Chem: chemical bonding

bonkysleuth SG

Using knowledge, principl and concepts from different areas of chemistry, explain and interpret, as fully as you can, the data given in the table below

You will ned to consider each type of info. separately and also to link this info. together

Compound Boiling point/deg.cel Electrical conductivity of a 0.1mol/dm3 solution

sodium chloride 1413 Good

ethanoic acid 118 slight

ethanol 79 poor

NaCl properties are explained using ionic bonding but are ethanoic acid and ethanol explained using covalent bonding? I do remember acids being explained regarding their propeties using covalent bonding. And what do the question mean by link this info. together? Asking me to compare why NaCl has highest b.p and can conduct electricity best? Well, i can explain the properties but am not sure how to explain these 3 all at once.

SBS2601D SG

Both ethanol and ethanoic acid display intermolecular hydrogen bonds....hydrogen bonds are stronger than covalent bonds, but weaker than ionic bonds.

This accounts for the relatively high boiling point of water as well.

A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, like nitrogen, oxygen or fluorine.

http://en.wikipedia.org/wiki/Hydrogen_bond

bonkysleuth SG

Both ethanol and ethanoic acid display intermolecular hydrogen bonds....hydrogen bonds are stronger than covalent bonds, but weaker than ionic bonds.

This accounts for the relatively high boiling point of water as well.

A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, like nitrogen, oxygen or fluorine.

http://en.wikipedia.org/wiki/Hydrogen_bond

Never learnt of this at the O level standard. Am I supposed to write this explanation? By the way, how then do i differentiate the bp and electrical conductivity of ethanoic acid and ethanol?

SBS2601D SG

Never learnt of this at the O level standard. Am I supposed to write this explanation? By the way, how then do i differentiate the bp and electrical conductivity of ethanoic acid and ethanol?

Oh... Laughing

That's A level stuff.

If I'm gonna explain about the electrical conductivity, it would be because ethanoic acid dissociates more readily in water than ethanol.

the reason is due to the presence of another O atom in ethanoic acid that causes increased polarity between the H and O atom.

Er....I think I've forgotten a lot of stuff, but that's the gist behind it....some people who are more current, help this poor soul here?

lavastar SG

er u sure this is O level question? cause to explain the differences in bp between ethanoic acid and ethanol u need knowledge of 'A' level chemistry. i think all that is required is just general comparison

NaCl has a giant ionic structure with strong electrostatic attraction between Na+ and Cl- ions, which require more energy to overcome compared to weaker intermolecular hydrogen bonding (A levels)/ covalent bonds(O levels) in ethanoic acid and ethanol. It being an ionic compound will dissociate completely in water, producing more mobile ions to carry charges across compared to ethanol and ethanoic acid which are covalent compounds and oni dissociate partially in water, producing lesser mobile ions, hence lesser electrical conductivity.

Now comparing ethanoic acid and ethanol will require 'A' level stuff, but i will just put it here.

The Bp of ethanoic acid is higher than ethanol because intermolecular hydrogen bonding is stronger in ethanoic acid, due to the extra electronegative O atom which pulls electron away from the hydroxy group, making the O-H bond more polar, hence more difficult to overcome.

In terms of electrical conductivity, ethanoic acid has higher electrical conductivity as its extent of dissocation is higher, producing more mobile ions. This is because the CH3COO- anion is further stabilised by resonance, where the negative charge is extensively delocalised between two electronegative O atoms, favouring dissociation. In comparison, the CH3O- ion is less stable because the electron donating methyl group intensify the electronegative charge on the O atom, destabilising it, hence it is less likely to dissociate and lesser mobile ions are produced.

TenSaru SG

Perhaps for O level you can just use larger Mr to explain bp.

Occam's Razor SG

Lavastar,shouldn't ethanoic acid have a higher bp than ethanol because of dimerisation?

UltimaOnline SG

Lavastar,shouldn't ethanoic acid have a higher bp than ethanol because of dimerisation?

The dimerization effect is caused by hydrogen bonding, so in one sense it is already taken into consideration under hydrogen bonding; in another sense, there is significantly stronger instantaneous dipole - induced dipole van der Waals interactions between the dimers, due to increased no. of electrons and polarizability (due to the size of molecule).

Advice to students in general (*not* directed at anyone in particular) :
Check out the boiling points of any compound (eg. comparing within and across carboxylic acids, alcohols, alkanes, etc) by looking up Wikipedia, to make your own intelligent reasonable deductions (as opposed to accepting/memorizing your school notes/textbooks/teachers' explanations unthinkingly; as Sg students are often criticized of lacking street smarts and only being exam smart by blindly memorizing notes; be better than that.).

bonkysleuth SG

Hmmm I do not have the slightest idea about all this even after reading some info. up from the net. My explanation is kinda scattered.

By the way, if we give too long an answer, the teacher wouldnt bother reading it and would give the comments ,"just a 2/3 marks answer. Why are you writing so much?" Of course there are times when i write something out of the O's syllabus and she was like "Do you know no marks are awarded for unnecessary contents that have not been taught even if they are correct?"

So i was wondering wherther I should simply explain ethnaol and ethanoic acid using covalent bonding

yiha093 SG

if me i will say.

acid need break more bonds as compared to ethanol, hence bp higher

acid dissocciate more then ethanol, hence electrical conductivity up.

dkcx SG

if me i will say.

acid need break more bonds as compared to ethanol, hence bp higher

acid dissocciate more then ethanol, hence electrical conductivity up.

Boiling point is a physical process so how does breaking of bonds factor into your explaination?

Molecular mass, hydrogen bonds etc shld have more effect on boiling point

yiha093 SG

Boiling point is a physical process so how does breaking of bonds factor into your explaination?

Molecular mass, hydrogen bonds etc shld have more effect on boiling point

perhaps the breaking of bonds requires more energy ( high temp able to provide energy ) ma. ? therefore higher b.p ?

or am i wrong? just offering my opinion ba.

SBS2601D SG

perhaps the breaking of bonds requires more energy ( high temp able to provide energy ) ma. ? therefore higher b.p ?

or am i wrong? just offering my opinion ba.

I can guarantee you, you'll need plenty of energy trying to break the inter-atomic bonds. Laughing

It's not the same as inter-molecular bonds.

dkcx SG

perhaps the breaking of bonds requires more energy ( high temp able to provide energy ) ma. ? therefore higher b.p ?

or am i wrong? just offering my opinion ba.

The bonds to be broken between acid and alcohol to vapourise them is the same cos its the intermolecular bonds that keep them as liquids. The only slight difference is due to acids being able to form more hydrogen bonds thus requiring more energy but your explaination without specifying intermolecular bonds is like saying you are breaking up the molecule. Its not the number of bonds but more the strength of the bonds that affects the it.