Acids and Bases 10.chapter
The term acid comes from the Latin acidus, which means "sour". The sour taste of vinegar and citrus fruits is due
to the presence of acids.
Eg: Sulphuric acid
Bases are characterized by their bitter taste and slippery feel. Solutions containing bases are often called
alkaline
Eg: sodium hydroxide
10.1 Acids donate protons and bases accept them.
Bronsted Lowry definition focuses on protons:
In his definition an acid is any chemical that donates hydrogen ions, H+, and a base is any chemical
that accepts hydrogen ions.
A hydroxide ion is made by removing a proton (hydrogen ion) from a water molecule
A hydrogen ion, H+ , formed by loss of an electron with presence of only proton in it.
That's why some times said than an acid is a chemical that donates a proton and base is a chemical
that accepts a proton.
Acronym :
BAAD
Base Accepts proton Acid Donates proton
Examples of acid: Ascorbic acid/vitamin C
in citrus fruits
Vinegar contains acetic acid used to preserve foods.
HCl hydrochloric acid used in toilet bowl cleaners.
Carbonated beverage contains carbonic acid or phosphoric acid.
When hydrogen chloride Is mixed into water
HCl donates a hydrogen ion to H2O , here Hcl behaves as an acid (proton donor) and H2O behaves as a base
(proton acceptor). Products are hydronium ion, H3O+ and chloride ion.
Examples of Base: sodium bicarbonate (NaHCO3) as baking soda
Sodium hydroxide ( NaOH) to drain cleaners.
When ammonia is mixed with water
Ammonia behaves as a base water behaves as an acid, products are OH- and NH4+
An important aspect of Lowry definition is that it uses a behavior to define as an acid or a base. For ex
water behaved as base when mixed with HCL and same time water behaved as acid when mixed with NH
3
The Lewis definition focuses on lone pairs:
In his definition , a molecule which donates a lone pair behaves as base and that lone pair accepted by
molecule called acid.
Acid
Base
The water is behaving as a base, because its lone pair seeks out and accepts a positive charge (the
proton) HCL behaves as an acid because it donates a positive charge to the lone pair.
Ex: Carbonic acid from water and CO2 is a acid base reaction that does not involve the transfer of a
proton
A base donates a lone pair and an acid accepts
CO2 behaves as an acid when it donates a slightly positive charge (usually proton) of its carbon atom to
the lone pair on the oxygen atom of a water molecule.
This reaction starts as a typical dipole –induced dipole attraction between the oxygen of the water and
the carbon of CO2..
As the carbon gains electrons from the lone pair of the water, it begins to lose electrons to one of its
two oxygen atoms which leads to form covalent bond between water and CO2 ( shown in brackets)
then it forms more stable carbonic acid.
A salt is the ionic product of an acid-base reaction
Salt is an ionic compound formed from the reaction between an acid and a base
In this reaction HCL (acid) reacts with Sodium hydroxide(base ) and it forms sodium chloride (salt) and
water.
In this reaction, HCL (acid) reacts with Potassium hydroxide(base ) and it forms potassium chloride
(salt) and water.
Neutralization : the reaction between acid and base is called neutralization
Hydrogen chloride and pseudoephedrine react to form the salt pseudoephedrine hydrochloride, used
as medicine for stuffy nasal, salt form is easily soluble in
water and readily
absorbed by body.
--
10.2 Some acids and bases are stronger than others.
The stronger an acid, more rapidly it donates hydrogen ions, the stronger a base, the more rapidly it
accepts hydrogen ions, corrosiveness is result of stronger.
Ex strong acid HCL
Strong Base NaOH
Strength is checked by adding acid or base in water, if water remains little it is strong acid/base, if
water remains more it is weak acid/base.
Water in HCL
Water not remained
--------------
HCL is Strong acid
Hydrogen chloride donates hydrogen ions to water forming chloride ions and hydronium ions, no
water remained. That’s why HCL is strong acid.
Water in Acetic acid
Water remained so
--------------------
Acetic acid is weak acid
Acetic acid less tendency to donate hydrogen ions to water, only a small portion of the acetic acid
molecules converted to ions,(The C_H bonds of acetic acid are unaffected by the water because of
their non polarity) majority of acetic acid molecules remain as un-ionized form that's why its weak acid.
10.3 Solutions Can Be Acidic, Basic, or Neutral
Amphoteric: A substance whose ability to behave as an acid is about the same as its ability to behave as
a base
Eg Water behaves as acid as well as base.
When a water molecule gains a hydrogen ion, second water must lose a hydrogen ion.
So
In pure water, the total number of hydronium ions must be same as total number of hydroxide ions.
Experiment reveals that concentration of OH- and H3O+ in pure water is extremely low 0.0000001
Molarity for each.
OH- = 0.0000001 M
H3O+ = 0.0000001 M
= 10-7 M
= 10-7 M
The concentration of hydronium ions in any aqueous solution multiplied by the concentration of the
hydroxide ions in the solution always equal the constant Kw which is very small number.
[OH- ] [H3O+ ] =
10
-7
*
10
-7
Kw =
0.0000000000000010
= 10-14
Any solution containing equal number of hydronium ions and hydroxide ions is said to be neutral.
If an acid added to neutral solution it increases hydronium ions leads to behave as acidic.
[OH- ]
10
-9
*
10
-5
=
10
[H3O+ ] =
Kw
-14
If an base added to neutral solution it increases hydronium ions leads to behave as basic.
[OH- ]
10
-3
*
10
-11
=
10
[H3O+ ]
=
Kw
-14
H
The P scale is used to describe acidity:
H
H
The P scale is a numeric scale to express to express acidity of a solution . Mathematically, P is equal to
the negative logarithm of the hydronium ion concentration.
H
+
P = -log [ H3O ]
Log (logarithm ) = The power to which 10 is raised
[ H3O+ ] = the molar concentration of hydronium ions
What is the pH of a neutral solution?
H
P
= -log H3O+
= -log 10-7
= - (-7)
= 7
What is the pH of an acidic solution?
H
= -log H3O+
= -log 10-5
= - (-5)
= 5
What is the pH of a basic solution?
P
P
H
=
=
=
=
-log H3O+
-log 10-9
- (-9)
9
Solution
Acidic
Neutral
Basic
H
P
<7
=7
>7
10.4 Buffer solutions Resist changes in PH
Buffer solution: A solution that resists changes in pH upon the addition of an acid or a base.A buffer
solution contains at least two components:1.A component to neutralize any incoming base.2.A component to
neutralize any incoming acid.
Buffer solutions can be prepared by mixing a weak acid and weak base.
When any strong acid like HCl added to the particular buffer solution is neutralized by the sodium
acetate(salt), here weak base sodium acetate as component in buffer solution changes into acetic acid
and sodium chloride, newly formed acetic acid is already another component of buffer solution, sodium
chloride is salt. So it proves buffer resists change in PH
When any strong base like NaOH added to the particular buffer solution is neutralized by the Acetic
acid(salt), here weak acid acetic acid as component in buffer solution changes into sodium acetate and
water, newly formed sodium acetate is already another component of buffer solution, water is neutral.
So it proves buffer resists change in PH
Eg: