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Chapter 7: Acids/Bases - LCHS Physical
Science |
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Objectives: The learner will...
...observe and list the physical and
chemical properties of water
...describe the difference between pure and potable water
...explain the differences between acids, bases and salts
...compute pH based on hydrogen ion [H+] concentration |
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Vocabulary:
potable : water that is fit
to drink
acid : substance that ionizes in water to give hydrogen ions
[H+]
base: substance that ionizes in water to give hydroxide ions
[OH-]
neutralize : when acids and bases completely combine to form
salt and water
litmus : A blue powder from lichen that changes red in acid
and blue in a base |
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WATER
| 71% of the earth's surface is water
with the oceans averaging 3.8 kilometers deep. If all the
continents could be plowed into the sea, 3 kilometers of water
would cover the entire earth. The glaciers and ice caps contain
2% of earth's water. The earth's atmosphere holds 5 cm of
water that is constantly recycled in the hydrologic cycle.
Water that is fit to drink is called "potable".
This means it might contain trace amounts of harmless minerals
or sediments. Water is densest just before freezing at
4°C. Water that is less than or greater than 4°C is
less dense. Water is unique in that as it gets
colder than 4°C, the molecules realign to make a crystalline
solid arrangement that is farther apart and less dense, expanding
about 10% at 0°C (which is why ice floats). |
| Pure water molecules consist only of
two atoms of hydrogen and one atom of oxygen (H2O).
The oxygen and the hydrogen atoms share two pairs of electrons
covalently. The hydrogen connect to the oxygen at 105 degree
angles (the Mickey Mouse look) which make the oxygen end of
the water molecule slightly more negatively charged than the
other. This property of water makes it attracted to itself
and other charged/polar substances. Water molecules will also
frequently break down into a [H+] ion and a [OH-] ion and
then recombine. |
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ACIDS, BASES, SALT, pH
An acid is a polar substance that dissolves/ionizes in water
to give excesss hydrogen ions [H+]. A base is a polar
substance that dissolves/ionizes in water to give excesss hydroxide
ions [OH-]. In an acid/base reaction an acid (compound
always starts with H) and a base (compound always
ends with [OH]) combine in a double replacement
fashion to form a salt and water. Another way to represent
water is HOH or [H+][OH-] (In a way, water is both
an acid and a base!).
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Acid/Base H+
+ [OH]- => H2O
HxN + M[OH]y=> MN + H2O
3H2[SO4] +
2Al[OH]3 => Al2[SO4]3
+ 6H2O
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| 1 < Acids < 7
| 7 < Bases < 14
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| HCl - hydrochloric acid
| Na(OH) - sodium hydroxide
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| H2(SO4) - sulfuric acid
| Ca(OH)2 - calcium hydroxide
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| H3(PO4) - phosphoric acid
| Al(OH)3 - aluminum hydroxide
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| lemon juice pH 4
| baking soda pH 8
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| vinegar pH 4
| borax pH 9
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| soda water pH 4
| ammonia pH 10
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| aspirin pH 5
| liquid plummer pH 12
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If every H+ is neutralized with an [OH-] then the pH of
the final solution is 7 (neutral). Excess acid will have
a pH less than 7 and excess base will have a pH greater than 7.
The pH of a solution is calculated by taking the negative base
10 log of the concentration of hydrogen ions [H+] in moles per
Liter. This sounds more complicated than it really is and you
can learn more details about pH calculation in a chemistry class.
pH = -log[H+]
e.g. a solution that contains .001 M/Liter of [H+]
pH = -log[.001] = -log [10-3] = --3 = +3 (acid)
The pH scale is not linear, meaning that an acid
with a pH of 2 is 10 times stronger than an acid with a
pH of 3! Certain naturally occuring plant pigments can indicate
the presence and strength of acids and bases by changing color.
Litmus is a blue powder extracted from a type of lichen
that changes red in acid and blue in base. Other pH indicators
are purple cabbage,
EX-Lax pills (phenolphthalein), and goldenrod paper dye.
Download and run this acid/base titration program.
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| Practice: |
Help: |
PB1
H2SO4 is an (A)cid, (B)ase or (S)alt?
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H2 |
PB2
Na(OH) is an (A)cid, (B)ase or (S)alt?
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OH |
PB3
A pH of 7 is (A)cid, (B)ase or (N)eutral?
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see above |
PB4
A pH of 3 is (A)cid, (B)ase or (N)eutral? |
see above |
PB5
A pH of 9 is (A)cid, (B)ase or (N)eutral? |
see above |
PB6
What is the formula of the salt for Mg(OH)2 + H2(SO4) |
find charges
of Mg & (SO4)
criss cross |
PB7
What is the base that forms the salt AlCl3
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find charge of Al
add OH and criss cross
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PB8
What is the acid that forms the salt MgCl2
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find charge of
Cl
add H and criss cross |
PB9
What is the pH of a solution that has a H+ concentration of .001
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-log(.001) |
PB10
How much stronger is an acid with a pH of 2 than 6?
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10^(6-2) |
answer bank:
(some are bogus)
acid
base
10000
Mg(SO4)
3 |
acid
base
neutral
Mg2(SO4)
2 |
Al(OH)2
Al(OH)3
HCl3
HCl
1 |
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Scientist Spotlight:
Henry
Cavendish 1731-1810 English chemist and physicist who was
shy and absent-minded... Cavendish perfected the technique of collecting
gases above water, publishing his techniques and new findings in On
Fractious Airs (1766).
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Resources (Study Links/Study Tips/Reading Lists)
The
pH Factor
Download and run
this acid/base titration program.
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Chapter 7:Acids/Bases - LCHS Physical
Science |
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