Chemistry: Post your doubts here!

[qwertypoiu]

Oh my lord!
idk wat the hell I hve been reading xP
Thankyew....I got it finally xD

Haha happens sometimes
And you're welcome

 

[forest822]

Can anyone explain this to me? Thank You.!

E. Change of formation of CO2= -395kjmol^-1
E. Change of formation of H2O= -297kimol^-1
E. Change of combustion of C2H6= -1620kJmol^-1

Calculate the enthalpy of the formation of ethane.
[-2(395)+3(-297)]-(-1620)
= -61kJmol^-1

WHY?

 

[asadalam]

Can anyone explain this to me? Thank You.!

E. Change of formation of CO2= -395kjmol^-1
E. Change of formation of H2O= -297kimol^-1
E. Change of combustion of C2H6= -1620kJmol^-1

Calculate the enthalpy of the formation of ethane.
[-2(395)+3(-297)]-(-1620)
= -61kJmol^-1

WHY?

Use Hess's Law and construct a cycle

2C + 3H2 ---> 2CO2 + 3H20 (These are the Del H of formation of CO2 and H20)
C2H6 + 3.5O2 ----> 2CO2 + 3H20 ( Del H of Comb of ethane)

As you can see both have the same products,hence you can construct a cycle and work out how C2H6 can be formed from 2C and 3H2.The cycle would give

2(-395) + 3(-297) = Del H of formation + (-1620)

Solve it and you'll get -61 kJ/mol

Read this too:http://www.chemguide.co.uk/physical/energetics/sums.html

 

[Metanoia]

Can anyone explain this to me? Thank You.!

E. Change of formation of CO2= -395kjmol^-1
E. Change of formation of H2O= -297kimol^-1
E. Change of combustion of C2H6= -1620kJmol^-1

Calculate the enthalpy of the formation of ethane.
[-2(395)+3(-297)]-(-1620)
= -61kJmol^-1

WHY?

Construct an equation for combustion of ethane.

C2H6 + 3.5O2 ----> 2CO2 + 3H20

Heat of combustion (reaction) = heat of formation of products - heat of formation of reactants

 

[fantastic girl]

Asak ppl
can someone tell me in Q4 (d) for reaction 2, is heating necessary ?
http://onlineexamhelp.com/wp-content/uploads/2014/08/9701_s14_qp_42.pdf

 

[zain ul abidin]

Asak ppl
can someone tell me in Q4 (d) for reaction 2, is heating necessary ?
http://onlineexamhelp.com/wp-content/uploads/2014/08/9701_s14_qp_42.pdf

Reaction is ES so will require Cl2+AlCl3/FeCl3

 

[fantastic girl]

Reaction is ES so will require Cl2+AlCl3/FeCl3

yea thats wat my miss taught me but ms says heat :/

 

[Mahnoorfatima]

U= C4H6O
Draw the displayed formula of two structural isomers of U that each contain the same functional groups as U:
IS THIS CORRECT??? If not can anyone explain WHY??
https://sketch.io/sketchpad/

 

[Studydayandnight]

HNE is a reactive compound.
CH3(CH2) 4CH(OH)CH=CHCHO
(c) Give the structural formulae of all of the carbon-containing compounds formed in each case when HNE is reacted separately with the following reagents.

(i) hot concentrated manganate(VII) ions in acid solution.

So what I came up with is: CH3(CH2)4CH(OH)CO2H + HO2CHO

but in the markscheme the OH- group and the CHO group have also been oxidised giving: CH3(CH2)4COCO2H and HO2CCO2H

What i don't get is that it says manganate ions, not dichromate, so how can the alcohol and the aldehyde group get oxidised as well?

 

[princess Anu]

Q what will be the result of oxidising this compound with Hot conc K2cR2O7.
Acc to me there should be no change!! any help will be appreciated!

 

[psychiatrist]

http://kinggsofchemistry.weebly.com/uploads/2/7/1/9/27197677/atomic_structure.pdf
Q4,6,17,18,27. ASAP.

 

[asadalam]

HNE is a reactive compound.
CH3(CH2) 4CH(OH)CH=CHCHO
(c) Give the structural formulae of all of the carbon-containing compounds formed in each case when HNE is reacted separately with the following reagents.

(i) hot concentrated manganate(VII) ions in acid solution.

So what I came up with is: CH3(CH2)4CH(OH)CO2H + HO2CHO

but in the markscheme the OH- group and the CHO group have also been oxidised giving: CH3(CH2)4COCO2H and HO2CCO2H

What i don't get is that it says manganate ions, not dichromate, so how can the alcohol and the aldehyde group get oxidised as well?

Well,isnt KMnO4 an oxidising agent?IT IS first and foremost,its the conditions that cause it to break the C=C,but along with that it will oxidise anything it can ,which include the OH and COH groups.

