1. 8.2 Factors That Affect Rate of Dissolving and Solubility

2. Factors that Affect the Rate of Dissolving
Rate of Dissolving – how quickly a solute dissolves in a solvent.
Depends on :
Temperature
Agitation
Particle Size

3. Temperature For most solid solutes the rate of
dissolving is greater at higher temperatures.
WHY?
At higher temperatures the solvent molecules have greater kinetic energy.
This causes them to collide with the undissolved solid molecules more frequently.
This increases their rate of dissolving.

4. Agitation
Agitating a mixture by stirring or by shaking the container increases the rate of dissolving.
Why?
This brings fresh solvent into contact with undissolved solid.

5. Decreasing Particle Size
A large lump of solid sugar dissolves slower than and equal mass of powdered sugar.
WHY?
When you break up the large mass into many small masses, you increase the surface area that is in contact with the solvent.
This allows the solid to dissolve faster, increases solubility.

6. 3 Steps for Dissolving
The Process of Dissolving at the Molecular Level 1. The forces between the particles in the solid must be broken. This step always requires energy. In an ionic solid, the forces that are holding the ions together must be broken (intramolecular forces). In a molecular solid, the forces between the molecules must be broken (intermolecular forces).

7. Continued…
2. Some of the intermolecular forces between the particles in the liquid must be broken. This step also requires energy. 3. There is an attraction between the particles of the solid and the particles of the liquid. This step always gives off energy.

8. Polar and Non-Polar Substances
Things to remember from last class…
Non-polar molecules can contain either:
non-polar bonds (from the ∆EN)
Polar bonds but the dipoles cancel out. Example CO2

9. Polar and Non-Polar Substances
Polar molecules can only be non-polar if:
If some of the polar bonds result in a net dipole (dipole ≠ zero)
b) This usually happens when the molecule is
unsymmetrical or

10. “Like Dissolves Like” NO! Polar compounds Polar Solvents
Non-Polar Compounds
Non-Polar Solvents
NO!

11. A. Solubility and Intermolecular Forces
Dipole-Dipole Attractions –
Occurs between the opposite charges on 2 different polar molecules.
This force is only 1% as strong as Ionic or covalent bonding.

12. Solubility and Intermolecular Forces
The Water Molecule…

13. B. Solubility and Intermolecular Forces
Hydrogen Bonding- Occurs between an H and a highly electronegative atom.
Examples H + N, O, F, Cl

14. Ion-Dipole Interactions

15. C. Solubility and Intermolecular Forces
Ion-Dipole Interactions-
Are attractive forces between ions and polar molecules. Crystal molecules are made up of cations and anions in a repeated fashion.
How does an Ionic Compound Dissolve?
The negative end of the water molecule attracts the + ions and the positive end of water attracts the – ions. Thus the crystal breaks down (dissolves).

16. Molecular Size and Solubility
Small molecules are often more soluble than larger molecules.
The more carbon atoms added, the less soluble it will be.

17. Homework