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Acid Strength

Acid strength is an acid's ability or tendency to lose a proton. Different acids should have different strengths. There are very few strong acids which are completely ionized in water or solution by loosing one proton. Strong acids are strong electrolytes. In other words, one mole of strong acid HA dissolves completely in water, yielding one mole of H+ ion and one mole of conjugate base A-. A strong acid does not have any non ionized acid of HA.

According to the equation,

HA(aq) $\to$ H+(aq) + A-(aq)

By contrast, weak acids are partially ionized in water or solution. In other words, one mole of weak acid HA dissolves partially in water and yields one mole of H+ ions and one mole of conjugate base A-. It has some non ionized acid and conjugate base in that solution.

Following are some of the examples for strong acids which dissolves or ionizes 100% in water.

HCl - Hydrochloric acid
HI - Hydro iodic acid
HNO3 - Nitric acid
H2SO4 - Sulfuric acid

When the different acids and bases are dissolved in water, they should yield different hydronium or hydroxyl ion (H+) concentrations.
  1. If an acid is strong, it dissolves completely in water or donates their proton 100% of time, to produce the higher concentration.
  2. If the acid is weak, it dissolves partially in water or donates their proton less than 100% of time to produce the lower concentration. Strong acid have weak H-A bond so that they can lose protons more easily.
The size of atom ‘A’ determines the strength of H-A bond. The term stability of the conjugate base is also used to discuss the acid strength. Stronger acid have a larger Ka and a more negative pka than compared to the weaker acids. The stronger acids should have ka value greater than one (ka >1). The acids which have ka value of less then one (ka<1) are considered weak.Super acids are acids which are stronger than 100% sulfuric acid. Examples for super acids is fluoro antimonic acid, magic and perchloric acid.

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Acid Base Strength

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  1. Acid base strength for all the acids and bases are not same, since they do not ionize or dissociate in water to the same extent.
  2. Acid base strength can be determined by the dissociation constant Ka. The terms strong and weak produce an indication of the strength of an acid and base. The term strong and weak describe the ability of acid and base solutions to conduct electricity.
  3. The acids and bases which conduct electricity very well are called as strong acid and strong base.
  4. The strong acids and bases contain a large number of ions. The acid and base which conduct electricity weakly or partially are called as weak acid and weak base. The weak acids and weak bases contains only a few ions.
  5. The acid and base bond strength is implied by the relative amount of molecules and ions present in the solution.
  6. Acid and base bond is represented by H-A, M-OH, respectively, where ‘A’ represents negative ions and ‘M’ the positive ions.
  7. For strong acids, they have more ions in solution, therefore the bonds holding ‘H’ and ‘A’ together must be weak.
  8. Strong acids easily dissociate into the respective ions. For weak acids and weak bases, they do not have more ions in the solution and they have only few ions in solution, therefore the bonds holding ‘H’ and ‘A’ together should have enough strength.
  9. The weak acid entities bond together as molecules and do not readily split into individual ions.
  10. For weak acids and base, strong bonds exist predominantly as molecules in solutions.
  11. For strong acids and bases, weak bonds easily dissociate into ions. Acid base strength is related to molecule and ions. So the strong acids and strong bases have weak bond strength. Weak acids and weak bases have strong bond strength.

pka and Acid Strength

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Ka is the acid dissociation constant and is also known as acidity constant or acid ionization constant. An acid dissociation constant is a quantitative measure of acid strength in a solution. It is the equilibrium constant for the acid base chemical reactions.
The equilibrium constant is a characteristic for a reaction.

HA (aq) $\to$ H+ (aq) + A- (aq)

Ka, the acid ionization constant is mainly used to measure the propensity of a compound to donate a proton.
The acid strength is represented as either equilibrium constant or as a percent dissociation. The equilibrium constant Ka is determined by dividing the equilibrium concentration of the produce of the reaction by the equilibrium concentration of the reactants of the reaction.

Ka = $\frac{[H^+][A^-]}{[HA]}$
More commonly a logarithmic measure of the acid dissociation constant is used in practice, due to many order of magnitude spanned by Ka value. Dissociation constant, denoted by pKa, is equal to –log10 Ka (negative log of dissociation constant). Some times it is referred to as an acid dissociation constant, which is a measure of the strength of an acid/base. The most convenient scale of acidity is pKa.

pKa = - log10ka

pKa value for acid and base is as follows,
  • Strong acid - pKa < 2
  • Weak acid - 7 > pKa > 2
  • Weak base - 10 >pKa > 7
  • Strong acid - pKa > 10
Generally a more positive value of pKa corresponds to a weaker acid and more negative value of pKa corresponds to a strong acid. So the low value of pKa indicates a stronger acid and higher value of pKa indicates a weaker acid. A strong acid has a pKa value of about less then -1.74.

Determining Acid Strength

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The factors such as polarity and inductive effect, atomic radius, and bond strength determine the acid strength. Determining acid strength, comparison with other acids with out use of pH calculation, the following important characteristics are observed.
  1. Electronegativity: The more acidic, the higher the electronegativity of a conjugate base in the same period. In other words, for more acid, the more electronegative A- (where HA (aq) ↔ H+ (aq) + A- (aq)).
  2. Atomic radius: If atomic radius is increases, the acidity also increases. For example HCl and HI acid both are stronger acids and both are 100% dissociated in solution. But HI is stronger than HCl, since the atomic radius for iodine atom is much larger than chlorine atom.
  3. Charge: The more positively charged a species is, the more acidic it is.
  4. Equilibrium: Determining acid strength can also be defined by the equilibrium position of its dissociation reaction.
HA (aq) + H2O (l) H3O + (aq) + A- (aq)

In a strong acid, equilibrium lies to the right which means that almost the entire original HA is dissociated at equilibrium. A strong acid yields a weak conjugate base (A-), so a strong acid is also described as an acid whose conjugate base is much weaker than water.

