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You are using an out of date browser. It may not display this or other websites correctly.You should upgrade or use an alternative browser. acid base question

• Thread starter Thread starter morrisol
• Start date Start date May 17, 2018

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morrisol

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7+ Year Member Joined Dec 9, 2014 Messages 201 Reaction score 27 Points 4,736

1. Pre-Medical

Members do not see this ad. i was just wondering why in this reaction ch3oh is the acid and nanh2 is the base? ch3oh + nanh2 <-> ch3o- +na + nh3 Sort by date Sort by votes S

sapientnarwhal

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Methanol is a weak acid and sodium amide is an extremely strong base. Note that the pKa of NH3 is 38 (making this an extremely weak acid), so in the rxn NH3 + H2O <-> NH2- + H3O+, the conjugate base of NH3 is very strong. Remember a weak acid has a stronger conjugate base and vice versa. Upvote 0 Downvote M

morrisol

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7+ Year Member Joined Dec 9, 2014 Messages 201 Reaction score 27 Points 4,736

1. Pre-Medical

sapientnarwhal said: Methanol is a weak acid and sodium amide is an extremely strong base. Note that the pKa of NH3 is 38 (making this an extremely weak acid), so in the rxn NH3 + H2O <-> NH2- + H3O+, the conjugate base of NH3 is very strong. Remember a weak acid has a stronger conjugate base and vice versa. Click to expand...

how would u figure that out without knowing the pkas? thats whats confusing me Upvote 0 Downvote S

sapientnarwhal

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More of an inference question, which would be more stable: CH3O- and NH3, or NaNH 1- and CH3OH2 +? Oxygen can stabilize the negative charge and is more electronegative, and ammonia is formed which is stable (also recall that you really only see ammonia and ammonium acid base rxns and rarely sodium amide). Oxygen has low affinity for and is unstable with + charge, N is unable to accommodate essentially a -2 charge. Alcohols are typically always weakly acidic. If you see an anionic salt as a reactant in an acid/base rxn, it's more than safe to assume it is the base. Upvote 0 Downvote M

morrisol

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7+ Year Member Joined Dec 9, 2014 Messages 201 Reaction score 27 Points 4,736

1. Pre-Medical

sapientnarwhal said: More of an inference question, which would be more stable: CH3O- and NH3, or NaNH 1- and CH3OH2 +? Oxygen can stabilize the negative charge and is more electronegative, and ammonia is formed which is stable (also recall that you really only see ammonia and ammonium acid base rxns and rarely sodium amide). Oxygen has low affinity for and is unstable with + charge, N is unable to accommodate essentially a -2 charge. Alcohols are typically always weakly acidic. If you see an anionic salt as a reactant in an acid/base rxn, it's more than safe to assume it is the base. Click to expand...

oh so it has to do with electronegativity Upvote 0 Downvote S

sapientnarwhal

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Yes to a certain extent, Eneg is the simplest way to explain this but there is a more complicated explanation. Forgot to mention that the sodium methoxide salt is definitely more stable than the NaNH1- thing (don't even know what to call it). You should already know that ammonia is basic/nucleophilic, so losing a proton would substantially increase its basicity. You should also know that alcohols are weak acids with strong conjugate bases. This + knowledge that titrations must involve strong acid or base => solve < 5 seconds. Upvote 0 Downvote M

morrisol

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7+ Year Member Joined Dec 9, 2014 Messages 201 Reaction score 27 Points 4,736

1. Pre-Medical

sapientnarwhal said: Yes to a certain extent, Eneg is the simplest way to explain this but there is a more complicated explanation. Forgot to mention that the sodium methoxide salt is definitely more stable than the NaNH1- thing (don't even know what to call it). You should already know that ammonia is basic/nucleophilic, so losing a proton would substantially increase its basicity. You should also know that alcohols are weak acids with strong conjugate bases. This + knowledge that titrations must involve strong acid or base => solve < 5 seconds. Click to expand...

im just starting to learn acids and bases again so im starting from scratch so i dont know much of that stuff yet Upvote 0 Downvote

CommyO

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Removed 5+ Year Member Joined Nov 23, 2016 Messages 220 Reaction score 108 Points 2,651 You guys are reading way to much into this. All this question requires you to know is the fundamental properties of acids and bases Acids donate proton Bases accept the proton methanol is deprotonated to alkoxide, therefore it is a acid sodium amide is protonated to ammonia, therefore it is a base You can also recall: pKa for alcohols is ~15.7 pKa for amine is ~40 pKa measures acidity, lower indicates stronger acid. This will tell you also why the methanol is the acid in this particular reaction Upvote 0 Downvote S

sapientnarwhal

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CommyO said: You guys are reading way to much into this. All this question requires you to know is the fundamental properties of acids and bases Acids donate proton Bases accept the proton methanol is deprotonated to alkoxide, therefore it is a acid sodium amide is protonated to ammonia, therefore it is a base You can also recall: pKa for alcohols is ~15.7 pKa for amine is ~40 pKa measures acidity, lower indicates stronger acid. This will tell you also why the methanol is the acid in this particular reaction Click to expand...

No I would disagree. If OP were only given sodium amide and methanol, how would they determine which is the acid and base? The reaction provided by OP in first post is the answer to a question that was asking for the products of a rxn b/w the NaNH2 and CH3OH. Upvote 0 Downvote M

morrisol

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sapientnarwhal said: No I would disagree. If OP were only given sodium amide and methanol, how would they determine which is the acid and base? The reaction provided by OP in first post is the answer to a question that was asking for the products of a rxn b/w the NaNH2 and CH3OH. Click to expand...

