15.1
Bronsted-Lowry Acids and Bases
The acid-base reaction class has been studied for quite some time. In 1680, Robert Boyle reported traits of acid solutions that included their ability…
An Arrhenius acid is a substance that produces a hydrogen ion when dissolved in water, and an Arrhenius base is a substance that produces an OH−, or hydroxide ion.
Hydrochloric acid is an Arrhenius acid as it dissociates into a hydrogen ion and a chloride ion when dissolved in water. Sodium hydroxide is an Arrhenius base as it dissociates into a sodium ion and a hydroxide ion when dissolved in water.
However, this definition cannot be used to describe acids and bases that are not in an aqueous solution or bases that do not contain hydroxide ions.
A broader definition by Brønsted and Lowry defines an acid as a hydrogen ion or proton donor, whereas a base is a proton acceptor.
When hydrochloric acid is dissolved in water, it acts as an acid by donating a proton to water, producing a hydronium ion and a chloride ion. When ammonia is dissolved in water, it acts as a base and accepts a proton from water, producing an ammonium ion and a hydroxide ion.
A Brønsted-Lowry acid will always react with a Brønsted-Lowry base and vice versa. When an acid, for example, acetic acid, donates its proton, water acts as a base and accepts the proton. Acetic acid is converted into a conjugate base, acetate, and water is converted into a conjugate acid, a hydronium ion.
Acids and bases that differ from each other due to the transfer of a proton are called conjugate acid-base pairs. In the reverse reaction, the conjugate acid, hydronium, acts as a proton donor and the conjugate base, acetate, will accept a proton.
The strength of an acid is determined by its ability to donate a proton whereas the strength of a base is determined by its ability to accept a proton. A stronger acid is more likely to donate a proton than a weaker acid. Likewise, a stronger base is more likely to accept a proton than a weaker base.
The strength of an acid and its conjugate base are inversely related. A strong acid dissociates completely in solution and the resulting conjugate base is too weak to accept a proton. The same applies in the case of a strong base and its conjugate acid.
On the other hand, a weak acid dissociates partially in solution. The conjugate base of a weak acid is also relatively weak; therefore, a mixture of the undissociated weak acid and its weak conjugate base will be present in equilibrium. The same phenomenon occurs in the case of a weak base and its weak conjugate acid.
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Q1: How does the Brønsted-Lowry definition differ from the Arrhenius definition of acids and bases?
The Arrhenius definition limits acids to substances producing hydrogen ions in water and bases to those producing hydroxide ions. The Brønsted-Lowry definition is broader, defining acids as proton donors and bases as proton acceptors, allowing acids and bases to exist outside aqueous solutions and bases without hydroxide ions. This expanded framework better describes acid-base behavior across diverse chemical systems.
Q2: What are conjugate acid-base pairs and how do they form?
Conjugate acid-base pairs are species that differ by one proton. When an acid donates a proton, it becomes its conjugate base; when a base accepts a proton, it becomes its conjugate acid. For example, acetic acid donates a proton to water, forming acetate (conjugate base) and hydronium ion (conjugate acid). These pairs are central to understanding reversible acid-base reactions.
Q3: How does acid strength relate to the strength of its conjugate base?
Acid strength and conjugate base strength are inversely related. A strong acid completely dissociates, leaving a very weak conjugate base unable to accept protons. Conversely, a weak acid partially dissociates, producing a relatively weak conjugate base that can accept protons in the reverse reaction. This inverse relationship determines whether acid-base reactions reach equilibrium or completion.
Q4: Why does ammonia act as a base when dissolved in water?
Ammonia acts as a base because it accepts a proton from water according to Brønsted-Lowry theory. When ammonia accepts a proton from water, it forms an ammonium ion while water becomes a hydroxide ion. This proton-accepting behavior defines ammonia as a base, demonstrating that bases need not contain hydroxide ions initially.
Q5: What determines whether an acid-base reaction reaches equilibrium or goes to completion?
Strong acids and bases dissociate completely, producing extremely weak conjugate bases or acids unable to reverse the reaction, causing it to go to completion. Weak acids and bases partially dissociate, producing weak conjugate acids or bases capable of reversing the reaction, establishing equilibrium. The relative strengths of the weak species determine the equilibrium position.
Q6: How does water function as both an acid and a base in Brønsted-Lowry theory?
Water acts as an acid by donating a proton to bases like ammonia, becoming a hydroxide ion. Water acts as a base by accepting a proton from acids like hydrochloric acid, becoming a hydronium ion. This amphoteric behavior demonstrates that substances can be classified as acids or bases depending on the other species present in the reaction.
Q7: What is the relationship between proton donation ability and acid strength?
A stronger acid more readily donates protons than a weaker acid. Acid strength is determined by its propensity to transfer protons to bases. Strong acids like hydrochloric acid donate protons completely in solution, while weak acids like acetic acid donate protons only partially, establishing equilibrium with their conjugate bases.