Chemistry: Post your doubts here!

[Rizwan Javed]

Solids W, X, Y and Z are compounds of two different Group II metals. Some of their applications are described below. Compound W is used as a refractory lining material in kilns. Compound X is used as a building material. It can also be heated in a kiln to form compound Y. When Y is hydrated, it forms compound Z which is used agriculturally to treat soils.
Which statements about these compounds are correct?

1 Adding W to water has less effect on pH than adding Y.
2 Adding Z to soil increases the pH of the soil.
3 The metallic element in Y reacts with cold water more quickly than the metallic element in W.

Answer A.
My Name? How please?

For this question, first you need to identify what those w, x, y and z compounds are.
By looking at their applications, you can figure out that w is MgO (since it has a high mp, there it used as a refractory material)
Compound x is CaCO3 (it is used in marbles etc)
When CaCO3, that is X is heated, it produces Y which is CaO like this:

CaCO3 ----> CaO + CO2

Adding Y, to water produces Ca(OH)2 which is alkaline solution.

Now you know what these mystery compounds are.
When W(MgO) is added to water, it does not cause much change of pH since it has very little solubility in water (solubility with water increases down the group), whereas Y which is CaO is soluble, thus causes an increase in pH. - Statement 1 correct.

Z is Ca(OH)2 is an alkali. So it will raise the pH of the soil. - Statement 2 is correct.

The metallic element in Y is Ca. I think you know that Ca reacts readily with cold water, while Mg does not react readily with cold water but it reacts with Steam. - Hence Statement 3 is also correct.

So the answer is A.

 

[Rizwan Javed]

Can you please give the whole question?
Enthalpy changes of K2CO3 & KHCO3 you mentioned refer to enthalpy changes of formation or combustion for these compounds?

wow, you just cleared my concept. thank you! Okay, just one more please
View attachment 60114

We have to draw the Hess's law here. In b(iii), we found enthalpy change of K2CO3, it was -32.6 kJ/mol and in c(iii), we found enthalpy change of KHCO3, it was +23.19 kJ/mol , can you please draw the Hess's cycle for me??

 

[Iris Dylan Lane]

For this question, first you need to identify what those w, x, y and z compounds are.
By looking at their applications, you can figure out that w is MgO (since it has a high mp, there it used as a refractory material)
Compound x is CaCO3 (it is used in marbles etc)
When CaCO3, that is X is heated, it produces Y which is CaO like this:

CaCO3 ----> CaO + CO2

Adding Y, to water produces Ca(OH)2 which is alkaline solution.

Now you know what these mystery compounds are.
When W(MgO) is added to water, it does not cause much change of pH since it has very little solubility in water (solubility with water increases down the group), whereas Y which is CaO is soluble, thus causes an increase in pH. - Statement 1 correct.

Z is Ca(OH)2 is an alkali. So it will raise the pH of the soil. - Statement 2 is correct.

The metallic element in Y is Ca. I think you know that Ca reacts readily with cold water, while Mg does not react readily with cold water but it reacts with Steam. - Hence Statement 3 is also correct.

So the answer is A.

W.O.W.
Thanks a lot

 

[ashcull14]

 

[The Sarcastic Retard]

View attachment 60115

Can I get the diagram of electrochemical cell?

 

[ashcull14]

Can I get the diagram of electrochemical cell?

oh sorry

 

[Eugene99]

Can you please give the whole question?
Enthalpy changes of K2CO3 & KHCO3 you mentioned refer to enthalpy changes of formation or combustion for these compounds?

Its winter 2011 p22; could u please check?

 

[The Sarcastic Retard]

View attachment 60115

Concentration of Pb2+ is less than 1 moldm^-3.
Pb2+ + 2e- <===> Pb
Decreasing the concentration of Pb2+, the equilibrium shifts to the reactants side.
Remember this :
In half equations, if the eq. shifts towards electrons side, the E value of that half cell decreases and vice versa. So here the eq. shifts in direction where there is e- so the E value will be decreasing so the value lesser than -0.13 that is -0.17 is your answer.

 

[The Sarcastic Retard]

Its winter 2011 p22; could u please check?

BEND FEXO
Bond breaking is endo
Bond forming is exo
we have equation : 2KHCO3 ------> K2CO3 + CO2 + H2O
So breaking is KHCO3 and forming is K2CO3
So +(2 * c(iii)) - (b(iii)) = Enthalpy change of this reaction.

 

[Eugene99]

BEND FEXO
Bond breaking is endo
Bond forming is exo
we have equation : 2KHCO3 ------> K2CO3 + CO2 + H2O
So breaking is KHCO3 and forming is K2CO3
So +(2 * c(iii)) - (b(iii)) = Enthalpy change of this reaction.

do we not need to draw any cycle here?

 

[Rizwan Javed]

Its winter 2011 p22; could u please check?

I think you're looking for this
Edit : the equation is not balanced . It should have been 2KHCO3 instead of KHCO3.

BEND FEXO
Bond breaking is endo
Bond forming is exo
we have equation : 2KHCO3 ------> K2CO3 + CO2 + H2O
So breaking is KHCO3 and forming is K2CO3
So +(2 * c(iii)) - (b(iii)) = Enthalpy change of this reaction.

^This method by The Sarcastic Retard is equally applicable here.

 

[Eugene99]

View attachment 60120
I think you're looking for this
Edit : the equation is not balanced . It should have been 2KHCO3 instead of KHCO3.

^This method by The Sarcastic Retard is equally applicable here.

yep, exactly!
thanks

 

[The Sarcastic Retard]

View attachment 60120
I think you're looking for this
Edit : the equation is not balanced . It should have been 2KHCO3 instead of KHCO3.

^This method by The Sarcastic Retard is equally applicable here.

From where that HCl came?

 

[Syed Umar]

thank u so much I get that...here is another
Question is :
Compound A is a liquid which does not react with 2,4-DNPH reagent or with aqueous bromine. Suggest two structural formula of A.
(We found the molecular formula in previous part which was C2H4O2) Here is what the markscheme says
View attachment 60113
It's (d)
I know ethanoic acid is formed but isn't HCO2CH3 just another way of writing CH3CO2H?

No it is an ester actually.

 

[My Name]

Solids W, X, Y and Z are compounds of two different Group II metals. Some of their applications are described below. Compound W is used as a refractory lining material in kilns. Compound X is used as a building material. It can also be heated in a kiln to form compound Y. When Y is hydrated, it forms compound Z which is used agriculturally to treat soils.
Which statements about these compounds are correct?

1 Adding W to water has less effect on pH than adding Y.
2 Adding Z to soil increases the pH of the soil.
3 The metallic element in Y reacts with cold water more quickly than the metallic element in W.

Answer A.
My Name? How please?

THE COMPOUNDS:

W has to be MgO because it has high mp.
X has to be CaCO3 which when heated will give you Y that is CaO.
Now about Y when hydrated forms Z which can be used to treat soil.
So Z will be the metal Ca(OH)2.Okay?

THE STATEMENTS:

1 Adding W to water has less effect on pH than adding Y.
2 Adding Z to soil increases the pH of the soil.
3 The metallic element in Y reacts with cold water more quickly than the metallic element in W.

MgO doesn't dissovle much in water so won't cause change BUT Y that is CaO will as it is easily soluble. :3
So point 1 check!
Z is alkaline so yep it will result in change in pH of soil.So point 2 check.
For point 3 you need to recall the equation for the elements Ca and Mg.
Mg does not dissolve in cold water but Ca does. So point 3 check.

So A.

 

[Eugene99]

From where that HCl came?

When you read the full question, you'd know. It's winter 11, 9701/22. They reacted K2CO3 and KHCO3 in two seperate experiments to determine the enthalpy change for the thermal decomposition of KHCO3...We could take any steps in order to reach it, as the overall energy change is independent of the path taken

 

[Iris Dylan Lane]

THE COMPOUNDS:

W has to be MgO because it has high mp.
X has to be CaCO3 which when heated will give you Y that is CaO.
Now about Y when hydrated forms Z which can be used to treat soil.That will be Ca(OH)2.
So Z will be the metal Ca.Okay?

THE STATEMENTS:

1 Adding W to water has less effect on pH than adding Y.
2 Adding Z to soil increases the pH of the soil.
3 The metallic element in Y reacts with cold water more quickly than the metallic element in W.

MgO doesn't dissovle much in water so won't cause change BUT Y that is CaO will as it is easily soluble. :3
So point 1 check!
Z is alkaline so yep it will result in change in pH of soil.So point 2 check.
For point 3 you need to recall the equation for the elements Ca and Mg.
Mg does not dissolve in cold water but Ca does. So point 3 check.

So A.

Thanks a lot xD RJ helped out xD But ty for your time <3

 

[My Name]

Thanks a lot xD RJ helped out xD But ty for your time <3

Welcome.
Ohh RJ replied to the Bio thread also same time as me.
It's good I am revising something but I did take time to know what the compounds are. :/

 

[Iris Dylan Lane]

Welcome.
Ohh RJ replied to the Bio thread also same time as me.
It's good I am revising something but I did take time to know what the compounds are. :/

xD
At least you got it..i'm stupid.
Thanks Again

 

[My Name]

xD
At least you got it..i'm stupid.
Thanks Again

Welcome
But I take time! >.<