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Edexcel AS Chemistry Unit 2 Thread

considering your Unit 1 results, you shouldn't seem so worried :smile:

I'm really dreading bonding angles - my turn for a question

What is the angle between the hydrogen bonds between water molecules?

I'm sure I did this in a recent past paper, but I can't find the answer anywhere in the books :confused:

Reply 41

x10

5

Melanie-v

considering your Unit 1 results, you shouldn't seem so worried :smile:

I'm really dreading bonding angles - my turn for a question

What is the angle between the hydrogen bonds between water molecules?

I'm sure I did this in a recent past paper, but I can't find the answer anywhere in the books :confused:

hydrogen bonds always make a 180 degrees angle and don't forget the dashed line while drawing them

Reply 42

tsanad

Can someone please tell me where i can find the jan 2010 unit 2 mark scheme.

Cheers

Reply 43

Tzarchasm

13

x10

hydrogen bonds always make a 180 degrees angle and don't forget the dashed line while drawing them

What?

It depends on how many lone pairs there are. For example, in a water molecule, there are two lone pairs. Each lone pair reduces the initial bonding angle by 2.5, thus because there's two, there'd be a total reduction of 5 and therefore the bonding angle would be 104.5 between hydrogen for a water molecule.

Reply 44

Patston

Tzarchasm

What?

It depends on how many lone pairs there are. For example, in a water molecule, there are two lone pairs. Each lone pair reduces the initial bonding angle by 2.5, thus because there's two, there'd be a total reduction of 5 and therefore the bonding angle would be 104.5 between hydrogen for a water molecule.

I think he means the bonding forces between two water molecules.

So: [H2O]---[H2O]

The dashed line is the hydrogen bond between molecules of water.
At least that is what I think is meant.

Reply 45

T.S

can anyonee tell me wts d most important parts to know for unit 2

Reply 46

tsanad

H----O-H
l
H

The angle between O---H is 180 degrees. That is the hydrogen bonding in water molecules. Thats what x10 was explaining

O
/ \
H H

In this water molecule, the angle <HOH (between the Hydrogen atoms) is 104.5 degrees, due to the 2 lone pairs present in the oxygen atom. More lone pairs the smaller the angle gets because increased repulsion. That is what Tzarchasm was explaining.

Reply 47

x10

5

tsanad

H----O-H
l
H

The angle between O---H is 180 degrees. That is the hydrogen bonding in water molecules. Thats what x10 was explaining

O
/ \
H H

In this water molecule, the angle <HOH (between the Hydrogen atoms) is 104.5 degrees, due to the 2 lone pairs present in the oxygen atom. More lone pairs the smaller the angle gets because increased repulsion. That is what Tzarchasm was explaining.

Yah thats what I meant, I thought he meant between two molecules....

A question...

When drawing a reaction profile (kinetics), for a catalysed reaction, do we draw one hump or two humps, because I'm not sure weather they expect us to understand the activated complex thing or not? I hope they don't

Thanks in advance

Reply 48

tsanad

x10

Yah thats what I meant, I thought he meant between two molecules....

A question...

When drawing a reaction profile (kinetics), for a catalysed reaction, do we draw one hump or two humps, because I'm not sure weather they expect us to understand the activated complex thing or not? I hope they don't

Thanks in advance

It depends on what catalyst you are using. If you are using a homogenous catalyst, a catalyst which is in the same state as the reactant, then you are expected to show the 2 humps - because there is the forming of an intermediate.

If its a non homogenous catalyst (eg, catalyst is solid and reactant is a gas) then theres no intermediate formed, therefore, only 1 hump which basically has a lower activation energy than the un-catalysed reaction.

Reply 49

Melanie-v

10

x10

Yah thats what I meant, I thought he meant between two molecules....

A question...

When drawing a reaction profile (kinetics), for a catalysed reaction, do we draw one hump or two humps, because I'm not sure weather they expect us to understand the activated complex thing or not? I hope they don't

Thanks in advance

Thanks, I did mean between molecules. :smile:

We're meant to draw with two hills, one to the complex, then another from there to the products (If you make a complex).

Btw, I'm a 'she' ^^

Reply 50

x10

5

Melanie-v

Thanks, I did mean between molecules. :smile:

We're meant to draw with two hills, one to the complex, then another from there to the products (If you make a complex).

