Chem Question Acidity

[osimsDDS]

This doesnt make sense to me:

Of the following, which is the strongest acid?
a) BH3
b) CH4
c) NH3
d) OH2
e) HF

I was looking for the least electronegative atom attached to the hydrogen....wouldnt it make sense because those are held less tightly...they say the answer is HF, but isnt the hydrogen held very tightly by the very electronegative F???? so it wouldnt give up the Hydrogen...

Also is A not the answer because the hydrogens in BH3 are anions? thanks

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[osimsDDS]

somebody out there!!!!

 

[Glycogen]

This doesnt make sense to me:

Of the following, which is the strongest acid?
a) BH3
b) CH4
c) NH3
d) OH2
e) HF

I was looking for the least electronegative atom attached to the hydrogen....wouldnt it make sense because those are held less tightly...they say the answer is HF, but isnt the hydrogen held very tightly by the very electronegative F???? so it wouldnt give up the Hydrogen...

Also is A not the answer because the hydrogens in BH3 are anions? thanks

Well,if we are comparing bad with worse! then HF is the strongest.Among these choices HF seems more logical.I choose BH3 right after I looked at the Q as BH3 is a lewis acid but then as you go across the periodic table acidity increases (except halogen which increases down the group) so F must be more acidic than B.I am not sure about it though,this just seems logical.

 

[osimsDDS]

I also chose BH3 but iam wondering maybe its because its a hydride anion? I mean the Boron is a cation and the Hydrogens are anions so it would give off H- into solution which is not the same as H+....is that correct?

 

[Mstoothlady2012]

why wouldn't it be D?

 

[Glycogen]

I also chose BH3 but iam wondering maybe its because its a hydride anion? I mean the Boron is a cation and the Hydrogens are anions so it would give off H- into solution which is not the same as H+....is that correct?

I can not follow your reasoning.We know for sure that BH3 is an acid,the only matter is that,there is no better option among choices other than HF.Yes,I'm agree that HF in a weak acid but this is when we are comparing it with other hydrogen halide.In this case,I think it does make sense if we assume that is stronger than BH3.I think it is all about the more electronegativity of F.
Check this out:
http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch01/ch1-8.html

 

[osimsDDS]

why wouldn't it be D?

water? not likely...more likely it would be either BH3 or HF....NH3 is a base

 

[Mstoothlady2012]

water? not likely...more likely it would be either BH3 or HF....NH3 is a base

I don't think BH3 would give up its hydrogen so easily and like you said F is very electronegative so doesn't give up its H easily either.

 

[Danny289]

This doesnt make sense to me:

Of the following, which is the strongest acid?
a) BH3
b) CH4
c) NH3
d) OH2
e) HF

I was looking for the least electronegative atom attached to the hydrogen....wouldnt it make sense because those are held less tightly...they say the answer is HF, but isnt the hydrogen held very tightly by the very electronegative F???? so it wouldnt give up the Hydrogen...

Also is A not the answer because the hydrogens in BH3 are anions? thanks

When comparing atoms within the same row of the periodic table, the more electronegative the anionic atom in the conjugate base, the better it is at accepting the negative charge.


HF ------------> H+ + F-
F the most electronegative---------> F- the most stable in this case
hope it helped
good luck

 

[drgreen]

I also chose BH3 but iam wondering maybe its because its a hydride anion? I mean the Boron is a cation and the Hydrogens are anions so it would give off H- into solution which is not the same as H+....is that correct?

i don't think they are hydrides, BH3 has 3 covalent bonds since B has 3 valence electrons. Hydrides are seen when elements in Group IA and IIA are used. eg. NaBH4 and LiAlH4.

 

[osimsDDS]

When comparing atoms within the same row of the periodic table, the more electronegative the anionic atom in the conjugate base, the better it is at accepting the negative charge.


HF ------------> H+ + F-
F the most electronegative---------> F- the most stable in this case
hope it helped
good luck

Question Danny, BH3, you have hydride correct? H- because the Boron is +3...so the hydride anion is not electronegative at all correct?? is this correct thanks

 

[HoangDDS]

You already know that HA + H20 <---> A- + H30+ and the more stable the conjugate base (A-) the more acidic HA is, therefore it drives the reaction towards the product more.

