Chemistry: Post your doubts here!

[johnsth]

www.chemguide.co.uk
contains notes classified according to our syllabus.

hey thanks man

 

[smzimran]

Assalamu alaikum,
How to identify dependent and independent variables in P5??

Independent variables are those which you can change on your will for example :
mass (you can select any amount),
concentration,
volume,
etc

Dependent variables are those which you cannot control or change directly at your will for example :
boiling point (liquid has different boiling point for different temperatures)
volume of gas given off during a reaction (depends on conc. of reactants)
volume of acid/alkali needed for neutralisation
etc

I hope u got it

 

[hassam]

well there was question in which i had to very temp.... to know its effect on rate of rxn. rate was being mesaured by timing the time for cross to disappear......thisoulfate with hcl rxn........can somebody give me the full plan for this....in particular how will i change the temperature and yet be able to use that cross disappearing method

 

[Mustehssun Iqbal]

Independent variables are those which you can change on your will for example :
mass (you can select any amount),
concentration,
volume,
etc

Dependent variables are those which you cannot control or change directly at your will for example :
boiling point (liquid has different boiling point for different temperatures)
volume of gas given off during a reaction (depends on conc. of reactants)
volume of acid/alkali needed for neutralisation
etc

I hope u got it

ok thanks loads! : )

 

[steel]

where can i get the answer?
chemistry application booklet question

 

[kimkimyap]

welcome sir... hey in fact i thought CO2 was formed by oxidation ... i just discovered the answer today after your post..

Hello, u r wrong there,
In CO, the oxidation state of carbon is +2 and NOT +4 so the whole compound is uncharged (+2 -2 = 0),
If It was charged we would have refered to it as carbon monooxide ion but we do not!

If u are in A2, you should know that Group IV elements have two oxidation states (+2 and +4)

Conclusion:
CO2 is formed by oxidation as one Oxygen atom is added and 'Addition of oxygen is OXIDATION'

Dont confuse yourselves people !

Ok . Tq for ur inform. So does anybody know why carbon is not one of the answer for this question?

In a car engine, non-metallic element X forms a pollutant oxide Y.
Further oxidation of Y to Z occurs in the atmosphere. In this further oxidation, 1 mol of Y reacts
with ½ mol of gaseous oxygen.
What can X be?
1 carbon
2 nitrogen
3 sulfur

 

[umarashraf]

Ok . Tq for ur inform. So does anybody know why carbon is not one of the answer for this question?

In a car engine, non-metallic element X forms a pollutant oxide Y.
Further oxidation of Y to Z occurs in the atmosphere. In this further oxidation, 1 mol of Y reacts
with ½ mol of gaseous oxygen.
What can X be?
1 carbon
2 nitrogen
3 sulfur

I still have problem with the word OXIDATION.... if C has an oxidation state of +2 in CO the +2-2=0.... and CO2= +2-4=-2... it is still reduction not oxidation.... and if C has oxidation state of +4 in CO2 the +4-4=0... its neither oxidation or reduction....

 

[umarashraf]

Independent variables are those which you can change on your will for example :
mass (you can select any amount),
concentration,
volume,
etc

Dependent variables are those which you cannot control or change directly at your will for example :
boiling point (liquid has different boiling point for different temperatures)
volume of gas given off during a reaction (depends on conc. of reactants)
volume of acid/alkali needed for neutralisation
etc

I hope u got it

I still have problem with the word OXIDATION.... if C has an oxidation state of +2 in CO then +2-2=0.... and CO2= +2-4=-2... it is still reduction not oxidation.... and if C has oxidation state of +4 in CO2 the +4-4=0... its neither oxidation or reduction.... there is no way in this world you can define it a oxidation if you consider oxidation states of CO and CO2 ..... say what....????

 

[smzimran]

I still have problem with the word OXIDATION.... if C has an oxidation state of +2 in CO then +2-2=0.... and CO2= +2-4=-2... it is still reduction not oxidation.... and if C has oxidation state of +4 in CO2 the +4-4=0... its neither oxidation or reduction.... there is no way in this world you can define it a oxidation if you consider oxidation states of CO and CO2 ..... say what....????

Sometimes, oxidation and reduction are not defined by oxidation states,
there are four definitions of oxidation and reduction,
in this case,we will check with the other definitions which are addition of oxygen and removal of hydrogen...
Whats with so many smileys?

 

[umarashraf]

Sometimes, oxidation and reduction are not defined by oxidation states,
there are four definitions of oxidation and reduction,
in this case,we will check with the other definitions which are addition of oxygen and removal of hydrogen...
Whats with so many smileys?

addition of hydrogen or oxygen are not the best of the ways to check redox reactions mate...

 

[smzimran]

addition of hydrogen or oxygen are not the best of the ways to check redox reactions mate...

In some cases such as this when there is no change in oxidation states,
they are the only ways to check...

 

[umarashraf]

hmmmm ... still a mystery for me...

 

[Mobeen]

Hi,
I need help as i am not able to understand fully the markscheme answer.
Q. the trend of solubility of group2 sulfates.
Q. the trend of stability of carbonates and nitrates of group2 metals.

thankyou...

 

[XPFMember]

Assalamoalaikum wr wb!
The 2009 er for paper 4 says:
Teachers and candidates are reminded that this paper is set on the whole A level syllabus – not just those learning outcomes printed in bold type in the syllabus document. Although most marks will be allocated to the “non-AS” sections, this does not mean that all marks will be so allocated.

 

[Pals_1010]

Hello I would need help with Nov 2003 MCQ Question 40 please? Why is the answer D and not B? Thanks

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_w03_qp_1.pdf

 

[smzimran]

Hello I would need help with Nov 2003 MCQ Question 40 please? Why is the answer D and not B? Thanks

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_w03_qp_1.pdf

The mixture is distilled...

 

[Pals_1010]

The mixture is distilled...

Aaaah ok.... I guess I missed that part Thanks

 

[Amy Bloom]

Hi,
I need help as i am not able to understand fully the markscheme answer.
Q. the trend of solubility of group2 sulfates.
Q. the trend of stability of carbonates and nitrates of group2 metals.

thankyou...

Hello there!
The trend of solubility of group 2 sulfates decreases down the group. remember that barium sulfate is sparingly soluble salt.
The stability of carbonates and nitrates increase down the grp because carbonates and nitrates have to be heated more strongly b4 they are completely decomposed.
Try to think this way: The carbonate ion has a small +2 charge into a small volume hence it has a high charge density. The distorting effect on a negative ion will increase.

 

[Mobeen]

Hello there!
The trend of solubility of group 2 sulfates decreases down the group. remember that barium sulfate is sparingly soluble salt.
The stability of carbonates and nitrates increase down the grp because carbonates and nitrates have to be heated more strongly b4 they are completely decomposed.
Try to think this way: The carbonate ion has a small +2 charge into a small volume hence it has a high charge density. The distorting effect on a negative ion will increase.

Can you explain it in terms of L.E and enthaloy of Hydration...

 

[Pals_1010]

http://www.xtremepapers.com/CIE/International A And AS Level/9701 - Chemistry/9701_w07_qp_1.pdf

Question 10 please?