Arrhenius
Summary
An Arrhenius acid raises the H⁺ (H₃O⁺) concentration in water; an Arrhenius base raises the OH⁻ concentration in water. It is the narrowest acid-base definition, so NH₃ is a Brønsted-Lowry base but not an Arrhenius one. The name also tags the rate equation k = Ae^(−Ea/RT).
Detail
Meaning: An Arrhenius acid increases the H⁺ (H₃O⁺) concentration when dissolved in water, as HCl does. An Arrhenius base increases the OH⁻ concentration in water, as NaOH does. Because the definition depends on water and on OH⁻ itself, it is the narrowest of the three models. Brønsted-Lowry widens it to proton donors and acceptors, and Lewis to electron-pair acceptors and donors.
Equation: k = A e^(−Ea/RT), the Arrhenius equation, where k is the rate constant (units set by reaction order), A is the frequency factor (same units as k), Ea is activation energy (J/mol), R is 8.314 J/(mol·K), and T is temperature (K).
On the MCAT: Acid-base items ask you to classify species: NH₃ is a Brønsted-Lowry base with no OH⁻ to release, and BF₃ is a Lewis acid with no H⁺ to give. Kinetics items ask you to predict that k rises when T rises or Ea falls, or to read a ln k versus 1/T line with slope −Ea/R.
Common confusion: Arrhenius and Brønsted-Lowry are not interchangeable. Arrhenius acids and bases also fit Brønsted-Lowry, but many Brønsted-Lowry bases fail the Arrhenius test because they contain no OH⁻ to release.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- General Chemistry (Ebbing and Gammon)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.