Water and Aqueous Solutions. Intermolecular Forces. These are the attractions between molecules not within the molecule These forces dictate what state the molecular compound is at a given temperature These forces are weaker than ionic and covalent bonds. Types of Forces:.
molecules for each other
1. Dispersion Force- the weakest of the forces. This is caused by the motion of the electron clouds of atoms as they pass each other
2. Dipole-dipole forces- a little stronger than dispersion forces, this is the attraction between polar molecules.
What are polar molecules? These are molecules that have polar bonds that do not cancel out each other. They usually have lone pairs on the molecule.
Polar bonds- the unequal sharing of electrons between elements
3. Hydrogen bonding: This is the strongest intermolecular force of the three.
A very strong attraction between H of one molecule and either F, O, or N of another molecule.
Since H bonding is strong, it takes more energy to break the bonds apart
How much energy is needed to change 86.0 g of water at 100o C to 86.0 g of steam at 100o C?
Equation: mass x heat of vaporization
86.0 g x 2.26 kJ/g
= 194 kJ
Likewise, when water condenses from steam at 100o C to liquid at 100o C,
2.26 kJ is given off.
Molecules of nearly the same molecular mass as water have lower boiling points and melting points because they do not have the hydrogen bonding that water has.
The heat that is absorbed when 1 g of water changes from a solid to a liquid is 334 J/g. This is the amount of energy needed to break the bonds that hold ice together
How much energy in kJ is needed to change 47.6 g of ice at 0o C to liquid water at the same temperature?
mass x heat of fusion
47.6 g x 0.334 kJ/g
= 15.9 kJ
Solutions- samples containing dissolved substances. Aqueous solutions- water is the solvent
Solvent- what the substance is dissolved in
Solute-the dissolved particles.
these can be ionic or molecular
Solutions are also called homogeneous mixtures
Ionic compounds and polar covalent molecules dissolve easily in water
Nonpolar covalent molecules do not dissolve in water
the negative and positive ions are separated by the solvent molecules
sometimes the ionic bonding is so strong that the solute cannot come between them, and the substance is insoluble
Oil and Gasoline are nonpolar, so they cannot be dissolved in water-
“Like dissolves Like”
Electrolytes- these conduct electric current in aqueous solution and when melted
those that are insoluble only conduct current when melted
Examples: NaCl, CuSO4 , NaOH
Weak electrolytes- only a portion of the solute exist as ions
Strong electrolytes- almost all the solute exists as ions
Nonelectrolytes- do not conduct current in aqueous solution or when melted
Examples: cane sugar, rubbing alcohol
Some compounds contain water within their crystal structure- the water within is called the water of hydration.
The compound is called a hydrate
The water can be lost if the compound is heated above 100o C- WHY?
A hydrate can effloresce if its vapor pressure is higher than that of water, the hydrate loses the water of hydration
These are used as desiccants or drying agents
silica gels are put in purses or shoes
Deliquescent compounds can remove enough water from the air so that they become solutions
Naming- You name the compound and then add the hydrate with a prefix to tell how many waters are in the crystal
Ex: CuSO4• 5H2O:
Copper (II) sulfate pentahydrate
Suspensions- mixtures where the particles can settle out
-clay in water
Colloids- particles are smaller than those in suspensions, but larger than those in solution
Many examples- whipped cream, milk, fog, dust in air
Tyndall Effect- when you shine a light through a colloid and the light scatters- don’t use high beams in the fog
Brownian Motion- the random chaotic motion of the particles in a colloid caused by the collision of water molecules with the particles
Emulsion- a colloidal dispersion of a liquid in a liquid. An emulsifying agent must be used to allow the liquids to mix.
Example- oil and water don’t mix, but when you add soap, the emulsion occurs