HF + H20 ----> F- + H3O+
HF is a weak acid b/c its conjugate base F- is a stronger base than H20 therefore it drives the reaction more to the product side than the reverse.

BH3 + H20 <---- BH2 - + H3O+
Since H20 is a stronger base than BH2- (not even stable) the reaction is gonna favor the reactant / left side.

But actually by looking at the choices, I already notice that none of the choices were an acid besides HF. Therefore I would've picked HF anyways.

 

[Danny289]

Question Danny, BH3, you have hydride correct? H- because the Boron is +3...so the hydride anion is not electronegative at all correct?? is this correct thanks

this question is not related to your opening thread. you can ask about "H" atom. and the answer is "H" has electronegativity as other elements. don't make it complicated. it is easy lets try for BH3:
BH3====> BH2- + H+
BH2- is not stable conjugate base because "B" is not strong electronegative( in this case comparing to HF). if you want to arrange these compands based on acidity you will have:
HF > H2O > NH3 > CH4>BH3

 

[osimsDDS]

this question is not related to your opening thread. you can ask about "H" atom. and the answer is "H" has electronegativity as other elements. don't make it complicated. it is easy lets try for BH3:
BH3====> BH2- + H+
BH2- is not stable conjugate base because "B" is not strong electronegative( in this case comparing to HF). if you want to arrange these compands based on acidity you will have:
HF > H2O > NH3 > CH4>BH3

So basically when looking at acidity you look for the most electronegative ion formed in the conjugate base (this is along the same period)

Now how about basicity? does the same go for basicity? thanks

 

[Danny289]

So basically when looking at acidity you look for the most electronegative ion formed in the conjugate base (this is along the same period)

Now how about basicity? does the same go for basicity? thanks

good question
A convenient way to look at basicity is based on electron pair availability more pair electrons available more basic:
then we will have stronger base:
BH2- > CH3- > NH2- > HO- > F-

 

[osimsDDS]

good question
A convenient way to look at basicity is based on electron pair availability more pair electrons available more basic:
then we will have stronger base:
BH2- > CH3- > NH2- > HO- > F-

wow this makes a lot of sense thanks alot man...you made my day

So basically F- has only 1 electron available, O has 2, N has 3, C has 4, and B has 5 correct?

 

[Danny289]

wow this makes a lot of sense thanks alot man...you made my day

So basically F- has only 1 electron available, O has 2, N has 3, C has 4, and B has 5 correct?

your welcome buddy,

Your resoning is not correct think about electronegativity again: "F" in F- is more electronegative than "B" in BH2- then BH2- has more electron pair available.

 

[joonkimdds]

why wouldn't it be D?

d) OH2
i think this is same as H2O so it has pH= 7 = neutral = neither acid nor base.

 

[Mstoothlady2012]

good question
A convenient way to look at basicity is based on electron pair availability more pair electrons available more basic:
then we will have stronger base:
BH2- > CH3- > NH2- > HO- > F-

ARe you sure this is right? I think it shoud be....

F- > OH- > NH2

 

[HoangDDS]

Just wanted to add that binary acid trend (ie. HCl):

increases left to right (across a row)
ie. NH3<H20<HF

and increases down a period
ie. HF<HCl<HBr<HI

HF is a weak acid because its more EN than the other halogen so it's gonna hold on to the H tighter. Which is why HF only dissociates about 10-15% H+ in water.

So the basicity trend would be the opposite of the above.

 

[Mstoothlady2012]

Just wanted to add that binary acid trend (ie. HCl):

increases left to right (across a row)
ie. NH3<H20<HF

and increases down a period
ie. HF<HCl<HBr<HI

HF is a weak acid because its more EN than the other halogen so it's gonna hold on to the H tighter. Which is why HF only dissociates about 10-15% H+ in water.

So the basicity trend would be the opposite of the above.

So what I wrote is wrong?

 

[Mstoothlady2012]

good question
A convenient way to look at basicity is based on electron pair availability more pair electrons available more basic:
then we will have stronger base:
BH2- > CH3- > NH2- > HO- > F-

Danny I am really confused by this...you have got to help me out

I thought since HF is a weak acid...F- would be a really strong base. Instead you have it as the least basic.

 

[HoangDDS]

Danny I am really confused by this...you have got to help me out

I thought since HF is a weak acid...F- would be a really strong base. Instead you have it as the least basic.

