Chemistry: Post your doubts here!

[Alice123]

How to calculate enthalpy change of a reaction when enthalpy change of formation or combustions r given??? is it enthalpy change of (products- reactants ) or is it the other way round??? can any1 explain me in details??? littlecloud11

 

[iKhaled]

How to calculate enthalpy change of a reaction when enthalpy change of formation or combustions r given??? is it enthalpy change of (products- reactants ) or is it the other way round??? can any1 explain me in details??? littlecloud11

can you give an example pls ?

 

[Alice123]

Q2b of http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s12_qp_22.pdf
Q3ai http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w09_qp_21.pdf iKhaled these were the best examples i cud find

 

[Alice123]

When drawing optical isomers, do we attach the groups to the C anywhere we like?? or is there sum other rule, like the heavier one shud b at the top?? iKhaled or littlecloud11

 

[iKhaled]

When drawing optical isomers, do we attach the groups to the C anywhere we like?? or is there sum other rule, like the heavier one shud b at the top?? iKhaled or littlecloud11

no anywhere u like it doesn't matter but dont forget if it asks u to show it displayed then u must show the bonds in CN and any functional group and u have to draw it in a 3D..i will look at the enthalpy question now for u

 

[littlecloud11]

How to calculate enthalpy change of a reaction when enthalpy change of formation or combustions r given??? is it enthalpy change of (products- reactants ) or is it the other way round??? can any1 explain me in details??? littlecloud11

When you have enthalpy of formation or combustion given it's not always enthalpy change of product- reactant. It's best if you draw the Hess's cycle and use the direction of arrows for your calculation. That way you'll never go wrong.


Here, for the enthalpy change of reaction it's the enthalpy change of formation of product (CH4) - the enthalpy change of formation of reactant (CO2)



Here, the enthalpy change of reaction is the = the enthalpy change of combustion of the reactants - the enthalpy change of combustion of products.

So you see, this is case specific. I would suggest you draw a fully labeled cycle before attempting to make calculations.

 

[iKhaled]

Q2b of http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s12_qp_22.pdf
Q3ai http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w09_qp_21.pdf iKhaled these were the best examples i cud find

for 3ai and these type of questions u need to draw a cycle that will help u to determine the enthalpy change from the given values like this one for example
to find H use vectors..like go against the left vector so u have to minus it then go with the right vector so u add it..did u get it ? i can explain more if u couldn't get a part of what i said

 

[Alice123]

When you have enthalpy of formation or combustion given it's not always enthalpy change of product- reactant. It's best if you draw the Hess's cycle and use the direction of arrows for your calculation. That way you'll never go wrong.

View attachment 24753
Here, for the enthalpy change of reaction it's the enthalpy change of formation of product (CH4) - the enthalpy change of formation of reactant (CO2)

View attachment 24752

Here, the enthalpy change of reaction is the = the enthalpy change of combustion of the reactants - the enthalpy change of combustion of products.

So you see, this is case specific. I would suggest you draw a fully labeled cycle before attempting to make calculations.

Thankyou soooooooo much.... u've made this very easy for me

 

[Alice123]

for 3ai and these type of questions u need to draw a cycle that will help u to determine the enthalpy change from the given values like this one for example
to find H use vectors..like go against the left vector so u have to minus it then go with the right vector so u add it..did u get it ? i can explain more if u couldn't get a part of what i said

May Allah bless u with good grades!!!! Thanks i got it, i thot we had to use formulas for this

 

[littlecloud11]

Thankyou soooooooo much.... u've made this very easy for me

No problem! =D

 

[Alice123]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s12_qp_22.pdf
3dii, My skeletal formulae don't match with the markscheme

 

[littlecloud11]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s12_qp_22.pdf
3dii, My skeletal formulae don't match with the markscheme

 

[iKhaled]

View attachment 24758

do u have to draw the carboxylic acid group in skeletal formula too or we can just add co2h ?

 

[syed1995]

do u have to draw the carboxylic acid group in skeletal formula too or we can just add co2h ?

We need to show everything attached to the chain so yeah =O would be different than -OH.. except for the C-H bonds.. and the vertices on the chain are C atoms..

 

[syed1995]

littlecloud11

You're awesome

Thanks for clearing that up!

 

[PhyZac]

Define dynamic equilibrium and reversible reaction.

 

[PANDA-]

Define dynamic equilibrium and reversible reaction.

Dynamic equilibrium:
A reversible reaction in which the rate of forward reaction is equal to the rate of backward reaction in a closed system, and there is no change in the concentration of reactants and products.

Reversible reaction:
A reaction that does not go to completion as the the reactants react to give products, which in turn react to form the inital reactants again.

 

[PhyZac]

Dynamic equilibrium:
A reversible reaction in which the rate of forward reaction is equal to the rate of backward reaction in a closed system, and there is no change in the concentration of reactants and products.

Reversible reaction:
A reaction that does not go to completion as the the reactants react to give products, which in turn react to form the inital reactants again.

Exactly what I wanted, Jazaka Allah Khairan!! Thanks aloot!!
May Allah S.W.T make the exam easy for you and all of us Aameen. May Allah S.W.T have mercy on you and your family.! Aameen.

 

[PANDA-]

Exactly what I wanted, Jazaka Allah Khairan!! Thanks aloot!!
May Allah S.W.T make the exam easy for you and all of us Aameen. May Allah S.W.T have mercy on you and your family.! Aameen.

Your Duaa made my day.
You're welcome

 

[snoonono]

I want exact definitions of relative atomic, isotopic, molecular and formula
masses, based on the 12C scale the way is should be answered ASAP! Thanks in advance