It isnt just for breaking the C=C,it oxidises too and that you shouldnt forget.Both k2cr207 and kmn04 oxidise equally,its that the conditions of kmn04 give some unique qualities to it,but if it oxidises in hot and conc form,it'll be most effective and oxidise all fully,as well as break the bond.

 

[asadalam]

A

Q what will be the result of oxidising this compound with Hot conc K2cR2O7.
Acc to me there should be no change!! any help will be appreciated!

Are you sure its Hot Conc K2Cr2O7 and not KMnO4?

 

[princess Anu]

Yeah, its not a CIE question..
I am just confused that is it correct to say that k2cr2O7 doesn't react with alkene under any conditions whatsoever? or can it react under hot conc conditions?

 

[Sarosh Jameel]

PLZ someone help me with this !!

 

[Xaptor16]

A mixture of 10cm3 of methane and 10cm3 of ethane was sparked with an excess of oxygen. After cooling to room temperature, the residual gas was passed through aqueous potassium hydroxide. All gas volumes were measured at the same temperature and pressure. What volume of gas was absorbed by the alkali?
A 15 cm3
B 20 cm3
C 30 cm3
D 40 cm3
the answer is C. i keep getting 36 cm3. now i know that their reaction would produce carbon dioxide and water. then the gases remaining would be excess O2 and CO2 at room temp and pressure. i think only CO2 would be absorbed by the alkali right? I'm not getting 30, could someone help me please?

 

[Xaptor16]

A solution of Sn2+ ions will reduce an acidified solution of MnO4 – ions to Mn2+ ions. The Sn2+ ions are oxidised to Sn4+ ions in this reaction. How many moles of Mn2+ ions are formed when a solution containing 9.5 g of SnCl 2 (Mr: 190) is added to an excess of acidified KMnO4 solution?
A 0.010
B 0.020
C 0.050
D 0.125
the answer is B. i dont get how?

 

[The Sarcastic Retard]

A mixture of 10cm3 of methane and 10cm3 of ethane was sparked with an excess of oxygen. After cooling to room temperature, the residual gas was passed through aqueous potassium hydroxide. All gas volumes were measured at the same temperature and pressure. What volume of gas was absorbed by the alkali?
A 15 cm3
B 20 cm3
C 30 cm3
D 40 cm3
the answer is C. i keep getting 36 cm3. now i know that their reaction would produce carbon dioxide and water. then the gases remaining would be excess O2 and CO2 at room temp and pressure. i think only CO2 would be absorbed by the alkali right? I'm not getting 30, could someone help me please?

CH4(g) + 2O2(g) -------> CO2(g) + 2H2O(l)
C2H6(g) + 3.5O2(g) --------> 2CO2(g) + 3H2O(l)
Residual gas is CO2 which is absorbed by KOH(aq)
CO2(g) + OH-(aq) ---------> CO3(-2)(aq) + H2O(l)
Volume of CO2 produced by CH4 and C2H6 is 10 and 20 cm^3 respectively, total volume of CO2 produced = 30 cm^3

 

[The Sarcastic Retard]

A solution of Sn2+ ions will reduce an acidified solution of MnO4 – ions to Mn2+ ions. The Sn2+ ions are oxidised to Sn4+ ions in this reaction. How many moles of Mn2+ ions are formed when a solution containing 9.5 g of SnCl 2 (Mr: 190) is added to an excess of acidified KMnO4 solution?
A 0.010
B 0.020
C 0.050
D 0.125
the answer is B. i dont get how?

Eqn 1 : Sn2+ = Sn4+ + 2e-
Eqn 2 : MnO4- + 8H+ + 5e- = Mn2+ + 4H2O
These equations are available in data booklet : http://www.cie.org.uk/images/164870-2016-specimen-data-booklet.pdf
Balance the number of electrons, SHOULD BE EQUAL IN BOTH THE EQN.
Eqn 1 : 5Sn2+ = 5Sn4+ + 10e-
Eqn 2 : 2MnO4- + 16H+ + 10e- = 2Mn2+ + 8H2O
Add Eqn 1 with Eqn 2 = 5Sn2+ + 2MnO4- + 16H+ = 5Sn4+ + 2Mn2+ + 8H2O
Ignore liquids, we are just taking ions into account.
2moles of MnO4- reacts with 5moles of Sn2+
2 Mn---> 5 Sn
0.4 Mn <- 1 Sn
Hence, 9.5/190 * 0.4 = 0.02mol

 

[psychiatrist]

http://kinggsofchemistry.weebly.com/uploads/2/7/1/9/27197677/atomic_structure.pdf
Q4,6,17,18,27. ASAP.
Metanoia asadalam The Sarcastic Retard