Bonding Test - AQA Mod. 1

Hi.

I just went through some exam questions and got stuck on a few.

1) Which of each pair of molecules has the most ionic character? Explain why?
(i) HF or HCl
(ii) H2S or H2O
(iii) F2 or FCl

2) Which of each pair of ionic compounds has the most covalent character? Explain why.
(i) NaCl or KCl
(ii) Li2O or Na2O
(iii) MgCl2 or AlCl3

3) What factors increases the degree of covalent character which an ionic compound has?

4) Decide which of each pair of molecules has the highest boiling point. Explain each answer.
(iii) CH3CH2OCH3 and CH3CH2CH2OH

For question 4) I said that propan-1-ol can from hydrogen bonds and CH3CH2OCH3 is a non polar molecule and can only form VdW forces
therefore propan-1-ol has the higher boiling point.
Is that explenation right?

and the last question.
5) Name the bonding and interaction which occurs between
(i) iodine molecules in a crytsal of iodine.
I said instanateous dipole-induced dipole. is that right?
(ii) molecules of methane?
I said VdW forces. is that right?

Thx for ur help
Kathrin

Reply 1

NathanM

Kathrin

1) Which of each pair of molecules has the most ionic character? Explain why?
(i) HF or HCl
(ii) H2S or H2O
(iii) F2 or FCl

For this you need to look at the Pauling scale of electonegativities. The molecule with the highest difference in electronegativity has the most ionic character.

i) HF; difference = 1.8, HCl; difference = 0.8...so HF has the most ionic character
ii) H₂S; difference = 2.5, H₂O; difference = 0.4...so H₂S has the most ionic character

etc

2) Which of each pair of ionic compounds has the most covalent character? Explain why.

Just use the method above. :smile:

3) What factors increases the degree of covalent character which an ionic compound has?

The distance between elements in the periodic table increases the degree of covalent character. ie. RbF is much, much more ionic than AsBr. As a rule of thumb, if the electronegative difference is greater than 1.5 then the compound is likely to be ionic rather than covalent

4) Decide which of each pair of molecules has the highest boiling point. Explain each answer.
(iii) CH3CH2OCH3 and CH3CH2CH2OH

For question 4) I said that propan-1-ol can from hydrogen bonds and CH3CH2OCH3 is a non polar molecule and can only form VdW forces
therefore propan-1-ol has the higher boiling point.
Is that explenation right?

sounds good to me

5) Name the bonding and interaction which occurs between
(i) iodine molecules in a crytsal of iodine.

I said instanateous dipole-induced dipole. is that right? not sure =S

(ii) molecules of methane?

I said VdW forces. is that right? yup :smile:

Reply 2

Kathrin

thank you so much.

But is there a simpler explenation for Q3?
I dont think what you said is AS stanard :biggrin:

Reply 3

Eau

Q3: "What factors increases the degree of covalent character which an ionic compound has?"

An ionic compound will have covalent character (and hence sharing of electrons) when the cation is polarising and/or when the anion is polarisable.

Small, highly charged (high charge density) cations are very polarising. Small means the electrons are close to the nucleus (source of protons) of the cation, and a high positive charge of the cation also helps draw electrons towards it.

Reply 4

Eau

Q5(i): Name the bonding and interaction which occurs between iodine molecules in a crytsal of iodine.

"I said instanateous dipole-induced dipole. is that right?"

Correct. This is exactly the same as van der Waal's forces (vdW's forces).