Chapter 20 - Acids, bases and pH Flashcards
What does the Arrhenius model suggest about acids?
They dissociate and release H+ ions when dissolved in water
What does the Arrhenius model suggest about alkalis?
They dissociate and release OH- ions when dissolved in water
What is an alkali?
A soluble base
What is a Brønsted-Lowry acid?
A proton donor
What is a Brønsted-Lowry base?
A proton acceptor
What is a conjugate acid-base pair?
Two species that can be interconverted by the transfer of a proton
Formula for the hydronium ion
H3O+
What does H+ really represent in an equation?
The hydronium ion
What type of scale is pH?
Logarithmic
What does a low value of H+ indicate?
A high pH
What does a high value of H+ indicate?
A low pH
How to work out pH?
pH = -log[H+]
How to work out [H+]?
10^-pH
What is important to remember about calculating the pH of strong acids?
They completely dissociate:
[H+(aq)] = [HA(aq)]
What happens to weak acids when dissolved in solution?
They partially dissociate
What is Ka?
The acid dissociation constant
What does pH measure?
Hydrogen ion concentration
What is the equation of Ka for a weak acid before assumptions?
[H+(aq)] x [A-(aq)]
[HA(aq)]
How to find pKa?
pKa = -log(Ka)
How to find Ka from pKa?
10^-pKa
What characteristics of pKa and Ka will a strong acid have?
Large Ka value, small pKa
What characteristics of pKa and Ka will a weak acid have?
Small Ka value, large pKA
What are the units for Ka?
mol dm^-3
What is produced when a weak acid dissociates?
An equilibrium
What is the first approximation we use when calculating pH for a weak acid?
You ignore the H+ ions produced from the dissociation of water because the dissociation from the acid is greater
You assume all H+ ions come from the acid, so [H+(aq)] = [A-(aq)]
What is the second approximation we use when calculating pH for a weak acid?
Because the dissociation is so small, you assume that only a tiny amount of H+ is produced
[HA(aq)] eqm = [HA(aq)] start
What is the simplified version of Ka we get?
Ka = [H+(aq)]^2
[HA(aq)] start
How to find [H+] using the simplified Ka expression?
[H+] = √(Ka x [HA(aq)]
What experiment would you conduct to measure Ka?
Prepare a standard solution of weak acid of unknown concentration
Measure the pH using a pH probe
When will approximation 1 break down?
If the pH of the acid is more than 6, [H+] from the dissociation of water is 10^-7, which is significant compared to the [H+] from the acid of 10^-6
It breaks down for very weak acids
When will approximation 2 break down?
When [H+] becomes significant and there is a large difference between [HA] eqm and [HA] start - [H+]
It breaks down for stronger weak acids with a Ka value of > 10^-2
What is Kw?
The ionic product of water
Equation for Kw
Kw = [H+][OH-]
What is the value of Kw at RTP?
1 x 10^-14
Units for Kw
mol^2 dm^-6
When will a solution be acidic?
When [H+] > [OH-]
When will a solution be neutral?
When [H+] = [OH-]
When will a solution be alkaline?
When [OH-] > [H+]
For a strong base, how do you calculate pH?
Use [base] as [OH-]
How do you calculate the pH of weak bases?
Don’t need to know
For a dibasic base, how do you calculate [OH-]?
2 x [dibasic base]
What is a di/tribasic acid?
Releases 2/3 protons
Acid + carbonate
Salt + water + carbon dioxide
The ionic equation for acid + carbonate
2H+ + CO3 2- -> H2O + CO2
Acid + base
Salt + water
The ionic equation for acid + base
2H+ + MgO -> Mg 2+ + H2O
Acid + alkali
Salt + water
The ionic equation for acid + alkali
H+ + OH- -> H2O
Acid + metal
Salt + hydrogen
The ionic equation for acid + metal
2H+ + Zn -> Zn 2+ + H2
What is important to remember about ionic equations?
An acid will always be represented by H+
What is a mono/di/tribasic acid?
1/2/3 protons can be replaced in a reaction
