WATER & COLLIGATIVE PROPERTIES IN SOLUTION

The Importance of Water

¨  Water is a polar molecule. ¤ Polarity creates many dipoles throughout the sample of

water.

¨  Because of this, it is a great solvent for various substances. ¤ This explains why water is known as the “universal

solvent”.

Hydrogen Bonds

¨  Intermolecular dipole interactions create a network of hydrogen bonds (H-bonds) ¤ Affect several properties

of water n Examples: Low vapor

pressure & high surface tension

Surface Tension

¨  Makes it possible for water striders to “walk” on water.

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Vapor Pressure

¨  Hydrogen bonding between water molecules also explains water’s unusually low vapor pressure. ¤ Because hydrogen bonds hold water molecules to one

another, the tendency of these molecules to escape is low, and evaporation is slow.

¨  This relates to water’s high boiling point.

Ice vs. Water

¨  The structure of ice is a regular open framework of water molecules arranged like a honeycomb

¨  When ice melts, the framework collapses, and the water molecules pack close together, making the liquid water more dense than ice.

Colligative Properties

¨  Properties of a solvent that are changed when a solute is added.

¨  The type of solute you add does not matter but how many particles you add does. ¤  For example: it only matters how many ions you add, not which

ones.

¨  The type of solvent matters ¤  Each solvent is different and is affected by different amounts of

solutes. ¤  Example: adding 10g of NaCl may affect water differently than

adding 10g of NaCl to ethanol.

What do Colligative Properties Affect?

¨  There are three major effects of colligative properties: 1.  Vapor Pressure lowering 2.  Boiling point elevation 3.  Freezing point depression

¨  The magnitude of each of these all depend on how many particles have dissolved, not upon their identity

¨  Magnitude of effect is based partially on the molality of the solution

¤  When it is molecular NOT ionic

What makes these things happen?

¨  In each process, energy is expended to either bring molecules together (freezing) or pull them apart (boiling).

¨  If a substance is present between (i.e. dissolved) the

solvent molecules, the solvent has to use more energy to complete the processes.

¨  This forces the solvent to deviate from its normal

freezing and boiling points.

Expression of Number of Particles

¨  To express the number of particles as a colligative property, chemists use molality.

¨  Molality (m) is the number of moles of solute dissolved in

1kg of solvent. ¨  Also known as “molal concentration”

Molality Example #1

¨  How many grams of sodium bromide, NaBr, must be dissolved in 400g of water to produce a 0.500 molal solution?

Freezing Point Depression

¨  Freezing point depression: ¤  Adding a solute to a liquid will

cause freezing point of the liquid to drop by a predictable amount

¤  Electrolytes (acids/bases and ionic

compounds) will have a greater effect based on the total number of ions placed into solution

¤  The constant, Kf, is the molal

freezing point depression constant n  Equal to the change in freezing

point for a 1-molal solution of a nonvolatile molecule solute

Freezing Point Depression

Freezing Point Depression Example

¨  If you use 3.68mol of sucrose (C12H22O12) and dissolve this into 2.50kg of water, what will be the change in the freezing point of your solution? Assume the Kf of water is -1.86oC/m.

Boiling Point Elevation

¨  Boiling point elevation: ¤  Adding a solute to a liquid will cause

the boiling point of that liquid to rise by a predictable amount.

¤  Electrolytes (acids/bases and ionic compounds) will have a greater effect based on the total number of ions placed into solution.

¤  The constant, Kb, is the molal boiling

point elevation constant n  equal to the change in boiling point for

a 1-molal solution of a nonvolatile molecular solute.

Boiling Point Elevation

Boiling Point Elevation Example

¨  If you use 5.76mol of sodium fluoride (NaF) and dissolve this into 3.62kg of water, what will be the change in the boiling point of your solution? Assume the Kb of water is 0.51oC/m.