Chemistry: Post your doubts here!

[Holmes]

Could anyone help me with this. Looks easy but my results don't match!!!!!

12 Use of the Data Booklet is relevant to this question.
This question should be answered using bond enthalpy data. The equation for the complete combustion of methane is given below.
CH4 + 2O2 → CO2 + 2H2O
What is the enthalpy change of combustion of methane?
A –1530kJ mol–1
B –1184kJ mol–1
C –770kJ mol–1
D –688kJ mol–1

To my inadequate knowledge the answer is "D"
CH4 + 2O2 → CO2 + 2H2O

Bond energy of reactants:
=4(410)+2(496)
= +2632 kj/mol

now look at The bond energy of Products:
= 2 (740)+ 4 (460)
= +3320 kj/mol

last step:
enthalpy change = B.E of Reactants - B.E. of Products
= 2632 -3320
= - 688 kj/mol

HOPE IT HELPS!

 

[Holmes]

anyone help
PLZ


nov 2015 (13)

 

[Holmes]

nov 2015 (13)

 

[Mstudent]

To my inadequate knowledge the answer is "D"
CH4 + 2O2 → CO2 + 2H2O

Bond energy of reactants:
=4(410)+2(496)
= +2632 kj/mol

now look at The bond energy of Products:
= 2 (740)+ 4 (460)
= +3320 kj/mol

last step:
enthalpy change = B.E of Reactants - B.E. of Products
= 2632 -3320
= - 688 kj/mol

HOPE IT HELPS!

Your answer is correct, but if you look in the Data Booklet, In the Bond Energies section in polyatomic molecules section: there are different values for C double bond C and C Double Bond C in CO2.

 

[Thelastmoment]

Does anyone have complete ORGANIC notes I lost mine I (need them for the MCQ exam because I forgot most of the reactions and important points)


I will be really grateful if someone shares their...

Try Z notes or chemguide there's plenty of notes on the internet <33

 

[Thelastmoment]

Does anyone have complete ORGANIC notes I lost mine I (need them for the MCQ exam because I forgot most of the reactions and important points)


I will be really grateful if someone shares their...

There's notes by a dude called Daniel Lim if i remember correct he got 4A*s it seems and he posted his notes
I'll give you some links hope they help you,

http://daniellimjj.blogspot.com/2014/06/full-notes-for-cambridge-international.html
http://www.chemguide.co.uk/mechmenu.html
http://znotes.org/as-chemistry/

 

[Mstudent]

The Answer for question 6 is D, 536kj mol, I suppose?

This is how I got it:

1. Write the equation down:

S8 + 8O2 ----------> 8 SO2

2. Write down the bond energies:

The Change in heat for the equation is -2376

The left side of the equation:
(2232) + (8 X 496)

The right side

let Y= S=O

SO, 8(2Y)

3. SOLVE!!
(2232)+(8 X 496) - (16Y) = -2376
Y = 536

HOPE THE HELPS!!!

 

[Holmes]

 

[Holmes]

yeah it did thanks

 

[insharah]

A sample of ethyl propanoate is hydrolysed by heating under reflux with aqueous sodium hydroxide. The two organic products of the hydrolysis are separated, purified and weighed. Out of the total mass of products obtained, what is the percentage by mass of each product?
A 32.4% and 67.6%
B 38.3% and 61.7%
C 42.3% and 57.7%
D 50.0% and 50.0%
answer is A
PLEASE HELP
ANY HELP WILL BE APPRECIATED

 

[Mstudent]

A sample of ethyl propanoate is hydrolysed by heating under reflux with aqueous sodium hydroxide. The two organic products of the hydrolysis are separated, purified and weighed. Out of the total mass of products obtained, what is the percentage by mass of each product?
A 32.4% and 67.6%
B 38.3% and 61.7%
C 42.3% and 57.7%
D 50.0% and 50.0%
answer is A
PLEASE HELP
ANY HELP WILL BE APPRECIATED

OHHHH This got me into a trap too!