Acid Strength Chart

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Below figure show the chart for acid strength as well as base strength. Acid strength chart is very useful to check the acid strength of different acids and different bases.

 $K_a$                    Acid
       Name          Formula 
Formula  Name
 Large  Perchloric acid  HCl$O_4$  Cl$O_4^{-}$  Perchlorate ion
 3.2*$10^9$  Hydroiodic acid  HI  I-  Iodide
 1.0*$10^9$  Hydrobromic acid  HBr  Br-  Bromide
 1.3*$10^6$  Hydrochloric acid  HCl  Cl-  Chloride
 1.0*$10^3$  Sulfuric acid  $H_2SO_4$  HS$O_4^{-}$  Hydrogen sulfate ion
 2.4*$10^1$  Nitric acid  HN$O_3$  N$O_3$  Nitrate ion
 --------------  Hydronium ion  $H_3$O+  $H_2$O   Water
 5.4*$10^{-2}$  Oxalic acid
 H$O_2C_2O_2$H  H$O_2C_2O_2^{-}$  Hydrogen oxalate ion
 1.3*$10^{-2}$  Sulfurous acid  $H_2$S$O_3$  HS$O_3$               Hydrogen sulfite ion
 1.0*$10^{-2}$  Hydrogen sulfate ion  HS$O_4^{-}$  S$O_4^{2-}$  Sulfate ion
 7.1*10^${-3}$  Phosphoric acid  $H_3$P$O_4$  $H_2$P$O_4^{-}$  Dihydrogen phosphate ion
 7.2*$10^{-4}$  Nitrous acid  HN$O_2$  N$O_3^{-}$  Nitrite ion
 6.6*$10^{-4}$   Hydrofluoric acid  HF  F-  Fluoride ion
 1.8*$10^{-4}$     Methanoic acid  HC$O_2$H  HC$O_2^{-}$  Methanoate ion
 6.3*$10^{-5}$   Banzoic acid  $C_6H_5$COOH  $C_6H_5$COO-  Benzoate ion
 5.4*$10^{-5}$  Hydrogen oxalate ion  H$O_2C_2O^{2-}$  $O_2C_2O^{2-}$  Oxalate ion
 1.8*$10^{-5}$  Ethanoic acid  C$H_3$COOH  C$H_3$COO  Ethanoate(acetate) ion
 4.4*$10^{-7}$  Carbonic acid  C$O_3^{2-}$  HC$O_3^{-}$  Hydrogen Carbonate ion
 1.1*$10^{-7}$  Hydrosulfuric acid  $H_2$S  HS-  Hydrogen sulfide ion
 6.3*$10^{-8}$  Dihydrogen phosphate ion  $H_2$P$O_4^{-}$  P$O_4^{2-}$  Hydrogen phophate ion
 6.2*$10^{-8}$  Hydrogen sulfite ion  H$S^{-}$  $S^{2-}$  Sulfite ion
 2.9*$10^{-8}$  Hypochlorous acid  HClO  Cl$O^{-}$  Hypochlorite ion
 6.2*$10^{-10}$  Hydrocyanic acid  HCN  C$N^{-}$  Cyanide ion
 5.8*$10^{-10}$  Ammonium ion  N$H_4^{+}$  N$H_3$  Ammonia
 5.8*$10^{-10}$  Boric acid  $H_3BO_3$  $H_2BO_3^{-}$  Dihydrogen Carbonate ion
 4.7*$10^{-11}$  Hydrogen carbonate ion  HC$O_3^{-}$  C$O_3^{2-}$  Carbonate ion
 4.2*$10^{-13}$  Hydrogen phosphate ion  HP$O_4^{2-}$  P$O_4^{3-}$  Phosphate ion
 1.8*$10^{-13}$  Dihydrogen borate ion  $H_2$B$O_3^{-}$  HB$O_3^{2-}$  Hydrogen borate ion
 1.3*$10^{-13}$  Hydrogen sulfide ion  HS-  $S^{2-}$  Sulfide ion
 1.6*$10^{-14}$  Hydrogen borate ion   HB$O_3^{2-}$  B$O_3^{3-}$  Borate ion
 ---------------------- Water
 $H_2$O   OH-  Hydroxide

In the acid strength chart, strong acids are listed at the top left hand corner of the table which have ka value >1. If the acids have a ka value of less then 1, it is considered a weak acid. The strong bases are listed at the bottom right of the chart and weaker bases are listed at the top right of the chart.

Acid strength chart helps the students to recognize the reciprocal nature of the acid conjugate base strength relationship from increasing acid strength and decreasing base strength.
More topics in Acid Strength
Strong and Weak Acids Nitric Acid
Uses of Nitric Acid Ostwald Process
Fuming Nitric Acid Buffer
Alkaline Buffer Buffer Solution
Acid Buffer Buffer Solution Examples
Buffer Solution Calculations Buffer Range
Phosphate Buffer Henderson Hasselbach
Acetate Buffer Preparation of Buffer Solution
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