Solutions. Water Think back to the structure of water: The charges on water can attract other...

Solutions

Water• Think back to the structure of water:

• The charges on water can attract other “things” which makes water the universal solvent.

Water

• Also, because of the different charges water molecules can bond to one another.

• This hydrogen bonding gives water many of its unique properties.

Properties of Water

• Hydrogen bonding causes water to have some unique properties:high surface tensionhigh specific heatlow vapor pressurehigh heat of vaporization

Evidence of Surface Tension

• Evidence of water's surface tension can be seen where the water strider's legs dent but do not break through the water's surface.

• The attraction of polar water molecules to each other helps create water's strong surface tension.

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Beading• The water molecules beneath

the surface are pulled in all directions.

• The molecules at the surface are pulled together and in.

• This creates a tighter arrangement of molecules at the surface and the round shape of a drop of water.



Alcohol vs. Water • Alcohol has one O–H bond which is polar

but a large portion of the molecule is made up of C–H bonds which are nonpolar.

• Alcohol molecules do not attract each other as strongly as water molecules and have a weaker surface tension.





Water of Hydration

• Hygroscopica compound that removes water from the

environment (hydrates)

• Desiccant a hygroscopic substance that removes water

from the atmosphere to keep the environment dry

used as a drying agent (shoes, electronics)

• Deliquescent substances that remove so much water from the

air that it turns into a solution, ZnCl2

Solutions

• Solutions are homogeneous mixtures• Solute is the dissolved substance

Seems to “disappear” or “take on the state” of the solvent

• Solvent is the substance the solute dissolves inDoes not appear to change state

• Solutions in which the solvent is water are called aqueous solutionsWater is often called the universal solvent

• When in question, the solvent is the substance that you have more of.

The Solution Process - Solvation

• When ionic compounds dissolve in water they dissociate into ionsions become surrounded by water molecules -

hydrated

• When solute particles are surrounded by solvent molecules we say they are solvated

• Solvation of molecular compounds dissolving• Solvation of ionic compounds dissociation• Solvation rate affected by: Agitation,

temperature, particle size



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Solubility• When one substance (solute) dissolves in another (solvent) it is

said to be soluble

• When one substance does not dissolve in another they are said to be insoluble

• Rule of Thumb: Like dissolves like (polarity)

• There is a limit as to how much solute can dissolve in a solvent. this is called solubilityEx. at 20oC 64.2 g NiCl2 in 100 g H2O

g/L with a gas solute and liquid solvent

Solution Composition

• A saturated solution contains as much solute as will dissolve at that temperature (solubility curve)

• An unsaturated solution does not contain as much solute as will dissolve at that temperature (under solubility curve)

• A supersaturated solution contains more dissolved solid than a saturated solution can hold at that temperature. (above solubility curve)Obtained by dissolving a solid to saturation limit at an

elevated temperature and then cooling the solution.Supersaturated Solutions are unstable.

Reading Solubility Curves: The amounts shown will result in saturated solutions of a salt at a given

temperature.• Which of the salts shown is the least soluble in water at 10°C?

• 30 grams of KCl are dissolved in water at 45°C. How many more grams are needed to make the solution saturated at 80°C?

• A saturated solution of NaNO3 in 300 g of water is cooled from 50°C to 10°C, how many grams of the salt will precipitate from the solution?

20 grams

345-240 = 105 g

Describing Solutions - Qualitatively

• A concentrated solution has a high proportion of solute to solution – lots of solute

• A dilute solution has a low proportion of solute to solution – little solute

• A saturated solution has the maximum amount of solute that will dissolve in the solventDepends on temp

• An unsaturated solution has less than the saturation limit• A supersaturated solution has more than the saturation

limitadding a seed crystal will initiate the crystallization of this

supersaturated solution

http://video.google.com/videoplay?docid=-8399372020786468866&q=supersaturated+solution&hl=en

Solutions & Solubility

• The solubility of the solute in the solvent depends on the temperatureHigher Temp = Larger solubility of solid in liquidLower Temp =Larger solubility of gas in liquid

• The solubility of gases depends on the pressure of the gas above the solutionHigher pressure = Larger solubilityHenry’s Law: S1

P1

= S2

P2

Ex: A gas has a solubility of 0.77 g/L at 3.5 atm. What is the solubility at 1.00 atm.?

As the temperature increases, the number of grams of carbon dioxide that can dissolve in

water decreases.



Mentos + Soda

• Although the candies seem smooth, they have microscopic rough spots.

• The carbon dioxide molecules in the soda attach to these rough spots or nucleation points on the surface of the candies.

• The carbon dioxide builds up into bubbles and the increased pressure pushes the gas and the soda out of the bottle.

