Warmup 2/10/16 Tell me a story about things cancelling or balancing out. Objective Tonight’s Homework To learn how buffers effect an acid or a base pp 621: 60

Notes on Buffers We’ve defined the pH of a solution as the number of H + ions that float about in solution. So what happens if we add more H + ? Our solution will become more acidic. But what if there’s something else in the solution that can soak up the additional H + and form bonds with them? We would call such an ion a buffer, and a solution with this ion a buffer solution. Buffer solutions can absorb moderate quantities of acid or base without the pH changing.

Notes on Buffers Nearly all buffers are made of either weak bases or weak acids. Remember, we said weak acids and bases don’t break up in water. This is important. Strong acids and bases do not make buffers. One other key thing here, buffers only work if there’s a fairly large amount of them in the solution. If you keep adding acid or base to a solution with a buffer, eventually you’ll use the buffer up.

Notes on Buffers How does this work? Let’s use an example. Let’s say we add some hydrochloric acid to water. The acid will follow the reaction below. HCl  Cl - + H +

Notes on Buffers How does this work? Let’s use an example. Let’s say we add some hydrochloric acid to water. The acid will follow the reaction below. HCl  Cl - + H + If we add a base to this solution, (say, NaOH), the OH - will combine with the H +, giving us less H + ions overall, which means a more neutral pH. H + + Cl - + Na + + OH -  H 2 O + Na + + Cl - Next, let’s see what happens if we have a buffer

Notes on Buffers Let’s say instead of just acid, we also have hydrogen carbonate (HCO 3 - ), a weak acid. Remember, we said weak acids don’t break into ions, so it just sits there. Waiting. HCl + HCO 3 -  Cl - + H + + HCO 3 -

Notes on Buffers Let’s say instead of just acid, we also have hydrogen carbonate (HCO 3 - ), a weak acid. Remember, we said weak acids don’t break into ions, so it just sits there. Waiting. HCl + HCO 3 -  Cl - + H + + HCO 3 - If we add a base to this (like NaOH), the buffer, hydrogen carbonate kicks in. (Remember that our strong base will ionize in water). Na + + OH - + HCO 3 -  H 2 O + CO Na + The buffer has absorbed all the OH -, keeping it from reacting with the H +. pH stays constant!

Notes on Buffers Our buffer works if we add an acid, too. Let’s say we add sulfuric acid, H 2 SO 4. (Remember, as a strong acid, only the first H will break off from the acid in water.) HSO H + + HCO 3 -  H 2 CO 3 + HSO 4 - In this case, the buffer has absorbed the extra hydrogen that broke off the H 2 SO 4. Either way, it has kept our concentration of H + in solution from changing.

Review We have time to review some of the older topics that you guys wanted to go over.

Exit Question What is the job of a buffer? a) To keep a solution from getting more acidic b) To keep a solution from getting more basic c) To change the pH of a solution slowly d) To absorb anything extra put in the solution e) A and B f) C and D