Transcript MW – Molecules and Solutions

MW – Molecules and Solutions
Electrical Conductivity and pH
Reminders….

Things that are due today:
1) Kool-Aid Concentration Lab is due today!
2) Please hand in your homework from last class on concentration/dilution problems

Things that are coming up:
1) You have your second TEST coming up next week on Thursday! Please STUDY!!!
2) There will be a recuperation session for the Test next week on Tuesday after school. There may be some
food/treats involved. Then again, there may not be…. ;)
3) Your binder organization checklist is due on Tuesday or Wednesday next week…. Check it out on the
school website for details.
And…. last but not least,
4) There will be recuperation times at lunch on days 2, 6 and 8 for the technological component of the
course. Please, go and get your exploded view drawings done! (go today at lunch!!)
Electrical Conductivity...
 What makes water conduct electricity?
It is the substances that are dissolved in water.
Some substances, when dissolved in water (or any other
solvent), allow the movement of electrons. This causes
electricity.
Ex: Table salt is a substance that helps water to conduct
electricity. We call these substances electrolytes.
Terms...
 The electric conductivity of a solution is its capacity to allow
the passage of electric current.
 Solutions containing one or more electrolytes are called
electrolytic solutions.
 non-electrolytes: Substances that are soluble in water but that
do not allow the passage of electric current.
Example: Sugar
Example...
 The electrolytes in electrolytic solutions have many uses.
Batteries are an example. Many batteries contain an
electrolytic solution that allows electric current to
circulate between two diodes, the electrodes.
Examples...
Many living organisms need electrolytes to be present in water
in order to live. Certain fish are adapted to life in salt water
and cannot survive in fresh water. The opposite is also true,
other organisms are adapted to life in fresh water and cannot
survive in ocean water.
Example...
 Conductors of life.
- All the fluids in the human body are electrolytic solutions.
One of their important functions is to allow the transmission
of nerve impulses between neurons, the specialized cells of
the nervous system.
Electrolytic Dissociation…
 Electrolytic dissociation: is the separation of a dissolved substance into
two ions of opposite charges: a positive ion and a negative ion.
NaCl(s) Na+(aq) + Cl-(aq)
(cation) (anion)
 Electrolytic dissociation is a physical transformation. It does not
change the nature of the solute.
For example, when sodium chloride is dissolved in water it retains all of its properties.
The sodium chloride can be recovered by evaporating the water.
Symbols...
NaCl(s) Na+(aq) + Cl-(aq)
The indications in parentheses correspond to the physical
state of each substance involved in the transformation.
The (s) means solid and the (aq) means aqueous solution.
Not shown: An (l) would indicate liquid, and (g) would
specify a gas.
Electrolytic Dissolution...
 The dissolution of non-electrolytes do not produce ions.
Ex; when sugar is dissolved in water, its molecules remain intact:
C12H22O11(s)  C12H22O11(aq)
 It is the ions formed during electrolytic dissociation that allow electric
current to circulate.
Electrolysis...
The electrode called the
Anode is considered to be
the positive end.
The electrode called the
Cathode is considered to
be the negative end.
Recall: Ions vs. Electrodes…
 The cation has a ________ charge.
 The anion has a ________ charge.
Therefore…..
The cations flow towards the negative cathode
And
The anions flow towards the positive anode
Ions vs. Electrodes…
So…..
 Cations are positive but the Cathode is negative
And….
 Anion are negative but the Anode is positive
Types of Electrolytes...
 Electrolytes can be divided into three main categories, which differ from each other
based on their distinct properties:
1) Acids: Substances that release H+ ions when dissolved in water.
2) Bases: Substances that produce OH- ions when dissolved in water.
3) Salts: Substance produced by chemical bonding of a metallic ion and a nonmetallic ion (other than H+ and OH- ions).
HW: Fill out the chart on Electrolytic properties using your text book pg’s 58 to 60. (note:
you are responsible to take these notes yourself and you will be tested on them so make
sure you get them done before the Chapter Test)
Classification...
 One common way in which we classify solutions is based on their
acidity or alkalinity.
 This method uses a pH scale, which stands for “power of
hydrogen”, introduced by Danish chemist Soren Sorensen in
1909.
pH Notes...
 pH is a property that identifies acids, bases and neutral solutions (salts).
There are many ways to measure the pH of a substance...
1) Indicator: a substance that changes color depending upon the pH of a
solution. Ex: Universal indicator paper
2) pH meter: A pH meter consists of a special measuring probe connected
to an electronic meter that measures and displays the pH of a substance.
3) Multiple different indicators: overlapping turning points (the pH range
over which the indicator changes colour).
The pH scale...
 The pH scale allows us to determine the degree of acidity or
basicity of a solution. It goes from 0 to 14
 if the pH is < (less than) than 7, the solution is acid
 if the pH is = (equal to), 7 the solution is neutral
 if the pH is > (greater than) 7, the solution is basic
Most common chemicals fall in a pH range between
0 and 14.
The pH scale (con’t)...
 The pH scale actually measures the concentration of H+ ions in
solution
It is calculated as follows:
pH = - log[H+]
 Logarithm: a difference of 1 unit between two substances indicates
that one substance is 10 times more acidic than the other.
Ex; a solution with a pH of 3 is ten times more acid than a solution with a pH of 4.
Furthermore, a solution with a pH of 2 is 100 times more acid than a solution with
a pH of 4.
Example…
 What is the pH of a solution who’s concentration of H+
ions is 0.001 ?
pH = - log [H+]
pH = - log [0.001]
pH = 3
Would this be Acidic or Basic?
ACIDIC