Chemistry: Post your doubts here!

[daredevil]

Just a tip for Organic Chemistry questions: My teacher always said "Draw the structure in a way you understand..." so the key is to draw the structure then solve, even if skeletal formula is given, write it in structural or displayed form then work out the answer...

yh my teacher told us to do it in our head and practice that way bcz we need to complete our paper and we don't have time for drawing but i can almost never solve such questions w/o drawing first so i just draaw up a rough displayed formula quickly to help clear it out.

 

[ahmed abdulla]

yh my teacher told us to do it in our head and practice that way bcz we need to complete our paper and we don't have time for drawing but i can almost never solve such questions w/o drawing first so i just draaw up a rough displayed formula quickly to help clear it out.

10 A student calculated the standard enthalpy change of formation of ethane, C2H6, using a method
based on standard enthalpy changes of combustion.
He used correct values for the standard enthalpy change of combustion of ethane
(–1560 kJ mol–1) and hydrogen (–286 kJ mol–1) but he used an incorrect value for the standard
enthalpy change of combustion of carbon. He then performed his calculation correctly. His final
answer was –158 kJ mol–1.
What did he use for the standard enthalpy change of combustion of carbon?
A –1432 kJ mol–1
B –860 kJ mol–1
C –430 kJ mol–1
D –272 kJ mol–1

 

[Student12]

If you know the method, then you should be able to do the working too.

If I knew how to solve I wouldn't be asking.. Could u please explain in detail or someone??

 

[daisyy]

quote="Alice123, post: 607701, member: 44056"]see da above post

can any of you help in the question i posted above? Thanks [/quote]
Pleaase answer my question?? :/

 

[Zarif009]

IMPORTANT QUESTION!

What types of enthalpy changes can only be obtained by application of the Hess's law?

 

[Aymen Ezazi]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w10_qp_11.pdf
Q5 plzz thanku

 

[daredevil]

10 A student calculated the standard enthalpy change of formation of ethane, C2H6, using a method
based on standard enthalpy changes of combustion.
He used correct values for the standard enthalpy change of combustion of ethane
(–1560 kJ mol–1) and hydrogen (–286 kJ mol–1) but he used an incorrect value for the standard
enthalpy change of combustion of carbon. He then performed his calculation correctly. His final
answer was –158 kJ mol–1.
What did he use for the standard enthalpy change of combustion of carbon?
A –1432 kJ mol–1
B –860 kJ mol–1
C –430 kJ mol–1
D –272 kJ mol–1

the answer is C.
make up a Hess's triangle. use x to be the emthalpy of conbustion of C used by the student.put in the enthalpies of combustion of ethane and hydrogen. balance the triangle and multiply the enthalpies accordingly. the resulting equation will be: -

3(-286) +2x - (-1560) = -158

solve it out and x = -430 = enthalpy of combustion of C used by the student.

 

[daredevil]

IMPORTANT QUESTION!

What types of enthalpy changes can only be obtained by application of the Hess's law?

all kinds i think

 

[syed1995]

daredevil is on fire

 

[daredevil]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_11.pdf
Q5 plzz thanku

the mass of nickel in the coin is 20% of 10 = 2 g
calculate the no. of moles of Ni in the coin: -
2/58.7
multiply that no of moles with the avogadro's constant (6*10^23)
the answer u get is 2.05*10^22

 

[daredevil]

daredevil is on fire

i am getting a confidence boost by solving these questions and i need it for tuesday so yuppp i'm on FiiRREE!!

 

[1357913579]

http://papers.xtremepapers.com/CIE/...d AS Level/Chemistry (9701)/9701_w09_qp_1.pdf
number-28
thnks

 

[Eshika]

An organic compound, X, will react with an excess of calcium metal to produce a salt with the
empirical formula CaC4H6O4.

What could be the identity of X?
1 ethanoic acid
2 butanedioic acid
3 methylpropanedioic acid

 

[AbbbbY]

Solve this question please
A student calculated the standard enthalpy change of formation of ethane, C2H6, using a method based on standard enthalpy changes of combustion.
He used correct values for the standard enthalpy change of combustion of ethane (–1560 kJ mol–1) and hydrogen (–286 kJ mol–1) but he used an incorrect value for the standard enthalpy change of combustion of carbon. He then performed his calculation correctly. His final answer was –158 kJ mol–1.
What did he use for the standard enthalpy change of combustion of carbon?
A –1432 kJ mol–1
B –860 kJ mol–1
C –430 kJ mol–1
D –272 kJ mol–1
Ans is C...i cant get the right answer :/

First things first,

Δ Hc = ΔHr - ΔHp
(Since it specifies he used combustion values)

Formation of Ethane:
2C + 3H2 -> C2H6

Combustion of Ethane:
C2H6 + 7/2O2 -> 2CO2 + 3H2O (-1560 kJ/mol)

Combustion of Hydrogen:
H2 + 1/2O2 -> H2O (-286 kJ/mol)

Combustion of Carbon:
C + O2 -> CO2 ( x kJ/mol)

Now, using the formula stated above,

(2x)+(-286*3)-(-1560) = -158
2x = -158-702
x= -430kJ/mol

I know it looks long, but takes no more than half a minute to solve.

 

[AbbbbY]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s06_qp_1.pdf
Q2

3 peaks; 35 35, 35 37, 37 37

 

[AbbbbY]

Oh my. Just saw I was wasting time on answered questions. Opened the thread and it took me a few pages back -___-

 

[Fma 07]

hey

Jazaka'Allah Thankyou

 

[daredevil]

An organic compound, X, will react with an excess of calcium metal to produce a salt with the
empirical formula CaC4H6O4.

What could be the identity of X?
1 ethanoic acid
2 butanedioic acid
3 methylpropanedioic acid

ethanoicc acid??

bcz 2 organic molecules will be needed to balance out the 2+ valency of Ca

 

[daredevil]

3 peaks; 35 35, 35 37, 37 37

i dont get this one

 

[Eshika]

ethanoicc acid??

bcz 2 organic molecules will be needed to balance out the 2+ valency of Ca

yes it is ethanoic acid but how do u get the empirical formula that they have given to us ?