9.3 - section 3
TRANSCRIPT
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9.3 - The Acidic Environment
Section #3
Acids & pH
Acids occur in many foods, drinks and even within our stomachs
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Acids & pH
Acids are classified as being either Strong Acids or Weak Acids.
This is based upon the nature of the equilibrium system that they establishwhen they are dissolved in water.
Classification of Acids
HA H+ + A-
Shows very high levels ofdissociation into ions.
i. Strong Acids
Shows very low levels ofdissociation into ions.
ii. Weak Acids
HA H+ + A-
100% dissociationin solution
Equilibrium favors Forward Reaction
HA H+ + A-
low% dissociationin solution
Equilibrium favors Reverse Reaction
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There are several acids that are classified as strong acids, however the mostcommonly encountered strong acids in the HSC are
Acids & pH
Strong Acids
i. Hydrochloric Acid
H - Cl
HCl - Molecular Weight = 36.46 g.mol-1
Also known as Muriatic Acid.
Small amounts of hydrochloric acid are secreted by cells inthe lining of the stomach to aid in food digestion.
Ionizes in solution according to the equation
HCl H+ + Cl-
It is classified as monoprotic acid
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H2SO4 - Molecular Weight =
98.08 g.mol-1SHO OH
O
O Ionizes in solution according tothe equations
ii. Sulfuric Acid
H2SO4 H
+
+ HSO4-
Acids & pH
HSO4- H+ + SO4
2-
Sulfuric acid undergoes two stages of ionisation. Hence it is classified as a
diprotic acid
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NO
H
O
O
HNO3 - Molecular Weight =
63.01 g.mol-1
Ionizes in solution according tothe equation
iii. Nitric Acid
HNO3 H
+
+ NO3-
Acids & pH
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HCOOH - Molecular Weight =46.03 g.mol-1
Also known as Formic AcidC
H
O
OH
There are several acids that are classified as weak acids, however the mostcommonly encountered weak acids in the HSC are
Acids & pH
Weak Acids
i. Methanoic Acid
HCOOH H+ + COOH-
Ionizes in solution according to
the equation
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CH3COOH - Molecular Weight
= 60.05 g.mol-1
Also known as Acetic AcidC
C
O
OHH
H
H
Ionizes in solution according tothe equation
ii. Ethanoic Acid
CH3COOH H+ + CH3COO
-
Acids & pH
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C6H8O7 - Molecular Weight = 192.13 g.mol-1
Systematic name is
iii. Citric Acid
Acids & pH
2-hydroxypropane-1,2,3-tricarboxylic
C3H5O(COOH)(COO)22-
(aq) H+
(aq) + C3H5O(COO)33-
(aq)
C3H5O(COOH)2COO-(aq) H+
(aq) + C3H5O(COOH)(COO)22-
(aq)
C3H5O(COOH)3(aq) H+
(aq) + C3H5O(COOH)2COO-(aq)
Ionizes in solution according to the equations
H2C
C
CH2
OH
COOH
COOH
COOH
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Acids & pH
Ions do not simply float around in liquid water.
Remember from the preliminary course, when NaCl crystals dissolve in water,the ions become surrounded by water molecules
Ions in Water
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Acids & pH
It is precisely due to this fact we use the subscript (aq) to indicate dissolved
ions in solution
NaCl(s) g Na+
(aq) + OH-(aq)
Similarly, the ions produced by the dissociation of acids are not simply floating
around.
In fact the H+ions piggy back upon water molecules, forming HydroniumIons H3O
+.
+
Hydronium Ion
H3O+
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Acids & pH
The H+ ion bonds to the water molecule through a special type of covalent
bonding called a Coordinate Bond(more on this in the Chemistry of Art).
Hence, H+(aq) and H3O+
(aq) can be used interchangeably.
