The
rate of dissolution of a solute in a solvent is dependent on three factors:
- temperature – at higher temperatures, the solvent molecules have a higher kinetic energy and will collide with the solute particles more often (rate ↑)
- agitation – stirring or shaking a solution brings more solvent particles into contact with the solute particles (rate ↑)
- surface area – crushing or breaking up the solute will increase the area of interface between solute and solvent (rate ↑)
Solubility & Particle Attractions
Solute
particles are held together by forces of attraction - there must be an energy
input to break these forces of attraction.
Solvent
particles are held together by forces of attraction - there must be an energy
input to break these forces of attraction.
When the solute and solvent are mixed to create a solution, there is the force of attraction between solute particles and solvent particles – this usually
results in an energy output.
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| A substance is more likely to dissolve if the energy output is greater than the energy input (i.e., an exothermic dissolution). |
Polar & Non-polar Substances
Polar
solutes dissolve in polar solvents & non- polar solutes dissolve in
non-polar solvents.
- This is why alcohol dissolves in water - they are both polar.
- This is why oil dissolves in gasoline - they are both non-polar.
- This is why oil and water don't mix - one is non-polar and the other is polar.
Just remember “like
dissolves like.”
Dipole-Dipole Attractions
A dipole-dipole attraction or dipole-dipole force operates between two neutral, polar molecules.
Recall that polar molecules have a ẟ+ end and a ẟ- end. When two polar molecules are in close proximity to each
other, the ẟ+ end of one molecule is attracted to the ẟ- end of its neighbour. This attraction is called a dipole-dipole force.
ẟ+ H-Cl ẟ-⎹⎹⎹⎹⎹ ẟ+ H-Cl ẟ-
Hydrogen bonding is a special form of dipole-dipole force. Although it operates in the same way as a dipole-dipole force, the molecule must contain a H atom bonded directly to a F, O or N atom. Water, as seen below, can undergo hydrogen bonding because it has H bonded to O. The atoms directly involved in the H-bond can be designated as the "hydrogen bond acceptor" (this is always a F, O or N from one molecule) or the "hydrogen bond donor" (this is always the H atom from a neighbouring molecule).
Tryit!: Show all the unique hydrogen bonds within a water and ammonia mixture. Label the hydrogen bond acceptor in each case. The answer is at the end of the lesson.
Ion-Dipole Attractions
When
an ionic compound comes in contact with water, the water molecules pull out the
ions and surround them in solution.
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| The positive ions are surrounded by the ẟ- ends of the water molecules (on the left). The negative ions are surrounded by the ẟ+ ends of the water molecules (on the right). This is referred to as a "sphere of hydration." |
Answer for the Tryit!
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| Notes: 1. There is no set amount of dashes used to represent the force of attraction (hydrogen bond). 2. The HBA (hydrogen bond acceptor) does not always have to be on the left, I just happened to draw them all that way. |
Homework: Review Questions, p. 370 # 2, 3, 5, 7, 14, 15
Student Questions:
1. Please explain p. 370 # 5, 15.
