Conjugate (acid-base theory)
Aconjugate acid,within theBrønsted–Lowry acid–base theory,is achemical compoundformed when an acidgives a proton(H+) to abase—in other words, it is a base with ahydrogen ionadded to it, as it loses a hydrogen ion in the reverse reaction. On the other hand, aconjugate baseis what remains after an acid has donated a proton during a chemical reaction. Hence, a conjugate base is a substance formed by theremoval of a protonfrom an acid, as it can gain a hydrogen ion in the reverse reaction.[1]Becausesome acidscan give multiple protons, the conjugate base of an acid may itself be acidic.
In summary, this can be represented as the followingchemical reaction:
Johannes Nicolaus BrønstedandMartin Lowryintroduced the Brønsted–Lowry theory, which said that any compound that can give a proton to another compound is an acid, and the compound that receives the proton is a base. A proton is a subatomic particle in the nucleus with a unit positive electrical charge. It is represented by the symbolH+because it has thenucleusof a hydrogenatom,[2]that is, ahydrogen cation.
Acationcan be a conjugate acid, and ananioncan be a conjugate base, depending on whichsubstanceis involved and whichacid–base theoryis used. The simplest anion which can be a conjugate base is thefree electron in a solutionwhose conjugate acid is the atomic hydrogen.
Acid–base reactions
[edit]In anacid–base reaction,an acid and a base react to form a conjugate base and a conjugate acid respectively. The acid loses a proton and the base gains a proton. In diagrams which indicate this, the new bond formed between the base and the proton is shown by an arrow that starts on anelectron pairfrom the base and ends at the hydrogen ion (proton) that will be transferred:
In this case, the water molecule is the conjugate acid of the basic hydroxide ion after the latter received the hydrogen ion fromammonium.On the other hand,ammoniais the conjugate base for the acidic ammonium after ammonium has donated a hydrogen ion to produce the water molecule. Also, OH−can be considered as the conjugate base ofH
2O,since the water molecule donates a proton to giveNH+
4in the reverse reaction. The terms "acid", "base", "conjugate acid", and "conjugate base" are not fixed for a certain chemical substance but can be swapped if the reaction taking place is reversed.
Strength of conjugates
[edit]The strength of a conjugate acid is proportional to itssplitting constant.A stronger conjugate acid will split more easily into its products, "push" hydrogen protons away and have a higherequilibrium constant.The strength of a conjugate base can be seen as its tendency to "pull" hydrogen protons towards itself. If a conjugate base is classified as strong, it will "hold on" to the hydrogen proton when dissolved and its acid will not split.
If a chemical is a strong acid, its conjugate base will be weak.[3]An example of this case would be the splitting ofhydrochloric acidHClin water. SinceHClis a strong acid (it splits up to a large extent), its conjugate base (Cl−
) will be weak. Therefore, in this system, mostH+
will behydroniumionsH
3O+
instead of attached to Cl−anions and the conjugate bases will be weaker than water molecules.
On the other hand, if a chemical is a weak acid its conjugate base will not necessarily be strong. Consider that ethanoate, the conjugate base of ethanoic acid, has abase splitting constant(Kb) of about5.6×10−10,making it a weak base. In order for a species to have a strong conjugate base it has to be a very weak acid, like water.
Identifying conjugate acid–base pairs
[edit]To identify the conjugate acid, look for the pair of compounds that are related. Theacid–base reactioncan be viewed in a before and after sense. The before is the reactant side of the equation, the after is the product side of the equation. The conjugate acid in the after side of an equation gains a hydrogen ion, so in the before side of the equation the compound that has one less hydrogen ion of the conjugate acid is the base. The conjugate base in the after side of the equation lost a hydrogen ion, so in the before side of the equation, the compound that has one more hydrogen ion of the conjugate base is the acid.
Consider the following acid–base reaction:
3+H
2O→H
3O+
+NO−
3
Nitric acid(HNO
3) is anacidbecause it donates a proton to the water molecule and itsconjugate baseisnitrate(NO−
3). The water molecule acts as a base because it receives the hydrogen cation (proton) and its conjugate acid is thehydroniumion (H
3O+
).
Equation | Acid | Base | Conjugate base | Conjugate acid |
---|---|---|---|---|
HClO 2+H 2O→ClO− 2+H 3O+ |
HClO 2 |
H 2O |
ClO− 2 |
H 3O+ |
ClO− +H 2O→HClO+OH− |
H 2O |
ClO− |
OH− |
HClO |
HCl+H 2PO− 4→Cl− +H 3PO 4 |
HCl | H 2PO− 4 |
Cl− |
H 3PO 4 |
Applications
[edit]One use of conjugate acids and bases lies in buffering systems, which include abuffer solution.In a buffer, a weak acid and its conjugate base (in the form of a salt), or a weak base and its conjugate acid, are used in order to limit the pH change during a titration process. Buffers have both organic and non-organic chemical applications. For example, besides buffers being used in lab processes, human blood acts as a buffer to maintain pH. The most important buffer in our bloodstream is thecarbonic acid-bicarbonate buffer,which prevents drastic pH changes whenCO
2is introduced. This functions as such:
Furthermore, here is a table of common buffers.
Buffering agent | pKa | Useful pH range |
---|---|---|
Citric acid | 3.13, 4.76, 6.40 | 2.1 - 7.4 |
Acetic acid | 4.8 | 3.8 - 5.8 |
KH2PO4 | 7.2 | 6.2 - 8.2 |
CHES | 9.3 | 8.3–10.3 |
Borate | 9.24 | 8.25 - 10.25 |
A second common application with an organic compound would be the production of a buffer with acetic acid. If acetic acid, a weak acid with the formulaCH
3COOH,was made into a buffer solution, it would need to be combined with its conjugate baseCH
3COO−
in the form of a salt. The resulting mixture is called an acetate buffer, consisting of aqueousCH
3COOHand aqueousCH
3COONa.Acetic acid, along with many other weak acids, serve as useful components of buffers in different lab settings, each useful within their own pH range.
Ringer's lactate solutionis an example where the conjugate base of an organic acid,lactic acid,CH
3CH(OH)CO−
2is combined with sodium, calcium and potassium cations and chloride anions in distilled water[4]which together form a fluid which isisotonicin relation to human blood and is used forfluid resuscitationafterblood lossdue totrauma,surgery,or aburn injury.[5]
Table of acids and their conjugate bases
[edit]Below are several examples of acids and their corresponding conjugate bases; note how they differ by just one proton (H+ion). Acid strength decreases and conjugate base strength increases down the table.
Acid | Conjugate base |
---|---|
H 2F+ Fluoroniumion |
HFHydrogen fluoride |
HClHydrochloric acid | Cl−Chlorideion |
H2SO4Sulfuric acid | HSO− 4Hydrogen sulfateion (bisulfateion) |
HNO3Nitric acid | NO− 3Nitrateion |
H3O+Hydroniumion | H2OWater |
HSO− 4Hydrogen sulfateion |
SO2− 4Sulfateion |
H3PO4Phosphoric acid | H2PO− 4Dihydrogen phosphateion |
CH3COOHAcetic acid | CH3COO−Acetateion |
HFHydrofluoric acid | F−Fluorideion |
H2CO3Carbonic acid | HCO− 3Hydrogen carbonateion |
H2SHydrosulfuric acid | HS−Hydrosulfideion |
H2PO− 4Dihydrogen phosphateion |
HPO2− 4Hydrogen phosphateion |
NH+ 4Ammoniumion |
NH3Ammonia |
H2O Water (pH=7) | OH−Hydroxideion |
HCO− 3Hydrogencarbonate(bicarbonate)ion |
CO2− 3Carbonateion |
Table of bases and their conjugate acids
[edit]In contrast, here is a table of bases and their conjugate acids. Similarly, base strength decreases and conjugate acid strength increases down the table.
Base | Conjugate acid |
---|---|
C 2H 5NH 2Ethylamine |
C 2H 5NH+ 3Ethylammoniumion |
CH 3NH 2Methylamine |
CH 3NH+ 3Methylammoniumion |
NH 3Ammonia |
NH+ 4Ammoniumion |
C 5H 5NPyridine |
C 5H 6N+ Pyridinium |
C 6H 5NH 2Aniline |
C 6H 5NH+ 3Phenylammoniumion |
C 6H 5CO− 2Benzoateion |
C 6H 6CO 2Benzoic acid |
F− Fluorideion |
HFHydrogen fluoride |
PO3− 4Phosphateion |
HPO2− 4Hydrogen phosphateion |
OH−Hydroxideion | H2OWater(neutral,pH7) |
HCO− 3Bicarbonate |
H 2CO 3Carbonic acid |
CO2− 3Carbonate ion |
HCO− 3Bicarbonate |
Br− Bromideion |
HBrHydrogen bromide |
HPO2− 4Hydrogen phosphate |
H 2PO− 4Dihydrogen phosphateion |
Cl− Chlorideion |
HClHydrogen chloride |
H 2OWater |
H 3O+ Hydroniumion |
Nitriteion | Nitrous acid |
See also
[edit]References
[edit]- ^Zumdahl, Stephen S., & Zumdahl, Susan A.Chemistry.Houghton Mifflin, 2007,ISBN0618713700
- ^"Brønsted–Lowry theory | chemistry".Encyclopedia Britannica.Retrieved25 February2020.
- ^"Strength of Conjugate Acids and Bases Chemistry Tutorial".www.ausetute.com.au.Retrieved25 February2020.
- ^British national formulary: BNF 69(69 ed.). British Medical Association. 2015. p. 683.ISBN9780857111562.
- ^Pestana, Carlos (7 April 2020).Pestana's Surgery Notes(Fifth ed.). Kaplan Medical Test Prep. pp. 4–5.ISBN978-1506254340.