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AS Chemistry P2 Prep.

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Drift_king

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has anybody how to calculate the bond energy in Standard Entalply change in combustion and formation.... ???
:(
 
Jaf

Jaf

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saudha said:
The central atom is C and it is surrounded by 4 hydrogens. First find the number of electrons being shared. In this case, it is 4 from carbon and 1 from each hydrogen, totaling up to 8 electrons. Now, each bond uses 2 electrons. There are 4 bonds, and therefore no lone pairs. So CH4 has 4 bonds and no lone pairs.

Another one: NH3. The central atom is N and it makes 2 bonds with hydrogen. The total number of electrons being shared are 5 from Nitrogen and 3 from the hydrogens. So total electrons = 8. Nitrogen makes 3 bonds (which use 6 electrons), so 2 electrons are left making 1 lone pair. So NH3 has 3 bonds and 1 lone pair.

When you have the bonds and lone pairs decided, learn this:

2 bonds, 0 lone pair = linear and 180 degrees (e.g. CO2)
2 bonds, 1 lone pair = angular and 117 degrees. (e.g. BF3)
2 bonds, 2 lone pair = angular and 104.5 degrees. (e.g. H2O)
3 bonds, 0 lone pair = trigonal planar and 120 degrees. (e.g. AlCl3)
3 bonds, 1 lone pair = trigonal pyramidal = 107 degrees. (e.g. NH3 as I did above)
4 bonds, 0 lone pair = tetrahedral = 109.5 degrees. (CH4 as I did above)
5 bonds, 0 lone pair = trigonal bipyramidal = 90 and 120 degrees. (PCl5)
6 bonds, 0 lone pair = octahedral and 90 degrees. (SF6)

The theory is basically that the electron pairs arrange themselves around the central atom to minimize the amount of repulson, so they try to be as far apart as possible. Also, you need to know that lone pair - lone pair repulsion > lone pair - bond repulsion > bond-bond repulsion. Lone pair repulsions are stronger because they are closer to the atom and therefore exert more pressure
Click to expand...

BF3 has a trigonal planar shape. The bond angle in it is 120 degrees. It has three bond pairs and no lone pairs. This seems all but natural considering it's in the same group as aluminium and thus has 3 outer electrons which are used for bonding.
117 degrees (the shape and examples) is not included in the syllabus (primarily because there's no solid rule for this one, the same number of bonding pairs and lone pairs can produce a range of angles). But just in case, examples would be SO2 and O3.

Angular is not an acceptable name for the molecule with 104.5 as the bond agle for CIE, AFAIK. I may be wrong, but this word has never been used in a marking scheme. The correct name of the shape of molecules like H2O and H2S is bent, non-linear and V-shaped. (for example, see question 2) (c) (ii) May 2005)
 
oldfashionedgirl

oldfashionedgirl

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Drift_king said:
has anybody how to calculate the bond energy in Standard Entalply change in combustion and formation.... ???
:(
Click to expand...
when ur given standard heat of formations of the reactants and products, then formula is: heat of formation of products - heat of formation of reactants
if ur asked to calculate enthalpy change by using values from data booklet, then formula is: reactants - products
if ur given enthalpy changes of combustion, then formula to use is: enthalpy change of combustion of elements - enthalpy change of combustion of products
 
Jaf

Jaf

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Question:
When one encounters a problem where we have to draw the dot and cross structure for a compound with more than 2 atoms do we use a different symbol (beside a 'dot' and 'cross') to represent the electrons of the third atom? This is what I did in IGCSEs. But one of my teachers said that a 'dot and cross' structure should only have dots and crosses. So keep the symbol for the electrons of every alternate atom the same. The mark schemes are inconclusive from what I've seen.

Please, no wild guesses. Only answer if you're sure.
 
angelgirl:)

angelgirl:)

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saudha said:
The central atom is C and it is surrounded by 4 hydrogens. First find the number of electrons being shared. In this case, it is 4 from carbon and 1 from each hydrogen, totaling up to 8 electrons. Now, each bond uses 2 electrons. There are 4 bonds, and therefore no lone pairs. So CH4 has 4 bonds and no lone pairs.

