Introduction to Acids and Bases

Name the roles from the event

A Brønsted–Lowry acid donates a proton, and a Brønsted–Lowry base accepts one. In \(NH_3+H_2O\rightleftharpoons NH_4^++OH^-\), ammonia accepts a proton from water. Ammonia is therefore the base and water the acid in the forward direction. These labels describe behavior in a reaction, not simply the presence of hydrogen or an electrical charge.

Find the conjugate partners

NH₃ and NH₄⁺ differ by exactly one H⁺, so they are a conjugate base–acid pair. Water and hydroxide form the other pair. Removing H⁺ decreases the charge by one unit; adding H⁺ increases it by one. Conjugate partners are found across the reaction arrow, not by pairing the two reactants together.

Some species can take either role

Such a species is amphiprotic. The carbonic acid in the first reaction can subsequently interconvert with dissolved carbon dioxide and water, so this table isolates the proton-transfer step.

Work through an amount calculation

An original solution contains 2.40 mmol bicarbonate and receives 0.60 mmol hydroxide. Under conditions where the displayed proton-transfer reaction is effectively complete and competing reactions are negligible, one mole of hydroxide consumes one mole of bicarbonate. The final accounting gives 1.80 mmol bicarbonate and 0.60 mmol carbonate, with no excess added hydroxide. Charge is conserved because −1 plus −1 becomes −2.

Water is a reactant, not just a backdrop

Water can exchange a proton with another water molecule, producing hydronium and hydroxide. At 25 °C in the dilute model,

Both ions are present in acidic and basic aqueous solutions. Calling a solution acidic means hydronium exceeds hydroxide, not that hydroxide has vanished.

Interpret observations without tasting samples

Indicators and calibrated pH probes provide evidence about acidity. Conductivity can reveal mobile ions, but a highly conducting solution is not necessarily acidic: a neutral salt solution can also conduct strongly. Use teacher-approved samples and appropriate protective equipment; tasting or directly touching a chemical is never a valid identification method.

Strength is different from amount

“Strong acid” describes extensive proton transfer to water under ordinary aqueous conditions. “Concentrated” describes how much solute is present per volume. A dilute strong acid can have less hydronium than a sufficiently concentrated weak acid. To predict pH, you need both the equilibrium behavior and the analytical concentration.