Therefore the equations for the ionisation of acids can be written (in a more correctform)as
HCl(g) + H2O(l) H3O+
(aq) + Cl-(aq)
i. Hydrochloric Acid
H2SO4(l) + H2O(l) H3O+
(aq) + HSO4-(aq)
ii. Sulfuric Acid
HSO4-(aq) + H2O(l) H3O
+(aq) + SO4
2-(aq)
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C3H5O(COOH)(COO)22-(aq) + H2O(l) H3O+(aq) + C3H5O(COO)33-(aq)
C3H5O(COOH)2COO-(aq) + H2O(l) H3O+
(aq) + C3H5O(COOH)(COO)22-
(aq)
C3H5O(COOH)3(aq) + H2O(l) H3O+
(aq) + C3H5O(COOH)2COO-(aq)
CH3COOH(l) + H2O(l) H3O+
(aq) + CH3COO-(aq)
iv. Methanoic Acid
HNO3(l) + H2O(l) H3O+
(aq) + NO3-(aq)
iii. Nitric Acid
HCOOH(l) + H2O(l) H3O+
(aq) + COOH-(aq)
Acids & pH
v. Ethanoic Acid
vi. Citric Acid
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Acids & pH
The ionisation of strong & weak acids can be easily visualized.
Weak acids exhibit a very small degree of dissociation in solution
Visualizing Strong & Weak Acids
100% of the HCl molecules in Hydrochloric Acid will dissociate in solution
Example
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Acids & pH
Weak acids exhibit a very small degree of dissociation in solution
Example
1% of the CH3COOH molecules in Acetic Acid will dissociate in solution
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Acids & pH
The terms Concentrated and Dilute refer to the amounts of substances (inthis case Acids & Bases) dissolved into certain volumes of a solvent (in the HSCthe solvent is mostly water).
Remember that the concentration of a solution is given by the equation
Concentrated & Dilute Acidic Solutions
# mol
Volume (L)
Concentration =
Therefore, a concentrated solution can be created when a large amount ofmaterial is dissolved in a relatively small amount of solvent.
Likewise, a dilutesolution can be created when a small amount of material isdissolved in a relatively large amount of solvent.
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Acids & pH
Example
Concentrated HCl(a strong acid)
Diluted HCl(a strong acid)
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Acids & pH
Example
Concentrated CH3COOH(a weak acid)
Diluted CH3COOH(a weak acid)
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When an acid dissolves in water anAcid Equilibriumis setup
TheAcid Dissociation Constantfor this system is
[H+] [X-]
[HX]
Ka =
The stronger an acid is, the more it dissociates producing more H+ and X-
Hence the stronger acid is the larger its Ka value will be.
Strong acids completely dissociate in solution so their Ka values approach
infinity!
Acids & pH
Acid Equilibria
HX H+ + X-
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Pure water can act an extremely weak acid
Therefore
[H+] [OH-]
[H2O]
Ka =
Exactly how chemists get around this is not on your course however usingmathematical processes we gain a special constant called the Ion Product
Constant of Water, Kw.
at 25C Kw = 1.0 10-14
What is the concentrationof water in water???
Kw = [H+] [OH-]
Hence the [H+] of pure water is 1.0 10-7, this is important when weconsider
Acids & pH
Self Ionisation of Water
H2O(l) H+
(aq) + OH-(aq)
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pH is a measure of the strength of an acidic solution.
Acids & pH
pH
pH = -log10[H+]
Hence the pH of pure water is
pH = -log10[1.0 10-7]
pH = 7
Calculation steps
1. Enter the concentration using theEXPbutton
making sure you pushequalsat the end.
2. Push thelogbutton
making sure you pushequalsat the end.
3. Push theplus/minusbutton
making sure you pushequalsat the end.
Examining the equation
[H+] = 1.0 10-7g pH = 7
[H+] > 1.0 10-7g pH < 7
[H+] < 1.0 10-7g pH > 7
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pOH is a measure of the strength of an alkaline solution.
pOH = -log10[OH-]
pH & pOH are related by
pH + pOH = 14
Hence the pH of 0.1M NaOH is
[OH-] = 0.1 pOH = 1
pOH = -log10[0.1]
pH = 13
Acids & pH
pOH
NaOH Na+(aq) + OH-(aq)
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So, the pH scale is as follows
It is possible to have pH values smaller that 0 and larger than 14!
Acids & pH
The pH Scale
It is very important to note that if a solution changes its pH by one unit on thepH scale, there is a ten fold change in the H+ concentration.
pOH
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Consider the following situation
Acids & pH
pH of Acids of Equal Concentrations
CH3COOH0.1M
HCl0.1M
pH = -log10[0.1]
pH = 1
[H+] = 0.1
100% dissociation
strong acid
pH = -log10[0.001]
pH = 3
[H+] = 0.001
1% dissociation
weak acid