1.8K: Define a buffer as relatively large amounts of a weak acid or base and its conjugate in equilibrium that maintain a relatively constant ph when

Transcription

1 1.8K: Define a buffer as relatively large amounts of a weak acid or base and its conjugate in equilibrium that maintain a relatively constant ph when small amounts of acid or base are added. 1

2 Chem 20 Review: A graph showing the continuous change of ph during an acid-base titration, which continues until the titrant is in great excess, is called a ph curve Endpoint refers to the point in a titration analysis where the addition of titrant is stopped. The endpoint is defined (empirically) by the observed colour change of an indicator. Equivalence Point refers to the point in any chemical reaction where chemically equivalent amounts of the reactants have combined. This point is determined by stoichiometry. 2

3 Titration Analysis Chem 20 Review: Selecting proper indicators Alizarin yellow is not a suitable indicator because it will change colour long before the equivalence point of this strong acid-strong base reaction, which theoretically has a ph of exactly 7. Orange IV is also unsuitable; its colour change would occur too late. The ph at the middle of the colour change range for bromothymol blue is 6.8, which very closely matches the equivalence point ph; so bromothymol blue should give accurate results. 3

4 Interpreting ph Curves Buffering: is the property of some solutions to resist any significant change in ph when an acid or base is added. Buffering region (nearly level portions of the graph) Why is it buffering? Initially, the solution is mainly all water and OH - ions. Any additional acid added (H 3 O + ) immediately reacts with OH - to become water which does not change the ph significantly. This leveling effect finally fails near the equivalence point, when the OH - is almost completely consumed. Once excess acid has been added, the solution consists of water and H 3 O + ions, so the ph has dropped to the acid range. Then any additional acid that is added, simple increase the H 3 O + concentration slightly, but does not change the ph much. 4

5 Acid Base Indicators Any acid base indicator is really two entities for which we use the same name: a Bronsted-Lowry conjugate acid-base pair. At least one of the entities is visibly coloured, so you can tell simply by looking when it forms or is consumed. Examples include: Phenolphthalein: conjugate acid is colourless, conjugate base is bright pink Bromothymol Blue: conjugate acid is yellow, conjugate base is blue, and when they are in equal quantities (appear green to the human eye) Litmus Paper red (HIn) to blue (In - ) We will use the designation HIn for the conjugate acid and In - for the conjugate base as their actual formulas can be very complex. Summary: An indicator is a conjugate weak acid-weak base pair formed when an indicator dye dissolves in water. 5

6 Polyprotic Entities Chem 20 Review: Polyprotic acids can lose more than one proton Polyprotic bases can gain more than one proton If more than one proton transfer occurs in a titration, chemists believe the process occurs as a series of single-proton transfer reactions. On a graph, this means there will be more than one equivalence point First proton transfer = 100% Second proton transfer = 100% Carbonate ion is a diprotic base 6

7 Polyprotic Entities A ph curve for the addition of NaOH to a sample of H 3 PO 4(aq) displays only two rapid changes in ph even though H 3 PO 4(aq) is triprotic. This is because only two of the transfers are quantitative. The third reaction never goes to completion, but instead establishes an equilibrium. General Rule: Only quantitative reactions produce detectable equivalence points in an acid-base titration. 7

8 ph Curves 8

9 General Rule Strong Acid to Weak Base: ph at equivalence point is always lower than 7 Strong Base to Weak Acid: ph at equivalence point is always higher than 7 9

10 10

11 ph Curve Shape SA-SB: water is the only acid or base present = neutral solution SA-WB: a weak acid (NH 4+ )is present along with water, at the equivalence point, so the solution is acidic (ph < 7) WA-SB: a weak base (CH 3 COO-) is present along with water, at the equivalence point, so the solution is basic (ph > 7) WA-WB: do not have detectable equivalence points, because the reactions are usually not quantitative. 11

12 Buffers A buffer is a relatively large amount of any weak acid and its conjugate base, in the same solution. In equilibrium, they maintain a relatively constant ph when small amounts of acid or base are added. I.e. The addition of a small amount of base produces more acetate ions. The very small change in the acid-base conjugate pair ration and the complete consumption of the OH - explains why the ph change is very slight The addition of a small amount of acid produces more acetic acid. The very small change in the acid-base conjugate pair ratio and the complete consumption of the H 3 O + explains why the ph change is very slight 12

13 Buffer Example: Blood Plasma Blood plasma has a remarkable buffering ability, as shown by the following table. This is very useful, as a change of more than 0.4 ph units, can be lethal. If the blood were not buffered, the acid absorbed from a glass of orange juice would likely be fatal. 13

14 Buffering Capacity The limit of the ability of a buffer to maintain a ph level. When one of the entities of the conjugate acid-base pair reacts with an added reagent and is completely consumed, the buffering fails and the ph changes dramatically. All of the CH 3 COOH (aq) is used up, OH - additions will now cause the ph to drastically increase All of the CH 3 COO - (aq) is used up, H 3 O + additions will now cause the ph to drastically decrease 14