Acids and Bases
Acids
Definition : latin meaning sour “Acidus”
Lots of acids
– Dangerous
Nitric Acid, Sulfuric Acid
– Essential
Ascorbic Acid
– Dietary
Acetic Acid (ethanoic acid) (vinegar), Citric Acid, Carbonic
Acid
– Odd
Stearic Acid, Benzoic Acid
info
Turns blue Litmus paper Red
React with metals to produce hydrogen
gas
H2SO4 + Zn
ZnSO4 + H2
React with Carbonates to produce CO2
Arrhenius Theory
An acid is a
substance that
dissociates in water to
+
produce H ions
– Svante Arrhenius (nobel prize 1903)
Add HCl to water and we get H+ ions and
Chlorine
Nitric and Sulfuric acid behave the same
HCl
H+
+
Cl
HNO3
H+
+
NO3
H2SO4
2H+ +
SO42-
When we add a solution of Hydrogen chloride
to water we call it Hydrochloric acid
Mono basic:
HCl
H+
Di Basic
H2SO4
2H+ +
SO42-
Tri Basic
H3PO4
3H+ +
PO43-
+
Cl
Dependant on the number of hydrogen ions
donated
Strong and weak acids
Sulfuric and nitric are
strong as they dissociate
fully in water but acetic
acid is weak as it only
partially dissociates
Hydronium Ion
The H+ ion ....just a proton
Cannot exist independently in solution so
the Hydronium ion is born
The oxygen atom allows an electron to
form a dative covalent bond with the
hydrogen atom
Hydrogen ion reacts with water to
form Hydronium Ion H3O+
H. +
H+
X
H
.O
..
.O.
X
H
HX
+
X
H
Hydronium Ion
For accuracy we should use
HA +
H 2O
H 3O +
+
A-
HCl +
H 2O
H 3O +
+
Cl-
Many chemists simply say the hydrogen ion
Importance of water
If we add pure Hydrogen Chloride or
acetic acid into a solvent like benzene (no
water) we do not get either H+ or
Hydronium ion formation and blue litmus
paper stays blue
Shows that Hydrogen Chloride or acetic
acid is only acid when water is present
Bases
Behave in the opposite manner to acids
Turn Red litmus paper blue
Example:
– Sodium Hydroxide (NaOH)
– Magnesium Hydroxide ([Mg(OH)2])
– Calcium Hydroxide ([Ca(OH)2])
Info
Many bases have a soapy feel
Used as degreasers in household
detergents
Act by converting the oil into soap
which washes away more easily
Arrhenius Theory
A base is a substance that dissociates in
water to produce OH- ions
The OH- ion is called the hydroxide ion
When added to water it dissociates
completely into Sodium ions and
hydroxide ions
NaOH
Na+ +
OH-
Arrhenius Theory
Similarly for Magnesium Hydroxide and
Calcium Hydroxide
[Mg(OH)2]
Mg2+ +
2OH-
[Ca(OH)2]
Ca2+ +
2OH-
Strong and Weak bases
Strong bases include Sodium
Hydroxide because they
dissociate almost completely in
water but ....Magnesium
Hydroxide and Calcium Hydroxide
do not and are referred to as
weak bases
A number of flaws to the Arrhenius
Theory
Hydronium ions are present not bare
hydrogen ions
Arrhenius theory limited to aqueous
solutions and not all acid base reactions
are water based
NH3 +
HCl
NH4Cl
Base
acid
ammonium
chloride
Brønsted -Lowry
Update needed to reflect more
accurately the reactions that
occur
1923 Johannes Bronsted
(Danish) & Thomas Lowry
(English) – working independently
came up with the same theory
Brønsted -Lowry
Definition of an Acid
–An acid is a proton donor
Definition of a base
–A Base is a proton acceptor
Detail
HCl
+ H2O
Acid
Donates
base
accepts
a proton
a proton
H3O + + Cl-
Therefore these obey the B/L definition
Ammonia Gas
NH3
+ H2O
base
accepts
acid
Donates
a proton
a proton
NH4 + + OH-
Therefore these obey the B/L definition
What to learn
Water can act
as an acid or
a base amphoteric
Can be applied to non aqueous
reactions
HCl
Acid
+
NH3
base
NH4+ +
