Comparing the Strength of Acids and Bases

Expert reviewed • 22 November 2024 • 7 minute read

Understanding how to compare the strength of acids and bases is fundamental in chemistry. This article explores methods to measure and compare acid-base strength using pH and electrical conductivity.

Understanding Acid Strength

Acid strength refers to the degree to which an acid ionizes in water to produce hydrogen ions (H+\text{H}^+) or more accurately, hydronium ions (H3O+\text{H}_3\text{O}^+). The more readily an acid donates protons, the stronger it is.

Using pH to Compare Acids

For a valid comparison of acid strength using pH measurements, two conditions must be met:

  • The acids must have equal concentrations (equimolar solutions)
  • The acids must be monoprotic (donate one proton per molecule)

When these conditions are met, the stronger acid will produce a lower pH due to greater ionization.

Example 1: Comparing Monoprotic Acids of Equal Concentration

Let's compare two acids at the same concentration:

  • Hydrochloric acid (HCl, strong acid):

    • Concentration: 1.0 mol L−11.0 \text{ mol L}^{-1}
    • Complete ionization: HCl(aq)→H(aq)++Cl(aq)−\text{HCl}_{(aq)} \rightarrow \text{H}^+_{(aq)} + \text{Cl}^-_{(aq)}
    • [H+]=1.0 mol L−1[\text{H}^+] = 1.0 \text{ mol L}^{-1}
    • pH = 0.00

  • Acetic acid (CH3COOH\text{CH}_3\text{COOH}, weak acid):

    • Concentration: $1.0 \text{ mol L}^{-1}$
    • Partial ionization: CH3COOH(aq)⇌H(aq)++CH3COO(aq)−\text{CH}_3\text{COOH}_{(aq)} \rightleftharpoons \text{H}^+_{(aq)} + \text{CH}_3\text{COO}^-_{(aq)}
    • [H+]=0.020 mol L−1[\text{H}^+] = 0.020 \text{ mol L}^{-1}
    • pH = 1.70

The lower pH of HCl correctly indicates it is the stronger acid.

Common Mistakes in Comparing Acid Strength

Comparing Polyprotic and Monoprotic Acids

When comparing sulfuric acid (H2SO4\text{H}_2\text{SO}_4, diprotic) with hydrochloric acid (HCl, monoprotic):

H2SO4(aq)→H(aq)++HSO4(aq)−⇌2H(aq)++SO4(aq)2−\text{H}_{2}\text{SO}_{4(aq)} \rightarrow \text{H}_{(aq)}^{+} + \text{HSO}_{4(aq)}^{-} \rightleftharpoons 2\text{H}_{(aq)}^{+} + \text{SO}_{4(aq)}^{2-}

The diprotic nature of H2SO4\text{H}_2\text{SO}_4 means it can produce more hydrogen ions per molecule, leading to a lower pH. This doesn't necessarily indicate greater acid strength.

Comparing Different Concentrations

Consider:

  • 0.001 mol L−10.001 \text{ mol L}^{-1} HCl: pH = 3.00
  • 1.0 mol L−11.0 \text{ mol L}^{-1} CH3COOH\text{CH}_3\text{COOH}: pH = 2.37

Despite being a weaker acid, the higher concentration of acetic acid results in a lower pH. This demonstrates why concentration must be equal for valid comparisons.

Electrical Conductivity Measurements

Electrical conductivity provides another method for comparing acid strength. In aqueous solutions, ions act as charge carriers, enabling electrical conduction.

For valid comparisons:

  • Solutions must be equimolar
  • Acids must be monoprotic

The stronger acid will show higher electrical conductivity due to greater ionization and thus more ions in solution.

Comparing Base Strength

The same principles apply when comparing bases:

  • pH Method:

    • For equimolar solutions, stronger bases have higher pH values
    • Must compare bases of equal concentration

  • Conductivity Method:

    • Stronger bases show higher electrical conductivity
    • Solutions must be equimolar for valid comparison

Key Points to Remember

  • Always ensure equal concentrations when comparing acid or base strength
  • Consider the number of ionizable protons (monoprotic vs. polyprotic)
  • pH and conductivity comparisons are only valid under controlled conditions
  • Higher concentration doesn't necessarily indicate greater strength

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Understanding pH and pOH in Chemical Solutions

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Calculating pH When Mixing Acids and Bases

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