R-AECHE-29

Intermolecular forces quiz:

1. For the substances $CH_3CH_{2}OH, H_{2}O, CH_3CH_{2}CH_{3}$ and $CH_{3}COCH_{3}$, order in decreasing order the strengths of intermolecular forces.

   ==2, 1==, 4 ,3.

2. The molar heat of sublimation (the heat required to convert 1 mole of solid directly to the gas state) of solid neon is 2.16 kJ mol-1 whereas that of ice is 46.9 kJ mol-1. Which of the following statements help to explain these facts?

   1. Only dispersion forces are present between neon atoms
   2. There is strong hydrogen bonding between water molecules in ice
   3. There are strong covalent bonds within water in ice
   4. There are weak covalent bonds between neon atoms.

   a (1 and 2 only)

3. Which of the following is the most polar bonds?

   1. C-N
   2. N-O
   3. O-F
   4. ==P-Cl==

4. Does water or ammonia have a higher boiling point? The molar masses of both substances are similar, so dispersion forces with be roughly equal. However, oxygen has a greater electronegativity than nitrogen, thus the hydrogen bonding between water molecules will be greater than between ammonia molecules, and more energy is required to overcome the stronger intermolecular forces. As energy is proportional to temperature, water will have a higher boiling point.

==Solubility of gasses decreases as temperature increases (inversely proportional)==

Solutions form when the solvent and solute mix. THis process involves breaking the bonds/attraction in these individual substances, which is endothermic, so requires energy.

In the solution, attractions form between the solvent and solute, and this bond formation is exothermic, releasing energy.

Whether the solution forms overall depends on the overall process requires too much energy to be favourable.

We want more energy to be released, then energy to be consumed, for a substance to be dissolved and thus create a solution.

Water - the universal solvent

• As a highly polar molecule, water is great at dissolving polar molecular substances and many ionic substances.

• Water can form a range of intermolecular forces to stabilise an solution:

Ion-dipole forces:

• Ion-dipole forces form between ions of an ionic compound and molecules of water. We say the ions are hydrated.

• If the ion-dipole forces formed between ions and water are strong enough, they can pull ions off the surface of an ionic lattice, and the ionic substance is soluble.
• Not all ionic substances dissolve. In insoluble substances, the ionic bonding is too strong, and the ion-dipole bonds formed are not strong enough to beak up the ionic lattice.

Solubility of ionic substances All ionic substances are soluble to some extent, so we categorise substances by the concentration of a saturated solution at 25 degrees celcius.

• A substance is soluble if a saturated solution is 0.1 mol L-1 or higher.

• A substance is described as slightly soluble if a saturated solution is 0.01 mol L-1 to 0.1 mol L-1

• A substance is described as insoluble if a saturated solution is below 0.01 mol L-1

• In most insoluble substances, the rate of reaction for ions to both be displaced and reform the ionic lattice is equal (without solvent). This leads to an equilibrium. When water is added, the rate of reaction for ions to become displaced increases. However, this increase is marginal. In theory, if you add enough solvent the substance will dissolve.

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