytes b. 1 mol Na3PO4 yields 4 product ions c. 5.0 x 10-3 mol Na3PO4 x 4 mol particles L 1 mol Na 3PO4 = 2.0 x 10-2 mol particles / L = 2.0 ◊ 10 –2 osmol 3. If 5.00 g glucose are dissolved in 1.00 x 102 mL of solution, calculate molarity, M, of the glucose solution a. M glucose = mol glucose / L solution b. M glucose = 2.78x10-2 mol glucose / 1.00 x 10-1 L solution

4.

5.

6.

7.

c. M glucose = 2.78 x 10-1 M Calculate the number of grams of glucose in 7.50 x 102 mL of a 15.0% solution a. 15.0% (m/V) = Xg glucose/7.50 ◊ 102 mL ◊ 100% b. Xg glucose ◊ 100% = (15.0% m/V)(7.50 ◊ 102 mL) c. Xg glucose = 113 g glucose Calculate % (m/m) of platinum in gold ring with 14.00 g Au and 4.500 g Pt a. [4.500 g Pt / (4.500 g Pt + 14.00 g Au)] ◊ 100% b. 4.500 g / 18.50 g ◊ 100% = 24.32% Pt Calculate the molarity of a solution made by diluting 0.050 L of 0.10 M HCl solution to a volume of 1.0 L a. M2 = M1V1 / V2 b. M2= 0.10 M x 0.050 L / 1.0 L c. M2 = 0.0050 L or 5.0 x 10−3L Calculate the molarity of 2.0 L of solution containing 5.0 mol NaOH a. MNaOH = 5.0 moles of solute / 2.0 L of solution = 2.5 M...