ACIDS
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 Acids are proton (hydrogen ion, H + ) donors
 Acids have a pH lower than 7
 Acids taste sour
 Acids effect indicators
 Blue litmus turns red
 Methyl orange turns red
 Acids react with active metals, producing H
2
 Acids react with carbonates
 Acids neutralize bases

H +
Strong acids are assumed to be 100% ionized in solution (good H + donors).
H Cl H
2
SO
4
H NO
3
Weak acids are usually less than 5% ionized in solution (poor H + donors).
H
3
PO
4
H C
2
H
3
O
2
Organic acids

Blue litmus paper turns red in contact with an acid.
Methyl orange turns red with addition of an acid

Acids react with active metals to form salts and hydrogen gas.
Mg + 2HCl  MgCl
2
+ H
2
(g)
Zn + 2HCl  ZnCl
2
+ H
2
(g)
Mg + H
2
SO
4
 MgSO
4
+ H
2
(g)

2HC
2
H
3
O
2
+ Na
2
CO
3
2 NaC
2
H
3
O
2
+ H
2
O + CO
2

Effects of Acid Rain on Marble
( calcium carbonate )
George Washington:
BEFORE
George Washington:
AFTER

Neutralization reactions ALWAYS produce a salt and water .
HCl + NaOH  NaCl + H
2
O
H
2
SO
4
+ 2NaOH  Na
2
SO
4
+ 2H
2
O
2HNO
3
+ Mg(OH)
2
 Mg(NO
3
)
2
+ 2H
2
O

 Bases are proton (hydrogen ion, H + ) acceptors
 Bases have a pH greater than 7
 Bases taste bitter
 Bases effect indicators
 Red litmus turns blue
 Phenolphthalein turns purple
 Solutions of bases feel slippery
 Bases neutralize acids

H +
 Sodium hydroxide (lye), NaOH
 Potassium hydroxide, KOH
 Magnesium hydroxide, Mg(OH)
2
 Calcium hydroxide (lime), Ca(OH)
2
OH (hydroxide) in base combines with H + in acids to form water
H + + OH  H
2
O

Red litmus paper turns blue in contact with a base.
Phenolphthalein turns bright pink in a base.

Milk of Magnesia contains magnesium hydroxide, Mg(OH)
2
, which neutralizes stomach acid,
HCl.
2 HCl + Mg(OH)
2
MgCl
2
+ 2 H
2
O

Main Idea: Titrations are an application of acidbase neutralization reactions that require the use of an indicator.
15

• The stoichiometry of an acid-base neutralization reaction is the same as that of any other reaction that occurs in solution (they are double displacement reactions, after all).
• For example, in the reaction of sodium hydroxide and hydrogen chloride, 1 mol of NaOH neutralizes 1 mol of
HCl:
NaOH (aq) + HCl (aq)  NaCl (aq) + H
2
O (l)
• Stoichiometry provides the basis for a procedure called titration, which is used to determine the concentrations of acidic and basic solutions.
16

• Titration is a method for determining the concentration of a solution by reacting a known volume of that solution with a solution of known concentration.
• If you wish to find the concentration of an acid solution, you would titrate the acid solution with a solution of a base of known concentration.
• You could also titrate a base of unknown concentration with an acid of known concentration.
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In the titration of an acid by a base, the pH meter measures the pH of the acid solution in the beaker as a solution of a base with a known concentration is added from the buret.
http://wps.prenhall.com/wps/media/objects/3
312/3392202/blb1703.html
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How is an acid-base titration performed?
• The figure on the previous slide illustrates one type of setup for the titration procedure outlined on the next slide.
• In the procedure pictured on Slide 4, a pH meter is used to monitor the change in pH as the titration progresses.
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1) A measured volume of an acidic or basic solution of unknown concentration is placed in a beaker. The electrodes of a pH meter are immersed in this solution, and the initial pH of the solution is read and recorded .
2) A buret is filled with the titrating solution of known concentration. This is called the standard solution , or titrant .
3) Measured volumes of the standard solution are added slowly and mixed into the solution in the beaker. The pH is read and recorded after each addition. This process continues until the reaction reaches the equivalence point , which is the point at which moles of H + ion from the acid equal moles of OH ion from the base.
20

In the titration of a strong acid by a strong base, a steep rise in the pH of the acid solution indicates that all of the H + ions from the acid have been neutralized by the OH ions of the base. The point at which the curve flexes is the equivalence point of the titration. Bromthymol blue is an indicator that changes color at this equivalence point.
Notice that phenolphthalein and methyl red don’t match the exact equivalence point, but the slope is so steep that it doesn’t matter.
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• The previous slide shows how the pH of the solution changes during the titration of 50.0 mL of 0.100 M HCl, a strong acid with 0.100 M NaOH, a strong base.
• The inital pH of the 0.100 M HCl is 1.00.
– As NaOH is added, the acid is neutralized and the solution’s pH increases gradually.
– When nearly all of the H + ions from the acid have been used up, the pH increases dramatically with the addition of an exceedingly small volume of NaOH.
– This abrupt change in pH occurs at the equivalence point of the titration.
– Beyond the equivalence point, the addition of more NaOH again results in the gradual increase in pH.
22

You can calculate the volume of base needed to reach the equivalence point using the formula: V·M = V·M.
There are three situations in which you determine pH.
•Initial strong acid concentration
(this is simply the –log[H+] which is based on the [Acid].)
•Equivalence point (or endpoint) when moles of OH- = moles of H+.
The pH is 7 (due to the autoionization of water.)
•Before and after the endpoint (calculate excess moles of H+ or OH-, divide by the total volume, and calculate the pH based on this value.)

• Many indicators used for titrations are weak acids.
– Each has its own particular pH or pH ranges over which it changes color.
• The point at which the indicator used in a titration changes color is called the end point of the titration.
– It is important to choose an indicator for a titration that will change color at the equivalence point of the titration.
– Remember that the role of the indicator is to indicate to you, by means of a color change, that just enough of the titrating solution has been added to neutralize the unknown solution.
• Equivalence point ≠ End point!
– BUT for strong-strong titrations , the pH change is so steep and so large, that the are approximately equal.
24

• Chemists often use a chemical dye rather than a pH meter to detect the equivalence point of an acid-base titration.
• Chemical dyes whose colors are affected by acidic and basic solutions are called acid-base indicators.
• Many natural substances act as indicators.
– If you use lemon juice in your tea, you might have noticed that the brown color of tea gets lighter when lemon juice is added.
– Tea contains compounds called polyphenols that have slightly ionizable hydrogen atoms and therefore are weak acids.
– Adding acid in the form of lemon juice to a cup of tea lessens the degree of ionization, and the color of the unionized polyphenols becomes more apparent.
• Chemists have several choices in selecting indicators.
– Bromthymol blue is a good choice for the titration of a strong acid with a strong base, and phenolphthalein changes color at the equivalence point of a titration of a weak acid with a strong base.
25

The equivalence point here is not at a pH of
7. Phenolphthalein is an indicator that changes color at this equivalence point.
Notice that the starting pH is different and the region of change is smaller.
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• You might think that all titrations must have an equivalence point at pH 7 because that is the point at which concentrations of hydrogen ions and hydroxide ions are equal and the solution is neutral.
• This is not the case – some titrations have equivalence points at pH values less than 7, and some have equivalence points at pH values greater than 7.
• These differences occur because of reactions between the newly formed salts and water – salt hydrolysis .
– Some salts are basic (weak acid, strong base) and some salts are acidic (strong acid, weak base).
• The previous slide shows that the equivalence point for the titration of HPr (a weak acid) with NaOH (a strong base) lies at pH 8.80.
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Titration of a weak acid with a strong base

Such as CH
3
COOH and NaOH
 Equivalence point occurs at a pH >7
 Should use an indicator that changes at a pH above 7, such as phenolphthalein.
•Notice that starting pH is higher and
•the Equivalence Point pH is higher.

