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Chem 2 - Acid-Base Equilibria I: The Basics of Acids and Bases

  1. 1. Acid-Base Equilibria (Pt. 1) The Basics of Acids and Bases By Shawn P. Shields, Ph.D. This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
  2. 2. Overview of Acid/Base Definitions There are three main classifications of acids and bases 1) The Arrhenius definition is the most restrictive. 2) The BrΓΈnsted-Lowry description is more broad. 3) The Lewis classification of acids and bases is the most general of all.
  3. 3. Arrhenius Acids and Bases Arrhenius Acids are compounds that produce hydronium H3O+ (also called β€œprotons” H+) in aqueous soln (water). 𝐇𝐂π₯ 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β†’ 𝐇 πŸ‘ 𝐎+ 𝐚πͺ + 𝐂π₯βˆ’ (𝐚πͺ) 𝐇𝐂π₯ 𝐚πͺ β†’ 𝐇+ 𝐚πͺ + 𝐂π₯βˆ’ (𝐚πͺ) hydronium β€œproton”
  4. 4. Arrhenius Acids and Bases Arrhenius Bases are compounds that produce hydroxide OHο€­ in aqueous solution (water). ππšπŽπ‡ 𝐚πͺ β†’ πŽπ‡βˆ’ 𝐚πͺ + 𝐍𝐚+ (𝐚πͺ)
  5. 5. BrΓΈnsted-Lowry Acids and Bases BrΓΈnsted-Lowry acids donate (or β€œgive away”) protons H+. 𝐇𝐂π₯ 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β†’ 𝐇 πŸ‘ 𝐎+ 𝐚πͺ + 𝐂π₯βˆ’ (𝐚πͺ) H+ on HCl is donated to water to produce hydronium hydronium
  6. 6. BrΓΈnsted-Lowry Acids and Bases BrΓΈnsted-Lowry bases accept protons H+. 𝐇𝐂π₯ 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β†’ 𝐇 πŸ‘ 𝐎+ 𝐚πͺ + 𝐂π₯βˆ’ (𝐚πͺ) 𝐍𝐇 πŸ‘ 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β‡Œ 𝐍𝐇 πŸ’ + 𝐚πͺ + πŽπ‡βˆ’ (𝐚πͺ) Water (H2O) accepts H+ from HCl (Water acts as a base) NH3 accepts H+ from water (H2O) (NH3 acts as a base)
  7. 7. Lewis Acids and Bases A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor. ππ…πŸ‘ 𝐚πͺ + 𝐍𝐇 πŸ‘ π₯ β†’ π…πŸ‘ 𝐁 βˆ’ 𝐍𝐇 πŸ‘ (𝐚πͺ) Lewis acid Lewis base
  8. 8. The pH Scale The acidity of a substance can be determined by measuring the concentration of hydronium H3O+ (or H+) in solution. pH is related to the concentration of hydronium H3O+. pH = ο€­ log [H3O+]
  9. 9. The pH Scale pH = ο€­ log [H3O+] The lower the pH, the more acidic the solution. The higher the pH, the more basic. acid 14neutral0 7 base
  10. 10. Strong versus Weak Acids Analyze the two solutions shown below. What is different about them? What is similar? H3O+ Clο€­ H3O+ Fο€­ HF
  11. 11. Strong versus Weak Acids The solution on the left (HCl) shows the acid dissolved completely into ions. The beaker on the right (HF) shows a solution with very few HF molecules dissolved in solution H3O+ Clο€­ H3O+ Fο€­ HF
  12. 12. Strong versus Weak Acids HCl is a strong acid 𝐇𝐂π₯ 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β†’ 𝐇 πŸ‘ 𝐎+ 𝐚πͺ + 𝐂π₯βˆ’ (𝐚πͺ) HF is a weak acid 𝐇𝐅 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β‡Œ 𝐇 πŸ‘ 𝐎+ 𝐚πͺ + π…βˆ’ (𝐚πͺ)
  13. 13. Strong and Weak Bases Bases can be categorized in exactly the same way. Strong bases dissolve completely into ions in solution Example: ππšπŽπ‡ 𝐚πͺ β†’ 𝐍𝐚+ 𝐚πͺ + πŽπ‡βˆ’ 𝐚πͺ
  14. 14. Strong and Weak Bases Weak bases only partially dissolve into ions in solution. Examples: 𝐌𝐠 πŽπ‡ 𝟐 𝐚πͺ β‡Œ 𝐌𝐠 𝟐+ 𝐚πͺ + 𝟐 πŽπ‡βˆ’ 𝐚πͺ 𝐍𝐇 πŸ‘ 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β‡Œ 𝐍𝐇 πŸ’ + 𝐚πͺ + πŽπ‡βˆ’ (𝐚πͺ)
  15. 15. Conjugate Acid-Base Pairs When an acid donates a proton, a conjugate base is formed as well. 𝐇𝐂π₯ 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β†’ 𝐇 πŸ‘ 𝐎+ 𝐚πͺ + 𝐂π₯βˆ’ (𝐚πͺ) 𝐇𝐅 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β‡Œ 𝐇 πŸ‘ 𝐎+ 𝐚πͺ + π…βˆ’ (𝐚πͺ) acid conjugate base for HCl acid conjugate base for HF
  16. 16. Conjugate Acid-Base Pairs When a base accepts a proton, a conjugate acid is formed as well. 𝐍𝐇 πŸ‘ 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β‡Œ 𝐍𝐇 πŸ’ + 𝐚πͺ + πŽπ‡βˆ’ (𝐚πͺ) base conjugate acid for NH3
  17. 17. Conjugate Acid-Base Pairs Conjugate acid-base pairs are two related species differing only by a proton (H+) 𝐇𝐅 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β‡Œ 𝐇 πŸ‘ 𝐎+ 𝐚πͺ + π…βˆ’ (𝐚πͺ) 𝐍𝐇 πŸ‘ 𝐚πͺ + 𝐇 𝟐 𝐎 π₯ β‡Œ 𝐍𝐇 πŸ’ + 𝐚πͺ + πŽπ‡βˆ’ (𝐚πͺ) base conjugate acid for NH3 acid conjugate base for HF
  18. 18. Neutralization Reactions An acid and a base react to form water and a β€œsalt”. 𝐇𝐂π₯ 𝐚πͺ + ππšπŽπ‡ 𝐚πͺ β†’ 𝐇 𝟐 𝐎 π₯ + ππšπ‚π₯(𝐚πͺ) 𝐇𝐅 𝐚πͺ + 𝐍𝐇 πŸ‘ 𝐚πͺ β‡Œ 𝐇 𝟐 𝐎 π₯ + 𝐍𝐇 πŸ’ 𝐅(𝐚πͺ) acid β€œsalt” acid β€œsalt” base base
  19. 19. What you Should Be Able to Do ο‚΄Recognize acids and bases. ο‚΄Write the chemical reaction for the dissolution of an acid in water. ο‚΄Write the chemical reaction for the dissolution of a base in water.
  20. 20. What you Should Be Able to Do ο‚΄Identify strong and weak acids and bases. (See the list of strong acids and bases to memorize.) ο‚΄Identify conjugate acid-base pairs. ο‚΄Write the chemical reaction for an acid reacting with a base (neutralization reaction).
  21. 21. Next up, The Autoionization of Water (Pt 2)

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Chem 2 - Acid-Base Equilibria I: The Basics of Acids and Bases

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