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Chem week 1 Pt.2

by: Darlene Franklin

Chem week 1 Pt.2 Che 212

Darlene Franklin
SUNY Oswego

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Solutions, Solubility, Concentrations and Colligative Properties
General Chem II
Stephany Rugg
Class Notes
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This 26 page Class Notes was uploaded by Darlene Franklin on Monday February 1, 2016. The Class Notes belongs to Che 212 at State University of New York at Oswego taught by Stephany Rugg in Spring 2016. Since its upload, it has received 41 views. For similar materials see General Chem II in Chemistry at State University of New York at Oswego.

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Date Created: 02/01/16
Chapter 14 Solutions: Solubility, Concentrations and Colligative Properties © 2015 Pearson Education, Inc. The mystery of wood frogs • Cold-blooded animals – Cytoplasm is ~80% water • Wood frogs survive very cold temperatures by hibernating in a suspended “frozen” state. – has no heartbeat, no blood circulation, no breath, and no brain activity. – Their vital functions can be revived within 1–2 hours of thawing • Wood frogs survive “flood their blood” with a large amount of glucose (sugar water solution) prior to going into hibernation. © 2015 Pearson Education, Inc. 2 Solutions • Homogeneous mixtures. – Two or more substances make up a mixture. – Ex. a solid and a liquid, a gas and a liquid • Solution formation is the result of the interaction of the Intermolecular forces of solute and solvent particles. – Likes dissolve in likes. • Nature has a tendency toward spontaneous mixing. – Generally, uniform mixing is more energetically favorable. © 2015 Pearson Education, Inc. 3 Common Types of Solutions © 2015 Pearson Education, Inc. 4 Solubility • When one substance (solute) dissolves in another (solvent), it is said to be Soluble Ex. Salt+Water • When one substance does not dissolve in another, it is said to be Insoluble. – Ex. Oil+water • The solubility of one substance in another depends on the following: – The types of Intermolecular attractive forces • When solutions with different solute concentrations come in contact, they spontaneously mix to result in a Uniform distribution of solute throughout the solution. © 2015 Pearson Education, Inc. 5 Solubility • The maximum amount of Solute that can be dissolved in a given amount of solvent is called the Solubility • There usually is a limit to the solubility of one substance in another. – Gases are Always soluble in each other. – Two liquids that are mutually soluble are said to be Miscible. • Alcohol and water are miscible. • Oil and water are immiscible. • The solubility of one substance in another varies with Temperature and Pressure. © 2015 Pearson Education, Inc. 6 Nature’s Tendency Toward Mixing: Entropy • In general: lower Potential energy. • But formation of a solution does not necessarily lower the potential energy of the system. • 2 ideal gases in same container, they spontaneously mix, even though the difference in attractive forces is negligible. • So why do they mix? © 2015 Pearson Education, Inc. 7 Nature’s Tendency Toward Mixing: Entropy • The gases mix because the energy of the system is lowered through the release of entropy. – the measure of energy dispersal throughout the system. • Energy spontaneously spreads out over as large a volume as it is allowed. – As each gas expands to fill the container, it spreads its energy out and lowers its entropy. © 2015 Pearson Education, Inc. 8 Solutions: Effect of Intermolecular Forces Energy changes in the formation of most solutions also involve differences in attractive forces between the particles. Non Both polar nonpolar © 2015 Pearson Education, Inc. Solutions: Effect of Intermolecular Forces • For the solvent and solute to mix, you must overcome 1. All solute attractive forces or 2. Some solvent-solvent Both processes are Endothermic • At least some of the energy to do this comes from making new solute–solvent attractions, which is an Exothermic process. © 2015 Pearson Education, Inc. Relative Interactions and Solution Formation Ex: Pentane heptane Nonpolar dispersion © 2015 Pearson Education, Inc. 11 Will It Dissolve? “Likes Dissolve in Likes” • A chemical will dissolve in a solvent if it has a structure similar to that of the solvent. – Compatible Intermolecular forces • Polar molecules and ionic compounds will be more soluble in Polar solvents. Ex: NaCl in H2O Ethanol in water • Nonpolar molecules will be more soluble in Nonpolar solvents. Ex: Grease in hexanes © 2015 Pearson Education, Inc. 12 Practice Problem: Solubility