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AS Chemistry- helping each other out!

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Reply 1380

frozo123

Original post by C0balt

No it's my first time

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Oh nvm then

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Reply 1381

chzm

Original post by frozo123

Well you're missing out enthalpy of atomisation, mean bond enthalpies, enthalpy of hydration, enthalpy of solution, lattice enthalpy
But yeah what you done so far is good

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it depends what exam board you are my specification doesn't cover enthalpy of atomisation, hydration or lattice enthalpy.

Reply 1382

Clovers

Original post by frozo123

make sure you actually understand what the definitions are on about
rather than just memorising them, it helps :smile:

Oh okay thanks

Reply 1383

Clovers

Original post by VeryLastMinute

Here are my notes on Hess' Law and Hess Cycles, hope they help, if you can't read any parts let me know and I can type them up.

Edit: Would appreciate some feedback on the notes, just to see if they're missing out on anything. I do international as chem so I sat my unit 1 in January (and wrote that before the exam) and I'm not sure if I've missed out anything there.

Thankyou, it hasd made certain elements a lot clearer

Reply 1384

wednesday_adams

guys....I genuinely do not understand Hess's Law cycles AT ALL.

I've tried and tried....

How did you guys manage to wrap your heads around it?

Reply 1385

Diligence

Hi,

I would appreciate it if someone could explain to me why some salts are soluble in water while others such as lithium fluoride are not?

Thanks

Reply 1386

C0balt

Original post by wednesday_adams

guys....I genuinely do not understand Hess's Law cycles AT ALL.

I've tried and tried....

How did you guys manage to wrap your heads around it?

After a couple of questions it clicked in
Leave it a couple of days and come back, a fresh mind does wonders for me

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Reply 1387

chantellerose

How are you guys revising for chemistry? Lots of people are telling me to do past papers but I've done some and they aren't improving my grade :frown:

Reply 1388

wednesday_adams

Original post by C0balt

After a couple of questions it clicked in
Leave it a couple of days and come back, a fresh mind does wonders for me

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thanks, that is good advice. i might keep coming back to it with a different mind set until i finally get it..

Reply 1389

langlitz

Original post by chantellerose

How are you guys revising for chemistry? Lots of people are telling me to do past papers but I've done some and they aren't improving my grade :frown:

I recommend making really condensed notes for everything, a bit like this (except for chemistry). It works for me :smile:

image.jpg526.8KB

Reply 1390

langlitz

Original post by wednesday_adams

guys....I genuinely do not understand Hess's Law cycles AT ALL.

I've tried and tried....

How did you guys manage to wrap your heads around it?

I've never really understood the point of drawing those diagrams, I find it easier to just reverse one of the equations to get the products on the right side etc. But you need to remember to flip the sign of the

\Delta H

Reply 1391

randlemcmurphy

Original post by wednesday_adams

guys....I genuinely do not understand Hess's Law cycles AT ALL.

I've tried and tried....

How did you guys manage to wrap your heads around it?

Don't worry, at AS level as long as you remember the specific equations for whether you have formation or combustion data you will be fine, I remember my teacher saying you could rely on them alone. Although at A2 you will have to have an understanding about it, however it should make more sense at that point in A2.

Reply 1392

S.Ahmad

Out of the three following Intermolecular forces, whats the order of strength?
-Electronegativity (permanent polar dipole-dipole)
-Van der Waals',
-Hydrogen Bonds

Reply 1393

C0balt

Original post by S.Ahmad

Out of the three following Intermolecular forces, whats the order of strength?
-Electronegativity (permanent polar dipole-dipole)
-Van der Waals',
-Hydrogen Bonds

Hydrogen bonds>dipole dipole>van der waals

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Reply 1394

frozo123

Original post by Diligence

Hi,

I would appreciate it if someone could explain to me why some salts are soluble in water while others such as lithium fluoride are not?

Thanks

the enthalpy of solution is positive and too endothermic, this means that there is a the entropy of the surroundings is negative, so despite the entropy of the stystem being postive, the total entropy of the system will be negative.
Basically, for AS, the ionic bonds are too strong lol

(edited 9 years ago)

Reply 1395

zara1234567

Hey please can someone help on this question.. I just don't understand or enthalpy changes of combustion in general or of anything. Like when you do energy released and then switch over and do the opposite sign for temp released.. well all that makes no sense to me. Pls help!

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1428619215289.jpg29.8KB

Reply 1396

Super199

http://filestore.aqa.org.uk/subjects/AQA-CHEM2-QP-JUN12.PDF

Quesiton 2C. Can someone explain this to me. If the delta h is positive does that mean the reaction is endothermic. How do you know the reaction is to the right is my question basically.

If it was negative how would you know which way was exothermic or endothermic.

Is the way you do it by looking at the sign and saying the reaction to the right if negative = endothermic. If delta h is positive reaction to right = endothermic?

Thanks.

Reply 1397

zara1234567

Also just tried doing it and got for bi) +56430 but I'm unsure whether it should be negative or not. For bii) 0.02 and for biii) 2820000 but that number seems so big so seems wrong
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Reply 1398

C0balt

Original post by zara1234567

First question is Q=mcAT. m is the mass of water, c is the specific heat capacity, delta T is the temperature change.

For second question divide the mass used by molar mass.

For third use the values above and calculate delta H.
deltaH=-Q/n

P.S don't forget to convert to kJ
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