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Water and Aqueous Solutions
Intermolecular ForcesThese 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:Van der Waals- the weak
attraction of 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.
The Water Molecule:A highly polar molecule that is
essential to lifeHeld together by strong hydrogen
bondingBecause of the hydrogen bonding,
water displays capillarity – the movement of water up a tube against gravity, and has surface tension- the inward pull of the water molecules so that the surface area of the liquid is minimized
Hydrogen bonding also allows water to:
- Have a low vapor pressure- the pressure inside a closed container caused by the evaporation and the condensation of water
- Have a high boiling point- Have a high heat of vaporization- Have high specific heat capacitySince H bonding is strong, it takes more energy to break the bonds apart
Specific Heat CapacityIt takes 4.18 Joules (1 cal) to
raise the temperature of 1 g of water 1o C
Water is a good medium for solar energy
The air temperature around a large area of water remains fairly constant due to the ability of water to absorb and give off the large amount of energy
Water Vapor and IceHeat of vaporization- the amount
of energy needed to convert 1 g of a substance from a liquid to a gas at the boiling point
Due to hydrogen bonding, it takes 2.26 kJ to convert 1 g of liquid water at 100o C to 1 g steam at 100o C. The bonds hold the molecules together, so it takes a lot of energy to break those bonds
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. This shows that the heat of
vaporization of water is equal to the heat of condensation of water
You can get a worse burn from steam than from water at the same temperature because of the energy absorbed by the water to make the steam
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.
IceMost liquids, when they become solids,
condense, so the volume decreases, thereby increasing its density
This does not happen with water As water cools, the density does increase until it reaches 4o C. At this temperature, there is a change in the structure of the molecules where they form an open structure that has a larger volume, decreasing the density
This is why ice floats when added to water.
This allows the creatures in water to be able to survive during winter
Heat of Fusion-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 fusion47.6 g x 0.334 kJ/g= 15.9 kJ
Aqueous Solutions:Solutions- samples containing dissolved
substances. Aqueous solutions- water is the solvent
Solvent- what the substance is dissolved inSolute-the dissolved particles.
these can be ionic or molecularSolutions are also called homogeneous
mixtures Ionic compounds and polar covalent
molecules dissolve easily in waterNonpolar covalent molecules do not dissolve
in water
During Solvation- (the process of dissolving a substance)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 and Nonelectrolytes:Electrolytes- these conduct
electric current in aqueous solution and when meltedthose that are insoluble only conduct current when melted
Examples: NaCl, CuSO4 , NaOH
Electrolytes can be weak or strong-
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
Water of Hydration:
Some compounds contain water within their crystal structure- the water within is called the water of hydration.
The compound is called a hydrateThe 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
Hygroscopic- compounds that can remove moisture from the air.
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
Heterogeneous Aqueous Systems:Suspensions- mixtures where the
particles can settle out -clay in water
Colloids- particles are smaller than those in suspensions, but larger than those in solutionMany 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
