6.10.2. the brönsted- lowry Acids and bases

The Danish chemist, Johannes Brönsted and the English chemist, Thomas M. Lowry gave a more general definition of acids and bases. According to Brönsted-Lowry theory, acid is a substance that is capable of donating a hydrogen ion H+ and bases are substances capable of accepting a hydrogen ion, H+. In short, acids are proton donors and bases are proton acceptors.

Brönsted-Lowry theory expands the definition of acids and bases beyond the traditional Arrhenius definitions. In this theory, an acid is identified by its ability to donate a proton (H+), while a base is defined by its ability to accept a proton. For example, hydrochloric acid (HCl) donates a proton to water, forming hydronium ions (H3O+), while ammonia (NH3) accepts a proton from water, forming ammonium ions (NH4+). This theory helps us understand acid-base reactions in a broader range of contexts, not limited to aqueous solutions.

Think of acids and bases as two dance partners in a ballroom. The acid, which is the proton donor, hands off a token (the hydrogen ion) to its partner, who is the base (the proton acceptor). This exchange allows them to create a new dance (the resulting chemical reaction), showcasing how they can interact in various environments, not just on the dance floor (in water).

Consider the example of dissolution of NH3 in H2O represented by the following equation: H2O + NH3 ⇌ NH4+ + OH–. In this reaction, water acts as a proton donor and ammonia acts as a proton acceptor, identifying them as Brönsted-Lowry acid and base, respectively.

In the reaction between ammonia (NH3) and water (H2O), ammonia accepts a proton from water. This results in the formation of ammonium ions (NH4+) and hydroxide ions (OH–). In this case, water donates a proton and is classified as the Brönsted-Lowry acid, while ammonia, which accepts the proton, is the Brönsted-Lowry base. This interaction highlights the dynamic nature of acid-base reactions and how reactants can switch roles depending on the context.

Imagine a game of catch where one player (water) throws a ball (the proton) to another player (ammonia). The player who catches the ball becomes a different character in the game (the ammonium ion) while the thrower, after throwing the ball, transforms into another version of themselves, now having one less ball to throw (the hydroxide ion). This illustrates how the roles of acids and bases can change in chemical reactions.

The acid-base pair that differs only by one proton is called a conjugate acid-base pair. Therefore, OH– is called the conjugate base of an acid H2O, and NH4+ is called conjugate acid of the base NH3.

Conjugate acid-base pairs are vital in understanding acid-base chemistry. They represent the relationship between an acid and a base that differ by a single proton. For instance, when a strong acid donates a proton, it forms a weaker base, known as its conjugate base. Conversely, when a base accepts a proton, it forms a weak acid, known as its conjugate acid. This dynamic highlights how substances can behave differently depending on their protonation state.

Consider a tennis game where a player can either serve (be an acid) or receive (be a base). If the player serves a ball (donates a proton), they are now like the conjugate base because they have one less ball to serve. When they receive a ball (accept a proton), they become the conjugate acid because they have that ball back to serve again. This cycle demonstrates how substances can switch between being acids and bases.

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