Acids and Bases

Classifying Acids and Bases

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Adi Manocha

Learning Objectives

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Electrolytes
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Electrolytes are substances that form ions in solution (via ionisation or dissociation)

All substances can be categorised into one of the three strengths:

  • Strong electrolytes ⇒ will fully form ions in solution (strong acids and ionic substances)

  • Weak electrolytes ⇒ will partially form ions in solution (weak acids and NH₃)

  • Non-electrolytes ⇒ will not form any ions in solution (most covalent molecular substances (excluding acids or NH₃))

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What's the Difference Between Ionisation and Dissociation
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Ionisation occurs when covalent bonds (in covalent compounds) are broken to form ions in solution (e.g. HCl 🠒 H⁺ + Cl⁻)


Dissociation occurs when ionic bonds (in electrovalent or ionic compounds) are broken to form ions in solution (e.g. NaOH 🠒 Na⁺ + Cl⁻)

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Acid and Base Strength
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Acid/Base Strength is effectively the extent to which the substance will dissociate/ionise

  • A strong acid fully ionises in solution

  • A weak acid partially ionises in solution

  • A strong base fully reacts with water to form OH⁻

  • A weak base partially reacts with water to form OH⁻

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Monoprotic and Polyprotic Acids
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Some acids are able to donate multiple protons (H⁺ ions), these are called "polyprotic acids"


An Acid molecule contains a polar bond between a hydrogen atom and an electronegative atom. This bond is disrupted when the acid is added to water,  producing a H⁺ ion.

  • A monoprotic acid contains just one of these bonds, as such it only produces one H⁺ ion when added to water.

  • Diprotic acids (such as H2SO4) contain two of these bonds, producing two H⁺ ions, while Triprotic acids produce three H⁺ ions.

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Salts
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Salts are:

  1. any ionic substance that does not contain H⁺, OH⁻ or O2⁻

  2. an ionic product derived in an acid-base reaction


Salts may be neutral, acidic or basic

  • neutral salts are formed from the reaction between a strong acid and a strong base; they do not hydrolyse in water

  • basic salts are formed from the reaction between a strong base and a weak acid; they undergo hydrolysis to produce a basic solution

  • acidic salts are formed from the reaction between a strong acid and a weak base; they undergo hydrolysis to produce an acidic solution

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Amphoteric and Amphiprotic Substances
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Amphoteric substances are molecules or ions that are able to react with acids and bases.


Amphiprotic salts are ionic substances that can act as an acid or a base. 


For example: HCO3⁻ (an amphiprotic salt) can react with the Water (an amphoteric substance) as either an acid or a base 

  • Acting as an acid, HCO3⁻ reacts with H₂O to produce a HCO⁻ ion and a H₃O⁺ion

  • Acting as a base, HCO3⁻ reacts with H₂O to produce H₂CO₃⁻ and an OH⁻ ion 

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Defining Acids and Bases
Classifying Acids and Bases
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