Btw, I'm a 'she' ^^

Sorry I usually have that habit of calling people "he's",,, didn't check your name lol

Reply 51

Narik

OP

13

I'm pretty confident about this exam. At the risk of sounding like a cocky bitch, I should get atleast 110+. :jive:

Reply 52

sapphire23X

I'm making notes on the whole book again for revision, only on chapter three so far though x]
Just something I was wondering about - it says that the fact that atoms with large electron clouds tend to be easily deformed favours the existence of an instantaneous dipole/the creation of an induced dipole - is this because the distortion of the electron cloud will result in the centres of positive and negative charge failing to coincide?

Just a simple yes or no question I figured I'd ask you guys haha, it would make a lot of sense for the answer to be yes but I thought I'd check :smile:

Reply 53

SK-mar

1

Narik

I'm pretty confident about this exam. At the risk of sounding like a cocky bitch, I should get atleast 110+. :jive:

just did the jan 2010 paper got 57/80 which should be 108 UMS..... buttttt it was a good paper i found, what did you think of the jan 2010 paper compared to the june 09 and spec papers?? hmmm i need to learn alot more still though!!

Reply 54

username297163

17

x10

4 Which of the following has dipole-dipole interactions between its molecules, but no
hydrogen bonding?

A Methane, CH4
B Methanol, CH3OH
C Ammonia, NH3
D Hydrogen iodide, HI

Why is the answer D?

Hydrogen bonding can only form between N, O, F and only those molecules.....CH4 doesn't have dipole dipole interactions because theyre electronegativites are the same so the electrons are spread evenly around the molecule and H-I has polar bond because I would be delta - and H would be delta +

Reply 55

username297163

17

Tzarchasm

What?

It depends on how many lone pairs there are. For example, in a water molecule, there are two lone pairs. Each lone pair reduces the initial bonding angle by 2.5, thus because there's two, there'd be a total reduction of 5 and therefore the bonding angle would be 104.5 between hydrogen for a water molecule.

Yeah that is true but i think you misunderstood what was meant

Hydrogen bonding is always 180 degrees so if a water molecule and another water molecule joined together the bond angle would be 180 but other than that your right

Reply 56

Narik

OP

13

SK-mar

just did the jan 2010 paper got 57/80 which should be 108 UMS..... buttttt it was a good paper i found, what did you think of the jan 2010 paper compared to the june 09 and spec papers?? hmmm i need to learn alot more still though!!

Awesome.

I did that paper last week - got 70/80 I think - it was an easy paper in that there wasn't anything which made you go :lolwut:

. June 2009 was tough - I had to guess a few but managed 69/80 so that was allright. I've done the spec paper - that was REALLY EASY. I got 76/80 or something like that. Erm, I still have to do the Jan 2009 though.

I would say that if you're getting above 55, you are sorted. Although I'd personally aim for over 60 to be on the safe side. :h:

Reply 57

Narik

OP

13

Mr_Muffin_Man

Yeah that is true but i think you misunderstood what was meant

Hydrogen bonding is always 180 degrees so if a water molecule and another water molecule joined together the bond angle would be 180 but other than that your right

Hydrogen fluoride isn't always 180*. You get a zig-zag pattern. The F-H-F is 180*, but the H-F-H isn't.
http://wpcontent.answers.com/wikipedia/commons/thumb/3/30/Hydrogen-fluoride-solid-2D-dimensions.png/400px-Hydrogen-fluoride-solid-2D-dimensions.png

:smile:

Reply 58

username297163

17

Group 1-2 and 7 reactions are a pain to learn...does anyone have any tips to share :P

Reply 59

SK-mar

1

Narik

Awesome.

I did that paper last week - got 70/80 I think - it was an easy paper in that there wasn't anything which made you go :lolwut:

. June 2009 was tough - I had to guess a few but managed 69/80 so that was allright. I've done the spec paper - that was REALLY EASY. I got 76/80 or something like that. Erm, I still have to do the Jan 2009 though.

I would say that if you're getting above 55, you are sorted. Although I'd personally aim for over 60 to be on the safe side. :h:

I did june 09 last week and found it pretty similar in difficulty, as some questions were easy yet others were quite challenging so there was a balance. I think i got 50 something in that one. But i agree you need to get above 60 to be on the safe side! above 60 should guarantee you 90%+ in UMS. I hope our exam is pretty standard though!!! :p:

Edexcel AS Chemistry Unit 2 Thread

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