Chapter 18 - pH and Indicators Flashcards
What is the self-ionisation of water?
When water dissociates into ions H⁺ and OH⁻.
Pure water conducts electricity when it dissociates into its ions
Where does the equilibrium lie in the dissociation of water into its ions?
The concentration of H⁺ and OH⁻ ions in pure water is very small so the equilibrium lies mainly on the left
What is the expression of Kc for the dissociation of water into its ions?
[H⁺][OH⁻]
Kc = ————-
[H₂O]
Why is the value of [H₂O] taken to be a constant?
Very little ionisation occurs
Kw = …
Kw = Kc.[H₂O] = ionic product of water = [H⁺][OH⁻]
What is the value of Kw at 25°C?
Kw = 1 x 10⁻¹⁴
What is the value of [H⁺]?
[H⁺] = 1 x 10⁻⁷
What happens if the conditions of a solution change (i.e, acid or base added) ?
If the conditions change, and the value of H⁺ increases (for example), then the value of OH⁻ must decrease by the same amount (since Kw is a constant)
For acidic solutions, [H⁺] >[OH⁻]
[H⁺] > [OH⁻]
For basic solutions [H⁺] > [OH⁻]
[H⁺] < [OH⁻]
Is Kw temperature dependent?
Yes → the self-ionisation of water will increase at higher temperatures (as the forward reaction is exothermic) and the pH of pure water decreases at higher temperatures
How do you calculate pH?
pH = -log₁₀[H⁺]
How do you calculate pOH?
pOH = -log₁₀[OH⁻]
How do you calculate the pH of a strong base?
pH = 14 - pOH
What are the limitations of the pH scale?
○ Only applies to dilute solutions → over 1M the accuracy decreases
○ Scale only runs from 0-14, even though values outside this range exist
○ Only applies to aqueous solutions
What is the difference between concentrated and strong?
A concentrated solution is one that contains a large amount of solute, whereas strong refers to the extent to which an acid can donate its H⁺ ions
What symbol is used to represent the strength of an acid?
Ka (acid dissociation constant) - it gives a measure of the extent to which an acid is dissociated
Do strong acids have a high or low value for Ka? Why?
Strong acids have a large amount of dissociation and a high value of [H⁺]. They therefore have a high value of Ka
Do weak acids have a high or low value for Ka? Why?
Weak acids have small amount of dissociation and a small value of [H⁺]. They therefore have a low value for Ka
What symbol is used to represent the strength of a base?
Kb - base dissociation constant
How do you calculate the pH of a weak acid?
First, find the value of [H⁺] ions:
[H⁺] = √(Ka x Macid)
then put this figure into the original pH formula
