THE BROWNSTED-LOWRY THEORY
A. The theory
· Bronsted-Lowry acid is
defined as anything that releases H1+ ions. An acid is a proton (hydrogen ion) donor.
· Bronsted-Lowry base is
defined as anything that accepts H1+ ions. A base is a proton (hydrogen ion) acceptor.
Example
HCl(l) + NH3 (aq)
--------> Cl -(l) + NH4+(aq)
Acid Base
<--------
H4 + (aq) +
H2O(l)
--------> NH3(aq)+ H3O+(aq)
Acid Base
<----------
· In this
reaction, NH4 +(aq) work as
the Acid, because it transfers proton (H+) to H2O(l).
· While H2O(l) work
as the base, because it receive proton from NH4 +(aq).
B. Special Concept in Bronsted-Lowry
· Bronsted Lowry theory does not only refer to reactions
where water is the solvent
· Not only
in form of molecul but also as kation and anion.
· Couple of
Asam-Basa Bronsted-Lowry should just have only one different H+ ion.
C. Conjugate Acid-Base Pairs
Conjugate acids and
conjugate bases are characterized as the acids and bases that lose or gain
protons. In an acid-base reaction, and acid plus a base reacts to form a
conjugate base plus a conjugate acid.
Acid
+ Base ---------> Conjugat Base + Conjugat Acid
The conjugate acid obaf a se is formed when the base gains a
proton. Refer to the following equation:
NH3(g) + H2O(l) ---------> NH4+(aq) +
OH-(aq)
- NH4+ is
the conjugate acid to the base NH3, because NH3 gained
a hydrogen ion to form NH4+, the conjugate acid.
The conjugate base of an acid is formed when the acid donates a
proton.
- OH- is
the conjugate base to the acid H2O, because H2O
donates a hydrogen ion to form OH-, the conjugate base.
Note: The stronger the acid or base, the weaker the conjugate. The
weaker the acid or base, the stronger the conjugate.
In summary:
- A
conjugate acid of the base
H+ +
Base - A
conjugate base of the acid Acid-
H+
D. How to identify Conjugate Pairs
HNO3 + H2O --------> H3O+
+ NO3-
1. HNO3 Is
an acid because it donates a proton to
water and its conjugate base is NO3- .An
easy way to identify the conjugate base is that it
differs from the acid by one proton.
2. H2O
is a base because it accepts a proton from HNO3 and
its conjugate acid is H3O+ .
Again to identify the conjugate acid (or
any conjugate pair) is that it differs from the base by
one proton.
Equation
|
Acid
|
Base
|
Conjugate Base
|
Conjugate Acid
|
HClO2 + H2O -----> ClO2- +
H3O+
|
HClO2
|
H2O
|
ClO2-
|
H3O+
|
H2O + OCl- ----->
OH- + HOCl
|
H2O
|
OCl-
|
OH-
|
HOCl
|
HCl + H2PO4- ----->
Cl- + H3PO4
|
HCl
|
H2PO4-
|
Cl-
|
H3PO4
|
E. Amfiprotik
• Sometimes
a molecule can donate a proton (act as an acid) and sometimes it can accept a
proton (act as a base).
• Molecules
that have this ability to act as both an acid and a base are called amphoteric or amphiprotic.
• Water is
the most common example of an amphoteric substance.
• The
ionitation of water include in Autoprotolisis
H2O (l) +
H2O (l) --------> H3O+(aq)+
OH – (aq)
<--------
F. Indicators
Acid-Base Indicator is a substance that can show different
colors in the acid or base solution.
Universal Indicator
Universal Indicator is a mixture of indicator dyes. Such an
indicator is useful because it gives a range of colors (a ‘spectrum’) depending
on the strength of the acid or alkali added. The solutions of different acids
produce different colors. Indeed, solution of the same acid with different
concentrations would also give different colors.
The more acidic solutions turn universal indicator
bright red. A less acidic solution will only turn it orange-yellow. There are
also colors differences produced with different alkali solutions. The most
alkaline solution give a violate color.
Natural Acid-Base Indicator
Natural Acid and Base Indicator is the indicators made from natural substances.
Example: Rose, etc.
That are some experiments to observe the acidity
of the substances by using some indicators.
A. Observe the acidity of the substances by
using red cabbage
1. Cut the cabbage until very small
2. At the same time, boill the water
3. Pour the water to the cabbage.
4. If the color of the cabbage has been dissolved,
filter the cabbage, wait until cold. After that, pour the substances (alkali or
acid) that would be observed into it.
5. If the solution turns into red, it means that
the substance is acid.
6. If the solution turns into blue, it means that
the substance is base.
B. Observe the acidity of the substances by
using Litmus Paper
When the litmus paper added to an acidic
solution, it turns red. The molecules of the indicator are actually changed in
the presence of the acid. When the litmus paper added to alkali solutions, it
turns blue. Note that litmus just gives a single color change. Therefore, it
can only used to find know whether the substances are acid or alkali. It cannot
show us the probability of the substances pH.
How to use Litmus Paper?
1. Prepare the acids and bases.
2. Drop it to litmus paper.
3. See the change of the litmus paper color.
4. If the blue litmus paper becomes red, the
substances are acid.
5. If the blue litmus paper becomes blue, the
substances are alkali.
6. If there is no change in color, the substances
are neutral.
G. “Trayek” of the acid and base color change
“Trayek” of the acid and base color change is a
range of the probability of the substances pH values based on the color change
of the substances color.
Strong : Provide the pH interval which help us to
determine the pH of the substance.
Weakness : Every indicator doesn’t have pH limitation for
each, therefore we need more than one indicators in our experiment.
Several Kinds Of Indicator That Can Be Used To
Find Know The “Trayek” Color Change.
Indicator
|
“Trayek” Of Color Change
|
Color Change
|
Litmus
|
5.5-5.8
|
Red-blue
|
Orange Methyl
|
2.9-4.0
|
Red-yellow
|
Red Methyl
|
4.2-6.3
|
Red-yellow
|
BTB
|
6.0-7.6
|
Yellow-blue
|
Phenolphthalein
|
8.3-10.00
|
No color-red
|
This is the most accurate way to measure the pH
of the substances. It uses an electrode to measure pH electrically. The pH
value would automatically show on the screen of the pH meter.