Another one: NH3. The central atom is N and it makes 2 bonds with hydrogen. The total number of electrons being shared are 5 from Nitrogen and 3 from the hydrogens. So total electrons = 8. Nitrogen makes 3 bonds (which use 6 electrons), so 2 electrons are left making 1 lone pair. So NH3 has 3 bonds and 1 lone pair.

When you have the bonds and lone pairs decided, learn this:

2 bonds, 0 lone pair = linear and 180 degrees (e.g. CO2)
2 bonds, 1 lone pair = angular and 117 degrees. (e.g. BF3)
2 bonds, 2 lone pair = angular and 104.5 degrees. (e.g. H2O)
3 bonds, 0 lone pair = trigonal planar and 120 degrees. (e.g. AlCl3)
3 bonds, 1 lone pair = trigonal pyramidal = 107 degrees. (e.g. NH3 as I did above)
4 bonds, 0 lone pair = tetrahedral = 109.5 degrees. (CH4 as I did above)
5 bonds, 0 lone pair = trigonal bipyramidal = 90 and 120 degrees. (PCl5)
6 bonds, 0 lone pair = octahedral and 90 degrees. (SF6)

The theory is basically that the electron pairs arrange themselves around the central atom to minimize the amount of repulson, so they try to be as far apart as possible. Also, you need to know that lone pair - lone pair repulsion > lone pair - bond repulsion > bond-bond repulsion. Lone pair repulsions are stronger because they are closer to the atom and therefore exert more pressure
Click to expand...

NH3 isnt it total of 3 e- shared by nitrogen and 1 by each 3 H...?and nitrogen has 1 lone pair...so total around N will be 8 e-......its shape is trianguler pyramidal..isntead of trigonal..
CO2 has 4 bonds thats 2 bond pairs which repel eachother equally but since the e-,it makes a lenear molecule and 180' angle...
Boron has 3 e- in its outermost shel so it will make 3 bonds with F total of 6 e-...it does doesnt achieve noble gas electronic configraion in its outer most shell..so it has 120' angle and is trigonal planar in shape...its on pg 35 and 39 of AS and A Level Chemistry book...and it can make a dative covalent bond with eg NH3...
PCL5 forms 120 and 180 degree angle not 90 degree...
H2O has a bend shape or V shape not angular...
SF6 has 90 and 180 degree angle...
hope iam right? if not plz correct ne as well?
 
saudha

saudha

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angelgirl:) said:
NH3 isnt it total of 3 e- shared by nitrogen and 1 by each 3 H...?and nitrogen has 1 lone pair...so total around N will be 8 e-......its shape is trianguler pyramidal..isntead of trigonal..
CO2 has 4 bonds thats 2 bond pairs which repel eachother equally but since the e-,it makes a lenear molecule and 180' angle...
Boron has 3 e- in its outermost shel so it will make 3 bonds with F total of 6 e-...it does doesnt achieve noble gas electronic configraion in its outer most shell..so it has 120' angle and is trigonal planar in shape...its on pg 35 and 39 of AS and A Level Chemistry book...and it can make a dative covalent bond with eg NH3...
PCL5 forms 120 and 180 degree angle not 90 degree...
H2O has a bend shape or V shape not angular...
SF6 has 90 and 180 degree angle...
hope iam right? if not plz correct ne as well?
Click to expand...

i have no idea somebody ...wanted info and i just copy pasted..... so you r going to xplain whn this topic comes/.....
 
littlecloud11

littlecloud11

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raamish said:
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w06_qp_2.pdf


http://www.xtremepapers.com/papers/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_w06_ms_2.pdf

In 4g) my answer is (ch2-ch2-ch2-ch2-ch2-ch2-)n But in marking scheme the structure is branched. Can somebdy please tell me how they made this branced structure?
Click to expand...

The structure of Z is drawn, so when you polymerize just break the double bond and draw two units together, and the structural formula becomes-
-CH(CH3)CH2CH(CH3)CH2-

Got it?
 

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angelgirl:)

angelgirl:)

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wat r the condition for the Hyber process...coz iam gething 2 different conditon from 2 different book...dont know which 1 is correct...
 
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