Quiz #5. Lecture 14. Professor Hicks General Chemistry (CHE132) Spring Copyright 2009 Charles Hicks. Copyright 2009 Charles Hicks

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1 Quiz #5 Lecture 14 Professor Hicks General Chemistry (CHE132) Spring

not react to form Hbut it can react with any + that is added HC 2 2 HC 2 2 C 2 2 H + H + HC 2 C 2 2 2 HC 2 2 H + H + H + HC 2 2 C 2 2 Free H +

2 Making a Buffer Buffers buffers = solutions that resist ph changes act by neutralizing added acid or base made by preparing a solution of a weak acid/base and the salt of its conjugate acid/base most of the weak acid does not dissociate into + but it can react with H most of the weak base does not react to form Hbut it can react with any + that is added HC 2 2 HC 2 2 C 2 2 H + H + HC 2 C HC 2 2 H + H + H + HC 2 2 C 2 2 Free H + determines the ph but all the H can react with added H the overall effect is that the ph will not change as much as water would if a strong acid or base was added 2

3 how buffers work against strong acid new HA H 2 HA + + HA A Added + new A how buffers work against strong base H 2 HA A HA A + + Added H 3

4 ICE tables for buffers What is the ph of a buffer that is 0.55 M HC 2 2 and 0.66 M NaC 2 2? initial HC 2 2 (aq) + H 2 C 2 2 (aq) + + (aq) change equil x +x +x 0.55x 0.66+x +x 1.76 x 10 5 = 1.76 x 10 5 = ( x)x (0.55x) 0.66x 0.55 acetic acid K a = 1.76 x 10 5 x is small compared to the molarity of acid and base because they are both weak x = 1.46 x 10 5 = [ + ] ph = 4.83 HendersonHasselbalch Equation used to calculate ph of buffer solution equation derived from the K a expression input = initial concentrations of the weak acid and conjugate base assumes the x is small approximation [conjugate base anion] ph = pka + log [weak acid] pk a = log(k a ) initial initial 4

5 Example: What is the ph of a buffer that is M HC 7 H 5 2 and M NaC 7 H 5 2? HC 7 H H 2 C 7 H [A ] ph = pk a + log [HA] ( 0.150) ph = log ph = 4.66 [H [H ] = 10 ] = 10 ph = % = 0.044% < 5% = log K a for HC 7 H 5 2 = 6.5 x 10 5 ( ) pk a = log K a ( ) = recognize chemical formula has 2 oxygen atoms it is a carboxylic acid weak acid so using the HHA equation is a valid approximation Effectiveness of Buffers a buffer will be most effective against acids and bases when the [base]/[acid] = 1 equal concentrations of acid and base reasonably effective when 0.1 < [base]/[acid] < 10 buffers are always more effective when both acid and base concentrations are higher 5

Example: Calculate the ph of a solution prepared so that it was 0.085 M in nitrous acid (HN 2 ) and 0.10 M in sodium nitrite (NaN 2 ).

6 Example: Calculate the ph of a solution prepared so that it was M in nitrous acid (HN 2 ) and 0.10 M in sodium nitrite (NaN 2 ). This solution is a buffer b/c we have a weak acid and its conjugate bases use the HH eqn 1) look up K a 2) calculate pk a 3) calculate [B]/[A] ph = pk a + log([b]/[a]) B = N 2 A =HN 2 K a = 4.6 x 10 4 ph = log (1.17) ph = [B]/[A] = 0.10/0.085 = 1.17 pk a = log (4.6 x10 4 ) = Example: Calculate the ph of a solution prepared so that it was 0.85 M in nitrous acid (HN 2 ) and 1.0 M in sodium nitrite (NaN 2 ). This solution is a buffer bc we have a weak acid and its conjugate bases use the HH eqn 1) look up K a 2) calculate pk a 3) calculate [B]/[A] ph = pk a + log([b]/[a]) B = N 2 A =HN 2 K a = 4.6 x 10 4 ph = log (1.17) ph = [B]/[A] = 1.0/0.85 = 1.17 pk a = log (4.6 x10 4 ) = [ + ] = = 3.93 x10 3 % diss = (3.93 x10 3 /0.86) x 100% = 0.04% approximation is K this buffer has same B/A ratio and therefore same ph as previous example 6

7 Buffering Capacity a concentrated buffer can neutralize more added acid or base than a dilute buffer selecting a conjugate acid/base pair for a buffer buffer most effective when ph = pk a select an acid with a pk a close to desired ph 7

8 Example: Which of the following acids would be the best choice to combine with its sodium salt to make a buffer with ph 4.25? Chlorous Acid, HCl 2 pk a = 1.95 Nitrous Acid, HN 2 pk a = 3.34 Formic Acid, HCH 2 pk a = 3.74 Hypochlorous Acid, HCl pk a = 7.54 pk a of 3.74 is closest to the desired ph of 4.25 Example a) Which acid and its conjugate base would be the best choice to make a ph 4.0 buffer? b) Calculate the [B]/[A] ratio be for this buffer ph = pk a + log ([B]/[A]) 4.0 = log ([B]/[A]).01 = log ([B]/[A]) = ([B]/[A]) [B]/[A] = 0.97 by adding a little less base than acid the ph will be lower than the pk a 8

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10 The Dangers of Antifreeze each year, thousands of pets and wildlife die from consuming antifreeze contains ethylene glycol sweet taste initial effect drunkenness metabolized in the liver to glycolic acid HCH 2 CH overwhelms the buffering ability of HC 3, causing the blood ph to drop when the blood ph is low, it ability to carry 2 is compromised acidosis ethylene glycol (aka 1,2 ethandiol) 19 10

Lecture 8. Making a Buffer. Buffers. Professor Hicks Inorganic Chemistry (CHE152)

Lecture 8. Making a Buffer. Buffers. Professor Hicks Inorganic Chemistry (CHE152) Lecture 8 Professor Hicks Inorganic Chemistry (CHE152) Making a Buffer Buffers buffers = solutions that resist ph changes act by neutralizing added acid or base made by preparing a solution of a weak acid/base