Lab Report HL: Intermolecular forces and physical properties

[nreyes94]

Hey guys, first post here

Chemistry teacher just left us a practical concerning intermolecular forces and physical properties.

The practical was divided into 3 aspects:

1. Dipoles:

we open the burette and a stream of the following liquids will come out, then we put an ebonite rod near the stream and observe the results

hexane, water, propanone, ethanol and tetrachloromethane.

Why were some substances repelled from the rod?

2. Solid's solubility:

prove the solubility of 5 solids on water and hexane:

NaCl, iodine, urea, wax and SO2

Why were some of the solids dissolved, why were some not dissolved? i got to comment on their ionic and covalent nature of the solids.

3. liquid's solubility:

prove the solubility of 5 liquids in water and hexane:

methano, ethanol, penthanol, propanone, hexane

Why were some of the liquids dissolved, why were some not dissolved? i got to comment on their intermolecular forces concerning solubility

I hope i can get some guidance as to what to write on the practical

Thank you!

[Drake Glau]

1. Think about the polarity of the molecules based off the electro-negativities of the atoms. Was the rod - or +? Are the molecules a net - or +? I know the tetrachloride has a net - charge all over it

2. Ionic bonds will break and the substance will ionize in water...covalent bonds tend to not do that

3. You need something to form a hydrogen bond and you need polarity. Does the polarity of the molecules change as stuff is added (such as your homologous alcohol group there)? Adding non-polar CH2 groups to the alcohol can't help the solubility any since it's non-polar right?

[nreyes94]

1. the ebonite rod was charged negatively, what do you mean by asking that of the molecules? im taking chemistry HL in spanish :$

2. so if the bonds of the solids are ionic they will break down in water?

[Drake Glau]

The electro-negativity of the atoms in the molecules will determine where the electrons are gather and thus where the negative pole it. The tetrachloromethane for example is a carbon currouned by 4 chlorines. The Cl is much more electro-negative than the carbon and thus it attracts the electrons away from the carbon and towards the Cl. Since they are all equally electronegative the electrons are spread out equally (excluding dipole moments and heisenburg uncertainty) so the molecule in general has a negative charge and is non-polar because it doesn't have a + pole at all.

Ionic bonds are formed because of the nature of Ions. They give electrons to create bonds and when you put them in water (due to the polarity of the water) they tend to break back apart forming a cation and anion floating around in the water

[ervo]

A better way to express the hydrophobic and non-polar nature of CCl₄ is that there is no overall dipole. That is the language you should use. It is misleading to say there is "no + pole" because there is - it's on the carbon atom. It is also misleading to say "the molecule in general has a negative charge" (Drake Glau's words), as it doesn't. There is simply an unequal distribution of charge.

Methanol is the most soluble of methanol, ethanol and propanol because it has the smallest hydrocarbon chain on it, which is the hydrophobic "end" of the molecule. After pentanol I think they become completely insoluble in water (this is something you can easily check with Wikipedia).

Propanone cannot hydrogen bond, as it has no hydroxyl (-OH) groups. However it is fairly polar due to its small size and the carbonyl group (C=O), which is why it is still soluble in water (but not as much as the lower alcohols). Hexane is of course insoluble in water.

Most ionic compounds do dissolve in water because of a phenomenon called solvation. The water molecules crown around the individual ions, with their positive ends attracted to anions and negative ends to cations. Some salts don't dissolve in water, though, notably most carbonates, barium sulfate, silver halides and most oxides, sulfides and hydroxides of metals that are not in groups 1 and 2 don't dissolve in water (or are sparingly soluble).

The ones that don't dissolve in water are those that have a highly endothermic enthalpy of solution and where the enthalpy of lattice formation is low.