Download Lesson 2: Electrolytes

Pure water does not conduct
electricity!

Def: substances that, when dissolved in water
(aqueous solution), conduct an electric current.

Therefore, an electrolytic solution allows electricity
to flow.

The electrical conductivity of a solution is a
measure of its ability to allow an electric current to
flow through it.

When an electrolyte is
dissolved in water, it
separates into two ions
of opposite charges
(positive and negative).

This is a physical
change.

3 conditions must be met in order for an electric
current to flow:
1. Electric charges (ions) must be present
 ions are found in ionic compounds
2. These charges must be mobile
 when dissolved in water, the ions are pulled apart
and are free to conduct electricity
3. The charges must move in a particular direction
 the electrodes on the electrical conductivity meter
direct the charges
Ionic compounds!!!!
When an ionic compound is placed in water,
the ions dissociate. This means that they
separate. The solution will now be filled with
ions.

Def: A solution that contains an
electrolyte
(hence, that conduct electricity!)
Applications: batteries!
solid
aqueous
• When NaCl dissolves in water, sodium and
chlorine ions are produced.
• A dissociation equation must be balanced (a
subscript on the reactant side becomes a
coefficient on the product side.)
• The charges on the ions must be present. (This is
determined by the periodic table)
• Subscripts must be written showing the state of
the compound or ion.
H2O
C12H22O11(s)


C12H22O11(aq)
When dissolved in water,
no ions are produced!
For example: when sugar
is dissolved in water, the
molecules remain intact,
although they become
detached!


Positive ions
Negative ions
negative electrode (cathode)
positive electrode (anode)
Acids
Bases
Salts
The following are all acids:
HCl, H2SO4, HBr, H2S
What are some properties of these compounds?
•
•
•
•
Turn blue litmus paper red.
Conduct electricity (electrolytic!)
React with metals
Sour in taste (but never taste an unknown acid!!)
What ions are produced when these acids are
dissolved in water?
HCl →H+1 + Cl-1
H2SO4 →2H+1 + SO4-2 Acids produce
hydrogen
ions
HBr → H+1 + Br-1
in solution.
H2S → 2H+1 + S-2
CH3COOH → H+1 + CH3COO-1

Acids always contain a “H” in the formula.

They may also end with “COOH” (exception!)

Combinations of C’s and H’s only are not acids.
They are hydrocarbons (CH4 and C3H8 ).

NH3 is also an exception: it is a base.
Bases
The following are all bases:
NaOH, Mg(OH)2,KOH
What are some properties of these compounds?
•
•
•
•
•
Turn red litmus paper blue.
Conduct electricity (electrolytic!)
Do not react with metals
Bitter
Slippery to the touch
Electrolytic Dissociation
What ions are produced when these bases are
dissolved in water?
Bases produce
NaOH →Na+1 + OH-1
Mg(OH)2→Mg+2 + 2OH-1 hydroxide ions
in solution.
+1
-1
KOH → K + OH
NH3 + H20 → NH4+ + OH-1 (exception!)
Bases contain “OH” in their formula.


A substance produced by the chemical bonding of
a metallic ion and nonmetallic ion (other than H+
and OH- )
Ionic compounds (NaCl, MgBr2 , CaSO4)
What are some properties of these compounds?
• Do not affect litmus paper (neutral!)
• Conduct electricity
• Do not react with metals
1.
2.
3.
4.
5.
6.
7.
8.
9.
NaOH
HClO4
CsOH
HI
HBr
HNO3
MgCl2
Sr(OH)2
H2SO4
10.CH3COOH
11.Ba(OH) 2
12.CH4
13.MgSO4
14.Ca(OH)2
15.HNO2
16.K3P
17.HSO418.KOH

The pH of a solution is actually an indication of the
concentration of H+ ions in that solution.

pH means the “power of Hydrogen”





Acids release hydrogen ions in solution (water).
The water already contains a mixture of hydrogen
and hydroxide ions.
H2O →2H+1 + OHTherefore in an acidic solution there will be both
hydrogen and hydroxide ions.
However there will be more hydrogen ions.
The pH of acids is less than 7.





Bases release hydroxide ions in solution (water).
The water already contains a mixture of hydrogen
and hydroxide ions.
H2O →2H+1 + OHTherefore in a basic solution there will be both
hydrogen and hydroxide ions.
However there will be more hydroxide ions.
The pH of bases is greater than 7
pH = -log[H+]


[H+] is the concentration of hydrogen ions
On the pH scale, a difference of 1 unit is
equivalent to 10 fold difference in acidity!
Ex: a solution with a pH of 3 is 10 times more
acidic than a solution with a pH of 4!
Each unit on the scale represents a factor of 10.
A substance with a pH of 9 is 10x10x10x10 more
basic than a substance with a pH of 5.
9-5=4
104 = 10x10x10x10 = 10 000
A substance with a pH of 2 is 10x10x10x10x10 more
acidic than a substance with a pH of 7.
7-2=5
105 = 10x10x10x10x10 = 100 000
Makes sense?