Solutions Chapter 6

What Are Solutions? Solution: homogeneous mixture of 2 or more substances Solution: homogeneous mixture of 2 or more substances –Solid, liquid, or gas –Solvent: dissolving medium –Solute: substance that dissolves –When in solution, you cannot distinguish solvent and solute

What is a Solution? Soluble – a substance that can dissolve in a given solvent Soluble – a substance that can dissolve in a given solvent –Miscible: two liquids that can dissolve in each other Example: water and antifreeze Example: water and antifreeze Insoluble – substance cannot dissolve Insoluble – substance cannot dissolve –Immiscible: two liquids that cannot dissolve in each other Example: oil & water Example: oil & water

Why Do Some Substances Dissolve and not Others? To dissolve, solute particles must dissociate from each other and mix with solvent particles To dissolve, solute particles must dissociate from each other and mix with solvent particles –Attractive forces between solute and solvent must be greater than attractive forces within the solute Process of surrounding solute particles with solvent particles is called SOLVATION Process of surrounding solute particles with solvent particles is called SOLVATION –In water, it is also called HYDRATION

Aqueous Solutions of Ionic Compounds Remember: Remember: –Water molecules are polar (+ and – ends) –Water molecules are in constant motion When you put salt in water, water molecules collide with surface of crystal When you put salt in water, water molecules collide with surface of crystal –Charged ends of water attract ions of salt –Dipole interaction (water/salt) is stronger than ions in crystal, so it pulls them away

Solvation

Aqueous Solutions of Molecular Compounds Water is also a good solvent for many molecular compounds (Example: sugar) Water is also a good solvent for many molecular compounds (Example: sugar) –Sugar has many O-H bonds (polar) –When water is added, the O-H bond becomes a site for hydrogen bonding with water –Water’s hydrogen bonds pulls the sugar molecules apart –Oil is not a good solute because it has many C-H bonds (not polar) and few or no O-H (polar) bonds

Factors that Affect Solvation Rate Increase Solvation Rate (Dissolve Faster) by: Increase Solvation Rate (Dissolve Faster) by: –Agitation (stirring) –Increase surface area (make particles smaller) –Temperature (make it hotter) All these increase the number of collision between water and the solute All these increase the number of collision between water and the solute

Solubility Only a limited amount of solute can dissolve in a given amount of solvent Only a limited amount of solute can dissolve in a given amount of solvent Every solute is unique for the solvent Every solute is unique for the solvent This is ‘Solubility’ – the amount of solute that can dissolve in a given amount of solvent at a specified temperature and pressure This is ‘Solubility’ – the amount of solute that can dissolve in a given amount of solvent at a specified temperature and pressure

Solubility Continued Solubility can also be understood at the particle level: Solubility can also be understood at the particle level: –As particles collide, some particles are deposited back to the solute –Some particles are removed from the solute. –When the rate of deposit equals the rate of solvation, then the solution is SATURATED Saturated Solution – no more solute can be dissolved in the solvent at this temperature and pressure Saturated Solution – no more solute can be dissolved in the solvent at this temperature and pressure Unsaturated Solution – there is still room for more solute to be dissoved Unsaturated Solution – there is still room for more solute to be dissoved

Factors that Affect Solubility Most substances are MORE soluble at high temperature than at low Most substances are MORE soluble at high temperature than at low –If you dissolve a substance until saturated at high temperature and then reduce the temperature, the solution becomes “supersaturated” –Supersaturated solutions are unstable A small change makes the solute reappear A small change makes the solute reappear Rock candy worked that way. How? Rock candy worked that way. How?

Gas Solubility is Opposite Solids Gas solubility DECREASES with temperature Gas solubility DECREASES with temperature Gas solubility decreases with shaking Gas solubility decreases with shaking

Colligative Properties of Solutions

Electrolytes and Colligative Properties When solutions are made, the physical properties of the solutions are affected by the number of particle dissolved When solutions are made, the physical properties of the solutions are affected by the number of particle dissolved –Colligative: depending on the collection

Colligative Properties 1. Electrolytes vs non-electrolytes –Ionic compounds ARE electrolytes because they form ions in solution that conduct electricity –Molecular compounds ARE NOT electrolytes because they do not conduct electricity 2. Vapor Pressure Lowering Adding a non-volatile solute lowers the vapor pressure of the solution (vs. solvent)Adding a non-volatile solute lowers the vapor pressure of the solution (vs. solvent) More solute  more vapor pressure loweringMore solute  more vapor pressure lowering

Colligative Properties 3. Boiling Point Elevation Because vapor pressure is lowered, it takes more energy to make it boilBecause vapor pressure is lowered, it takes more energy to make it boil Boiling point temperature is raised Boiling point temperature is raised Boiling point elevation is directly proportional to solution molality Boiling point elevation is directly proportional to solution molality What is the benefit of adding salt to boiling water for pasta? What is the benefit of adding salt to boiling water for pasta? 4. Freezing Point Depression Freezing point temperature is loweredFreezing point temperature is lowered Solute particles interfere with attractive forces of solventSolute particles interfere with attractive forces of solvent Freezing point of a solution is always lower than the freezing point of a pure solventFreezing point of a solution is always lower than the freezing point of a pure solvent FP Depression is directly proportional to molalityFP Depression is directly proportional to molality

Colligative Properties 5. Osmosis and Osmotic Pressure Diffusion: mixing of gasses or liquids through random motions Diffusion: mixing of gasses or liquids through random motions Osmosis is diffusion of solvent through a semi permeable membrane from high solvent concentration to lower solvent concentration Osmosis is diffusion of solvent through a semi permeable membrane from high solvent concentration to lower solvent concentration –Living cells use this to get materials in/out of cells

Colligative Properties Example: Salt/water Example: Salt/water –During Osmosis, Water molecules move both directions through membrane But only water can move through the membrane But only water can move through the membrane So pure water builds up on one side of the membrane So pure water builds up on one side of the membrane Water/salt builds up on the other. Water/salt builds up on the other. –Higher concentration of water on one side creates: Osmotic pressure Osmotic pressure A pressure or push to equalize the water/salt concentrations A pressure or push to equalize the water/salt concentrations Pressure depends on concentration of solute Pressure depends on concentration of solute

Colligative Properties - Antifreeze

Heterogeneous Mixture