What Are Brønsted-Lowry Acids and Bases?
In 1923, the Brønsted-Lowry Acids and Bases concept was put forth individually by Johannes Nicolaus Brønsted and Thomas Martin Lowry. Up until this point, the only definition that was given for defining acids and bases was the Arrhenius theory of acids and bases. Although this theory holds true as well, it is limited in its ability to properly demonstrate the functions of an acid and base.
The theory put forth by Johannes Brønsted and Thomas Lowry was essential to developing modern chemistry because it enabled people to examine reactions between acids and bases outside of reactions involving water. It also provided a rule for substances that were able to act as both acids and bases depending on the other substances they reacted with.
Pre-Brønsted-Lowry Acids and Bases
Before the Brønsted-Lowry acids and bases were proposed, the Arrhenius theory for acids and bases was used to determine what a substance was. It was proposed by Svante Arrhenius in 1884 based off of his observations of certain substances and the ions they produced when they reacted with water. While this theory still holds true, it was limited because it only examined acids and bases that could have aqueous reactions.
- Arrhenius Acids: any species that increases the concentration of H+ in an aqueous solution.
- Arrhenius Bases: any species that increases the concentration of OH– in an aqueous solution.
Although these rules were enough for a time, scientists began to look for a rule that could be used to define all acids and bases.
What is an Acid?
The study of chemical reactions makes being able to differentiate between acids and bases essential. There are several characteristics that help to differentiate acids from bases.
Most acids can be quickly distinguished by their characteristic sour flavor. Citrus fruits are a great example of this principle. In addition to this, acids are known to react with bases to form a salt and a water.
Acidic solutions are those with a pH that is less than 7. The lower a pH value is, the more acidic the substance or solution is. An example of this can be seen with distilled white vinegar which has an approximate pH of 2.4, while water (which can have a wide range of pH measurements) can have a pH of 7.
What is a Brønsted-Lowry Acid?
A Brønsted-Lowry acid is a compound that is able to donate a proton during a chemical reaction. Conversely, a Brønsted-Lowry base is a compound that is able to accept a proton during a chemical reaction. These observations took Arrhenius’ rule of acids and bases to the next level and allowed for more thorough classifications to be made.
- Brønsted-Lowry Acid is a proton / hydrogen ion (H+) donor in chemical reactions
- Brønsted-Lowry Base is a proton / hydrogen ion (H+) acceptor in chemical reactions
Why Are Hydrogen Ions and Protons Used Interchangeably For Determining Acids and Bases?
When describing a Brønsted-Lowry acid, it is often said that an acid is the element that donates a proton or hydrogen ion in the reaction. While they may appear to be very different properties, they are actually quite similar.
A hydrogen atom (H) in its simplest form can be shown as having a proton in its nucleus and one electron orbiting in its electron cloud.
When the hydrogen atom becomes ionized, however, it loses its electron and simply keeps its proton. Because an ionized hydrogen atom is essentially just a proton, the two terms are often used interchangeably.
Conjugate Acids and Bases
The Brønsted-Lowry acids and bases also allowed for the classifications of conjugate acids and conjugate bases. The idea of conjugate acids and bases is determined by the reactants in a chemical reaction. When a Brønsted-Lowry acid donates its proton during a chemical reaction, for example, the new product is the conjugate base. Likewise, when a base accepts a proton during a chemical reaction, the new product is the conjugate acid.
It is also important to note that a cation can be a conjugate acid, while an anion is usually a conjugate base. This, however, is dependent on which substance is involved in the chemical reaction.
Brønsted-Lowry Reaction Example
When using the Brønsted-Lowry theory of acids and bases, reactions are examined in the following manner:
Acid + Base => Conjugate Base + Conjugate Acid
Because of the nature of the equation, the reaction can also be expressed in the following manner:
Conjugate Base + Conjugate Acid => Acid + Base
Let’s examine the following reaction between hydrochloric acid and ammonia:
HCl (aq) + NH3 (aq) => NH4+ (aq) + Cl– (aq)
When looking at the above reaction, its apparent that the hydrochloric acid donates a proton to ammonia. The resulting products in this reaction are a positively charged ammonia ion (NH4+) and a negatively charged chlorine ion (Cl–).
When considering the implications of the reaction, we can see that the hydrochloric acid is the Brønsted-Lowry acid in this equation because it donated a proton to ammonia. Likewise, because ammonia accepted the proton, it is the Brønsted-Lowry base.
It can also be determined that because hydrochloric acid donated a proton, the negative chlorine ion is the conjugate base. Additionally, because ammonia accepted the proton from hydrochloric acid, the positive ammonia ion is the conjugate acid of the equation.
Brønsted-Lowry Acids and Bases in Water
When Brønsted-Lowry acids and bases come into contact with water, they follow the same rules as Arrhenius’ theory of acids and bases. This is only helps to validate the theory that was determined by Brønsted and Lowery.
When a Brønsted-Lowry acid is dissociated in water, it helps to increase the amount of hydrogen ions found in the solution (H+) because it donates protons to whatever it reacts with.
Likewise, when a Brønsted-Lowry base is dissociated in water, it takes (or accepts) protons that are already present in water to form hydroxide ions (OH–). This is because Brønsted-Lowry bases are known to take protons in a reaction, thus explaining why Arrhenius’ model works with acids and bases involved in aqueous reactions.