Well F- is not really a stronger base. It's just a stronger base than H20. Which why the reaction will slightly favor the right more than the left when we're looking at :

HF + H2O -> F- + H3O+

Because it only slightly favoring the right more than the left, there will not be as many H+ in solution. However because it Does dissociate slightly, that is why we still consider it a weak acid.

 

[dleodyddlek]

Okay. There are different reasoning for acidity trend.

If you are comparing the ones on the same row, electronegativity works here

HF > H2O > NH3 > CH4

but if you are on the same period, size comes into work

HI > HBr > HCl > HF

some of your reasoning that HF is the most electronegative so that F holds onto H, NO. the more electronegative, the stronger for the atom to hold onto e- not proton. What you need to understand is now the H+ will be lost. If you have electron cloud covering the H+, H+ is less likely to run away. But by having stronger EN so that the electron cloud is weaker on H+, now H+ has higher chance of running away from F than I.
However, for halogens, you need to consider their size. But really, it's the same concept. I is larger than F. That means in HI, H is further apart from I than in HF. This means H is easier to leave from halogen. That's all there is.


Another thing to keep in mind is that if the conjugate base is stable, the acid is stronger because if can live happily ever after even without H+ with him. [a little exaggeration lol] so what plays role in stability. The first thing will be EN. If it has stronger EN, it will be good to have electron cloud after loosing its H+ so more stable to contain the e-. Second thing but really more important thing is, does it has alot, if any, of reasonance structure. If it has more reasonance structure, then it has more chance of distributing its electron cloud = more stable.

Here's the example:

RCO2H > ROH



Basicity will be exactly the opposite so I don't think i need to mention it.
Hope this helps :]

 

[PooyaH]

Danny I am really confused by this...you have got to help me out

I thought since HF is a weak acid...F- would be a really strong base. Instead you have it as the least basic.

HF is a weak acid but it is the strongest acid out of the ones listed! HCl<HBr<HI are all strong acids but not HF since it is very electronegative! It is a lot harder to take that H away from F than Cl due to its atomic radius. And F- is not a strong base!

 

[Danny289]

Danny I am really confused by this...you have got to help me out

I thought since HF is a weak acid...F- would be a really strong base. Instead you have it as the least basic.

everything got take care of here. read the Previous posts.

 

[drgreen]

Okay. There are different reasoning for acidity trend.

If you are comparing the ones on the same row, electronegativity works here

HF > H2O > NH3 > CH4

but if you are on the same period, size comes into work

HI > HBr > HCl > HF

some of your reasoning that HF is the most electronegative so that F holds onto H, NO. the more electronegative, the stronger for the atom to hold onto e- not proton. What you need to understand is now the H+ will be lost. If you have electron cloud covering the H+, H+ is less likely to run away. But by having stronger EN so that the electron cloud is weaker on H+, now H+ has higher chance of running away from F than I.
However, for halogens, you need to consider their size. But really, it's the same concept. I is larger than F. That means in HI, H is further apart from I than in HF. This means H is easier to leave from halogen. That's all there is.


Another thing to keep in mind is that if the conjugate base is stable, the acid is stronger because if can live happily ever after even without H+ with him. [a little exaggeration lol] so what plays role in stability. The first thing will be EN. If it has stronger EN, it will be good to have electron cloud after loosing its H+ so more stable to contain the e-. Second thing but really more important thing is, does it has alot, if any, of reasonance structure. If it has more reasonance structure, then it has more chance of distributing its electron cloud = more stable.

Here's the example:

RCO2H > ROH



Basicity will be exactly the opposite so I don't think i need to mention it.
Hope this helps :]

You're on the money, man! good job. Not one thing i disagree with.

 

[Mstoothlady2012]

ok thanks guys! one more question...

Btw NH2- & NH3 which one is a stronger base?

 

[joonkimdds]

ok thanks guys! one more question...

Btw NH2- & NH3 which one is a stronger base?

NH2 is a stronger base
whoever agrees with me give me a thumb up~

 

[dleodyddlek]

NH2- is much more basic. NH3 itself has one lone pair which will love to accept a H+ and now since NH2- lost another H+ from it NH2- is so lonely that it'll try harder to accept a H+ lol