| Chem 110 |
General Chemistry |
Summer 2006 |
| Lecture Notes: Lec 1 30 May |
© R. Paselk 2006 |
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Introduction
Syllabus
Lecture Schedule
If you want to read ahead you may want to look at my lecture notes for Chem 110, Fall 2003 in my Course Archive. These notes will be very similar, with the caveat that each leacture in Summer is 1.5 lectures in Fall!
Review of Aqueous Equilibria
Acid-Base Chemistry
Brønsted-Lowry Acid/Base Theory:
- Acids are proton donors.
- Bases are proton acceptors.
- Note that there is no restriction as to solvent, and many
substances besides hydroxide ion can contribute basicity.
- Although I will signify protons in water as H+,
you should realize that naked protons do not exist in water -
they are always hydrated. At a minimum we see the hydronium ion,
H3O+. But hydronium ion is in fact also
generally thought to be hydrated, so you will sometimes see hydrogen
ion represented as H5O2+, H7O3+,
etc.
- A consequence of the Brønsted definition is that all
acids and bases are related to one or more conjugate bases
or acids. That is, when an acid dissociates to give a proton,
it also generates a conjugate base which can react with (accept)
a proton in the reverse reaction. For example, in the case of
water:
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H2O |
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H+ |
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OH- |
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acid |
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conj. base |
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H+ |
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OH- |
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H2O |
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base |
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conj. acid |
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| H3O+ |
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H+ |
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H2O |
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OH- |
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H+ |
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conj. acid
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acid
base
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conj.
base |
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Strong vs. Weak Acids & Bases
Remember, these terms refer to the degree of dissociation of
an acid or base, not its concentration:
- A Strong Acid is 100% dissociated at all concentrations
up to 1M. Common strong acids include:
- Nitric acid (HNO3)
- Hydrochloric acid (HCl)
- Sulfuric acid (H2SO4) for the first
dissociation only: H2SO4
HSO4- + H+. The second dissociation
is weak, that is it hardly dissociates at 1M.
- A Weak Acid is only partly dissociated at 1M. The
degree of dissociation varies widely, from a few percent to an
infinitesimal degree. Common weak acids include:
- Acetic acid (HC2H3O2 or
CH3CO2H, etc.)
- Formic acid (HCO2H)
- Hydrofluoric acid (HF)
- Most acids of biological origin such as amino acids, fatty
acids, metabolites, nucleic acids etc.
- A Strong Base is 100% dissociated at all concentrations
up to 1M. Common strong bases include:
- Sodium hydroxide (NaOH)
- Potassium hydroxide (KOH)
- A Weak Base only partly reacts at 1M. The degree of
dissociation varies widely, from a few percent to an infinitesimal
degree. Common weak bases include:
- Ammonia (NH3)
- Aluminum hydroxide (Al(OH)3)
- Magnesium hydroxide (Mg(OH)2)
The pH Scale
The concentration of hydronium ion in water is extremely influential on all kinds of chemistry. The range of hydronium ion concentration in water is also vast, with extremes of about 10M to about 10-15M, and commonly ranging from 1M - 10-14M. In order to accomodate this vast range of concentration conveniently we look instead at the logarithm of [H+] (recall that log 10x = x) and define a new term,
pH = -log[H+]
Let's look at some general characteristics of pH.
- Range: pH = -1 to pH = 15 (10M -10-15M)
- At midrange [H+] = [OH-] = 10-7M. The solution is said to be "neutral."
- Low pH means acidic:
- For 1M strong acid, pH = 0.0 (log 1 = 0)
- For 0.1M strong acid, pH = 1.0
© R A Paselk
Last modified 30 May 2006
