Mistake Master
Acid-base reactions
Strip an acid-base reaction to its essence and it is one thing: a hydrogen ion changing hands. Find who gives the proton and who takes it, and the acid, the base, and their conjugates all fall into place.
§1
Proton donors and acceptors.
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In the Brønsted-Lowry model, an acid-base reaction is a proton (H⁺) transfer. The acid donates a proton; the base accepts one. Every such reaction has a donor and an acceptor.
When ammonia reacts with water, NH₃ (the base) accepts a proton to become NH₄⁺, and H₂O (the acid) donates it, becoming OH⁻. The same substance can act as an acid in one reaction and a base in another.
After the transfer, the products are the conjugate acid and base: the base gains a proton to become its conjugate acid, and the acid loses one to become its conjugate base.
§2
Following the proton.
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Track the H⁺ to name every role.
- Find who donates the proton. The species that loses an H⁺ is the acid.
- Find who accepts the proton. The species that gains an H⁺ is the base.
- Identify the conjugates. The acid minus H⁺ is its conjugate base; the base plus H⁺ is its conjugate acid.
- Remember roles can switch. Water and other amphoteric species act as an acid or a base depending on the partner.
§3
The pieces you'll meet.
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Proton transfer, four roles.
§4
Worked example: ammonia in water.
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Reaction. NH₃ + H₂O → NH₄⁺ + OH⁻.
Base. NH₃ accepts a proton, so it is the base; it becomes NH₄⁺, its conjugate acid.
Acid. H₂O donates a proton, so it is the acid; it becomes OH⁻, its conjugate base.
Note. Here water acts as the acid, but with an acid partner (like HCl) water accepts a proton and acts as a base. The role depends on the partner — water is amphoteric.
§5
Mistakes that cost real points.
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"A substance is always an acid or always a base."
Roles depend on the reaction. Water donates a proton (acts as an acid) to ammonia but accepts one (acts as a base) from HCl. Amphoteric species switch roles with their partner, so you cannot fix a label in advance.
Fix. Determine acid or base per reaction by following the proton, not by a fixed label on the substance.
"The base is whatever contains OH."
In the Brønsted-Lowry model a base is any proton acceptor, not only hydroxide-containing species. Ammonia is a base because it accepts a proton, despite having no OH. Defining a base by OH misses many bases.
Fix. Identify a base as a proton acceptor; look at whether it gains an H⁺, not whether it contains OH.
"Conjugate acid-base pairs differ by more than one proton."
A conjugate pair differs by exactly one proton (H⁺). NH₃ and NH₄⁺ are a pair; H₂O and OH⁻ are a pair. If two species differ by more than one H⁺, they are not a conjugate pair.
Fix. Check that the two members of a conjugate pair differ by exactly one hydrogen ion.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.