Unit 8 – Solutions – Properties of Water as Universal Solvent Chapter 17

Properties of Water The Water Molecule Simple triatomic molecule Polarity Each O-H bond is highly polar O: δ- H: δ+ (partial charges) Region around oxygen has a slight negative charge; region around the hydrogens has a slight positive charge Refresh memory with the symbols of polarity

Properties of Water Hydrogen bonding occurs because of polarity Hydrogen bonding causes: High surface tension Low vapor pressure High specific heat capacity High heat of vaporization High boiling point Strongest intermolecular force was hydrogen bond- review other forces Heat capacity reflects on how fast molecules move. Heat of vaporization- in order to evaporate you have to break the hydrogen bond- huge amount to break the bonds for vaporization temperature

Properties of Water Surface Properties Surface tension Explained by water’s ability to form hydrogen bond Water molecules experience uneven attractions at the surface, pulling inward and minimizing surface area Holds a drop of liquid in a spherical shape while being flattened by gravity Pull toward the center- that is why a water droplet is sphereical Why insects with long legs can walk on top of water If u just use water to wash dishes it will take a lot of water to clean a dish, because they want to form together- we have to loosen the pressure between the surface tension- we can do this by using…

Properties of Water Surfactant – short for Surface Active Agent Decreases the surface tension of water Used to interfere with bonding between hydrogen molecules, causing beads of water to collapse Example: detergent in water Surfactant= detergent Makes the molecules move more freely- and can do the job better and scrub the “grime” off (also used to wash clothes)

Using Surfactants to Clean Oil Spill Effects To remove oil from birds , it needs to be suspended in water, so it can be rinsed away. That's not an easy task, since oil and water are notoriously hard to mix. In order to achieve it, Dawn detergent uses a group of chemicals known as "surfactants”. Surfactants reduce the surface tension of water and can weaken the barrier that automatically forms between oil and water, allowing them to unnaturally mix. Dawn specifically uses anionic surfactants -- such as alkyl dimethyl amine oxide – it does not convert molecules of water into ions.

Properties of Water Low vapor pressure Specific Heat Capacity Strong hydrogen bonds prevent water from escaping easily Prevents bodies of water from drying up Specific Heat Capacity C = 4.184 J/(g×°C) Caused by hydrogen bonding Heat released by water during winter; heat absorbed by water in summer

Properties of Water Evaporation Condensation Vaporization that occurs at the surface of a liquid that is not boiling Occurs as the result of absorption of heat of vaporization Involves breaking intermolecular hydrogen bonds between water molecules Explains high heat of vaporization and high boiling point Condensation Opposite of evaporation Occurs as the result of releasing of heat of vaporization Involves forming intermolecular hydrogen bonds between water molecules Think of this processes from the point of view of bonding

Properties of Water Ice Usually liquids contract as cooling occurs. Ice is very unique and different from other substances. For most of the substances - Density will increase as temperature decreases because of less volume FOR ICE: Below 4°C, density begins to decrease because water begins to no longer behave as a liquid. Insulates heat under the ice Less dense than water, allowing it to float in water Prevents freezing of bodies of water Water does the opposite than most substances- when you freeze it- it expands Important for marine life

Properties of Water When it freezs the hydrogen bond becomes stronger- forms a hexagonal structure which takes more space

Properties of Water When you dissolve something, you create a solution. Solutions are made up of two things, a solute and a solvent. Solute – substance that is being dissolved Solvent – substance that does not dissolves, i.e is what the solute is dissolving into (H2O) When you dissolve you have a solution- this is the outcome of this process

The Solution Process Solvation: process that occurs when a solute is dissolved Insoluble: attractions between the ions in the crystals are stronger than the attractions exerted by water (the solute will NOT DISSOLVE in the solvent)

What is, or is not, soluble in H20? “Like dissolves like” Polar solvents dissolve ionic compounds and polar molecules Water is polar therefore it can dissolve NaCl Copper (II) sulfate NaOH Nonpolar solvents dissolve nonpolar compounds Oil is nonpolar, which is why oil and water separate

Electrolytes and Non-electrolytes 1. Electrolytes: compounds that conduct an electric current in aqueous solution or the molten state 2. Non-electrolytes: compounds that do not conduct an electric current in aqueous solution or the molten state 3. Strong vs. Weak Electrolytes (page 485)

Hydrates Water in a crystal Also called water of crystallization The “Do not eat” packets in some foods such as beef jerky are hydrates They prevent moisture from entering the food Example: CuSO4  5H2O implies 5 moles of water to every copper and sulfate pair Tendency to pull moisture from the air Don’t eat packets- are hydrates- it prevents the electronic device from getting moisture- it pulls the moisture

Naming Hydrates MgSO4  7H2O Ba(OH)2  8H2O CaCl2  2H2O Magnesium sulfate heptahydrate Ba(OH)2  8H2O Barium hydroxide octahydrate CaCl2  2H2O Calcium chloride dihydrate