Acids and Bases Flashcards
Whats an acid according to the Arrhenius Concept?
Acids are substances that when dissolved in water increase the hydrogen ion (H+) concentration. Examples: HCl, HNO3, H2SO4
Whats a base according to the Arrhenius Concept?
Bases are substances that when dissolved in water increase the hydroxide ion (OH-) concentration. Examples: NaOH, KOH
Whats an acid according to the Bronsted Lowry theory?
Acid: substance that can donate protons (H+) to another substance Examples: HCl, HNO3, H2SO4, H2O
Whats a base according to the Bronsted Lowry theory?
substance that can accept H+ from another substance
Examples: NH3+, H2O
Bronsted Lowry have a pair of…
…non bonding electrons.
WHat is the pair of non bonding electrons in Bronsted Lowry bases used to do?
Brønsted Lowry bases have a pair of nonbonding e-, which is used to form a covalent bond with a H+ from a Brønsted Lowry acid.
Whats an acid in the Lewis theory?
substance that can accept e- from another substance Examples: H+, H2O
Whats a base in Lewis theory?
substance that can donate e- to another substance Examples: NH4+, H2O
Using Lewis theory, any…
…compound can be classified as acid or
base
Whats a Monoprotic acid?
transfer one H+ per molecule during an acid-base reaction
Whats a Diprotic acid?
transfer two H+ per molecule during an acid-base reaction
Whats a triprotic acid?
transfer three H+ per molecule during an acid-base reaction
In general, polyprotic acids transfer…
…two or more H+ during an acid-base reaction
The auto-ionisation of water allows it to…
…act as either a proton acceptor (base) or proton donor (acid)
In the presence of acid…
…water acts as a base
In the presence of base…
…water acts as an acid.
Water is a b…
…buffer
One water molecule can donate…
…a proton to another water molecule.
The auto-ionisation of water is an…
…equilibrium process - (forward and reverse
reactions)
What is Kw?
The ion product constant for water
What makes the ion product constant / ionisation constant useful?
it applies to any dilute aqueous solution, and can be used to calculate either the [H+] (if [OH-] is known) or [OH-] (if [H+] is known)
A solution where [H+] = [OH-] is said to be…
…neutral.
In most solutions, H+ and OH- are not equal, and as one of these ions increases…
the other MUST DECREASE so that the product of their concentrations equals 1 x 10^-14
At 25 degrees celcius, what is the Kw (ion product constant/ionization constant)?
KW = [H3O+][OH-] = 1.0 x 10-14
When is [H3O+] = [OH-] equal?
At equilibrium
What are [H3O+] = [OH-] at equilibrium?
[H3O+] = 1x10^-7
[OH-] = 1x10^-7
Because both together equal 1x10^-14
How to work out pH using -log and [H3O+]?
pH = -log [H3O+]
If [H3O+] = 1 x 10-7 then: -log 1 x 10-7 = 7 (neutral)
Why do we express molar concentration of H+ in terms of pH?
molar concentration of H+
in an aqueous solution is very small
In any acid-base equilibrium,…
…both the forward and reverse reactions involve proton
transfers
Every acid has a…
…conjugate base
Every base has a…
…conjugate acid.
Some acids are better…
…proton donors than others
Likewise, some bases are better…
…proton acceptors than others
The stronger the acid…
…the weaker its conjugate base.
The stronger the base…
…the weaker its conjugate acid.
HCl completely…
…ionizes in water - it is a strong acid.
If the affinity of a conjugate base for a H+ ion is low…
…it is a weak conjugate base
The chemistry of an aqueous solution often depends critically on the…
…pH of the solution
What are strong acids or bases?
Acids and bases that completely ionise in solution.
The pH is a measure of the…
…H+ ion concentration expressed as:
pH = -log10[H+]
What is pOH?
The pOH is a measure of the OH- ion concentration expressed as:
pOH = -log10[OH-]
Why do we subtract
pOH from 14? (pH scale 1 – 14)
As pH + pOH = 14, then pH is: 14 – 1.55 = 12.45
When must we convert pOH into pH?
When calculating the pH of strong bases since the initial answer is in pOH. We do not want this. We want the answer in pH. So we subtract pOH from 14 which converts it to pH to give final answer.
The amount that a weak acid dissociates is given by the…
… acid-dissociation constant Ka
What is Ka?
Acid-dissociation constant
ion product constant / ionization constant full equation?
KW = [H3O+][OH-] = 1.0 x 10-14
The acid-dissociation constant for this reaction can be written as:
Ka = [Hydrogen ion] * [conjugate base]
________________________________________
[Weak acid]
= [H3O+] * [A-]
_________________
[HA]
The larger the value of Ka…
…the stronger the acid.
Just as weak acids have an equilibrium constant Ka,…
weak bases have the
equilibrium constant Kb
What is Kb?
base-dissociation constant
The amount that a weak base dissociates is given by the…
…base-dissociation constant Kb
The base-dissociation constant for this reaction can be written as…
Kb = [Hydroxide ion] * [conjugate acid]
____________________________________
[Weak base]
= [OH-]*[HB+]
____________
[B]
The larger the value of Kb,…
…the stronger the base
The –log10 of Kb is called the…
pKb
What is the pKb?
The –log10 of Kb
pKa + pKb = ?
= 14.00
What is the value of Ka?
1.8 x 10^-5
wat do buffers do?
Buffer solutions can resist drastic changes of pH upon the addition of small amounts of strong acid or base
Buffers are generally…
…weak acid-base conjugate pairs
A buffer resists changes in pH because it contains:
- an acidic species to neutralise OH-
- a basic species to neutralise H+
How to figure out the H+ from a pH value?
use this equation:
[H+] = 10^(-pH value)
What is the Henderson-Hasselbalch equation used for?
calculating the pH of a buffer
What is the henderson-hasselbalch equation?
pH - pKa + log10 [base] / [acid]