Determine whether each compound is soluble in hexane: Water- Polar (not soluble in hexane) NH -3Polar (not soluble in hexane) CH CH CH - Nonpolar (soluble in hexane) 3 2 3 HCl- Polar (not soluble in hexane) © 2015 Pearson Education, Inc. 13 Heat of Solution • When some compounds, such as NaOH, dissolve in water, a lot of heat is released. (Exothermic) • When other compounds, such as NH NO , diss4lve 3n water, heat is absorbed from the surroundings. (Endothermic) • Why is this? © 2015 Pearson Education, Inc. 14 The Enthalpy of Solution • To make a solution, the following needs to occur: 1. all attractions between the solute particles need overcome; therefore, ΔH is Endothermic. solute 2. some attractions between solvent molecules need overcome; therefore, ΔH solvents Endothermic. 3. new attractions formed between solute particles and solvent molecules; therefore, ΔH mix is Exothermic. • The overall ΔH for making a solution depends on the relative sizes of the ΔH for these three processes. ΔH soln ΔH solute ΔH solvent ΔH mix © 2015 Pearson Education, Inc. 15 Solution Process Step 1: Separating the solute into its constituent particles Step 2: Separating the solvent particles from each other to make room for the solute particles Step 3: Mixing the solute particles with the solvent particles © 2015 Pearson Education, Inc. 16 Energetics of Solution Formation: Exothermic ΔH soln ΔH solute ΔH solvent ΔH mix 1. If the sum of (breaking attractions between solute) and (breaking attractions between 2. solvent ) is Less than the energy released in making the 3. new attractions between the solute and solvent, the overall process will be Exothermic. © 2015 Pearson Education, Inc. 17 Energetics of Solution Formation: Endothermic ΔH soln ΔH solute ΔH solvent ΔH mix 1. If the sum of (breaking attractions between solute) and 2. (breaking attractions between solvent ) is More than the energy released in making the new attractions between the solute and solvent, the overall process will be Endothermic. © 2015 Pearson Education, Inc. 18 Heats of Hydration • Seawater: ion in water – Attractive forces between ions = lattice energy Na Cl Na Cl – ΔH solute –ΔH lattice energy Cl Na Cl Na Na Cl Na Cl – Attractive forces in water = H bonds – Attractive forces between ion and water = ion–dipole – ΔH = heat released when 1 mol of gaseous ions dissolves in hydration water = ΔH solventΔH mix © 2015 Pearson Education, Inc. 19 Comparing Heat of Solution to Heat of Hydration • The lattice energy is always exothermic • If the heat of solution is large and endothermic, then the amount of energy it costs to separate the ions is more than the energy released from hydrating the ions. ΔH hydrationΔH latticeen ΔH solnis Endothermic • If the heat of solution is large and exothermic, then the amount of energy it costs to separate the ions is less than the energy released from hydrating the ions. Solvent ΔH hydrationΔH latticeen ΔH solnis Exothermic H solute © 2015 Pearson Education, Inc. 20 Heats of Solution for Ionic Compounds • For an aqueous solution of an ionic compound, the solutionthe difference between the heat of hydration and the lattice energy. ΔH Solution= ΔH Solute + ΔH Solvent + ΔH ΔH Solvent +ΔHydration +2kj/mol=821kj/mol+(-819kj/mol) Endothermic © 2015 Pearson Education, Inc. 21 Clicker: When NaCl is mixed with water, what is the cation? a) Na b) Na 1+ c) Na 1- d) Cl 1- e) Cl © 2015 Pearson Education, Inc. 22 Ion–Dipole Interactions • When ions dissolve in water, they become Hydrated • Each ion is surrounded by water molecules. • The formation of these ion–dipole attractions causes the heat of hydration to be very Exothermic FeCl3 * XH2O © 2015 Pearson Education, Inc. 23 Solution Equilibrium • The dissolution of a solute in a solvent is an Equilibrium process. • Initially, when there is no dissolved solute, the only process possible is Dissoluton (NaCl---- Na+ Cl-) • Shortly after some solute is dissolved, solute particles can start to recombine to reform solute molecules (Na+ Cl- ----- Nacl) , but the rate of dissolution >> the rate of deposition, and the solute continues to dissolve. • Eventually, the rate of dissolution = the rate of deposition – The solution is Saturated with solute and no more solute will dissolve. © 2015 Pearson Education, Inc. 24 Solution Equilibrium: NaCl in H O 2 © 2015 Pearson Education, Inc. 25 Solubility Limit • A solution that has the solute and solvent in dynamic equilibrium is Saturated • If you add more solute, it will Not Dissolve • The saturation concentration depends on the Temperature and Pressure of gases. © 2015 Pearson Education, Inc. 26


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