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Strong and weak acids. Strong acids dissociate fully in water to produce the maximum number of H + ions. This means if you had one mole of hydrochloric acid (HCl) molecules, they would...
Apr 20, 2023 · Acids and bases are classified as either strong or weak, based on their ionization in water. A strong acid or base is one which is completely ionized in an aqueous solution. A weak acid or …
Sep 16, 2022 · Acids are classified as either strong or weak, based on their ionization in water. A strong acid is an acid which is completely ionized in an aqueous solution. A weak acid is an …
Water is the base that reacts with the acid \(\ce{HA}\), \(\ce{A^{−}}\) is the conjugate base of the acid HA, and the hydronium ion is the conjugate acid of water. By definition, a strong acid yields 100% of \(\ce{H3O+}\) and \(\ce{A^{−}}\) when the acid ionizes in water.
- Overview
- Key points
- Introduction
- Brønsted-Lowry theory of acids and bases
- Identifying Brønsted-Lowry acids and bases
- Strong and weak acids: to dissociate, or not to dissociate?
- Strong and weak bases
- Example 1: Writing an acid-base reaction with hydrogen phosphate
- Conjugate acid-base pairs
- Example 2: Dissociation of a strong acid
•A Brønsted-Lowry acid is any species that is capable of donating a proton—H+ .
•A Brønsted-Lowry base is any species that is capable of accepting a proton, which requires a lone pair of electrons to bond to the H+ .
•Water is amphoteric, which means it can act as both a Brønsted-Lowry acid and a Brønsted-Lowry base.
•Strong acids and bases ionize completely in aqueous solution, while weak acids and bases ionize only partially.
•The conjugate base of a Brønsted-Lowry acid is the species formed after an acid donates a proton. The conjugate acid of a Brønsted-Lowry base is the species formed after a base accepts a proton.
•The two species in a conjugate acid-base pair have the same molecular formula except the acid has an extra H+ compared to the conjugate base.
•A Brønsted-Lowry acid is any species that is capable of donating a proton—H+ .
•A Brønsted-Lowry base is any species that is capable of accepting a proton, which requires a lone pair of electrons to bond to the H+ .
•Water is amphoteric, which means it can act as both a Brønsted-Lowry acid and a Brønsted-Lowry base.
•Strong acids and bases ionize completely in aqueous solution, while weak acids and bases ionize only partially.
•The conjugate base of a Brønsted-Lowry acid is the species formed after an acid donates a proton. The conjugate acid of a Brønsted-Lowry base is the species formed after a base accepts a proton.
•The two species in a conjugate acid-base pair have the same molecular formula except the acid has an extra H+ compared to the conjugate base.
In a previous article on Arrhenius acids and bases, we learned that an Arrhenius acid is any species that can increase the concentration of H+ in aqueous solution and an Arrhenius base is any species that can increase the concentration of OH− in aqueous solution. A major limitation of Arrhenius theory is that we can only describe acid-base beha...
The Brønsted-Lowry theory describes acid-base interactions in terms of proton transfer between chemical species. A Brønsted-Lowry acid is any species that can donate a proton, H+ , and a base is any species that can accept a proton. In terms of chemical structure, this means that any Brønsted-Lowry acid must contain a hydrogen that can dissociate as H+ . In order to accept a proton, a Brønsted-Lowry base must have at least one lone pair of electrons to form a new bond with a proton.
Using the Brønsted-Lowry definition, an acid-base reaction is any reaction in which a proton is transferred from an acid to a base. We can use the Brønsted-Lowry definitions to discuss acid-base reactions in any solvent, as well as those that occur in the gas phase. For example, consider the reaction of ammonia gas, NH3(g) , with hydrogen chloride gas, HCl(g) , to form solid ammonium chloride, NH4Cl(s) :
NH3(g)+HCl(g)→NH4Cl(s)
This reaction can also be represented using the Lewis structures of the reactants and products, as seen below:
In this reaction, HCl donates its proton—shown in blue—to NH3 . Therefore, HCl is acting as a Brønsted-Lowry acid. Since NH3 has a lone pair which it uses to accept a proton, NH3 is a Brønsted-Lowry base.
Note that according to the Arrhenius theory, the above reaction would not be an acid-base reaction because neither species is forming H+ or OH− in water. However, the chemistry involved− a proton transfer from HCl to NH3 to form NH4Cl − is very similar to what would occur in the aqueous phase.
In the reaction between nitric acid and water, nitric acid, HNO3 , donates a proton—shown in blue—to water, thereby acting as a Brønsted-Lowry acid.
HNO3(aq)+H2O(l)→H3O+(aq)+NO3−(aq)
Since water accepts the proton from nitric acid to form H3O+ , water acts as a Brønsted-Lowry base. This reaction highly favors the formation of products, so the reaction arrow is drawn only to the right.
