Chemistry: Post your doubts here!

[geek101]

Can anyone please explain question 16 of this paper? why isnt the answer D? since its the enthalpy change of formation the bond energy of the reactants should matter most not of the products.
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s07_qp_1.pdf

inorder to form a H-I bond lesser energy is required since the bond energy is low and so the enthalpy of formation is high, and the H-Cl bond requires a higher value of energy since it has a greater bond energy value so the enthalpy of formation is low. so it basically depends on the bond energy values, which will show the overall energy of formation, which is more or less indirectly proportional to the bond energy values. does it sound confusing?

look at it this way....if you find out the enthalpy change of formation it is reactants - products. If the product lets suppose is H-I so the product has low energy. so the reactants-products gives a positive value. If the product is H-Cl it has a high energy so reactants-products gives you a low value. like in the question, so we know that it is defined using bod energies.

 

[hmlahori]

inorder to form a H-I bond lesser energy is required since the bond energy is low and so the enthalpy of formation is high, and the H-Cl bond requires a higher value of energy since it has a greater bond energy value so the enthalpy of formation is low. so it basically depends on the bond energy values, which will show the overall energy of formation, which is more or less indirectly proportional to the bond energy values. does it sound confusing?

look at it this way....if you find out the enthalpy change of formation it is reactants - products. If the product lets suppose is H-I so the product has low energy. so the reactants-products gives a positive value. If the product is H-Cl it has a high energy so reactants-products gives you a low value. like in the question, so we know that it is defined using bod energies.

I guess it makes sense now thanks.

 

[Student12]

Someone Describe the use of lime in agriculture ?

 

[Student12]

Someone Describe the use of lime in agriculture ?

is it to neutrilise acid and to lower the scidity of the soil?

 

[geek101]

Someone Describe the use of lime in agriculture ?

cheated of wikipedia but these are quite a few uses :

The effects of agricultural lime on soil are:

• it increases the pH of acidic soil (the higher the pH the less acidic the soil); in other words, soil acidity is reduced and alkalinity increased
• it provides a source of calcium and magnesium for plants
• it permits improved water penetration for acidic soils
• it improves the uptake of major plant nutrients (nitrogen, phosphorus, and potassium) of plants growing on acid soils.

 

[geek101]

can someone pleeease explain why Q34 is B??

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w02_qp_1.pdf

 

[Student12]

cheated of wikipedia but these are quite a few uses :

The effects of agricultural lime on soil are:

• it increases the pH of acidic soil (the higher the pH the less acidic the soil); in other words, soil acidity is reduced and alkalinity increased
• it provides a source of calcium and magnesium for plants
• it permits improved water penetration for acidic soils
• it improves the uptake of major plant nutrients (nitrogen, phosphorus, and potassium) of plants growing on acid soils.

Thanks

 

[Zishi]

can someone pleeease explain why Q34 is B??

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w02_qp_1.pdf

Because 1 will decompose to give metal oxide and CO2 gas, 2 will also decompose to give Metal Oxide, NO2 and O2. 3 is already a metal oxide, so it won't decompose. As gases escape, we notice a decrease in mass.

 

[Student12]

Can anyone help me how to find angles??

 

[Amy Bloom]

H-F----------------------- ▲Hf = -271 Kjmol-1
H-Cl---------------------- ▲Hf = -92 Kjmol-1
H-Br---------------------- ▲Hf= -36 Kjmol-1
H-I------------------------ ▲Hf = +26.5 Kjmol-1

So the bond enthalpy for HCL is +431 HBr is +366 and HI is +299 which is a given data.
Hydrogen halides formed are all simple molecular compounds which are gaseous at room temp. As the size oh the Hydrogen atom increases down the Gp, the H-X bond length also increase.Consequently, the H-X bond enthalpy decreases down the Gp.As you notice that the decrease in bond enthalpies means that the stabilities of the Hydrogen halides decrease down the group. HCl is stable at 1500 degrees, HBR decomposes at 800 decrease and HI decomposes at 500 thus HI is least stable of these three Hydrogen halides.

