Water and aqueous systems

Water is unique  High polarity Forms extensive hydrogen bonds with other molecules Forms extensive hydrogen bonds with other molecules High surface tension High surface tension Low vapor pressure Low vapor pressure High specific heat High specific heat High heat of vaporization High heat of vaporization High melting point High melting point  All because of hydrogen bonds!

Properties of water  Specific heat of water: J/g°C J/g°C  Evaporization and condensation: ∆H evp = 2.26 kJ/g ∆H evp = 2.26 kJ/g ∆H cond = kJ/g ∆H cond = kJ/g  High boiling point: 100°C Most molecules with a low molecular mass have low boiling points. Most molecules with a low molecular mass have low boiling points. Ex: CH 4 boils at -164°C, and ammonia at -33.3°C Ex: CH 4 boils at -164°C, and ammonia at -33.3°C

Why does ice float?  When most liquids cool they contract and become more dense.  Water does this until about 4°C, then arranges itself like a honey comb.  Ice is less dense than water so it floats.

Aqueous solutions  Anything dissolved in water.  Solvent: what does the dissolving Ex: water, paint thinners, mineral spirits. Ex: water, paint thinners, mineral spirits.  Solute: what gets dissolved Ex: salt, the paint, etc. Ex: salt, the paint, etc.  Solvation: the process that occurs when a solute dissolves in a solution.

How does this work?  Water is polar (positive on one end and negative on the other)  The positive part of the water is attracted to the negative part of the solute and vise versa.  It surrounds the solute and breaks it apart.

Why can’t I dissolve oil in water?  I can dissolve it in gasoline.  “like dissolves like”  Water is polar, it will only dissolve polar molecules and ionic compounds (because of the difference of charges.  Gasoline is non-polar, and so is oil.

Electrolytes  Electrolytes conduct electricity when dissolved in water. All ionic compounds are electrolytes.  Nonelectrolytes do not.

Water of hydration  Water of hydration: Water is an important part of many different crystals. This is the water in the crystal.  Hydrate: any compound that contains water.  Effloresce: when a hydrate has a higher vapor pressure, and loses it’s water

Water of hydration (cont)  Hydroscopic: remove moisture from the air.  Desiccants: hydroscopic substances used to keep things dry. (the little packet that says “do not eat”)  Deliquescent: remove enough water from the air to dissolve completely

Calculating the formula of a hydrate  I have a g sample of hydrated barium iodide (BaI 2 ). The sample is heated to dry off the water. The dry sample has a mass of g. What is the mole ration between barium iodide and water (H 2 O)? What’s the formula of the hydrate?

Solve:  Find the difference of the two masses, so you can find the mass of the water. mass of hydrate: g mass of dry sample: g mass of water = g

Convert to moles 9.520g BaI 2 1 mol BaI2 391 g BaI2 = mol BaI 2 = mol H 2 O = mol H 2 O g H 2 O 1 mol H 2 O 18.0 g H 2 O

Mole ratio Divide each number by the smallest one / = / = 2.03 about 1BaI 2 : 2 H 2 O about 1BaI 2 : 2 H 2 O Because there are 2 mol of water for every one mole of Barium Iodide, we name it “Barium Iodide dihydrate”

Heterogenous systems  Suspensions: mixtures from which particle settle out upon standing  Colloids: Particles are smaller than a suspension, but larger than a solution. These cannot be filtered out and will not settle out.  Tyndall effect: Scattering of visible light in all directions

More!  Brownian Motion: Chaotic movement of colloidal particles, it keeps them from settling.  Emulsions: colloidal dispersions of liquids on liquids. There must be an emulsifying agent to form the mixture and keep it stable.

Solubility  Saturated solution: contains the maximum amount of solute for a given amount of solvent at a constant temperature. You can’t add any more sugar to your kool- aid, because it won’t dissolve. You can’t add any more sugar to your kool- aid, because it won’t dissolve.  Unsaturated solution: Contains less solute than a saturated solution. You could still add more sugar to your kool- aid if you wanted to. You could still add more sugar to your kool- aid if you wanted to.

 Solubility: the amount of substance that will dissolve in a given quantity of solvent at a given temperature to produce a saturated solution. How much sugar can you possibly put in your kool-aid? How much sugar can you possibly put in your kool-aid?

 Miscible: two liquids that can dissolve in each other. Ex: oil & gas Ex: oil & gas  Immiscible: two liquids that are insoluble with each other (won’t dissolve) Ex: oil & water Ex: oil & water