1. Acids, Bases and the pH scale
DP CHEMISTRY
Rob Slider

2. ACIDS Acids are compounds that: 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.

3. BASES Bases are compounds that: 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?

4. Common ACIDS Some common laboratory acids
HCl hydrochloric acid HClO4 perchloric acid
HBr hydrobromic acid HNO3 nitric acid
HI hydroiodic acid H2SO4 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.

5. Common BASES Some common laboratory bases
NaOH sodium hydroxide Ba(OH) calcium hydroxide
KOH potassium hydroxide Na2CO sodium carbonate
NH3 ammonia NaHCO 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

6. Equilibrium constant of water Kw
Note that water slightly dissociates in equilibrium with hydrogen and hydroxide ions:
H2O  H+ + OH-
So, 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

7. Neutralisation From the previous slide, Kw = [H+][OH-]
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+]

8. 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

9. 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:
If an acid has an H+ concentration of M, find the pH.
Number Representation and Logarithms
 Number
 Exponent Notation
 Log of the Number
 1000
 103  3
 100
 102  2
 10
 101  1  0
 0.1
 10-1
 -1
 0.01
 10-2
 -2
 0.001
 10-3
 -3
 0.0001
 10-4
 -4
Solution: First convert the number to exponential notation, find the log, then solve the pH equation.
H+ = M = 10-4; log of 10-4 = -4;
pH = - log [ H+] = - log (10-4) = - (-4) = +4 = pH

10. 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

11. pH exercises 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.?

12. pH exercises (solutions)
Condition
Answer
Most H+ ions: pH = 4; or pH = 5.
pH = 4
 Most OH- ions: pH = 10; or pH = 13.
pH = 13
 Least H+ ions: pH = 12; or pH = 13.
 Least OH- ions: pH = 8; or pH = 9.
pH = 8
 If acid was added to a solution of pH 4, the pH would increase or decrease?
Decrease
 If acid was removed from a solution of pH 3, the pH would inc. or dec.?
Increase
 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.?

13. pH of some common substances
Notice that an increase in 1 pH value means a ten fold increase in hydrogen ion concentration

14. 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.

15. pH Indicators Some specific examples:
Litmus red5 8blue
Phenolphthalein colourless red
Methyl orange red yellow
Bromothymol blue yellow6 7.6blue
Note: colour changes within these ranges are gradual

16. A pH problem - indicators
Colour
Litmus
Blue
Phenolphthalein
Colourless
Methyl orange
Yellow
Bromothymol blue
An unknown solution produces the colours above. What is the pH range of this solution?
8-8.3

17. 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.