Acid–Base Titration Curves and Indicators

Acid-Base Background

• Naming Acids:1) Name ionically first2) a. Ends in “ide” è hyrdo ic acid

b. Ends in “ate” è ic acidc. Ends in “ite”è ous acid

• Naming Bases:1) Name ionically* Note that all bases will end in hydroxide

Acid-Base Background• Acids in solution:

• Taste • Conduct electricity ( )• React with zinc and magnesium to produce ( )• Turn blue litmus paper

• Bases in solution:• Taste• Conduct • Slippery• Turn red litmus paper

Arrhenius Acids And bases• Acids are substances which increase theconcertation in aqueous solutions

• Bases are substances which increase theconcentration in aqueous solutions

Modified Arrhenius Acids and Bases• Modified Arrhenius Acids: A substance thatto produce

• Modified Arrhenius Base: substance that reacts with water to produce

Conjugate Acid-Base Pairs

Your Task

• Complete the acid-base conjugate pairs worksheet• You will find it in the folder labelled “Assigned Work” under the

stoichiometry section

pH Scale

Additional Items to Be Aware Of• Note that “ ” and “ ”

mean the same thing• Acids are typically represented by the ( )• There are 6 strong acids Anything ending in “OH” is a strong base

Note that these are just examples of strong bases. Thereare far more than 6.Anything ending in “OH” is a strong base

Monoprotic vs. Polyprotic• A monoprotic strong acid/base like HCl(aq) will ionize in water to produce 1

H+è one equivalence point

• A polyprotic strong acid/base like sulfuric acid H2SO4(aq) will produce 2 H+ and have 2 equivalence points

Titration Curves• You must always identify what is the

and what is the

Titration Curves• A is a graph of the pH (

) versus the of the reagent progressively added to the original sample.

• As the is approached, there is a

• When a titration is done to create a , the addition of is not stopped at the endpoint, but is continued

until a has been added.

• So what is happening?

• The of the (in the burette) to the acid produce large changes. This relatively flat region of the pH curve is where a occurs.

• As the titration proceeds, and base is added, some of the acid is reacted with the added base, but anywhere point some excess acid will remain, so the pH stays relatively low.

• Very near the equivalence point, a small excess of acid becomes a small excess of base with the addition of a few more drops, so the pH

changes.

• The equivalence point is the of this change, where

the curve is the.

• The point at which thebefore at past

End Point

What reaction is happening?• HCl(aq) + NaOH(aq) à HOH(l) + NaCl(aq)

• Net-ionic equation? Do you remember how?

• Why was the equivalence point 7? • Remember water has a ,

and the are neutral, so a strong acid-strong

base titration must have a pH of 7 at the equivalence point.

Equivalence points• It is important to note, that the equivalence point pH

is reactions.

• For every other acid-base reaction, the solution will contain ions or molecules that are– so titration curves must be done to determine the equivalence point .

General Rule

Strong Acid to Weak Base: pH at

Strong Base to Weak Acid: pH at

Titration Curves

Strong Acid (HCl)– Weak Diprotic Base (Na2CO3)• The equivalence point is still but do you notice anything

else?• If you observe the curve closely, you see that there are

where the curve steepens as the titration proceeds.• This happens because the base is ,

meaning that it will react with ; so the hydrogen ions attach to the carbonate one at a time.

• We use the second reaction equivalence point, because we want the

Why does the curve start at the top?Because now a strong acid is being added to the weak base, not vice versa

Strong Acid (HCl)– Weak Diprotic Base (Na2CO3)

• Net reaction:

You will want to choose an

indicator that changes color at

the second equivalence point

Strong Acid (HCl)– Weak Diprotic Base (Na2CO3)

• Why is the equivalence point less than 7???

• Balanced reaction: Na2CO3(aq) + 2HCl(aq) à 2NaCl(aq) + H2CO3(aq)

• Net ionic equation:

• 2Na+ (aq) + CO32-

(aq) + 2H+(aq) + 2Cl-(aq) à 2Na+ (aq) + 2Cl-(aq) + H2CO3(aq)

So at the equivalence point, there is neutral

water, neutral spectator ions, and some weak

acid H2CO3(aq). What do you think keeps the pH

below 7?

Why do we care about titration curves?

• Acid base reaction pH curves provide a wealth of information:• Initial pH levels

• Equivalence point volume of titrant

• Number of

• Equivalence point pH for ; so the endpoint observed for the indicator chosen will closely match the equivalence point of the reaction

• When choosing an indicator the endpointChoosing an Indicator

• Thymol blue is an

for this titration because it

(pH 7).

• Alizarin yellow is also

because it changes colour the equivalence point.

• Bromothymol blue is because its endpoint pH of 6.8 (assume the middle of its pH range) the reaction 7, and the colour change is completely on the vertical portion of the pH curve.

• It is also critical that the

• Some of the is used to react with the indicator to make it change color. But if the amount of indicator is small, the

, and the accuracy of the titration will not be affected.

Final Tips

Summary• An indicator for an acid–base titration analysis must be

(change of colour) at very nearly the same pH as the pH at the equivalence point of the reaction solution.

• The pH of the solution at the equivalence point for a–strong monoprotic base reaction will be

7.

• The pH of the solution at the equivalence point for any other acid–base reaction must be , by plotting a titration pH curve.