Aqa chem 4/ chem 5 june 2016 thread

Original post by DDan123

Agreed. But I have no idea what to expect for Chem 5 now. I felt confident for both units, but the exam on Tuesday completely shocked me so idek...

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Just get the basics of chem5 down and memorise those damn colours

Reply 1742

lahigueraxxx

Hi guys I have a question about the chem5 june 2015 paper

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN15.PDF

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-W-MS-JUN15.PDF

for 2b, why can't you use the equation

2H2 + O2 -----> 2H20 ??? Any help would be much appreciated!!

Reply 1743

26december

Original post by SirRaza97

Just get the basics of chem5 down and memorise those damn colours

I think CHEM5 is the slightly easier one (please don't let it be jinxed) because it's mainly about what you can remember and very little application questions can be asked

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Reply 1744

26december

Original post by lahigueraxxx

Hi guys I have a question about the chem5 june 2015 paper

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN15.PDF

http://filestore.aqa.org.uk/subjects/AQA-CHEM5-W-MS-JUN15.PDF

for 2b, why can't you use the equation

2H2 + O2 -----> 2H20 ??? Any help would be much appreciated!!

The enthalpy of formation is define as "when ONE mole of a substance is formed" therefore you need to only make ONE mole of Water/H2O

(edited 7 years ago)

Reply 1745

lahigueraxxx

Original post by 26december

The enthalpy of formation is define as "when ONE mole of a substance is formed" therefore you need to only make ONE mole of Water/H2O

Oh yeah! Thank you! :smile:

Reply 1746

ahsan_ijaz

4

Original post by SirRaza97

Generally ionic character is stronger than covalency but it really depends what you are comparing with what. Some covelant bonds are much stronger than ionic bonds and vice versa.

I think to answer your question (i am taking a guess) is that the covelant character in aliminum oxide is little compared to the ionic character. This slightly weakens the ionic bonding which is prescedent and causes it to have a lower melting point. Or, the idea of shielding and ionic radius of each element comes into play to effect the attraction between the two. Aluminium has more shielding due to an electron being in the 3p orbital.

Hi thanks,

I predicted a hard chem 4 paper ages ago lol - btw another Q - in AG= AH-TAS expression why is the entropy the one /1000 and not enthalpy as the enthalpy is kj/mol

Reply 1747

rolla01

11

Does anyone have a diagram with the colours ions we need to learn for CHEM 5

Reply 1748

Suits101

https://getrevising.co.uk/resources/colours-of-ions-in-solution-table-with-formulas-and-observations

Original post by ahsan_ijaz

Hi thanks,

I predicted a hard chem 4 paper ages ago lol - btw another Q - in AG= AH-TAS expression why is the entropy the one /1000 and not enthalpy as the enthalpy is kj/mol

Hi can I jump in?

When you calculate entropy change, AS (where A is delta, the triangle) it's in units of J/K/mol

When using AG = AH - TAS:

AG is Gibbs free energy change in kJ/mol
AH is enthalpy change in kJ/mol
T is temperature in K
AS is entropy change in kJ/k/mol

Therefore you have to divide AS (when you calculate it from entropy data only) by 1000 to convert from J/K/mol into kJ/K/mol

Reply 1749

joebush

1

Original post by rolla01

Does anyone have a diagram with the colours ions we need to learn for CHEM 5

Reply 1750

12284

8

Original post by Suits101

Hi can I jump in?

When you calculate entropy change, AS (where A is delta, the triangle) it's in units of J/K/mol

When using AG = AH - TAS:

AG is Gibbs free energy change in kJ/mol
AH is enthalpy change in kJ/mol
T is temperature in K
AS is entropy change in kJ/k/mol

Therefore you have to divide AS (when you calculate it from entropy data only) by 1000 to convert from J/K/mol into kJ/K/mol

does a reaction become spontaneous when AG is negative or positive?
Thanks

Reply 1751

Suits101

Original post by 12284

does a reaction become spontaneous when AG is negative or positive?
Thanks

Reaction is feasible if AG <= 0

(where <= means less than or equal to)

Reply 1752

faizah007

5

does anyone have any predictions for chem 5?

Reply 1753

Original post by ahsan_ijaz

Hi thanks,

I predicted a hard chem 4 paper ages ago lol - btw another Q - in AG= AH-TAS expression why is the entropy the one /1000 and not enthalpy as the enthalpy is kj/mol

My answer is what I think so it might not be right. I can ask my teacher and see what they say about it.

The reason you have to divide it by 1000 is so that it is in KJ mol^-1 K^-1 and not J mol^-1 K^-1 (You're converting the Joules into kilo-joules).

Think of it like taking away a measurement that's in metres from a measurement that's in kilometres. Converting both units to kilometres (or both to metres) makes the calculation easier. You could just do it keeping the units in the form they are in, but it will be quite fiddly.

Also it's important to note KJ/Mol is a standard unit so most answers are in this form. Gibbs free energy is usually in KJ/mol.

Reply 1754

Original post by Suits101

Hi can I jump in?

When you calculate entropy change, AS (where A is delta, the triangle) it's in units of J/K/mol

When using AG = AH - TAS:

AG is Gibbs free energy change in kJ/mol
AH is enthalpy change in kJ/mol
T is temperature in K
AS is entropy change in kJ/k/mol

Therefore you have to divide AS (when you calculate it from entropy data only) by 1000 to convert from J/K/mol into kJ/K/mol

Just a quick note. ΔS is not measured in J/K/Mol it's measured in J / K mol. Just being a bit of a neat freak here xD

(edited 7 years ago)

Reply 1755

Original post by faizah007

does anyone have any predictions for chem 5?

You can't really predict what's going to be on the paper (especially since what chem4 was like). Go through all of it. Leave nothing out.

Reply 1756

lahigueraxxx

Getting stressed now :///

Reply 1757