Acid, Bases and Kw Flashcards
Bronsted-Lowry acids are what?
Bronsted-Lowry acids are proton donors - they release hydrogen ions (H+) when they’re mixed with water. You never get H+ ions by themselves in water, they’re always combined with H2O to form hydroxonium ions H3O+
Bronsted-Lowry bases are what?
Bronsted-Lowry bases are proton acceptors. When they’re in solution, they grab hydrogen ions from water molecules
How do strong acids and bases react in water?
Strong acids dissociate (or ionise) almost completely in water - nearly all the H+ will be released
Strong bases ionise almost completely in water too
How do weak acids and bases react in water?
Weak acids dissociate only very slightly in water - only a small amount of H+ ions are formed
Weak bases only slightly ionise in water
Where does the equilibrium lie in strong and weak acid/base reactions with water?
In strong acids and bases the equilibrium lies extremely far to the right
In weak acids and bases the equilibrium lies extremely far to the left
What happens when acids and bases react?
Acids can only donate their protons if there’s a base to accept them - in this reaction the acid, HA, transfers a proton to the base, B:
HA + B —– BH+ + A-
It’s an equilibrium, so if you add more HA or B, the position of equilibrium moves to the right. If you add more BH+ or A-, the equilibrium will move to the left
How does an acid donate protons in water?
When acid is added to water, water acts as a base and accepts the proton:
HA + H2O —– H3O+ + A-
Does water dissociate?
Water dissociates slightly into hydroxonium ions and hydroxide ions. So this equilibrium exists in water:
H2O —– H+ + OH-
You can write an expression for Kc using this equilibrium reaction
Where does the equilibrium lie in water?
Water only dissociates a small amount so equilibrium lies well over to the left
Explain Kw
There is so much water compared to the amounts of H+ and OH- ions that the concentration of water is considered to have a constant value. So, if you multiple the expression for Kc by H2O you get the ionic product of water
Give the expression for Kw
Kw = Kc x [H2O] = [H+][OH-]
or simply: Kw = [H+] [OH-]
How does temperature affect Kw?
Kw always has the same value for an aqueous solution at a given temperature. The value of Kw changes as temperature changes
What can you say about Kw in pure water?
In pure water, there is always one H+ ion for each OH- ion. So, [H+] = [OH-]. This means that if you are dealing with pure water, then you can say that Kw = [H+]^2
What are the units of Kw?
mol^2 dm^-6