Describing Solutions Quantitatively

• Solutions have variable composition

• To describe a solution accurately, you need to describe the components and their relative amounts

• Concentration = amount of solute in a given amount of solutionOccasionally amount of solvent

Solution ConcentrationMolarity (M)

• moles of solute per 1 liter of solution

• used because it describes how many particles of solute in each liter of solution

• If a sugar solution concentration is 2.0 M , 1 liter of solution contains 2.0 moles of sugar, 2 liters = 4.0 moles sugar, 0.5 liters = 1.0 mole sugar, etc.

molarity = moles of solute

liters of solution

Examples - Molarity

1. An aqueous solution has a volume of 2.0 L and contains 36.0 g of glucose (C6H12O6). What is its molarity?

2. If you want to make 250. mL of a .500 M solution of copper (II) chloride in water, how many grams of solute will you need?

Dilution

• Dilution is adding solvent to decrease the concentration of a solution

• The amount of solute stays the same, but the concentration decreases

• Dilution Formula

Ms x Vs = Md x Vd

• Concentrations and Volumes can be most units as long as consistent

Examples - Dilution

1. How much stock NaCl (aq), which is 1.00 M, is required to make 100.0 mL of a 0.200 M NaCl(aq)?

2. How would you prepare 500. mL of a 0.100 M solution of MgSO4 from a stock solution of 2.00 M MgSO4?

Molality (m)

• molality (m) = mol solute

kg solvent

• How many grams of KI must be dissolved in 500. g of H2O to produce a 0.600 m solution?

Mole Fraction

• mole fraction (χ) = mol solute

mol solution

• What is the mole fraction of solute and solvent in a 0.150 m solution of KCl in H2O?

Heat of Solution

• Heat can be absorbed or released with ionic substances are dissolved in water. Sodium hydroxide in water becomes hot

(exothermic). • Calcium Chloride is found in hot packs.

Ammonium nitrate in water become cold (endothermic).

• Found in cold packs. Vinegar and Baking Soda???

Heat of Solution: Water + Alcohol

• * When alcohol dissolves in water, water molecules “bond” to alcohol molecules and separate them from other alcohol molecules.

• The energy released when alcohol and water molecules attract and bond is greater than the energy required to separate alcohol molecules from each other.

• Since the energy released is greater than the energy absorbed, the dissolving of alcohol in water is exothermic

Heat of Solution: Sodium Acetate

• The liquid in the hand warmer is a very concentrated solution of a salt called sodium acetate.

• The sodium and acetate ions are ready to bond with each other and with water molecules to form a crystal.

• Then these ions and molecules attract and bond to each other, energy is released.

Vapor Pressure Depression

• Vapor pressure is caused by solvent evaporation• Adding a non-volatile solute ALWAYS lowers the

vapor pressuremore solute = less solvent at surface = ↓ evaporation =

↓ vapor = ↓ vapor pressure

• Amount of solute is the only thing that alters this property – colligative property 1 mol glucose 1 mol glucose1 mol NaCl 1 mol Na+ and 1 mol Cl- b/c of

dissociation

• van’t hoff factor would be 2 (count of # of “disturbing” particles in solvent)

Boiling Point Elevation

• For something to boil the vapor pressure = atmospheric pressure.

• Adding solute = ↓ vapor pressure = ↑ boiling point

• Example – adding salt to water before cooking spaghetti noodles causes that water to boil at a hotter temperature, which leads to the noodles cooking faster

Freezing Point Depression

• During freezing, the particles of a solid take on an orderly pattern.

• Adding solute disrupts this pattern, so even more energy must be removed for the solution to solidify.

• Examples – applying salt to icy roads helps prevent the water from freezing and the use of antifreeze in vehicles.

BPE and FPD Calculations(boiling point elevation & freezing point depression)

• BPE/FPD = k x m x i• k = a different constant for each solvent

= positive for BP (.512 kg0C/mol for H2O)

= negative for FP (-1.86 kg0C/mol for H2O)

• m = molality• i = van’t hoff factor

(only matters for ionic compounds, for molecules i always = 1)

Examples – Colligative Calculations

• What is the boiling point of a solution that contains 1.25 mol of CaCl2 in 1400. mL of H2O?

• What is the freezing point of 72.3 g of magnesium sulfate in 1350 mL of H2O

• The freezing point for H2O is lowered to

-0.3900C when 3.90 g of a non-volatile molecular solid is dissolved in 475 g H2O. Calculate the molar mass of the substance.

Osmotic Pressure Elevation

• Osmosis is the diffusion of a solvent across a semi-permeable membrane.

• Osmotic pressure is the amount of pressure needed to stop osmosis.

• Adding more solute = ↑ flow of solvent = ↑ osmotic pressure

Homogeneous Aqueous Systems• Two types:

1. Suspension- a mixture from which particles settle out- suspension differs from a solution because the

particles of a suspension are much larger & do not stay suspended (> 1000 nm)

- Particles are too large to pass through filter paper- Ex. – muddy water, gravy

2. Colloid- a mixture containing intermediate-sized particles

larger than those in solutions but smaller than those in a suspension (1-1000 nm)

- Particles are too large to pass through SP membrane.- Ex. – glue, paint, smoke, milk, fog

Tyndall Effect

• The scattering of visible light by colloidal particles is called the Tyndall effect.

• Good test to distinguish between a solution and a colloid.

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