Nonpolar substances do not noticeably dissolve in water
A thin stream of liquid did not bend towards a charged rod
Polar substances dissolve in water
A thin stream of water net towards a charged rod. Other polar liquids bent slightly towards charged rod.
Polar solutes dissolve in polar water. Dipole-dipole attractions form between the polar molecules. The covalent bonds between atoms remain in tact while the attractions between molecules are broken and formed. The ethylene glycol molecules move between the water molecules due to the attractions these molecules.
Nonpolar molecules are not attracted to polar water molecules. Like dissolves Like. Nonpolar solutes dissolve in nonpolar solvents. Since the induced dipole attractions between the molecules is relatively weak, random mixing is responsible for the dissolving process.
The nonpolar iodine dissolved to a small extent in the water. Weak induced dipole-dipole attractions form between the molecules. Upon shaking the iodine leaves the water and becomes dissolved in the nonpolar carbon tetrachloride. Weak induced dipole-induced dipole attractions form between the molecules.
Predicting the polarity of a bond Use electronegativity
Between nonpolar molecules– weak induced dipole-induced dipole attractions act between the molecules. There are no oppositely charged ends, so temporary dipoles may be induced.
Between polar molecules– dipole-dipole attractions act between the molecules. Oppositely charged ends or dipoles are attracted.
Dipole-dipole attractions form between oppositely charged ends of different polar molecules polar molecule
Dipole-dipole attractions form between polar water molecules and polar alcohol molecules. The molecules have moved between each other. The individual molecules remain intact. METHANOL DISSOLVES IN WATER. Notice: The covalent bonds between the atoms remain intact the molecules have simply moved between each other.
Dipole-dipole attractions between ethanoic acid molecules. Also known as glacial acetic acid.
Induced dipole – Induced dipole Attractions Temporary shifts in electron density lead to temporary dipoles
Packing peanuts will dissolve in acetone Nonpolar Nonpolar Due to random motion and and mixing (entropy driven- increasing disorder) the molecules mix between each other. Since there is little attraction between any of the molecules, they simple mix Weak induced dipole – induced dipole attractions
Styrofoam in acetone Styrofoam in water Is styrofoam polar or nonpolar?
Boiling occurs when the vapor pressure (collisions of the molecules that have escaped the liquid) equals the external or outside pressure
On the previous graph
Find the “normal” boiling point of ethanol and heptane
Find the boiling point of three liquids at 500 mm Hg
Which liquid has greater attractions between the molecules? If the outside pressure is 200 mmHg what is the BP temp? If the outside pressure is 400 mmHg and the temperature is 10 C, will CS 2 be a liquid or gas?
Notice the difference in the axis- pressure compared temperature This is external pressure to show the state given different combinations of pressure and temperature S ℓ ℓ g S g
Energy Change Associated with Dissolving Ionic bond break Ion-dipole attractions form Net change +2 kJ/mol Note: the net change in energy +9 kJ/mol Energy absorbed to break ionic bonds Energy released as ion-dipole attractions form
Energy must be transferred out of the system to the surroundings increasing the kinetic energy (must have been PE before the reaction)
Is the reaction endothermic or exothermic?
An exothermic reaction
What bonds must be broken?
Energy must be transferred in to break ionic bonds
What attractions are formed?
Energy is transferred out as the ion-dipole attractions break
Is more energy transferred in or out?
Since the net energy change is negative, more energy was transferred out than in for an overall net transfer out
Calcium chloride dissolves in water Energy in to break ionic bonds Energy out as ion-dipole attractions form Net change in enthalpy Enthalpy CaCl 2 (s) Ca 2+ (aq) + 2Cl - (aq)
Ionic solid with ionic bonds between the ions Water with dipole-dipole attractions between the water molecules Ionic bonds break Ions dissociate Ions move between water molecules Ion-dipole attractions form Solution conducts electricity since charged particles (the ions) are free to move
Visualize the solution with ions and water molecules in motion
Ion-dipole attraction is the attraction between charges therefore an electrical force
Electrical force depends upon both
(increase charge – increase force)
(decrease distance- increase force)
With a greater positive charge, the Mg 2+ ion is more attracted to the water molecule than Li +1 . Remember: electrical force depends on magnitude of charge- greater charge greater electrical force With a greater size, the K +1 ion cannot get as close to the water molecule as the smaller Li +1 ion. With a lesser distance, the electrical force is greater Remember: electrical force depends on distance- greater distance lesser electrical force