CommyO said: You guys are reading way to much into this. All this question requires you to know is the fundamental properties of acids and bases Acids donate proton Bases accept the proton methanol is deprotonated to alkoxide, therefore it is a acid sodium amide is protonated to ammonia, therefore it is a base You can also recall: pKa for alcohols is ~15.7 pKa for amine is ~40 pKa measures acidity, lower indicates stronger acid. This will tell you also why the methanol is the acid in this particular reaction Click to expand...

thanks. but if i wasnt given the pkas and on the exam couldnt remember what they were i would look at the equation and think "ch3oh and nanh2 have H" so how would i know which does which job? i just want to figure out how i can answer this question if i forget certain concepts or whatnot on the exam. Upvote 0 Downvote

CommyO

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Removed 5+ Year Member Joined Nov 23, 2016 Messages 220 Reaction score 108 Points 2,651

morrisol said: thanks. but if i wasnt given the pkas and on the exam couldnt remember what they were i would look at the equation and think "ch3oh and nanh2 have H" so how would i know which does which job? i just want to figure out how i can answer this question if i forget certain concepts or whatnot on the exam. Click to expand...

If you weren't given the products like the other guy said, you might have to think about the valence electrons and how the NaNH2 will readily dissociate into their respective ions Na+(aq) and NH2-(aq). From there, we can say that the only way the methanol would be the base is if the alcohol was protonated to water which would be a good leaving group and yield water. That is very unlikely considering you have a NH2- ion ready to accept the proton Upvote 0 Downvote M

morrisol

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1. Pre-Medical

CommyO said: If you weren't given the products like the other guy said, you might have to think about the valence electrons and how the NaNH2 will readily dissociate into their respective ions Na+(aq) and NH2-(aq). From there, we can say that the only way the methanol would be the base is if the alcohol was protonated to water which would be a good leaving group and yield water. That is very unlikely considering you have a NH2- ion ready to accept the proton Click to expand...

also, why does Cl- have no affinity for H+ in this reaction NH4Cl (s) + h2o--> nh4+ + cl- Upvote 0 Downvote S

sapientnarwhal

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Cl- has achieved its most stable electron configuration (noble gas). In other words, it is a poor nucleophile. Review solutions and solubility rules. btw this is a decomposition rxn, a salt dissolving in water. NH4Cl <=> [NH4+] + [Cl-] ammonium salts are highly soluble in aqueous solutions HCl completely dissociates in water, best example of the stability of Cl- Last edited: May 20, 2018 Upvote 0 Downvote M

morrisol

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7+ Year Member Joined Dec 9, 2014 Messages 201 Reaction score 27 Points 4,736

1. Pre-Medical

sapientnarwhal said: Cl- has achieved its most stable electron configuration (noble gas). In other words, it is a poor nucleophile. Review solutions and solubility rules. btw this is a decomposition rxn, a salt dissolving in water. NH4Cl <=> [NH4+] + [Cl-] ammonium salts are highly soluble in aqueous solutions HCl completely dissociates in water, best example of the stability of Cl- Click to expand...

thanks. if you dont mind i have a few questions about how best to approach certain things that i would love opinions on. there are certain concpts that i tend to struggle the most on. 1) how do i know when im dealing with an acid/base or redox reaction separately 2) how do i know if a reaction is multiple steps or not 3) do salts only form when you have a strong acid and strong base reacting? and if not how do i know when a salt forms and how do i identify a salt? 4) also how do i know if something will hydrolyze? 5) if a compound is a weak acid or base then how can it completley dissociate in water? i know some of this stuff i will learn better when doing orgo but if you had any suggestions that would be great. Last edited: May 21, 2018 Upvote 0 Downvote M

morrisol

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7+ Year Member Joined Dec 9, 2014 Messages 201 Reaction score 27 Points 4,736

1. Pre-Medical

dryorku said: whenever you see sodium in a compound, dissociate that molecule and focus on the anion that results. here, it's NH2- comparing NH2- to CH3OH in solution, think about which one wants a proton more, or in other words think about which one would be more stable holding a negative charge. since O is more electronegative than N, O will carry a negative charge and N will hold the proton Click to expand...

thanks so much! this is the kind of answer i was looking for! much appreciated!!! Upvote 0 Downvote M

morrisol

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7+ Year Member Joined Dec 9, 2014 Messages 201 Reaction score 27 Points 4,736

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dryorku said: no problem buddy Click to expand...

can i ask you one more question. when do you use an ICE table? i can never seem to know when to use one and when i dont need to use one Upvote 0 Downvote S

sapientnarwhal

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5+ Year Member Joined Jan 17, 2018 Messages 85 Reaction score 36 Points 1,601

1. Pre-Medical

morrisol said: thanks. if you dont mind i have a few questions about how best to approach certain things that i would love opinions on. there are certain concpts that i tend to struggle the most on. 1) how do i know when im dealing with an acid/base or redox reaction separately 2) how do i know if a reaction is multiple steps or not 3) do salts only form when you have a strong acid and strong base reacting? and if not how do i know when a salt forms and how do i identify a salt? 4) also how do i know if something will hydrolyze? 5) if a compound is a weak acid or base then how can it completley dissociate in water? i know some of this stuff i will learn better when doing orgo but if you had any suggestions that would be great. Click to expand...

These are topics you should learn on your own time or with a tutor. Like I said, review this content and do practice problems. If you still have questions we are happy to answer. I can send you excellent content review materials if you PM me. Upvote 0 Downvote You must log in or register to reply here. Share: Facebook X Bluesky LinkedIn Reddit Pinterest Tumblr WhatsApp E-mail Share Link

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