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 are spectator ions?
Ions that do not change during reaction
General equation for neutralisation of acids with carbonates
Acid + carbonate —> salt + water + carbon dioxide
General equation for neutralisation of acid’s with metal oxides
Acid + base —> salt + water
General equation for neutralisation of acid’s with alkalis
Acid + alkali —> salt + water
What characteristics of pKa and Ka will a weak acid have?
Smaller ka value, lager pka value
What are the units for Ka?
Mol dm^-3
What is produced when a weak acid dissociates?
HA ->
What is the first approximation we use when calculating pH for a weak acid?
Monobasic so [H+]=[A-]
On the side there is a very small concentration of H plus ions from the dissociation of water however it is extremely small and will be neglected
What is the second approximation we use when calculating pH for a weak acid?
[HA]eqm=[HA]start-[H+]eqm
Weak dissociation so [H+] small and neglected so…
[HA]eqm=[HA]start
What is the simplified version of Ka we get?
Ka = [H+]^2/[HA]
How to find [H+] using the simplified Ka expression?
[H+] = (square root) ka X [HA]
What experiment would you conduct to measure Ka?
- preparing a standard solution of a weak acid of known concentration
- measuring the pH of the standard solution using a pH meter
When will approximation 1 break down?
Very weak acids or very dilute solutions
When will approximation 2 break down?
Not justified for stronger weak acids with Ka > 10^-2 moldm^-3 and very dilute solutions
What is Kw?
The ionic product of water
Equation for Kw
Kw = [H+][OH-]
What is the value of Kw at RTP?
1X10^-14 mol^2dm^-6
Units for Kw
mol^2dm^-6
When will a solution be acidic?
[H+]>[OH-]
When will a solution be neutral?
[H+]=[OH-]
When will a solution be alkaline?
[H+]
For a strong base, how do you calculate pH?
[H+] = kw(1X10^-14)/[OH-]
then -log(H+)
For a dibasic base, how do you calculate [OH-]?
Each mole of dibasic base releases 2 moles of OH-ions
What is monobasic, dibasic, and tribasic acid?
The total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction
