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Proton-Transfer Reactions in Aqueous and Non-aqueous Solutions

Key Concepts

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Proton-Transfer Reactions in Aqueous Solutions

Dissolving Acids in Water

  1. When an acid, HA, dissolves in water, H2O, it dissociates to form an anion, A-, and a proton, H+:

    H.
    .
    A
    .
    .
    A-
    + H+

    Notice that the electron contributed by the hydrogen atom to the bonding pair of electrons in the acid HA remains behind to form the anion A- and that the hydrogen "atom" no longer has an electron, it is the hydrogen ion H+ (a proton, or, hydron).

  2. The proton bonds to the oxygen atom of a water molecule to form the hydronium (or hydroxonium) ion, H3O+:

    H+ +
    H
    ..
    :O:H
    ..
    [H
    ..
    ]+
    H:O:H
    ..

    Notice that the central oxygen atom contributes both electrons to the bonding pair of electrons between the oxygen atom and the proton.
    This type of covalent bond is known as a coordinate covalent bond (or a dative bond).

  3. We can then write an overall chemical equation to represent the transfer of the proton from the acid to a water molecule:

    acid dissociates producing a proton : HAH++A-
    water accepts the proton : H++H2OH3O+    

    overall equation for the transfer of a
    proton from an acid to water:
    HA+H2OH3O++A-

    By definition, water is acting as a Brønsted-Lowry base because it has accepted a proton to form the hydronium (or hydroxonium) ion, H3O+.
    H3O+ is the conjugate acid of water, H2O.

    acid + base conjugate base
    (of acid HA)
    + conjugate acid
    (of base H2O)
    H.
    .
    A
    +
    H
    ..
    :O:H
    ..
    .
    .
    A-
    +
    [H
    ..
    ]+
    H:O:H
    ..
    acid + water conjugate base + hydronium (or hydroxonium) ion

Examples of proton transfer from an acid to water:

acid + water
(a base)
conjugate base
of the acid
+ conjugate acid
of water
HCl + H2O Cl- + H3O+
HNO3 + H2O NO3- + H3O+
H2SO4 + H2O HSO4- + H3O+
HSO4- + H2O SO42- + H3O+
NH4+ + H2O NH3 + H3O+

Dissolving Bases in Water

  1. Water molecules, H2O, exist in equilibrium with protons, H+, and hydroxide ions, OH-:
    H2O H+ + OH-

    Water can therefore act as a Brønsted-Lowry acid by donating a proton to a base.

  2. When a Brønsted-Lowry base dissolves in water, it accepts a proton from water molecules:
    B- + H+ → HB
  3. Overall reaction for the transfer of a proton from water (an acid) to a base:

    Dissociation of water produces proton:H2O H+ + OH-
    Base accepts protonB-+H+ HB

    Proton-transfer reaction:B-+H2O HB + OH-

    base + acid
    (water)
    conjugate acid
    of base
    + conjugate base of water
    (hydroxide ion)
    :B- +
    H
    .
    .
    :O:H
    ..
    H:B +
    ..
    :O:H-
    ..

    Water, H2O, has donated a proton, H+, to the base B-
    By definition, water is acting as a Brønsted-Lowry acid.
    OH- is the conjugate base of water, H2O.

    Examples of proton transfer from water to a base:

    base + water
    (an acid)
    conjugate acid
    of the base
    + conjugate base
    of water
    NH3 + H2O NH4+ + OH-
    CH3NH2 + H2O CH3NH3+ + OH-
    OH- + H2O H2O + OH-

Neutralisation as a Proton-Transfer Reaction

In a neutralisation reaction, an Arrhenius strong acid, HA, reacts with an Arrhenius strong base, MOH, to form water, H2O, and a salt, MA,:

HA + MOH → H2O + MA(aq)

Strong Arrhenius acids and strong Arrhenius bases dissociate completely in water:

acid dissociatesHAH++A-
base dissociatesMOHM++OH-

The species which react in aqueous solution are the ions:

H+ + A- + M+ + OH- → H2O + M+ + A-

Since M+ and A- exist on both the left hand side and the right hand side of the equation, they do not take part in the reaction, they are spectator ions, so we can ignore them:
H+ + OH- → H2O

We can see that an Arrhenius neutralisation reaction involves the transfer of a proton from the acid to the hydroxide ion of the base to form a water molecule.
In Brønsted-Lowry terms:

acid dissolves in water producing
hydronium (or hydroxonium) ion:
HA + H2OH3O+ + A-
base (hydroxide ion) accepts proton
from hydronium (or hydroxonium) ion:
OH- + H3O+H2O + H2O
base + acid conjugate acid
of base OH-
+ conjugate base
of acid H3O+

Examples of neutralisation reactions:

strong acid + strong base conjugate base
of the acid
+ conjugate acid
of base
HCl(aq) + NaOH(aq) H3O+ + OH- H2O + H2O
HNO3(aq) + NaOH(aq) H3O+ + OH- H2O + H2O
HCl(aq) + KOH(aq) H3O+ + OH- H2O + H2O
HNO3(aq) + KOH(aq) H3O+ + OH- H2O + H2O

Non-Arrhenius Aqueous Acid-Base Reactions

Amines, general formula R-NH2, are examples of non-Arrhenius bases because they do not dissociate in water to produce hydroxide ions, OH-, but they can act as Brønsted-Lowry bases by accepting a proton to form the R-NH3+ ion.

Examples:

acid + base
(amine)
conjugate base
of the acid
+ conjugate acid
of base (amine)
HCl(aq) + CH3NH2 Cl- + CH3NH3+
HBr(aq) + CH3NH2 Br- + CH3NH3+
CH3COOH(aq) + CH3NH2 CH3COO- + CH3NH3+

In each of these examples a proton, H+, has been transferred from the Brønsted-Lowry acid to the amine.
The amine is acting as a Brønsted-Lowry base by accepting the proton.

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Non-aqueous Solution Proton-Transfer Reactions

The Brønsted-Lowry definition of acids and bases is useful because it helps explain acid-base reactions that are not neutralisation reactions, and those that do not occur in aqueous solution.

  1. Ammonia gas, NH3(g), is not an Arrhenius base because it does not dissociate to produce hydroxide ions, but it can act as a proton acceptor in gaseous reactions so it is a Brønsted-Lowry base.

    Hydrogen chloride gas, HCl(g) is not an Arrhenius acid because it does not dissociate, but it can donate a proton in gaseous reactions so it is a Brønsted-Lowry acid.

    Ammonia gas will react with hydrogen chloride gas in a proton-transfer reaction:

    acid + base conjugate base
    of acid
    + conjugate acid
    of base
    HCl(g) + NH3(g) Cl- + NH4+

    A proton has been transferred from the Brønsted-Lowry acid, HCl(g), to the Brønsted-Lowry base, NH3(g).

  2. The Brønsted-Lowry definitions of acids and bases is useful when describing reactions that take place in solvents other than water, for example, in ethanoic (acetic) acid solutions.

    Many acids will donate a proton to ethanoic (acetic) acid, this makes the ethanoic (acetic) acid, by definition, a Brønsted-Lowry base in these solutions:

    acid + base
    (acetic acid)
    conjugate base
    of acid
    + conjugate acid
    of base
    HCl + HC2H3O2 Cl- + H2C2H3O2+
    HNO3 + HC2H3O2 NO3- + H2C2H3O2+
    H2SO4 + HC2H3O2 HSO4- + H2C2H3O2+

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Reactions that are NOT Proton-Transfer Reactions*

The defining characteristic of a proton-transfer reaction is that at some point in the reaction a hydrogen atom must lose an electron to form a proton (or hydron), and that this proton is then transferred to a different species.
If a proton (or hydron) is not formed, or if a proton is not transferred to a different species, then the reaction is not a proton-transfer reaction.

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*While these reactions are not covered by the Brønsted-Lowry acid-base theory, they can often be covered by the more general Lewis theory of acid-base behaviour.