SO basically You have to know that HYDROLYSIS OF ESTERS WITH NAOH ------> SODIUM SALT OF CARBOXYLLIC ACID + THE ALCOHOL

so make the equation : CH3CH2COOCH2CH3 + NAOH -------> CH3CH2COONA + CH3CH2OH

THEN assume you have one mole of ethyl ethanoate, so you will have 1 mole of all the products, the salt and the alcohol

multiply the moles with the MR of the products to find mass, you will get 96g of the salt and 46g of the alcohol

DON'T FORGET TO FIND TOTAL MASS WHICH IS 142G

THEN just find percentage by mass of total mass : { 46/142} X 100 = 32.4% and for the other one 67.6%

HOPE THIS HELPS YOU

 

[amina1300]

anyone help
PLZ

View attachment 62328
nov 2015 (13)

N2(g) + 3H2(g) 2NH3(g)
N2 = 7.2g/6dm3 = 1.2 mol/dm3
H2 = 12g/6dm3 = 2 mol/dm3

Kc= (NH3)^2/((H2)^3*(N2))
6*10^-2= x^2/ (2^3* 1.2)
x=0758=0.76 which is B

 

[amina1300]

View attachment 62332

View attachment 62333

volume of M = V
volume of N = 3 V.
Total volume = 4V.
P1V1=P2V2.
(1 x 10^5)(V)=P2(4V)
P2 = 25000 Pa

Change in temp ; 20 to 100 degree Celsius.
New pressure P2/T1=P3/T2.
25000/293=P3/373
Hence, P3 is 3.18 x 10^4 Pa.

 

[Brisingr]

Which compound contains two different elements with identical oxidation states?
A HCl O B Mg(OH)2 C Na2SO4 D NH4Cl
someone please help

 

[insharah]

OHHHH This got me into a trap too!

SO basically You have to know that HYDROLYSIS OF ESTERS WITH NAOH ------> SODIUM SALT OF CARBOXYLLIC ACID + THE ALCOHOL

so make the equation : CH3CH2COOCH2CH3 + NAOH -------> CH3CH2COONA + CH3CH2OH

THEN assume you have one mole of ethyl ethanoate, so you will have 1 mole of all the products, the salt and the alcohol

multiply the moles with the MR of the products to find mass, you will get 96g of the salt and 46g of the alcohol

DON'T FORGET TO FIND TOTAL MASS WHICH IS 142G

THEN just find percentage by mass of total mass : { 46/142} X 100 = 32.4% and for the other one 67.6%

HOPE THIS HELPS YOU

THANK YOU SO MUCH FOR THIS HELP

 

[Mstudent]

THANK YOU SO MUCH FOR THIS HELP

You are welcome anytime!!!

 

[amina1300]

Which compound contains two different elements with identical oxidation states?
A HCl O B Mg(OH)2 C Na2SO4 D NH4Cl
someone please help

A is the answer as H + and Cl + and O 2- (overall 0 so this is correct )
Mg +2 OH - (wrong)
Na + So4 -2 (wrong)
NH4+ Cl - (wrong)

 

[amina1300]

https://papers.gceguide.com/A Levels/Chemistry (9701)/9701_w16_qp_12.pdf
Q11

 

[amina1300]

https://papers.gceguide.com/A Levels/Chemistry (9701)/9701_w16_qp_12.pdf
Q15 Shouldnt the answer to this be D?
Cl +7 O -8 overall -1 ??

 

[Dukula Jayasinghe]

https://papers.gceguide.com/A Levels/Chemistry (9701)/9701_w16_qp_12.pdf
Q11

Well, lets look upon (D) first, shall we?
Now when the tap B is open, what will happen is pressure decreases as well as N2 concerntration decreases cos Li+N2 will occur. Position of equilibrium shifts to the left, causing % of NH3 to decrease. (D) is wrong.
Looking upon (C)
When A is open, pressure decreases, yield of ammonia decreases as position of equilibrium shifts to the left.
Looking upon (B)
When both are open, oh my god, the pressure will be even lower,....
Looking upon (A)
Now this is the answer. This is because pressure is higher so position of equilibrium is shifted to RHS so % of ammonia increases