The conjugate base of a weak acid is a strong base.
 CH
3
COOH = Weak acid
 CH
3
COO = Strong Base
 Once all of the acid has been converted to CH
3
H
2
COO , it starts to take the H+ from
O in solution, creating more OH- ions thus making the pH >7

 When a weak acid (such as HC
2
H
3
O
2
) is neutralized by a strong base (such as NaOH), the graph varies in two ways:
 the equivalence point is not at pH = 7 and
 a buffer region exists as you approach the endpoint.
 You can still calculate the volume of base needed to reach the equivalence point using the formula: V·M = V·M. Weak acids require the same amount of base for neutralization as strong acids because they dissociate as they are neutralized

Halfway to equivalence point pH = pKa

 1. Initial weak acid concentration (this is an ICE box calculation.) The shortcut can be used here.
 2. Equivalence point (endpoint) is when all of the weak acid has been neutralized and turned into the conjugate base (C
2
H
3
O
2
- in this case.) This is a hydrolysis problem. Calculate the [C
2
H
3
O
2
-] and then do an ICE box problem knowing that Kb = . Calculate the [OH-], the pOH, and then the pH.
 3. Halfway to the equivalence point (as in a halftitration) the pH = pKa. This is because at this point, there is a perfect buffer as the [HA] = [A-]. At this point, you can determine the Ka of an unknown weak acid… very useful.

 4. Before and after the half-way point , the pH can be calculated using the Henderson-Hasselbach equation (or an ICE box, if you want.) Use stoichiometry to determine the [HA] and [A-]. pH = pKa + log
 5. Finally, after the equivalence point , the pH depends on the excess strong base that has been added. As in the strong acid-strong base titration, calculate excess moles of OH-, divide by the total volume, and calculate the pOH and then pH based on this value. The effect on the pH by the A- is negligible compared to the excess
OH-.

•When a sample of a weak base is titrated with a strong acid, the curve resembles an inverted
Weak Acid – Strong Base titration curve.
Note that the pH at the equivalence point is less than 7 .
•An indicator such as phenolphthalein that changes at pH of 9 would change when only
6 mL of acid had been added even though the equivalence point is reached at around 11 mL.
The acid-base indicator must be chosen with a Ka near to the [H+] of the equivalence point; that is the pKa of the indicator must match the pH of the equivalence point.

Weak Diprotic Acid – Strong Base Titrations
When a weak diprotic acid (examples:
H
2
C
2
O
4 or H
2
CO
3
) is titrated, there are two equivalence points.
•The curve is not as distinct because of the various proton donors and proton acceptors in solution.

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What’s the Point of a Titration
Again?
• To find the unknown concentration of an acid or a base.
• So you perform the actual titration noting the volume you started with and how much volume of the titrant you added and then...
• Math! (Oh no! Not math! Anything but math!)
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Titration Calculations: An Example
The balanced equation of a titration reaction is the key calculating the unknown molarity. For example, sulfuric acid is titrated with sodium hydroxide according to this equation:
H
2
SO
4
(aq) + 2 NaOH (aq)  Na
2
SO
4
(aq) + 2 H
2
O (l)
1) Calculate the moles of NaOH in the standard from the titration data: molarity of the base (M volume of the base (V
(mol/L)(L) = mol NaOH in standard
2) From the equation, you know that the mole ratio of
NaOH to H mol NaOH)
3) M
2
SO
4
B
). In other words, M
B
B
) and the
V
B
= is 2:1. Two moles of NaOH are
2
SO
4
. mol H represents the molarity of the acid and V represents the volume of the acid in liters.
H
2
SO
4 titrated/V
A
2
M
A
A
SO
SO
4
4
/ 2
= mol
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M
A
V
A
= M
B
V
B
(mol acid/mol base)
This is the mole ratio
Does this make sense? Let’s find out using the definition of molarity (mol/L) and dimensional analysis...
M
A
V
A
= M
B
V
B
(mol acid/mol base)
(mol acid/L acid)(L acid) = (mol base/L base)(L base) (mol acid/mol base) mol acid = mol base (mol acid/mol base) mol acid = mol acid
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