AAMC 8: Na2CO3 + H20 -- will be basic or acidic?

[zut212]

On practice exam 8 for AAMC:

Also, if red litmus paper is dipped into the Na2CO3 solution, which was mixed with H2O, the litmus paper would:

A. Remain red, because carbonate is an acidic salt.
B. Remain red, because sodiam carbonate is neutral.
C. Turn blue, because carbonate reacts with water to produce OH-.
D. Turn blue, because sodium ions form sodium hydroxide in water.

ANS: C because "in water, carbonate will undergo the following reaction: CO3(2- superscript)(aq) + H2O(l) --> HCO3(-1 superscript)(aq) + OH-(aq). Red litmus paper will turn blue in a base."

One way a person can view this is as follows: The reaction is Na2CO3 + 2H2O --> 2NaOH + H2CO3. The NaOH is definitely basic, and the H2CO3 is definitely acidic. I suppose that this is ultimately basic, because NaOH is more basic than H2CO3 is acidic. Is this the logic of the correct answer is following? That was my logic in answering this problem. Please help me in understanding their logic.

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[TTfutureMD]

On practice exam 8 for AAMC:

Also, if red litmus paper is dipped into the Na2CO3 solution, which was mixed with H2O, the litmus paper would:

A. Remain red, because carbonate is an acidic salt.
B. Remain red, because sodiam carbonate is neutral.
C. Turn blue, because carbonate reacts with water to produce OH-.
D. Turn blue, because sodium ions form sodium hydroxide in water.

ANS: C because "in water, carbonate will undergo the following reaction: CO3(2- superscript)(aq) + H2O(l) --> HCO3(-1 superscript)(aq) + OH-(aq). Red litmus paper will turn blue in a base."

One way a person can view this is as follows: The reaction is Na2CO3 + 2H2O --> 2NaOH + H2CO3. The NaOH is definitely basic, and the H2CO3 is definitely acidic. I suppose that this is ultimately basic, because NaOH is more basic than H2CO3 is acidic. Is this the logic of the correct answer is following? That was my logic in answering this problem. Please help me in understanding their logic.

Note that since there are 2 sodium atoms attached to the NA2CO3, the C03 will have a -2 overrall charge. Each sodium has a +1 charge. Since you are releasing something negative (C03) into water, that negative charge will have an affect. It will not turn C03 H2Co3 because this would be more acidic than water. Look up the pka. Also, if you got H2C03, that would mean that water gave up 2 hydrogens. And that just aint gonna happen without some hardcore chemistry. But water does give up 1 hydrogen many times and will accept another hydrogen other times. So, if you put Na2C03 into water it will break up into 2NA+ and C03(2-). That negative charge will be able to take a hydrogen off of water, which will give you OH- in solution.

I think the trickiest thing for me ( i took this exam on sat) was remembering when litmus paper turns blue and when it turns red.




I hope this helped a little.

 

[loveoforganic]

Your answer is right, but there are flaws in your reasoning.

1) Sodium has charge too - why doesn't it do anything? Sodium does not react with water because it is the counterion of a strong base. Carbonate reacts with water because it is the conjugate base of a weak acid (and is thus either a weak base or strong base itself - weak in this case).

2) Forming H2CO3 would not necessarily mean you formed O2- from water. It didn't say you have a stoichiometrically equivalent amount of water and base, and it's thus implied that you have excess water. To form H2CO3, you'd just have to form some more OH- from the water available.

3) Water gives up protons not hydrogens (nitpicky, but to be clear)

 

[TTfutureMD]

Your answer is right, but there are flaws in your reasoning.

1) Sodium has charge too - why doesn't it do anything? Sodium does not react with water because it is the counterion of a strong base. Carbonate reacts with water because it is the conjugate base of a weak acid (and is thus either a weak base or strong base itself - weak in this case).

2) Forming H2CO3 would not necessarily mean you formed O2- from water. It didn't say you have a stoichiometrically equivalent amount of water and base, and it's thus implied that you have excess water. To form H2CO3, you'd just have to form some more OH- from the water available.

3) Water gives up protons not hydrogens (nitpicky, but to be clear)

Thanks Frank... Question: I learned to treat Na+ is a spectator ion. Im confused about Na+ being the counterion of a strong base. What do you mean by that? If we were to look at H2C03, the 2H+ would have an affect on the acidity of the solution- right? Would the 2H+ also be considered a counterion of a strong base?

 

[loveoforganic]

I was trying to make it simpler to understand but I think I failed. It's not charge alone that causes co3-- to act as a base, it's te instability of that charge held as is relative to how it would be held by oh-. Similarly, na+ does not act as an acid because of the stability of the charge held as is relative to how it would be held by h3o+

 

[catinthehat1234]

Hi, I know this thread is from a while ago, but I have a question. Would Na2CO3 solution be considered slightly acidic? Because it's coming from a weak acid whose conjugate base is weak. I definitely agree with the correct answer, but I'm just wondering if A could be the answer? Or it's less correct in this case? Thank you in advance!

 

[Mr. otcoD]

Hi, I know this thread is from a while ago, but I have a question. Would Na2CO3 solution be considered slightly acidic? Because it's coming from a weak acid whose conjugate base is weak. I definitely agree with the correct answer, but I'm just wondering if A could be the answer? Or it's less correct in this case? Thank you in advance!

