Transcript Topic 8.4 Acids and Bases The pH Scale

8.4 The pH Scale
Starter
 Write down examples of these items in your house
 Strong Acid
 Strong Base
 Weak Acid
 Weak Base
Objectives
 8.4.1 Distinguish between aqueous solutions that are
acidic, neutral or alkaline using the pH scale.
 8.4.2 Identify which of two or more aqueous solutions
is more acideic or basic, using the pH scale.
 8.4.3 State that each change in one pH unit
represents a ten-fold change in the hydrogen ion
concentration.
 8.4.4 Deduce changes in hydrogen ion concentration
when the pH of a solution changes by more than one
pH unit.
Terms
 pH
 Acidic
 Basic
 Neutral
Textbook
 Topic 8 on Moodle
The Dissociation of Water

H2O(l) ↔ H+ (aq) + OH-(aq)
Reversible
pH 7
In pure water at 25oC the [H+] is 10-7 mol dm-3.
( 1 molecule of water in 10 million is dissociated)
The pH Scale
 Stands for power of Hydrogen (it is the concentration
of hydrogen ions and equal to the power of ten with
the sign reversed)
Scale between 1 and 14
pH 7 is neutral
Acids are from 0 to 6.99
 As scientists we don’t like really big or really small
numbers. So we use scientific notation. When that
gets a bit tiresome, we go even further.
 Sorensen proposed the pH scale to simplify the
comparison of concentrations of H+ ions in
solution
A Brief History of the pH Scale
 The pH scale, the standard measurement of acidity, was
developed by the head of Carlsberg Brewery Laboratory’s
Chemical Department in 1909.
 Dr Søren Sørensen (1868-1939) developed the pH scale during
his pioneering research into proteins, amino acids and enzymes the basis of today’s protein chemistry. Basically meaning ‘the
power of hydrogen’, the scale provides a simple and universal
measurement of the amount of hydrogen ions in a solution, which
affects its acidity and how it reacts chemically.
Adding an Acid………..
 Increases the hydrogen ion concentration.
The pOH Scale
pH
[H+]
[H+]
(mol dm-3) (mol dm-3)
0
1
2
100
10-1
10-2
1
0.1
0.01
3
4
5
6
10-3
10-4
10-5
10-6
0.001
0.0001
0.00001
0.000001
7
10-7
0.0000001
What is
happening to the
hydrogen ion
concentration
with each
decrease in pH
unit?
Question……………..
 What is happening to the hydrogen ion
concentration when the pH changes by 2?
 What is happening to the hydrogen ion
concentration when the pH changes by 3?
Adding a Base………..
 If we add more OH- what is going to happen to the
[H+]
(Hint: remember Le Chatelier’s Principle)
pH
[H+]
[H+]
(mol dm-3) (mol dm-3)
8
9
10
10-8
10-9
10-10
0.0000001
0.00000001
0.000000001
11
12
13
14
10-11
10-12
10-13
10-14
0.0000000001
0.00000000001
0.000000000001
0.0000000000001
1. Black coffee has a pH of 5 and toothpaste has a pH of
8. Identify which is more acidic and deduce how many
times the [H+] is greater in the more acidic product.
(Total 2 marks)

A solution of acid A has a pH of 1 and a solution of
acid B has a pH of 2. Which statement must be correct?
A.
Acid A is stronger than acid B
B.
[A] > [B]
C.
The concentration of H+ ions in A is higher than in B
D.
The concentration of H+ ions in B is twice the
concentration of H+ ions in A
(Total 1 mark)
 Water is neutral, duh!!
 It’s neutral because it has equal amounts of both
 [H+] = 1x10-7
 [OH-] = 1x10-7
 pH = 7
 pOH = 7
 This allows us to use water as a reference point
 pH + pOH = pKw
 [H+][OH-] = K
Remember
It is a good idea at the moment to reread the
assessment statements and make sure you can do
these tasks.
Questions
 Worksheet
Plenary
 Will the pH change if I dilute an
acid??????
 Write your answer in as much detail as
you can
 Provide an example calculation