Leveling effect

{{short description|Effect of solvents on properties of acids and bases}} framed|right|Acid-base discrimination windows of common solvents<ref name="Shriver/Atkins fifth" />

'''Leveling effect''' or '''solvent leveling''' refers to the effect of solvent on the properties of acids and bases. The strength of a strong acid is limited ("leveled") by the basicity of the solvent. Similarly the strength of a strong base is leveled by the acidity of the solvent. When a strong acid is dissolved in water, it reacts with it to form hydronium ion (H₃O⁺).<ref>Zumdahl, S. S. “Chemistry” Heath, 1986: Lexington, MA. ISBN 0-669--04529-2.</ref> An example of this would be the following reaction, where "HA" is the strong acid: :HA + H₂O → A⁻ + H₃O⁺

Any acid that is stronger than H₃O⁺ reacts with H₂O to form H₃O⁺. Therefore, no acid stronger than H₃O⁺ exists in H₂O. For example, aqueous perchloric acid (HClO₄), aqueous hydrochloric acid (HCl) and aqueous nitric acid (HNO₃) are all completely ionized, and are all equally strong acids.<ref name=Skoog>{{cite book |last1=Skoog |first1=Douglas A. |last2=West |first2=Donald M. |last3=Holler |first3=F. James |last4=Crouch |first4=Stanley R. |title=Fundamentals of Analytical Chemistry |date=2014 |publisher=Brooks/Cole |isbn=978-0-495-55828-6 |pages=201–202 |edition=9th}}</ref>

Similarly, when ammonia is the solvent, the strongest acid is ammonium (NH₄⁺), thus HCl and a super acid exert the same acidifying effect.

The same argument applies to bases. In water, OH⁻ is the strongest base. Thus, even though sodium amide (NaNH₂) is an exceptional base (pK_a of NH₃ ~ 33), in water it is only as good as sodium hydroxide. On the other hand, NaNH₂ is a far more basic reagent in ammonia than is NaOH.

The pH range allowed by a particular solvent is called the acid-base discrimination window.<ref name="Shriver/Atkins fifth">{{cite book|last=Atkins|first=P.W.|title=Shriver and Atkins' Inorganic Chemistry, Fifth Edition|url=https://archive.org/details/shriveratkinsino00atki|url-access=limited|year=2010|publisher=Oxford University Press|isbn=978-1-42-921820-7|pages=[https://archive.org/details/shriveratkinsino00atki/page/n329 121]}}</ref> __TOC__ ==Leveling and differentiating solvents== Strong bases are ''leveling solvents'' for acids, weak bases are ''differentiating solvents'' for acids. In a ''leveling solvent'', many acids are completely dissociated and are thus of the same strength. All acids tend to become indistinguishable in strength when dissolved in strongly basic solvents owing to the greater affinity of strong bases for protons. This is called the leveling effect.{{cn|date=January 2024}}

In a ''differentiating solvent'' on the other hand, various acids dissociate to different degrees and thus have different strengths. For example, anhydrous acetic acid (CH₃COOH) as solvent is a weaker proton acceptor than water. Strong aqueous acids such as hydrochloric acid and perchloric acid are only partly dissociated in anhydrous acetic acid and their strengths are unequal; in fact perchloric acid is about 5000 times stronger than hydrochloric acid in this solvent.<ref name=Skoog/> A weakly basic solvent such as acetic acid has less tendency than a more strongly basic one such as water to accept a proton. Similarly a weakly acidic solvent has less tendency to donate protons than a strong acid. {{cn|date=January 2024}}

Because of the leveling effect of common solvents, studies on super acids are conducted in more differentiating solvents that are very weakly basic such as sulfur dioxide (liquefied) and SO₂ClF.<ref name=Olah>{{cite encyclopedia | authorlink = George Olah | author = Olah, G. A. |author2=Prakash, G. K. S. |author3=Wang, Q. |author4=Li, X. | title = Hydrogen Fluoride–Antimony(V) Fluoride | encyclopedia = Encyclopedia of Reagents for Organic Synthesis | editor = Paquette, L. | year = 2001 | publisher = J. Wiley & Sons | location = New York | doi = 10.1002/047084289X.rh037m | url = http://onlinelibrary.wiley.com/o/eros/articles/rh037m/frame.html | isbn = 978-0471936237 | url-access = subscription }}</ref>

==Types of solvent on the basis of proton interaction== On the basis of proton interaction, solvents are of four types,

(i) Protophilic solvents: Solvents which have greater tendency to accept protons, i.e., water, alcohol, liquid ammonia, etc.

(ii) Protogenic solvents: Solvents which have the tendency to produce protons, i.e., water, liquid hydrogen chloride, glacial acetic acid, etc.

(iii) Amphiprotic solvents: Solvents which act both as protophilic or protogenic, e.g., water, liquid ammonia, ethyl alcohol, etc.

(iv) Aprotic solvents: Solvents which neither donate nor accept protons, e.g., benzene, carbon tetrachloride, carbon disulfide, etc.

HCl acts as a strong acid in H₂O, an even stronger acid in NH₃, a weak acid in CH₃COOH, neutral in C₆H₆ and a weak base in HF. NaHSO₃ acts as a weak acid in DMSO, a strong acid in NH₃, a weak base in glacial acetic acid, and a strong base in sulfuric acid.

==References== <references />

{{DEFAULTSORT:Leveling Effect}} Category:Acid–base chemistry