1 / 13

Acid-base Dissociation

Acid-base Dissociation. For any acid, describe it’s reaction in water: H x A + H 2 O  x H + + A - + H 2 O Describe this as an equilibrium expression, K (often denotes K A or K B for acids or bases…)

erv
Download Presentation

Acid-base Dissociation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Acid-base Dissociation • For any acid, describe it’s reaction in water: • HxA + H2O  x H+ + A- + H2O • Describe this as an equilibrium expression, K (often denotes KA or KB for acids or bases…) • Strength of an acid or base is then related to the dissociation constant  Big K, strong acid/base! • pK = -log K  as before, lower pK=stronger acid/base!

  2. pKx? • Why were there more than one pK for those acids and bases?? • H3PO4 H+ + H2PO4- pK1 • H2PO4-  H+ + HPO42- pK2 • HPO41-  H+ + PO43- pK3

  3. Geochemical Relevance? • LOTS of reactions are acid-base rxns in the environment!! • HUGE effect on solubility due to this, most other processes

  4. Dissociation of H2O • H2O  H+ + OH- • Keq = [H+][OH-] • log Keq = -14 = log Kw • pH = - log [H+] • pOH = - log [OH-] • pK = pOH + pH = 14 • If pH =3, pOH = 11  [H+]=10-3, [OH-]=10-11 Definition of pH

  5. pH • Commonly represented as a range between 0 and 14, and most natural waters are between pH 4 and 9 • Remember that pH = - log [H+] • Can pH be negative? • Of course!  pH -3  [H+]=103 = 1000 molal? • But what’s gH+?? Turns out to be quite small  0.002 or so…

  6. Henderson-Hasselbach Equation: • When acid or base added to buffered system with a pH near pK (remember that when pH=pK HA and A- are equal), the pH will not change much • When the pH is further from the pK, additions of acid or base will change the pH a lot

  7. BUFFERING • When the pH is held ‘steady’ because of the presence of a conjugate acid/base pair, the system is said to be buffered • In the environment, we must think about more than just one conjugate acid/base pairings in solution • Many different acid/base pairs in solution, minerals, gases, can act as buffers…

  8. Buffering example • Let’s convince ourselves of what buffering can do… • Take a base-generating reaction: • Albite + 2 H2O = 4 OH- + Na+ + Al3+ + 3 SiO2(aq) • What happens to the pH of a solution containing 100 mM HCO3- which starts at pH 5?? • pK1 for H2CO3 = 6.35

  9. Think of albite dissolution as titrating OH- into solution – dissolve 0.05 mol albite = 0.2 mol OH- • 0.2 mol OH-  pOH = 0.7, pH = 13.3 ?? • What about the buffer?? • Write the pH changes via the Henderson-Hasselbach equation • 0.1 mol H2CO3(aq), as the pH increases, some of this starts turning into HCO3- • After 12.5 mmoles albite react (50 mmoles OH-): • pH=6.35+log (HCO3-/H2CO3) = 6.35+log(50/50) • After 20 mmoles albite react (80 mmoles OH-): • pH=6.35+log(80/20) = 6.35 + 0.6 = 6.95

  10. Bjerrum Plots • 2 D plots of species activity (y axis) and pH (x axis) • Useful to look at how conjugate acid-base pairs for many different species behave as pH changes • At pH=pK the activity of the conjugate acid and base are equal

  11. Bjerrum plot showing the activities of reduced sulfur species as a function of pH for a value of total reduced sulfur of 10-3 mol L-1.

  12. Bjerrum plot showing the activities of inorganic carbon species as a function of pH for a value of total inorganic carbon of 10-3 mol L-1. In most natural waters, bicarbonate is the dominant carbonate species!

  13. Carbonate System Titration • From low pH to high pH

More Related