DP CHEMISTRY
Rob Slider
ACIDS
Acids are compounds that:
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have a low pH (below 7)
taste sour
turn blue litmus paper red
react with bases to neutralize them and
produce salts
release H2 gas in reactions with active
metals
aqueous solutions conduct electricity
furnish H+
burn the skin if strong
dissolve carbonates
These are some common acids. What others can you
think of? Make a list in your notes.
BASES
Bases are compounds that:
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have a high pH (above 7)
taste bitter
turn red litmus paper blue
react with acids to neutralize them and
produce salts
are slippery feeling
aqueous solutions conduct electricity
burn skin if strong
react with fats to form soap
furnish OH-
What other bases/alkalis do you know?
Common ACIDS
Some common laboratory acids
HCl
HBr
HI
hydrochloric acid
hydrobromic acid
hydroiodic acid
HClO4
HNO3
H2SO4
perchloric acid
nitric acid
sulfuric acid
Five of these acids are classified as monoprotic acids. They only have one
hydrogen which they are able to donate. Sulfuric acid is classified as a
diprotic acid because it has two acidic hydrogens that it can donate.
Similarly, an acid which has three donatable hydrogens would be
classified as triprotic.
Common BASES
Some common laboratory bases
NaOH
KOH
NH3
sodium hydroxide
potassium hydroxide
ammonia
Ba(OH)2
Na2CO3
NaHCO3
calcium hydroxide
sodium carbonate
sodium hydrogen carbonate
Bases are often found in everyday products such as many cleaning products
(sodium hydroxide), antacid products (magnesium hydroxide )and fertilisers
(ammonia). It is a common misconception that bases are not as dangerous as
acids. In fact, many bases can be as much or more corrosive than many acids.
Note that an alkali is a base that is soluble in water
Equilibrium constant of water Kw
Note that water slightly dissociates in equilibrium with hydrogen and hydroxide ions:
H2O  H+ + OHSo, the equilibrium constant is:
Kw = [H+][OH-]
Experimentally, it has been determined in a neutral solution at 250C:
[H+] = [OH-] = 10-7
(a very small amount!)
So,
Kw = (10-7)(10-7) = 10-14
This shows how
far to the left this
equilibrium is
Neutralisation
From the previous slide,
Kw = [H+][OH-]
Kw = (10-7)(10-7) = 10-14
This value (10-14) is constant for water so,
addition of H+ means a decrease in [OH-]
an increase in OH- means a decrease in [H+]
pH
pH is a measure of the acidity of a solution. More accurately
stated it is a measure of the hydrogen ion concentration on
a logarithmic scale. pH stands for ‘potential of hydrogen’
Mathematically this is ,
pH = - log [H3O+]
or
pH = - log [H+]
The inverse of the above expression is
[H+] = 10-pH
So, in a neutral solution, the concentration of hydrogen is 10-7 M:
pH = -log[H+] = -log(10-7)
pH = 7
The logarithmic scale
The log scale is based on multiples of
ten where the log is the value of
exponent when the number is written
as an exponent.
To calculate pH, just use the log
button on your calculator
You try:
Number Representation and Logarithms
Exponent
Number
Log of the Number
Notation
1000
103
3
2
100
10
2
1
10
10
1
1
100
0
-1
0.1
10
-1
0.01
10-2
-2
-3
0.001
10
-3
-4
0.0001
10
-4
If an acid has an H+ concentration
of 0.0001 M, find the pH.
Solution:
First convert the number to exponential
notation, find the log, then solve the pH
equation.
H+ = 0.0001M = 10-4; log of 10-4 = -4;
pH = - log [ H+] = - log (10-4) = - (-4) = +4 = pH
pH scale
The pH scale, (0 - 14), is the full set of pH numbers which indicate the concentration of H+
and OH- ions in water.
pH Scale Principle:
H+ ion concentration and pH relate inversely.
OH- ion concentration and pH relate directly.
The following statements may be made about the
pH scale numbers.
Complete the last two
a. Increasing pH means the H+ ions are decreasing.
b. Decreasing pH means H+ ions are increasing.
c. Increasing pH means OH- ions are
d. Decreasing pH means OH- ions are
pH exercises
Condition
Most H+ ions: pH = 4; or pH = 5.
Most OH- ions: pH = 10; or pH = 13.
Least H+ ions: pH = 12; or pH = 13.
Least OH- ions: pH = 8; or pH = 9.
If acid was added to a solution of pH 4, the pH would increase or decrease?
If acid was removed from a solution of pH 3, the pH would inc. or dec.?
If base were added to a solution of pH 9, the pH would inc. or dec.?
If base were added to a solution of pH 2, the pH would inc. or dec.?
If acid were added to a solution of pH 13, the pH would inc. or dec.?
If base were removed from a solution of pH 12, the pH would inc. or dec.?
Answer
pH exercises (solutions)
Condition
Answer
Most H+ ions: pH = 4; or pH = 5.
Most OH- ions: pH = 10; or pH = 13.
Least H+ ions: pH = 12; or pH = 13.
Least OH- ions: pH = 8; or pH = 9.
If acid was added to a solution of pH 4, the pH would increase or decrease?
If acid was removed from a solution of pH 3, the pH would inc. or dec.?
If base were added to a solution of pH 9, the pH would inc. or dec.?
If base were added to a solution of pH 2, the pH would inc. or dec.?
If acid were added to a solution of pH 13, the pH would inc. or dec.?
If base were removed from a solution of pH 12, the pH would inc. or dec.?
pH = 4
pH = 13
pH = 13
pH = 8
Decrease
Increase
Increase
Increase
Decrease
Decrease
pH of some common substances
Notice that an increase
in 1 pH value means a
ten fold increase in
hydrogen ion
concentration
pH Indicators
 Many substances change colour as they
are exposed to different pH levels. These
can be used to “indicate” the pH of
substances when the colour ranges are
known
 Some natural products such as litmus,
cabbage, grapes and tea are natural
indicators while others such as
phenolphthalein and methyl orange are
synthetic
 pH indicators are themselves acids or
bases as they donate or accept protons
Notice the variety of ranges where different
indicators change colours. Some have more than one
change.
pH Indicators
Some specific examples:
1.
2.
3.
4.
Litmus
Phenolphthalein
Methyl orange
Bromothymol blue
red5
8blue
colourless8.3 10red
red3.1
4.4yellow
yellow6
7.6blue
Note: colour changes within these ranges are gradual
A pH problem - indicators
Indicator
Colour
Litmus
Blue
Phenolphthalein
Colourless
Methyl orange
Yellow
Bromothymol blue
Blue
An unknown solution produces the colours above. What is the pH range of this
solution?
8-8.3
pH meters
A pH meter is used to accurately
measure pH and takes the ‘guesswork’
out of judging colours using indicators.
Basically, they work by very precisely
measuring the voltage. This voltage is
then converted to a pH value by the
meter.