Liquid Force Flask

Help with Physical Chemistry! A few quick questions. Attention: CHAD?

1. Some biomolecules are big enough that they can have a polar end, and a nonpolar end. Cell membranes are made from phospholipids, which have polar heads & nonpolar tails. Explain why the two-layer arrangement of the phospholipids is vital to the cell structure in a water-based environment.

Zee answered question 2 correctly.

The other questions:

1) Polar molecules stick together and also repel nonpolar molecules. Nonpolar molecules also stick together and repel polar molecules. A good example is oil, which is nonpolar, and water, which is polar. When you try to mix them together they separate again. This is the key to how phopholipids form cell walls. One end of the molecule is polar and those polar ends like to stick together. The other end is nonpolar and those ends also like to stick together. That behavior makes the phospholipids form large sheets where the molecules stack up neatly with all the polar ends together and all the nonpolar ends together. The sheet of lined up molecules with then have a polar surface and a nonpolar surface. If that sheet is in a water based environment the polar surface will be happy to be in contact with water, which is also polar, but the nonpolar surface will try to repel water. The way that happens is that two sheets bond together with the polar faces touching and the nonpolar faces exposed to water. Basically that is what a cell wall is. The two-layer arrangement is naturally stable in a water-based environment making a cell a practical structure.

3) Generally the forces holding liquids and gasses together will be related to charge separation within the molecule creating a dipole and thus electrostatic attractive forces. In cases were the dipole is the result of permanent charge separation for example in water the result is a polar molecule. This type of bond is relatively strong. When it involves hydrogen atoms it is called a hydrogen bond. In molecules with no inherent charge separation because of inherent movement of the electron cloud there are momentary or instantaneous dipoles formed which result in a much weaker bond. This type of force is called a Van Der Waal’s force. All molecules exhibit Van Der Waals forces. Using this information we can make the determinations. For nonpolar molecules Van Der Waal’s forces will be the strongest intermolecular force. For polar molecules that do not contain hydrogen the molecular dipole force will be the strongest force. For polar molecules that contain hydrogen, in cases where the hydrogen is bonded nonsymmetrical to the other atom, hydrogen bonds will be the strongest molecular force. For polar solutions that contain ions ionic bonds between the dissolved ions and the solvent will be the strongest bonds.

Strongest bonds for each substance:
CH3OH – hydrogen bond – polar molecule with hydrogen bonded nonsymmetrical to oxygen creating charge separation.
BF3 – Van Der Waal’s – nonpolar molecule
NH3 – hydrogen bond – polar molecule with hydrogen strongly bonded nonsymmetrical to nitrogen.
CH3Cl and Ca2+ (aq). Ionic between dissolved ions and water, ion dissolved in a polar solvent causes very large charge separation.
CO2 – Van Der Waal’s – nonpolar molecule
HBr (aq) – ionic between dissolved ions and water, ion dissolved in a polar solvent causes very large charge separation.
Paraffin – Van Der Waal’s – nonpolar molecule

4) Candle wax, vegetable oil, and carbon dioxide are all nonpolar molecules. Therefore the intermolecular bonding is due to Van Der Waal’s forces in all cases. The reason one is a solid, one is a liquid and one is a gas all at room temperature is because of the differences in molecular size. Van Der Waal’s forces are roughly the same per unit of surface area between each of these molecules but the bigger molecule has a much bigger surface area and therefore a much bigger total amount of force holds them together.

Hand vs. Liquid Nitrogen – Revisited