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Understanding Acid Strength and pH Equations

Learn about strong and weak acids, pH expressions, equilibrium constants, and predicting pH changes. Understand the concept of acidity constants and how to calculate pH for weak acids. Explore the relationship between Ka, pKa, and strength of acids.

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Understanding Acid Strength and pH Equations

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  1. Questions • What is a strong acid? • What is a weak acid? • Write expression for pH • HX(aq) + H2O(l) X-(aq) + H3O+ For a strong acid like one above, what can you say about the concentration of [HX] and [H3O+]?

  2. Water dissociation constant, Kw Kw = [H3O+] x [OH-] Increases with increased temperature At 25 deg Kw = 1X10-14

  3. Weak Acids Do not fully dissociate so: • Are in equilibrium biased to left • Have a low concentration of ions • Are poor conductors of electricity • React slowly as [H3O+] is low CH3COOH + H2O n CH3COO- + H3O+ As they are in equilibrium, when they are diluted water concentration is increased and there is a shift to the right to compensate. This means predicting pH increase is harder.

  4. Acidity Constant, Ka Equilibrium constant Kc = But water is a solvent so Kc = So Ka = [H3O+] [A-] [HA] [H2O] [H3O+] [A-] [HA] [H3O+] [A-] [HA]

  5. Weak acid pH Because So Ka = Rearrange and pH = -log[H3O+] Now try Exercise 13C [H3O+] = [A-] [H3O+]2 [HA] [H3O+]= Ka x [HA]

  6. pKa pKa= -logKa As the strength of acid increase Ka gets bigger pKagets smaller Conjugate base gets weaker

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