Question

How to tell whether a substance is an acid or a base depending on its chemical formula. I know that it depends on how it dissociates; a chemical is an acid if it dissociates into a H+ and a base if it dissociates into a OH-.

But how do you know what it dissociates into? How do you know that HNO3 dissociates into H+ and NO3- making it an acid?

How do you know that CH₃NH₂ is the base and CH₃NH₃⁺ is the conjugate acid for use in the Hendersen Hasselbach equation?

 

Answer 1

first, elt us define acid/(bases

Arrhenius Acid/Base

Arrhenius acid/base is the first theory on acid/base definitions, it is mostly true for general applications.

Arrhenius acid is any type of substance that will dissociate in water to form hydrogen ions [H+]. That is, an acid increases the concentration of H+ ions in an aqueous solution.

Arrhenius base is any type of substance that will form hydroxide [OH−] ions. It typically must increase pH since OH- increases in concentration.

Examples of Arrhenius acid

HCl, H2SO4, H3PO4 à they will donate 1, 2, and 3 protons to solution i.e. H+

Examples of Arrhenius base

NaOH, Ca(OH)2, and; avoid NH3, since it is basic, yet will not donate OH from its NH3 equation.

Bronsted Lowery Acid/Bases

First, let us define Bronsted Lowry acid/base:

Bronsted Lowry acid: any species that will donate H+ (protons) in solution, and makes pH lower (i.e HCl)

Bronsted Lowry base: any species that will accept H+ (protons) in solution, and makes pH higher (NH3 will accept H+ to form NH4+)

Typically, acid/bases are shown in the left (reactants)

when we write the products:

Bronsted Lowery conjugate base = the base formed when the B.L. acid donates its H+ proton ( i.e. HCl -> Cl-

Bronsted Lowery conjugate acid = the acid formed when the B.L. base accept its H+ proton ( i.e. NH4+ has accept H+ proton)

Note that, typically conjugate bases/acids are shown in the right (product) side

So, from your reaction:

H2Y- transforms to H3Y; it is gaining and H+, so it is a BASE

H2Z- transforms to HZ2-; it is losing H+, so it is an ACID

then,

H3Y is the conjugate ACID (since it could donate H+ to form H2Y-)

and HZ2- is the conjugate BASE ( since it could accept H+ to form H2Z-)

now...

Typically, you need to KNOW the reactions

NO3 --> nitrate, NO3- is very electrnoegative since both, N and O are electongative, they will dissocaite H+ readily.

Then, CH3NH2 is an amine, if you see, the only electronegative species is Nitrogen, no other

then

NH2 group can accept H+ since nitrogen ahs a lone pair

therefore,

amines are most likely to act as a base

special note...

theoretically speaking,

NH3 is a base, but will act as an acid if NH3 --> NH2- + H+ is formed, which "typically" we will not see, but it might happen.... As well

NH2- wil act as a base, to accept H+ , stronger than NH3