Topic 4.8 Introduction to Acid-Base Reactions: apply the Bronsted-Lowry model to identify acids, bases and conjugate acid-base pairs, and write acid-base reactions as proton transfers.

What this topic is asking

The College Board (Topic 4.8) wants you to apply the Bronsted-Lowry model of acids and bases: an acid is a proton ($\text{H}^+$) donor and a base is a proton acceptor. You should identify the acid and base in a reaction, recognize the conjugate acid-base pairs, and distinguish strong from weak acids and bases. This is the conceptual foundation for the titrations of Topic 4.6 and the full acid-base unit (Unit 8).

The Bronsted-Lowry definitions

This model is broader than the older Arrhenius one (which defined acids as producing $\text{H}^+$ and bases as producing $\text{OH}^-$ in water). The Bronsted-Lowry view focuses on the proton transfer itself, so it covers reactions in which no hydroxide is produced, such as ammonia accepting a proton from water. To identify the acid and base, ask which species loses an $\text{H}^+$ (the acid) and which gains it (the base).

Conjugate acid-base pairs

For example, in $\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$, HCl (acid) and $\text{Cl}^-$ (its conjugate base) form one pair, and $\text{H}_2\text{O}$ (base) and $\text{H}_3\text{O}^+$ (its conjugate acid) form the other. Spotting the pairs is a matter of matching each species with the one that differs from it by a single proton.

Strong versus weak

A strong acid (such as HCl) or strong base (such as NaOH) transfers protons essentially completely, ionizing fully in water, so it is a strong electrolyte. A weak acid (such as acetic acid) or weak base (such as ammonia) transfers protons only partially, reaching an equilibrium in which most of the substance remains as intact molecules. This is why a weak acid solution is drawn as mostly molecules with only a few ions (Topic 3.8), and it is the key idea behind the different equivalence-point behavior in titrations. Water is amphoteric, able to act as an acid (donating a proton, as to ammonia) or a base (accepting one, as from HCl).

Try this

Q1. Identify the conjugate base of $\text{H}_2\text{SO}_4$ when it donates one proton. [1 point]

• Cue. $\text{HSO}_4^-$ (the acid minus one $\text{H}^+$).

Q2. Explain why ammonia is a Bronsted-Lowry base. [2 points]

• Cue. It accepts a proton (for example from water) to form $\text{NH}_4^+$, so it acts as a proton acceptor.