Transcript (activity) of hydrogen ions

Advanced
Biology
Matter and Energy
Chemistry
Molecules of Life
(1) A known amount of acid is pipetted into a conical flask and universal
indicator added. (2-3) The acid is titrated with the alkali in the burette
until the indicator turns green and the volume of alkali noted. (1-3) are
repeated with both known volumes mixed together BUT without the
contaminating indicator. (4) The solution is transferred to an evaporating
dish and heated to partially evaporate the water. (5) The solution is left
to cool to complete the crystallization. (6) The residual liquid can be
decanted away and the crystals can be carefully collected and dried by
'dabbing' with a filter paper OR the crystals can be collected by
filtration and dried.
(b) Reacting an acid with a metal or with an insoluble base (eg a
metal oxide, hydroxide or carbonate, often of a transition metal like
iron, zinc or copper).
(1)The metal, oxide, hydroxide or carbonate is stirred with the acid
and maybe heated to speed up the reaction (2). When no more of
the solid dissolves the solution can be partly evaporated with
further heating(2). The hot solution (with care!) is filtered (3), to
remove the excess solid, into an evaporating dish and left to cool
and crystallise (4). Then collect and dry.
pH scale
pH
pH represents the effective
concentration (activity) of hydrogen
ions (H+) in water.
This concentration could be expressed in
the same kind of units as other
dissolved species, but H+
concentrations are much smaller than
other species in most waters.
pH and logarithmic units
The activity of hydrogen ions can be
expressed most conveniently in logarithmic
units. pH is defined as the negative
logarithm of the activity of H+ ions:
– pH = -log [H+]
– where [H+] is the concentration of H+ ions in
moles per liter (a mole is a unit of measurement,
equal to 6.022 x 1023 atoms). Because H+ ions
associate with water molecules to form hydronium
(H3O+) ions, pH is often expressed in terms of
the concentration of hydronium ions.
pH 0 - 14
– Since pH is a log scale based on 10,
the pH changes by 1 for every power
of 10 change in [H+].
– A solution of pH 3 has an H+
concentration 10 times that of a
solution of pH 4. The pH scale ranges
from 0 to 14.
– However, pH values less than 0 and
greater than 14 have been observed in
very rare concentrated solutions.
Pure water
– In pure water at 22° C (72° F), H3O+ and
hydroxyl (OH-) ions exist in equal
quantities; the concentration of each is
1.0 x 10-7 moles per liter (mol/L).
– Therefore, pH of pure water = -log (1.0
x 10-7) = -(-7.00) = 7.00. Because pH is
defined as –log [H+], pH decreases as [H+]
increases (which will happen if acid is
added to the water).
Dehydration synthesis
Sucrose is common
disaccharide which
functions as a transport
sugar in plants.
The production of sucrose
by means of a
dehydration synthesis is
shown here.
Each sucrose molecule is
made by chemically
combining a glucose and
a fructose molecule.
Enzyme action
All proteins associated with the human body, or
with any living system, do something.
Some of them form structures, like fingernails;
others, like hormones, participate in chemical
reactions.
More on enzymes…
Enzymes are proteins
which stimulate
chemical reactions
between other
proteins. They are
present in every living
thing, and may
continue to function
long after the
organism is no longer
technically alive.
tenderness of meat
Beef, like fine wine, improves with age. The term
aging simply means the length of time beef cuts
are stored under controlled conditions of
temperature and humidity before they are
packaged for the meat counter. Aging allows
naturally occurring enzymes within the meat to
slowly break down some of the connective tissues
that contribute to toughness.
Beef aging significantly increases tenderness.
Aging times vary considerably from 3 to 21 days.
For the best eating quality, look for beef aged at
least 10 to 14 days.