I've given a try

How can the size of the hydrogen atom increase down the group? It is the H-X bond which increases down the group as atomic radius of the halogen atom increases.
I agree that the H-X bond gets weaker as you go down Group VII and so accounting for the decreasing bond energies. What about the enthalpy of formation of H-X?
Enthalpy of formation: the heat change that occurs when 1 mole of H-X is formed from its elements at std conditions. Hey, thanks for helping me.

 

[Amy Bloom]

Can anyone help me how to find angles??

You have to consider the valence shell electron pair repulsion theory.

• The theory gives the following order of repulsion of electron pairs: Lone pair/ lone pair > lone pair/bond pair > bond pair/ bond pair

From this you can deduce the shape and hence the angle.
Hope my answer has been helpful to you.

 

[geek101]

Can anyone help me how to find angles??

an addition to what Amy Bloom has said. Draw the compound identify the no. of bond pairs and lone pairs on the central atom of your bond. If:
4 bond pairs only: 109.5 >tetrahedral
3 bond pairs and 1 lone pair: 107 > triangular pyramidal
2 bond pairs and 2 lone pairs: 104.5 >V-shape
3 bond pairs only: 120 >triagonal planar
2 bond pairs only: 180 >linear
6 bond pairs only: 90 >octahedral

the arrow follows the type of structure. good luck with these!

 

[Amy Bloom]

an addition to what Amy Bloom has said. Draw the compound identify the no. of bond pairs and lone pairs on the central atom of your bond. If:
4 bond pairs only: 109.5 >tetrahedral
3 bond pairs and 1 lone pair: 107 > triangular pyramidal
2 bond pairs and 2 lone pairs: 104.5 >V-shape
3 bond pairs only: 120 >triagonal planar
2 bond pairs only: 180 >linear
6 bond pairs only: 90 >octahedral

the arrow follows the type of structure. good luck with these!

Maybe i can add something to this:
2 bond pairs and 3 lone pairs: Square based bi-pyramidal, but i don't know exactly the angle. You can see that in SF4. I've got this as a test.

 

[Student12]

How can the size of the hydrogen atom increase down the group? It is the H-X bond which increases down the group as atomic radius of the halogen atom increases.
I agree that the H-X bond gets weaker as you go down Group VII and so accounting for the decreasing bond energies. What about the enthalpy of formation of H-X?
Enthalpy of formation: the heat change that occurs when 1 mole of H-X is formed from its elements at std conditions.

Yea i meant the size of the HALOGEN atom increases down the Group. The formation part would be helpful from someone else nw

 

[Student12]

Can anyone Define bond polarity ?

 

[Amy Bloom]

Yea i meant the size of the HALOGEN atom increases down the Group. The formation part would be helpful from someone else nw

Okay. Thanks for trying to help me though. I appreciate it.

 

[Student12]

Okay. Thanks for trying to help me though. I appreciate it.

Aww.. thank you So how is your chemistry going ? Paper1 ?

 

[Amy Bloom]

Aww.. thank you So how is your chemistry going ? Paper1 ?

Chemistry..... I don't know for now, for i haven't completed the syllabus yet & hv not worked out any past papers coz i'm appearing for November. Still do you have doubts?

 

[Daniyal890]

Guys is the application booklet extensive enough to rely upon and can we study only from there or should we buy another book also for application part of p4

 

[Student12]

Maybe i can add something to this:
2 bond pairs and 3 lone pairs: Square based bi-pyramidal, but i don't know exactly the angle. You can see that in SF4. I've got this as a test.

isnt SF4 trigonal bi-pyramidal ? SF6 is octahedral or square bipyramidal or for that any 5 bonding pair and 1 lone pair, 4 bonding pair and 2 lone pair will be square bipyramidal.