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Definitions of Acids and Bases Tutorial

Key Concepts

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Defintions of Acids and Bases in Chronological Order

  ACIDS BASES
Lavoisier
(1779)
Definition:
An acid contains oxygen.
Limitations:
Does not account for substances with acidic properties that do not contain oxygen, for example, hydrochloric acid, HCl.
Davy
(1810)
Definition:
An acid contains hydrogen.
Limitations:
Does not account for substances which contain hydrogen but do not display acidic properties.
Arrhenius Definitions
(1884)
Definition:
An acid dissociates (or ionises) in water to produce hydrogen ions, H+(aq) (protons or hydrons).
Definition:
A base dissociates (or ionises) in water to produce hydroxide ions, OH-(aq).
Examples:
acid proton + anion
hydrochloric acid:
HCl H+(aq) + Cl-(aq)
nitric acid:
HNO3 H+(aq) + NO3-(aq)
Examples:
base hydroxide
ion
+ cation
sodium hydroxide:
NaOH OH-(aq) + Na+(aq)
potassium hydroxide:
KOH OH-(aq) + K+(aq)
Limitations:
Only accounts for acids that:

does not account for amphoteric substances
(those that can act as an Arrhenius acid or an Arrhenius base)

Limitations:
Only accounts for bases that:

  • are aqueous solutions (bases that are also known as alkalis)
  • have OH- already in their structures, eg, NaOH

does not account for amphoteric substances
(those that can act as an Arrhenius acid or an Arrhenius base)

Definition:
An amphoteric substance can act as either an Arrhenius acid or as an Arrhenius base, that is:
An Arrhenius acid will react with an Arrhenius base.
An Arrhenius base will react with an Arrhenius acid.

Example:
According to the Arrhenius definition of acids and bases, zinc hydroxide, Zn(OH)2(s), should be a base, that is, it dissolves in water to produce hydroxide ions, OH-(aq).3
However, zinc hydroxide can be shown to be amphoteric, it can act as both an Arrhenius acid by reacting with an Arrhenius base, and, it can act as an Arrhenius base by reacting with an Arrhenius acid!

(i) Zinc hydroxide will act like an Arrhenius acid when it dissolves in aqueous Arrhenius base (OH-(aq)):

Zn(OH)2(s) + OH-(aq) → Zn(OH)3-(aq)

(ii) Zinc hydroxide will act like an Arrhenius base when it dissolves in aqueous Arrhenius acid (H+(aq)):

Zn(OH)2(s) + H+(aq) → Zn(OH)+(aq) + H2O(l)

Brønsted-Lowry Definitions
(1923)
Definition:
An acid is a species that donates a proton (H+).
Definition:
A base is a species that accepts a proton (H+).
Examples:
HB H+ + B-

HB is acting as an acid by donating a proton, H+.
B- is the conjugate base of the acid HB .
Examples:
B- + H+ HB

B- is acting as a base by accepting a proton, H+ .
HB is the conjugate acid of the base B- .
Examples:

acid conjugate
base
+ proton
HCl Cl- + H+
HNO3 NO3- + H+
H2SO4 HSO4- + H+
HSO4- SO42- + H+
H2O OH- + H+

Examples:

base + proton conjugate
acid
OH- + H+ H2O
NH3 + H+ NH4+
CO32- + H+ HCO3-
HCO3- + H+ H2CO3
H2O + H+ H3O+

Definition:
An amphiprotic substance can act as a proton donor and as a proton acceptor.
An amphoptoric substance can act as either a Brønsted-Lowry acid or as a Brønsted-Lowry base.

Example:
hydrogen carbonate (bicarbonate) ion (HCO3-) is amphiprotic

(i) hydrogen carbonate (bicarbonate) ion acting as a Brønsted-Lowry acid:

HCO3- → CO32- + H+

(ii) hydrogen carbonate (bicarbonate) ion acting as a Brønsted-Lowry base:

HCO3- + H+H2CO3

Limitations:
The solvent system must be protonic, for example, the solvent system can be water (H2O(l)), or liquid ammonia (NH3(l)).

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Sample Questions with Worked Solutions

Question 1. What is the conjugate base of H2S ?

  1. Define "conjugate base":
    A Brønsted-Lowry acid donates (loses) a proton to form a conjugate base.
  2. Identify the Brønsted-Lowry acid in the question:
    H2S must be the Brønsted-Lowry acid because we are asked to find its conjugate base
  3. Write the balanced chemical equation to represent the Brønsted-Lowry acid donating (losing) a proton (H+) to produce its conjugate base:

    acid proton + conjugate base
    H2S H+ + HS-

  4. State your answer:
    HS- is the conjugate base of the acid H2S

Question 2: Explain why H2PO4- is said to be amphiprotic.

  1. Define "amphiprotic":
    An amphiprotic substance is one that can either:
    (i) donate (lose) a proton
    or
    (ii) accept (gain) a proton
  2. Write a balanced chemical equation to show H2PO4- donating (losing) a proton:
    H2PO4- → H+ + HPO42-
  3. Write a balanced chemical equation to show H2PO4- accepting (gaining) a proton:
    H2PO4- + H+ → H3PO4
  4. H2PO4- is amphiprotic because :
    H2PO4- can either:
    (i) lose a proton to form HPO42-
    or
    (ii) gain a proton to form H3PO4

Question 3. Explain why the carbonate ion, CO32-, is a Brønsted-Lowry base but not an Arrhenius base.

  1. Define each term:
    A Brønsted-Lowry base accepts (gains) a proton (H+) to form a conjugate acid.
    An Arrhenius base dissociates in water to produce hydroxide ions (OH-)
  2. Write a balanced chemical equation to show NH3 acting as a Brønsted-Lowry base:
    CO32- + H+ → HCO3-
  3. Demonstrate why CO32- is not an Arrhenius base:
    CO32- does not contain any hydroxide ions (OH-).
    Therefore CO32- cannot dissociate in water to produce hydroxide ions.
  4. Write an answer to the question:
    CO32- is Brønsted-Lowry base because it can accept a proton to form its conjugate acid, HCO3-.
    CO32- is not an Arrhenius base because it cannot dissociate in water to produce hydroxide ions.

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1. There are lots of other definitions of acids and bases, for example,

  1. Solvent-system definition extends the Arrhenius concept beyond water to other self-dissociating solvents, an acid increases the concentration of cations related to the solvent, a base increases the concentration of anions related to the solvent.
  2. Lewis (1923) acids are electron-pair acceptor, Lewis bases are electron-pair donors.
  3. Usanovich (1939) acids react with bases, give up cations, or accept anions or electrons, and Usanovich bases react with acids, give up anions or electrons, or combines with cations.
  4. Lux-Flood (1947) acids are oxide acceptors, Lux-Flood bases are oxide donors.

2. The solvent does not have to be water, but it does have to be protonic, for example, the solvent could be liquid ammonia or sulfuric acid.

3. Zinc hydroxide, Zn(OH)2, is only sparingly soluble in water at 25°C so much so that it is usually referred to as insoluble.
Ksp is very, very small, so very little Zn(OH)2(s) will dissolve in water.
Ksp = 5 × 10-17