[chstudent24]

Hey everyone I'm taking a General Chem 2 glass via correspondence. Chem has always been a tough subject so I'm glad I found this place. I'll post my questions here with my thought process.


1. In which of the following mixtures do you encounter ion-dipole forces: CH3OH in water or CA(NO3)2 in water? 

I'm thinking the way to solve this question is to figure out what molecule is the polar one? I'm confused on the process of figuring out which molecule is polar. (if i'm completely off please let me know as well)

2. For which substances are the dipole dipole attractive forces greatest?

Propane
Dimethyl ether
Methyl chloride
Acetaldehyde
Acetonitrile

Would adding up the weight of each of these molecules and finding the one with the least atomic mass be the way to solve this?


3. List the substances of CCl4, CBr4 and CH4 in order of increasing (a) polarizability, (b) strength of dispersion forces

For (a) I'm confused on how to figure out the polarizability for the molecules.  For (b) since dispersion forces to to increase in strength with increasing molecular weight....I should find the atomic weight for each and the heaviest has the the strongest dispersion force?

[Phlogiston]

Hey everyone I'm taking a General Chem 2 glass via correspondence. Chem has always been a tough subject so I'm glad I found this place. I'll post my questions here with my thought process.


1. In which of the following mixtures do you encounter ion-dipole forces: CH3OH in water or CA(NO3)2 in water? 

I'm thinking the way to solve this question is to figure out what molecule is the polar one? I'm confused on the process of figuring out which molecule is polar. (if i'm completely off please let me know as well)

Before worrying about polarity, think about what kinds of substances both of them are.  Think back to what the difference is between electrolytes and non-electrolytes is, and how to identify a solute that is an electrolyte. 

2. For which substances are the dipole dipole attractive forces greatest?

Propane
Dimethyl ether
Methyl chloride
Acetaldehyde
Acetonitrile

Would adding up the weight of each of these molecules and finding the one with the least atomic mass be the way to solve this?

Although some light molecules have strong dipole-dipole forces, that by itself is not enough to figure this one out.  What makes a molecule have a dipole in the first place?  Start with the structural formula or lewis structure, and look at bond polarity.

3. List the substances of CCl4, CBr4 and CH4 in order of increasing (a) polarizability, (b) strength of dispersion forces

For (a) I'm confused on how to figure out the polarizability for the molecules.  For (b) since dispersion forces to to increase in strength with increasing molecular weight....I should find the atomic weight for each and the heaviest has the the strongest dispersion force?

Characteristics of atoms / molecules that are highly polarizable include lots of electrons, and large size. 
For (b), you're on the right track, and you could alternatively think about how it relates to part (a) in terms of how polarizability affects strength of dispersive interactions.

[chstudent24]

Ok so an


1. electrolyte= any substance containing free ions that behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions


(a non electrolyte is a substance that doesn't contain an ion from my understanding)

So with this definition I'm thinking to identify if a solute is an electrolyte, i would need to see if it contains free ions?

Ion-an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge

so in this case NO3 is an Ion therefore making Ca(NO3)2 an electrolyte, thus this is the compound where ion-dipole forces are encounter. (please let me know if I'm wrong with my reasoning at any stage)


2. A dipole is made in the moment of a positive charge encountering a negative charge (or vice versa).  I looked at the structural formulas, but just the whole polarity concept confuses me.

3.  (a) Ok so lots of electrons and large size that would make me think CH4, CCl4, CBr4 (weakest to strongest) going from size and electrons.

(b) so going with what i found out from (a) i'm thinking that polarizability relates to the strength of dispersion forces. So the order would be the same (weak to strong) CH4, CCl4, CBr4


I appreciate your style of asking questions to help me find the answer. I feel i've learned a lot just from these 3 basic questions. Look forward to your response. And will post many more questions in the future.


 

[Phlogiston]

Ok so an


1. electrolyte= any substance containing free ions that behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions


(a non electrolyte is a substance that doesn't contain an ion from my understanding)

So with this definition I'm thinking to identify if a solute is an electrolyte, i would need to see if it contains free ions?

Ion-an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge

so in this case NO3 is an Ion therefore making Ca(NO3)2 an electrolyte, thus this is the compound where ion-dipole forces are encounter. (please let me know if I'm wrong with my reasoning at any stage)

Yep, you got it.  Ca(NO₃)₂ is a salt, so it will break up into ions in water.  Methanol won't.

2. A dipole is made in the moment of a positive charge encountering a negative charge (or vice versa).  I looked at the structural formulas, but just the whole polarity concept confuses me.

Have you gone through the Lewis structure / covalent bonding chapter yet?  Because some of the molecules you list don't have dipoles at all and some do, and in order to be able to tell the difference you need to be able to determine if a molecule has a dipole or not.  This is normally done via

For a molecule,
1) come up with a good Lewis structure
2) determine the molecular shape via VSEPR
3) determine whether there are bond dipoles
4) determine whether the bond dipoles (if present) cancel or add together to give rise to a dipole for the whole molecule

For example, for carbon dioxide this would look like

1) ::O=C=O:: (two lone pairs on each oxygen atom, and two double bonds)
2) since the central atom (carbon) has two "electron groups" it's linear
3) since O and C have different electronegativities, yes there are bond dipoles, each points from the C to the O as in
O <--- C ---> O
4) since these two dipoles are equal and in opposite directions, they cancel and there is no molecular dipole, so the molecule is non-polar.

3.  (a) Ok so lots of electrons and large size that would make me think CH4, CCl4, CBr4 (weakest to strongest) going from size and electrons.

(b) so going with what i found out from (a) i'm thinking that polarizability relates to the strength of dispersion forces. So the order would be the same (weak to strong) CH4, CCl4, CBr4


I appreciate your style of asking questions to help me find the answer. I feel i've learned a lot just from these 3 basic questions. Look forward to your response. And will post many more questions in the future.

(a) and (b) sound good.  Glad to be of *delete me*

[chstudent24]

Have you gone through the Lewis structure / covalent bonding chapter yet? Yes I've done this before but it was nearly 3 years ago so I'm rusty I've been going back to review

Because some of the molecules you list don't have dipoles at all and some do, and in order to be able to tell the difference you need to be able to determine if a molecule has a dipole or not.  This is normally done via

For a molecule,
1) come up with a good Lewis structure
2) determine the molecular shape via VSEPR

I was able to do this for each molecule

3) determine whether there are bond dipoles

This is the step that confuses me, I don't know how to figure out if there are bond dipoles.

[Yggdrasil]

This is the step that confuses me, I don't know how to figure out if there are bond dipoles.

Look at the electronegativity differences between the two atoms involved in the bond.  If the electronegativities are equal (e.g. in a C-C bond) or roughly equal (e.g. C-H), the bond will have no appreciable dipole.  If the electronegativity difference is greater than about 0.5 (e.g. in a C-O or C-N bond), then one atom in the bond (the more electronegative atom) will attract more electron density than the other.  This will result in a partial negative charge on the more electronegative atom and a partial positive charge on the less electronegative atom.  A separation of charges like this is known as a dipole and represented as a vector (arrow) pointing from the partial positive charge to the partial negative charge.