Chapter 20 - Acid-base Equilibria Flashcards
what was the Arrhenius model of an acid
Acids dissociate and release H+ ions in aqueous solutions
What was the Arrhenius model of an alkali
Alkalis dissociate and release OH- ions in aqueous solutions
What is the Bronsted Lowry model for an acid
Acids are proton donors
What is the Bronsted Lowry model for an alkali
Alkalis are proton acceptors.
Define conjugate acid-base pairs
They contain 2 species that can be interconverted by transfer of a proton
What is a Conjugate acid
If it has lost a H+.
What is a Conjugate alkali
If it has gained a H+
Define monobasic, dibasic, tribasic
They refer to the number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction.
What plays an active role in an acid
the H+ ion
acid + metal
salt + H2
acid + carbonate
salt + water + CO2
acid + base
salt + water
acid + alkalis
salt + water.
Equation for pH
ph = -log [H+]
Equation for [H+]
[H+] = 10^ -pH.
What is neutral on the pH scale
7.
What is acid on the pH scale
Anything below 7.
What is alkali on the pH scale
Anything above 7.
pH of strong acids
In aqueous solutions, a strong mono basic acid will complete dissociate.
So, for any strong acid, the [H+] is equal to the concentration of the acid [HA].
Define a strong acid
One that completely dissociates in aqueous solutions
Define a weak acid
One that partially dissociates in aqueous solutions.
What is the Ka
The acid dissociation constant for weak acids only.
It is equal to the Kc in some ways, but for an equilibrium equation
The larger the Ka
The further the equilibrium has shifted to the right side, so the greater the dissociation and the acid strength
Equation for pKa
pKa = -log Ka
Equation for Ka
10 ^ -pKa
Strong acid =
Large Ka = Small pKa
Weak acid =
Small Ka = Large pKa
What does a strong acid do in water
Completely dissociate
What happens when a weak acid in mixed with water
It will only partially dissociate and instead, an equilibrium will take place.
[HA] [A-] + [H+]
What does the [H+] depend on?
Concentration of the acid, [HA]
And the acid dissociation constant, Ka.
What are the 2 important approximations to make for the calculation of the Ka?
- You can assume that the concentration of H+ = Concentration of A-, because they are equal at first.
- The equilibrium concentration of HA is smaller than the undissociated concentration. As the dissociation of weak acids is small, you can assume that it is so small, it’s negligible, so that [HA] start = [HA] eqm
Equation to calculate Ka for a weak acid
Ka = [H+]^2 / [HA]
Experimentally, how can you calculate Ka (2 ways)
- Preparing a standard solution of a weak acid of known concentration.
- Measuring the pH of the standard solution using a pH meter.
What are the approximations made when calculating with weak acids?
- The dissociation of water is negligible.
2. The concentration of the acid is much greater than the H+ concentration at equilibrium
What is Kw
Ionic product of water. The ions in water multiplied together. At 25 degrees C, it is 1 x 10^-14.
For whole number pH values, ensure that the indices for [H+] and [OH-] add up to:
-14
Define an alkali
A soluble base that releases OH - ions in aqueous solutions.
