Water and Aqueous Systems

Water and Aqueous Systems Part I

Water Molecule • Triatomic (3 atoms) • Bent Shape (104.45°) • Polar Molecule δ- δ+ δ+

δ- δ+ δ+ δ+ δ+ δ+

Important Water Propertiesresulting from hydrogen bonding • high surface tension • low vapor pressure

Surface Properties • a glass so filled with water that the surface isn’t flat but bulges above the rim • Meniscus • water forms nearly spherical droplets

The surface of water acts like a skin, as the water strider demonstrates. • The molecules within the liquid form hydrogen bonds with other molecules that surround them on all sides.

surface tension • The inward force, or pull, that tends to minimize the surface area of a liquid

surfactant • is any substance that interferes with the hydrogen bonding between water molecules and thereby reduces surface tension

Vapor pressure • of a liquid is the result of molecules escaping from the surface of the liquid and entering the vapor phase. • Water has a relatively low vapor pressure

Higher the vapor pressure, the faster a liquid evaporates

Measurement of energy • Energy in various forms may be converted to heat, • heat units (calories or Joules) are used to measure the energies involved in chemical reactions.

Joule – DON’T copy ‘til I explain • the amount of heat required to raise the temperature of one gram of water one Kelvin is 4.18 Joules. • 4.18 Joules = 1 calorie • 1 kilocalorie is equivalent to 1000 calories • 1 food Calorie = 1 kilocalorie = 1000 calories • Q = m c ΔT • where Q = heat measured in calories or Joules • where m = mass measured in grams • where c = Specific Heat measured in Joules/g0Cor Joules/gK(Regents have changed over to all Joules and Kelvin degrees). • where ΔT = change in temperature

Water and Aqueous Systems Part II

Specific Heat • Is the amount of heat required to change the temperature of a 1 gram sample 1 K (°Celsius). • is the amount of heat a sample can hold. • The Specific Heat for water is: • 4.18 Joules / gram K • See reference table B

Evaporation vs. Vaporization • Evaporation is the process that describes a change from a liquid phase to a gaseous phase below the substance’s boiling point. • Vaporization is the process of liquid to gas at the boiling point. • These changes are Endothermic (requires energy in)

Condensationis the process that describes a change from a gaseous phase to a liquid phase. • This change is Exothermic (releases energy out)

Heat during a phase change • During a phase change temperature remains the same • Heat of vaporization (use at liquid-gas phase change) • Heat of Vaporization for water is 2260 Joules / gram • See reference table B • Heat of fusion (use at solid-liquid phase change) • Heat of Fusion for water is 334 Joules / gram • See reference table B

Solvents and Solutes • Solute - dissolved particles in a solution • Solvent - the dissolving medium in a solution • aqueous solution- water that contains dissolved substances

When a salt is added to water it dissociates and forms aqueous ions in solution. • Solvation- a process that occurs when an ionic solute dissolves; in solution, solvent molecules surround the positive and negative ions.

Electrolytes and Non-electrolytes • Electrolyte- conducts an electric current when it is in an aqueous solution or in the molten state. • all ionic compounds are electrolytes, but most covalent compounds are not • Non-electrolyte- does not conduct an electric current in aqueous solution or in the molten state (i.e. pure water)

Suspensions • a mixture where some of the particles settle out slowly upon standing. • differs from a solution because the particles of a suspension are much larger and do not stay suspended indefinitely. • are heterogeneous because at least two substances can be identified. • Ex. Mud water

Colloids • a heterogeneous mixture whose particles are intermediate in size (particles that range in size from 1 nm to 1000 nm) between those of a suspension and a solute solution. • The particles are spread throughout the medium, which can be a solid, liquid, or gas. • The first substances to be identified as colloids were glues. • Other colloids include such mixtures as gelatin, paint, aerosol spray, ice cream

Emulsions • a colloidal dispersion of a liquid in a liquid. • An emulsifying agent forms an emulsion and maintains its stability. • e.g.oilsand greases are not soluble in water. • form a colloidal dispersion if soap or detergent is added to the water. • Soaps and detergents are emulsifying agents. • Mayonnaise is a heterogeneous mixture of oil and vinegar. • quickly separate without the presence of egg, which is the emulsifying agent.

Soaps and Detergents • One end of a large soap or detergent molecule is polar and is attracted to water molecules. “hydrophilic” • The other end of the soap or detergent molecule is non-polar and is soluble in oil or grease. “hydrophobic” • Soaps and other emulsifying agents thus allow the formation of colloidal dispersions between liquids that do not ordinarily mix.

Water and Aqueous Systems Part III

Water and Aqueous Systems