Let's now look at a reaction involving ammonia, NH3 , in water:
NH3(aq)+H2O(l)⇌NH4+(aq)+OH−(aq)
In this reaction, water is donating one of its protons to ammonia. After losing a proton, water becomes hydroxide, OH− . Since water is a proton donor in this reaction, it is acting as a Brønsted-Lowry acid. Ammonia accepts a proton from water to form an ammonium ion, NH4+ . Therefore, ammonia is acting as a Brønsted-Lowry base.
A strong acid is a species that dissociates completely into its constituent ions in aqueous solution. Nitric acid is an example of a strong acid. It dissociates completely in water to form hydronium, H3O+ , and nitrate, NO3− , ions. After the reaction occurs, there are no undissociated HNO3 molecules in solution.
By contrast, a weak acid does not dissociate completely into its constituent ions. An example of a weak acid is acetic acid, CH3COOH , which is present in vinegar. Acetic acid dissociates partially in water to form hydronium and acetate ions, CH3COO− :
CH3COOH(aq)+H2O(l)⇌H3O+(aq)+CH3COO−(aq)
Notice that in this reaction, we have arrows pointing in both directions: ⇋ . This indicates that dissociation of acetic acid is a dynamic equilibrium where there will be a significant concentration of acetic acid molecules that are present as neutral CH3COOH molecules as well as in the form of the dissociated ions, H+ and CH3COO− .
A common question is, “When do you know when something is a strong or a weak acid?” That is an excellent question! The short answer is that there are only a handful of strong acids, and everything else is considered a weak acid. Once we are familiar with the common strong acids, we can easily identify both weak and strong acids in chemistry problems.
The following table lists some examples of common strong acids.
A strong base is a base that ionizes completely in aqueous solution. An example of a strong base is sodium hydroxide, NaOH . In water, sodium hydroxide dissociates completely to give sodium ions and hydroxide ions:
NaOH(aq)→Na+(aq)+OH−(aq)
Thus, if we make a solution of sodium hydroxide in water, only Na+ and OH− ions are present in our final solution. We don't expect any undissociated NaOH .
Let's now look at ammonia, NH3 , in water. Ammonia is a weak base, so it will become partially ionized in water:
NH3(aq)+H2O(l)⇌NH4+(aq)+OH−(aq)
Some of the ammonia molecules accept a proton from water to form ammonium ions and hydroxide ions. A dynamic equilibrium results, in which ammonia molecules are continually exchanging protons with water, and ammonium ions are continually donating the protons back to hydroxide. The major species in solution is non-ionized ammonia, NH3 , because ammonia will only deprotonate water to a small extent.
Hydrogen phosphate, HPO42− , can act as a weak base or as a weak acid in aqueous solution.
What is the balanced equation for the reaction of hydrogen phosphate acting as a weak base in water?
Since hydrogen phosphate is acting as a Brønsted-Lowry base, water must be acting as a Brønsted-Lowry acid. This means that water will donate a proton to generate hydroxide. The addition of a proton to hydrogen phosphate results in the formation of H2PO4− :
HPO42−(aq)+H+(aq)→H2PO4−(aq)
Since hydrogen phosphate is acting as a weak base in this particular example, we will need to use equilibrium arrows, ⇌ , in our overall reaction to show that the reaction is reversible. That gives the following balanced equation for the reaction of hydrogen phosphate acting as a weak base in water:
HPO42−(aq)+H2O(l)⇌H2PO4−(aq)+OH−(aq)
Now that we have an understanding of Brønsted-Lowry acids and bases, we can discuss the final concept covered in this article: conjugate acid-base pairs. In a Brønsted-Lowry acid-base reaction, a conjugate acid is the species formed after the base accepts a proton. By contrast, a conjugate base is the species formed after an acid donates its proton...
Let's reconsider the strong acid HCl reacting with water:
HCl(aq)+H2O(l)→H3O+(aq)+Cl−(aq)
acid base acid base
In this reaction, HCl donates a proton to water; therefore, HCl is acting as a Brønsted-Lowry acid. After HCl donates its proton, the Cl− ion is formed; thus, Cl− is the conjugate base of HCl .
Conjugate pair 1=HCl and Cl−
Because water accepts a proton from HCl , water is acting as a Brønsted-Lowry base. When water accepts a proton, H3O+ is formed. Therefore, H3O+ is the conjugate acid of H2O .
When a weak base and a strong acid are mixed, they react according to the following net-ionic equation: B (aq) + H₃O⁺ (aq) → HB⁺ (aq) + H₂O (l). If the acid and base are equimolar, the pH of the resulting solution can be determined by considering the equilibrium reaction of HB⁺ with water.
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- Jay
OH- is actually considered to be a strong base, as its conjugate acid, water (H2O), is a weak acid.
