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high school Acid–Base Reactions

Acid-Base Reaction

An acid releases hydrogen ions H⁺ when it dissolves in water, and a base releases hydroxide ions OH⁻. pH expresses the hydrogen ion concentration on a logarithmic scale: at 25 °C a solution is acidic below 7, neutral at 7 and basic above 7. Mixing an acid and a base starts a neutralization reaction in which H⁺ and OH⁻ combine into water, and the solution becomes neutral where the amounts of the two ions are equal. In the pictures, changing the hydrogen ion concentration moves the point on the pH scale, and changing the volume of hydrochloric acid added changes the number of ions and water molecules in the beaker.

What Acids and Bases Are

💡 The Arrhenius Definition
①Acid: a substance that releases H⁺ (hydrogen ions) when it dissolves in water
②Base: a substance that releases OH⁻ (hydroxide ions) when it dissolves in water
③HCl → H⁺ + Cl⁻ (acid)
④NaOH → Na⁺ + OH⁻ (base)

The pH Scale

10⁻³
Definition of pH
pH = -log[H⁺]
[H⁺] is the molar concentration of hydrogen ions (M)
Self-Ionization of Water
[H⁺] × [OH⁻] = Kw = 1.0 × 10⁻¹⁴ (25℃)
In pure water [H⁺] = [OH⁻] = 10⁻⁷ M, so pH = 7
🔍 Reading the Picture
①pH < 7 is acidic: there is more [H⁺] than [OH⁻]
②pH = 7 is neutral: [H⁺] and [OH⁻] are equal
③pH > 7 is basic: there is more [OH⁻] than [H⁺]
④When pH drops by 1, [H⁺] becomes 10 times larger, and the two bars always add up to the same length because [H⁺] × [OH⁻] is constant

Neutralization

5 mL
Neutralization Reaction
H⁺ + OH⁻ → H₂O
H⁺ from the acid and OH⁻ from the base combine to form water
Condition at the Equivalence Point
nacid × valence = nbase × valence
Molar concentration × volume × number of ions per formula is equal for the acid and the base: MVa = M′V′b
💡 The Key to Titration
①Adding 10 mL of HCl of the same concentration to 10 mL of NaOH turns all the H⁺ and OH⁻ into water, so pH = 7
②With less HCl than that, OH⁻ is left over and pH > 7; with more, H⁺ is left over and pH < 7
③Neutralization gives off heat, so the solution is hottest at the equivalence point
④Titration finds the unknown concentration of an acid or a base

Work It Out

Example 1
Find the pH of a 0.01 M hydrochloric acid (HCl) solution. (HCl is a strong acid, fully ionized)
1
HCl is a strong acid and ionizes completely, so [H⁺] equals the concentration of the acid.
[H⁺] = 10⁻² M
2
Substitute into pH = −log[H⁺].
pH = −log(10⁻²) = 2
▸ pH = 2
For a strong acid the concentration is [H⁺], so writing it as a power of 10 and flipping the sign of the exponent gives the pH.
Example 2
Find the pH of a 0.001 M sodium hydroxide (NaOH) solution. (25℃)
1
NaOH is a strong base, so [OH⁻] = 10⁻³ M and pOH = 3.
[OH⁻]=10⁻³ M, pOH=3
2
Use pH + pOH = 14.
pH = 14 − 3 = 11
▸ pH = 11
For a base, find pOH first and subtract it from 14 to get pH.

Summary

Calculating pH
pH = -log[H⁺]
At 25 °C, pH + pOH = 14
2023 KICE mock Chemistry I type, adapted
What volume of 0.1 M NaOH solution is needed to neutralize 100 mL of 0.1 M HCl solution?
①50 mL
②100 mL
③150 mL
④200 mL
⑤250 mL
▸ ② 100 mL
1
At the equivalence point, n(H⁺) = n(OH⁻).
2
Find the moles of HCl and work out the volume of NaOH with the same number of moles.
n(HCl)=0.1×0.1=0.01 mol → V(NaOH)=0.01/0.1=0.1 L
🎯 Exam Points
①pH = −log[H⁺]
②A difference of 1 in pH means a factor of 10 in [H⁺]
③Net ionic equation of neutralization: H⁺ + OH⁻ → H₂O
④At the equivalence point, molar concentration × volume × number of ions per formula is equal for the acid and the base
⑤A strong acid with a strong base gives pH = 7 at the equivalence point, and a weak acid with a strong base gives pH > 7
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Oxidation-Reduction (Redox) Reaction
Next →
Gas Laws

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