SCH3USolubility Mid-Unit Review

Knowledge and Understanding

  1. Name three ways to increase the rate of dissolving.

Increase temperature, increase surface area, agitation.

  1. Name the 3 factors that affect the solubility of a substance. For each factor, describe how it affects solubility.

Temperature / Increasing temperature increases kinetic energy of particles; this increases the solubility of solids and decreases solubility of gases.
Pressure / Increased pressure above a gas increases the solubility of that gas
Particle Size / Smaller particles dissolve easier than larger particles, even non-polar substances that are small can dissolve somewhat in polar solvents
  1. A molecule is found to have a solubility of 3 g/100ml (25oC) is it classified as soluble, sparingly soluble or insoluble? Soluble ( >1.0 g/ 100 mL)
  1. Describe, in your own words, the steps to dissolving. Include a diagram for each step.
  1. Energy is used to break the attractive forces between the solute molecules. (ex. Ionic bonds)
  2. Energy is used to break the attractive forces between the solvent particles (ex. Hydrogen bonds between water molecules)
  3. New attractive forces form between the individual solute particles and the solvent particles. This process releases energy.
  1. Why does increasing the temperature have a greater effect in increasing the solubility of solids compared to liquids?

Since the bonds between particles in a liquid are already weaker than the bond between particles in a solid, increasing temperature does not generally affect the solubility of most liquids.

  1. What is it about the structure of water that makes it such a good polar solvent?

Due to its bent shape and polar covalent bonds, water is a polar molecule. Since it is a liquid a room temperature, this polar molecule forms intermolecular forces with itself – hydrogen bonding. Water’s shape and polarity make it a good solvent.

Application

  1. How could global warming affect oxygen sensitive species such as salmon and trout?

Since increasing temperature decreases the solubility of gases, global warming could result in less dissolved oxygen, which would be detrimental to oxygen sensitive species such as salmon and trout.

  1. Instant heat and cold packs work by mixing different solutes and solvents together. Why is one reaction exothermic and the other endothermic?

The solution process can be endothermic or exothermic.

If the energy needed to break the attractive forces of the solute and solvent is greater than the energy released when the solution forces are made then the process is endothermic (more energy absorbed – i.e. cold packs).

If the energy needed to break the attractive forces of the solute and solventis less than the energy released when the solution forces are made then the process is exothermic (more energy released – i.e. instant heat).

  1. What is about the structure of soap molecules that allows makes them such good cleaning agents?

A soap molecule has a nonpolar end and a polar end. The nonpolar end allows it to mix with nonpolar substances that are hard to rinse with just water – like grease and oil. The polar end allows the soap to interact with polar solvents – like water. The soap allows the nonpolar “grime” to be dispersed in the water, and therefore able to be rinsed away.

  1. The solubility of MgF2 is 87mg/L and the solubility of NaF is 40g/L explain the difference in these values.

The solubility of MgF2 is much lower than the solubility of NaF, since 87 mg/L x 1 g/ 1000mg is 0.087g/L (compared to 40 g/L).

If you put both values in terms of g / 100 mL you can compare them to the solubility continuum.

0.87 g / L x 0.1 L / 100mL = 0.087 g/ 100mL *Note don’t divide by 100mL , just do the multiplication by 0.1 to get rid of litres and leave the 100 in the bottom to get the units / 100mL.

40 g/ L x 0.1 L / 100mL = 4 g / 100mL

Therefore MgF2 is insoluble (< 0.1 g/ 100mL ) and NaF is soluble (> 1.0 g/ 100mL).

Complete p.301 # 1, 2, 4, 6abcd

#1. When the attraction between the partially charged atoms in water are strong enough to pull the oppositely charged ions of sodium and chlorine out of the ionic crystal, the ionic bonds are broken. The attractive forces between water molecules are broken and new attractive forces are made between the partially negative atoms oxygen of water molecules and the positive Na ions, as well as between the partially positive hydrogen atoms of water molecules and the negative Cl ions.

#2 When water vaporizes the intermolecular attractions are broken (forces between molecules). Water vapour is still water molecules, but in vapour form, ie, they are further apart than when a liquid. Therefore the hydrogen bonding that occurs in water as a liquid is not formed in water as a vapour.

#4Hydrated ions can move through a solution and conduct electricity. A salt solution is made by dissolving an ionic compound ( a salt), which therefore forms hydrated ions. Sugar is a covalent compound; it does not dissolve as ions but as individual sugar molecules (no charge).

#6 in text page 327