The way I see it. What is an acid? A compound that can give up a Hydrogen. Na2CO3 has no hydrogens. I wouldn't really call it an acid, if anything H2CO3's weak conjugate base. (really the weakest)
IMO, You get carbonic acid, but carbonic acid is a strong acid, which means itll dissociate almost as soon as its made, so you can form bicarb and H+. The bicarb is considered a base, which is why it turns blue, not red

 

[BerkReviewTeach]

What makes you believe Na2CO3 is acidic? There is no H+ present, so it cannot be a proton donor. So you must reason about what will happen to the ions when added to water (Na+ and CO32-). Na+ is essentially inert, so it will act like a spectator ion. There is an outside chance that one to two molecules out of the gazillions may take an OH- from water to from NaOH and H+, but that will be so minimal that the solution would not be affected. CO32- (carbonate), on the other hand, can readily pick up a proton from water to form HCO3- (bicarbonate) and OH-. A solution with CO32-, HCO3-, and OH- has more OH- than H+, which defines it as being a basic solution. The best answer is choice C.

 

[catinthehat1234]

The way I see it. What is an acid? A compound that can give up a Hydrogen. Na2CO3 has no hydrogens. I wouldn't really call it an acid, if anything H2CO3's weak conjugate base. (really the weakest)
IMO, You get carbonic acid, but carbonic acid is a strong acid, which means itll dissociate almost as soon as its made, so you can form bicarb and H+. The bicarb is considered a base, which is why it turns blue, not red

Thank you! That makes sense, you've certainly cleared some things up. But just on a side note, I thought carbonic acid was thought of as a weaker acid?

 

[catinthehat1234]

What makes you believe Na2CO3 is acidic? There is no H+ present, so it cannot be a proton donor. So you must reason about what will happen to the ions when added to water (Na+ and CO32-). Na+ is essentially inert, so it will act like a spectator ion. There is an outside chance that one to two molecules out of the gazillions may take an OH- from water to from NaOH and H+, but that will be so minimal that the solution would not be affected. CO32- (carbonate), on the other hand, can readily pick up a proton from water to form HCO3- (bicarbonate) and OH-. A solution with CO32-, HCO3-, and OH- has more OH- than H+, which defines it as being a basic solution. The best answer is choice C.

Thank you! It definitely cleared things up in my head. That's all I needed

 

[catinthehat1234]

Thank you! It definitely cleared things up in my head. That's all I needed

Oh, and I forgot to mention that I'm almost certain that someone (sometime during my college career) said weak conjugate bases are weak because they're not very basic...which means they're slightly acidic? I think that's where my confusion is coming from, but I'm not really sure if that's correct at all.

 

[BerkReviewTeach]

Being a weak base does not necessarily make a compound an acid. Think about ammonia, NH3. It is a weak base (pKb around 4.7), but it is an even weaker acid (pKa around 20ish).

What is true is that as a base gets weaker it, conjugate becomes a stronger acid. I really don't like the idea of plugging, but if you could get a hold of the acid-base chapters for BR, it explains it better than anywhere. There are several excellent examples.

 

[catinthehat1234]

Being a weak base does not necessarily make a compound an acid. Think about ammonia, NH3. It is a weak base (pKb around 4.7), but it is an even weaker acid (pKa around 20ish).

What is true is that as a base gets weaker it, conjugate becomes a stronger acid. I really don't like the idea of plugging, but if you could get a hold of the acid-base chapters for BR, it explains it better than anywhere. There are several excellent examples.

Thank you for your input! I just came across a question (I got wrong) where the answer stated how HSO4 is slightly acidic and could lower pH. Since H2SO4 is a strong acid, it's conjugate base should be stable and weak, right? But then things like these confuse me and I'm not sure how to think anymore...Help?

 

[aldol16]

Thank you for your input! I just came across a question (I got wrong) where the answer stated how HSO4 is slightly acidic and could lower pH. Since H2SO4 is a strong acid, it's conjugate base should be stable and weak, right? But then things like these confuse me and I'm not sure how to think anymore...Help?

Acidity and basicity are only useful in relative terms. People rarely talk about acidity in isolation and when they do, it's with the classical strong acids and bases (HCl, NaOH, H2SO4, etc.). We talk about acidity and basicity on a spectrum because that's what matters in a reaction. If one substance has a pKa that's lower than that of another substance, the first substance will donate a proton to the second substance. Water has a pKa of 14 and HSO4 has a pKa of about 7. So if you put HSO4 in water, it's gonna dissociate into protons and sulfate anion. But compared to H2SO4, HSO4 is relatively less acidic and is the conjugate base of the former.

 

[catinthehat1234]

Acidity and basicity are only useful in relative terms. People rarely talk about acidity in isolation and when they do, it's with the classical strong acids and bases (HCl, NaOH, H2SO4, etc.). We talk about acidity and basicity on a spectrum because that's what matters in a reaction. If one substance has a pKa that's lower than that of another substance, the first substance will donate a proton to the second substance. Water has a pKa of 14 and HSO4 has a pKa of about 7. So if you put HSO4 in water, it's gonna dissociate into protons and sulfate anion. But compared to H2SO4, HSO4 is relatively less acidic and is the conjugate base of the former.

Oh my gosh, that makes so much more sense...just thinking in terms of relative pKa really cleared things up. Thank you

 

[catinthehat1234]

Could someone clarify whether strong acids, such as HBr, have strong or weak bonds? Is there a certain principle/rule I should know or follow? Much thanks (sorry this is a slightly unrelated topic)

 

[BerkReviewTeach]

If a reactant is strong, be it an acid or something else, we take this to mean highly reactive. In general, a highly reactive species forms more stable bonds when forming the product than it had as a reactant. So it is a safe conclusion that the reactant has weak bonds and the product has strong bonds, if the reactant is highly reactive.

HBr is a strong acid, so the H-Br bond must be weak relative to the hydronium O-H bond and the hydration 'bonds' of water to Br-. H-Br readily dissociates in water to form a more stable species.

This is ignoring the effects of entropy, which for HBr are not as significant as enthalpy.