TOPIC 8 ACIDS AND BASES 8.3 The pH scale

ESSENTIAL IDEA The pH scale is an artificial scale used to distinguish between acid, neutral and basic/alkaline solutions. NATURE OF SCIENCE (2.7) Occam’s razor – the pH scale is an attempt to scale the relative acidity over a wide range of H + concentrations into a very simple number.

THEORY OF KNOWLEDGE Chemistry makes use of the universal language of mathematics as a means of communication. Why is it important to have just one “scientific” language?

UNDERSTANDING/KEY IDEA 8.3.A pH = -log[H + ] and [H + ] = 10 -pH

UNDERSTANDING/KEY IDEA 8.3.B pH values distinguish between acidic, neutral and alkaline solutions.

pH (power of Hydrogen) The acidity of a solution is based on the [H + ] or hydrogen ion concentration. Typically [H + ] is very small so pH is a compact way to represent solution acidity. pH = -log 10 [H + ] pH decreases as [H + ] increases

pH = 7 Neutral solutions [H + ]=[OH - ] pH [OH - ] pH > 7 Alkaline or basic solutions [H + ]<[OH - ]

pH values are usually positive and do not have units. The pH value is inversely related to [H + ] which means that the lower the pH, the higher the [H + ] concentration and acidity. Solutions with lower pH’s are more acidic than higher pH’s.

APPLICATIONS/SKILLS Be able to solve problems involving pH, [H + ] and [OH - ].

RELATIONSHIPS OF pH and pOH pH = -log [H + ] pOH = -log [OH - ] pH + pOH = 14 [H+] = 10 -pH [OH-] = 10 -pOH Sig Fig Rule for pH – the number of decimal places in the pH figure is equal to the number of sig figs in the original [H + ].

pH’s of Strong Acids and Bases Since the 6 strong acids and the 7 strong bases dissociate or ionize completely, you can calculate the pH directly from the given concentration. 2mol/dm 3 HCl ↔ 2mol/dm 3 H mol/dm 3 Cl- 1mol/dm 3 Ba(OH) 2 ↔ 1mol/dm 3 Ba mol/dm 3 OH -

If you have one value, you can determine the other three values (pH, pOH, [H + ], [OH - ]) by the previous equations.

EXAMPLE 1 [H + ] = 1 x M What is the [OH - ]? What is pH? What is pOH? Acidic or Basic?

EXAMPLE 2 pOH = 3 What is the [OH - ]? What is pH? What is [H + ] ? Acidic or Basic?

CALCULATE THE pH 1. [H + ] = 5.92 x M 2. [OH - ] = 8.32 x M 3. pOH = [H + ] = 9.30 x M 5. pOH = 2.03

UNDERSTANDING/KEY IDEA 8.3.C A change of one pH unit represents a 10-fold change in the hydrogen ion concentration [H + ].

If you increase the pH by one unit, the [H + ] concentration decreases by 10 times and if you decrease pH by one unit [H + ] increases by 10 times. The pH scale is logarithmic so it compresses a very wide range of [H + ] into a much smaller scale of numbers.

[H+] = 1 x has a pH of 5 and [H+] = 1 x has a pH of 12. If the number preceding the “x 10 -y ” is “1”, then the pH is equal to the value of the exponent.

If the pH of a solution changes from 3 to 5, deduce how the [H + ] changes. If pH = 3, then [H + ] = 1 x mol/dm 3 If pH = 5, then [H + ] = 1 x mol/dm 3 So the [H + ] changed by or decreased by 100. An observation would be that changing pH from 3 to 5 decreased acidity or [H + ].

UNDERSTANDING/KEY IDEA 8.3.D The ionic product constant, K w =[H + ][OH - ] = 1 x at 298K.

APPLICATIONS/SKILLS Be familiar with the use of a pH meter and universal indicator.

Citations International Baccalaureate Organization. Chemistry Guide, First assessment Updated Brown, Catrin, and Mike Ford. Higher Level Chemistry. 2nd ed. N.p.: Pearson Baccalaureate, Print. Most of the information found in this power point comes directly from this textbook. The power point has been made to directly complement the Higher Level Chemistry textbook by Catrin and Brown and is used for direct instructional purposes only.