2.3 Shapes of molecules and ions
Posted by: intechemistry on: September 23, 2010
26 Responses to "2.3 Shapes of molecules and ions"
April 8, 2011 at 1:03 am
Salam.
Ionic bonds cover a range of strengths as do covalent bonds and both are regarded as being strong. There is no definite rule.
I find this data interesting
Sodium ethanoiate boiling point = 881.4 °C
NaCl boiling point = 1413 °C
Silicon boiling Point = 2355 °C
Carbon boiling point = 3825 °C
Indicates those chosen covalent bonds are stronger than the chosen ionic ones. However some covalent bonds can be weak. E.g. Cl-Cl bond 243 kJ mol-1 and O-O in organic peroxides [used as radical initiatiors - can't remember the strength :s ] and NaCl bond strength is something like 425 kJ mol-1, so, really, there is no clearcut answer, as unsatisfying as that may seem.
Because of the C and Si, I personally gave covalent compounds – on average – the edge over ionics, but really, it’s not important to consider this particular comparison at A-level.
Variations within bond strength (LE. dH(f) etc) amongst ionic compounds and variaitions of bond strength amongst the different covalent moleces are inportant considerations.
April 8, 2011 at 1:12 am
Yes, electronegativity does have a bearing on bond strength e.g. the O-H bond in trichloroethanoic acid (see Unit 4 trial paper) is quite low because of the electronegative Cl’s pulling e- density from the carboxylic acid group and hence the O-H bond. [although for the halogenalkanes, increasing polarisaion does NOT weaken the R-X bond !!]
And for ionics, polarising ability is important. The greater the polarization, you can think of it as having the ‘more bonding’, hence greater bond strength i.e. it’s ionic with some covalent character {ineterestingly, that data also implies covalent bondng is stronger, as the more covalent character it has the more exo the lattice enthalpy}
May 11, 2011 at 5:50 pm
Hi sir
[chemistry unit 2 june 2010, section A, Q3]
Why is B wrong? i.e. which has higher melting temperature – graphite or buckminsterfullerene? I thought carbon would have higher m.t. since it is a giant covalent network. But it turned out to be the other way round.
May 11, 2011 at 6:13 pm
Graphite and buckminsterfullerene are both allotropes of carbon.
Buckminsterfullerene is molecular but graphite is a giant structure. molecular species have lower mpt and bpt than comparable giant structures.
The Q is looking for a true answer, but answer B says “The melting temperature of buckminsterfullerene is higher than that of graphite.” which is false.
January 12, 2012 at 11:49 pm
Hi Sir, Qs :
#1 Bonding pair-bond pair repulsion is said to be greater than lone pair-bond pair repusion.Why? does the location of lone pair closer to nuclues of central atom has an effect ? How ?
# accurate Terminology fr edexcel ppr ,
does the term ‘trigonal pyramidal seen = pyramidal’ .accepted? and whats the proper term to be used ‘V-shaped’ or V-bent or bent alone is accepted ….for molecules wt 3 electrn pairs (2 B.P + 1 L.P) and 4 electrn pair (2 B,p and 2 l,p)
#3 is it a must to drw all the structures (shapes) of molecules in 3D formed asked in xams.
Thanks
January 13, 2012 at 12:43 am
jus a random Ques, how fdoes tetrahedral got a bond angle of 109.5.Cant figure out why .
January 13, 2012 at 2:32 am
@ 7, Michael’s sword, January 12, 2012 at 11:49 pm
#1 You have it the wrong way around. Lone pair (lp) repulsion with another lp is greater than bond pair (bp) with another bond pair.
i.e. in terms of repulsion, lp-lp > bp-bp
The lp’s are not bonded (obviously), hence they are not ‘spread out’ between the nuclei of the bonded atoms as is the case for a bond pair. lp’s are in the valence shell of the atom carrying them, so lone pairs will be very close together (in the same valence shell !) and repel much more strongly than is the case for bond pairs.
#2 “trigonal pyramidal seen = pyramidal” – I’m not sure what you mean by that, i.e. the “seen” bit. I would be hesitant to say just pyramidal as you can have 3 or 4 sided pyramids, trigonal and square based pyramids respectively. Use the accepted term “trigonal pyramidal which is the shape of ammonia.
V-shaped or bent is accepted for the shape of water. I prefer the term ‘V shaped’.
#3 Always draw shapes with 3D bonds, unless they happen to be flat, e.g. SO2, SO3, H2O AlCl3 etc., as the 3 dimensions make up their shape.
January 13, 2012 at 2:59 am
8, Earthling, January 13, 2012 at 12:43 am
If you are good in 3D geometry/trigonometry, you could mathematically calculate the maximum distance 4 points can evenlyget away from each other in three dimensions.
