Arrhenius Theory – this is the simplest of all acid/base theories. Arrhenius’ theory states the following:�

Arrhenius Acid: A substance that forms the hydrogen ion (H⁺) / hydronium ion (H₃O⁺) in water.

HNO₃ + H₂O 🡪 H₃O⁺ _(aq) + NO₃^– _(aq)

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OR

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HNO_3(aq) 🡪 H⁺_(aq) + NO₃^- _(aq)

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Arrhenius Base: A substance that forms the hydroxide ion OH^- in water

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NaOH 🡪 Na⁺ (aq) + OH^– (aq)

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Other Examples:

• Mg(OH)_{2 (aq)}
• Ba(OH)_2(aq)
• Al(OH)_3(aq)

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Brönsted-Lowry Theory – It addresses some of the issues that exist with the funky nitrogen bases! NH₃ (ammonia) is a base, but it doesn’t fit Arrhenius’ theory. �� **Proton = hydrogen ion H⁺

Brönsted-Lowry Acid: A substance that

loses / donates a proton (H⁺). The substance remaining after a proton is donated is then called a conjugate base.

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NH_3(aq) + HF_(aq) → NH₄⁺_(aq) + F^-_(aq)

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HCl + H₂O 🡪 H₃O⁺_(aq) + Cl^–_(aq)

Brönsted-Lowry Base: A substance that accepts a proton (H⁺). The substance remaining after a proton is accepted is then called a conjugate acid.

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NH₃ + H₂O ⇄ NH₄⁺ (aq) + OH^– (aq)

Will it donate or accept a proton?

Most of the time, we use Brönsted-Lowry’s theory to define an acid or base.

• If it loses an H⁺, it’s an acid…
• If it gains an H⁺, it’s a base.

In undergoing these processes, we end up forming conjugates of the original substances.

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Some important things to remember…�

The acid/base will always be reactants, and the conjugate pairs will always be products.

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Acid + Base 🡪 Conjugate base + Conjugate acid

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Amphiprotic / Amphoteric Substances

• These are substances that can act as an acid or a base, based on Brönsted-Lowry’s definition. The most common Amphiprotic substance is water!

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• In which of the following reactions is water behaving like an acid, and in which one is water behaving like a base?

• HCl + H₂O 🡪 H₃O⁺ + Cl^–

• NH₃ + H₂O 🡪 NH₄⁺ + OH ^–

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base

acid

HPO₄^2─ + H₂O 🡪 PO₄^3─ + H₃O⁺

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HPO₄^2─ + H₂O 🡪 H₂PO₄^─ + OH^─

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