Water and Aqueous Solutions

1. Water and Aqueous Solutions

2. 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

3. 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

4. 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

5. 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.

6. The Water Molecule: • A highly polar molecule that is essential to life • Held together by strong hydrogen bonding • Because 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

7. 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 capacity Since H bonding is strong, it takes more energy to break the bonds apart

9. Specific Heat Capacity • It 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

10. Water Vapor and Ice • Heat 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

11. 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

12. 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

13. 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.

14. Ice • Most 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

15. This is why ice floats when added to water. • This allows the creatures in water to be able to survive during winter

16. 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 fusion 47.6 g x 0.334 kJ/g = 15.9 kJ

17. Aqueous Solutions: 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

18. 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”

19. Electrolytes and Nonelectrolytes: Electrolytes- these conduct electric current in aqueous solution and when melted those that are insoluble only conduct current when melted Examples: NaCl, CuSO4 , NaOH

20. 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

21. 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 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

22. 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

23. 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 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

24. 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