8.4 The pH Scale

Starter Write down examples of these items in your house Strong Acid Strong Base Weak Acid Weak Base

Objectives Distinguish between aqueous solutions that are acidic, neutral or alkaline using the pH scale Identify which of two or more aqueous solutions is more acideic or basic, using the pH scale State that each change in one pH unit represents a ten-fold change in the hydrogen ion concentration 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 H 2 O(l) ↔ H + (aq) + OH - (aq) In pure water at 25 o C the [H+] is mol dm -3. ( 1 molecule of water in 10 million is dissociated) Reversible pH 7

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 Dr Søren Sørensen ( ) 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+] (mol dm -3 ) [H+] (mol dm -3 ) 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+] (mol dm -3 ) [H+] (mol dm -3 )

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