In two dimensions, like on a flat piece of paper, they can move a maximum of 90 degrees from each other, but In three dimensions, you get more ‘room’ and they can move apart a little further to 109.5 degrees.
January 16, 2012 at 6:05 pm
queries
Thanks for the explanation,but i was actually referring to the strength of bondingpair -bonding pair repulsion to bondingpair-bondingpair repulsion. ( my bad, pls just swap my sentence around).
and about why strength of lp-lp > b.p-b.p yea I got what you meant,but can we write those ans in the edexcel ppr Unit 2 exam? cz i remembr explaining everything i knw in the ans sheet and getting a perfect O.tq
January 16, 2012 at 8:37 pm
@ 11, Michael’s sword, January 16, 2012 at 6:05 pm
“i was actually referring to the strength of bondingpair -bonding pair repulsion to bondingpair-bondingpair repulsion.” –
I’m sorry, I can’t see any difference between
A) bondingpair -bonding pair repulsion
versus
B) bondingpair-bondingpair repulsion.
I guess for that “perfect zero” (nice expression) you were not answering the question that was asked of you, despite (perhaps) virtually everything you wrote was factually correct. It sometimes happens. Edexcel advise RTQ3, meaning Read The Question Three times. 1) To get an idea of what’s being asked of you. Then read it again CAREFULLY so that you fully understand (and perhaps make a helpful sketch/note to assist your composition of the answer) then read the question at the end followed by your answer to check your answer is what’s required. The fact they issue this advice makes me think the see too many students answering a different question.
January 16, 2012 at 11:55 pm
“i was actually referring to the strength of bondingpair -bonding pair repulsion to bondingpair-*LONE pair repulsion.”.typo sory sir im turnin blind i guess,may you can change the background page (colour) , hard to read at times.
but ur ans was mainly deals about ”Lone pair (lp) repulsion with another lp is greater than bond pair (bp) with another bond pair.”.
tq
January 18, 2012 at 1:10 am
bp-bp repulsion is lower than bp-lp repulsion*. bp electrons are located in the (covalent)bonds, obviously, and hence are in-between two atoms. A lp is situated on an atom, so a bp on an atom with a lp will experience greater repulsion than is the case for bp-bp interaction.
The lp (cannot move – it’s stuck on an atom) pushes the bp away from it.
* Double bonds and triple bonds will be different, but for A-level (as far as I am aware) double bonds are taken to be the same as a single bp.
I hope that helps. Does it?
May 19, 2012 at 3:25 am
Sir, is diamond a good or bad conductor of heat? Pearson says it is a bad conductor of heat but hodder says it is a good conductor of heat.
And sir, are fullerenes good conductors of electricity? pearson says they are good conductors because of delocalised electrons, but hodder says they are not because the delocalised electrons cannot move between molecules.
thank you sir
May 19, 2012 at 4:00 am
Kosher
Diamonds thermal conductivity:
“Most diamonds are electrical insulators but extremely efficient thermal conductors” source Also this source shows diamond having a higher conductivity than aluminium. Diamond is the exception to non-metals by the fact diamond is a good conductor.
Re: Electrical conductivity of fullerines. Read from here onwards.
May 19, 2012 at 12:56 pm
sir, the link for electrical conductivity of fullerene is not functioning… Pls attach another sir. Thank you once again
May 19, 2012 at 2:43 pm
Sorry Kosher. The correct link is this: http://intechemistry.wordpress.com/general-questions/#comment-3349
May 23, 2012 at 5:49 am
Hi ther sir,how does the N-O bond in the NO3 (nitrate ion is formed) ..Im puzzled. cz O has 6e- bt if N only shares 1 e- with it ?
May 23, 2012 at 12:27 pm
IF you mean NO3- as opposed to NO3 then just apply the usual rules:
1) Every O will have a double bond with the central atom except….
2) Each minus charge means an O will have a single bond to the central atom and the e- causing the negative charge will be on the O
3) If octet (period 2 atoms) is exceed, further bonds involving central atoms and O can occur by dative bonds from the central atoms.
4) Due to resonance, X=O nd X-O- bonds are equivalent.
See the pic.
May 23, 2012 at 12:36 pm
Ok thanks sir.so thers dative covalent bond of N atom to O atom? .is the diagram (that is the dative arrow used valid to show in xam if the Q is about drawing a dot and cross diagram
May 23, 2012 at 12:58 pm
The pic at the top of this page is meant to show HOW the bonds are formed. Using the arrow with STRAIGHT line/shaft is standard notation for a dative bond and is almost certainly going to be accepted.
For dot and cross diagram, as always, simply place the bonding electrons half way between the atoms.
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April 8, 2011 at 12:24 am
salam sir..
which of these is the strongest; the ionic bond or the covalent bond?
and does electronegativity and polarising ability similar?
thanks