Cl-
Brønsted –Lowry broadens the definition of
what can act as an acid or a base
Homework
Questions 12.1 in textbook and 12.1 in
workbook
Conjugate acid-base pairs
Add acetic acid (ethanoic acid) to water
CH3COOH
H3O
+ H 2O
CH3COO- +
The reaction works in both directions
Conjugate acid-base pairs
Add acetic acid (ethanoic acid) to water
CH3COOH
+ H 2O
H3O
Donates accepts
A proton a proton
CH3COO- +
Conjugate acid-base pairs
Add acetic acid (ethanoic acid) to water
CH3COO- + H3O
H2O
Accepts Donates
A proton a proton
CH3COOH
The water is behaving like an acid
+
Conjugate acid-base pairs
Add acetic acid (ethanoic acid) to water
CH3COOH
H3O
+ H 2O
CH3COO- +
The reaction works in both directions so we
call each conjugate pairs
CH3COOH is a conjugate acid of CH3COO-
CH3COO- is a conjugate base of CH3COOH
An acid changes into a conjugate base
when it donates a proton
CH3COOH
CH3COOH+
acid
Conjugate
proton
base
A base changes into a conjugate acid
when it accepts a proton
CH3COO- H+
CH3COOH
base
proton
conjugate acid
Every acid has a conjugate base and
every base has a conjugate acid...
We call this an conjugate acid - base pair
A conjugate acid – base pair is any pair
consisting of an acid and a base which
differs by one proton
Important
The definition says that the pair must involve
a single proton so...the conjugate pair of
H2SO4
HSO4
Example
What is the conjugate acid and the
conjugate base of HC2O4To change into its conjugate the base
must accept a proton...so
HC2O4base
+
H+
H 2C 2O 4
conjugate acid
Example
What is the conjugate acid and the
conjugate base of HC2O4To change into its conjugate the acid must
donate a proton...so
HC2O4Acid
C 2O 4 2 - +
H+
conjugate base
Tell me
Which is the acid and which is the base
HNO3
+
H2F2
Also mark out the pairs
H2NO3
+
HF2
HNO3
Base
+
H2F2
Acid
H2NO3
+
Acid
Conjugate acid-base pair
Conjugate acid-base pair
HF2
Base
Neutralisation
Neutralisation is the
reaction between an
acid and a base to
form salt and water
Neutralisation
Acids and base in the correct proportions
cancel each other out
This means they lose their
characterisitics,the acid is no longer
behaving like and acid and vice versa
The resulting solution has a neutral effect
on litmus paper
example
HCl + NaOH
NaCl+ H2O
The word salt simply refers to the fact that
the hydrogen in the acid is replaced by a
metal or ammonium ion
What’s Happening?
HCl + NaOH
NaCl+ H2O
In soln:
H+ + Cl - + Na + + OH - = H O
2
Lets ignore the spectator ions
Na + + Cl
H+ + Cl - + Na + + OH - = H O
2
Na + + Cl
The hydrogen (hydronium) ions
from the acid are reacting with
the hydroxide ions from the
base to form water which
results in a neutral solution
Everyday examples
Stomach HCL neutralised by NaHCO3
– In other words...by an antacid (bioSodol)
containing sodium hydrogencarbonate
HCl + NaHCO3
CO2
Another example (Maalox)
2HCl + Mg(OH)2
NaCl + H2O +
MgCl + H2O
Everyday examples
Calcium Oxide (lime) spread on soil to
neutralize the acid conditions
CaO + H2O
Lime + water
Ca(OH)2
Calcium Hydroxide
Calcium Hydroxide can neutralize acidity as
follows
Ca(OH)2 + H2SO4
2H2O
CaSO4 +
Calcium hydroxide plus Sulfuric Acid becomes Calcium Sulfate and
Everyday examples
Limestone added to lake water to
neuralise the effects of acid rain
CaCO3 + H2SO4
2H2O + CO2
CaSO4 +
Calcium carbonate plus Sulfuric Acid becomes Calcium Sulfate and
Water and Carbon Dioxide
Homework
Questions 12.2 – 12.6 in textbook and
12.2 – 12.4 in workbook