ACIDS AND BASES
Bases
▪ Bases are ionic compounds
▪ contain a metal and a polyatomic ion (acts like
a non-metal).
▪ Bases, like ionic compounds, undergo a
process of dissociation where there is a
separation of the ions in the compound
when it is dissolved in water.
KOH (s)
K+ (aq) + OH- (aq)
Molecular Bases
▪ Molecular compounds that have an unbound
pair of electrons
▪ Tend to be weak bases
▪ The unbound electron pair can accept (bind
with) a hydrogen ion (H+)
▪ Classic example is ammonia (NH3)
▪ The structure of ammonia has an unbound
electron pair
3
H+
Where does the H+ come from?
- A water molecule
NH3(aq) + H2O(l) → NH4+(aq) + OH-(aq)
Produces hydroxide ions = basic solution
4
Acids
▪ Acids differ from bases, as they are
molecular compounds.
▪ Acids undergo a process of ionization in
which a neutral atom or molecule is
converted into an ion when placed in
water.
▪ The ions then dissociate from one
another.
HBr(g)
H+ (aq) + Br- (aq)
Strong Acids and Weak Acids
▪ Acids can be considered either strong or weak
▪ Strong acids completely ionize to give off all of
the available H+ ions
▪ Strong acids have close to 100% percent
ionization, which is the % of molecules that will
form ions in solution
▪ Weak acids do not ionize well when placed in
water, and have less than 50% ionization
▪ Thus, when a weak acid is placed in water, the
form of hydrogen exists more in the form of the
molecular acid and not as H+ (aq)
▪ Strong acids have high electric conductivity while
weak acids have low electric conductivity
The pH Scale
▪ How can you tell how acidic or basic a substance
is?
▪ The answer is through the use of the pH scale
The pH scale works on the basis of a logarithmic scale
▪ A logarithmic scale means that for every change of
one unit on the scale represents a ten-fold
increase/decrease in the concentration of H+ ions
▪ Even basic, or alkaline, solutions contain H+ ions and
acidic solutions contain OH- ions
▪ Acidic solutions have more H+ ions than OH- ions.
▪ Basic/alkaline solutions have less H+ ions than OH- ions
▪ Neutral solutions have equal numbers of H+ ions and
OH- ions
▪ A solution with a pH of 3 is 10 X more acidic than a
solution with a pH 4 and 100 X less acidic than a
solution with pH 1
▪ Using the equation pH = -log10[H+] one can
determine the pH of a solution with a known
H+ concentration
▪ Note: [H+] is the concentration of H+ ions as
brackets indicates concentration (mol/L).
▪ Neutral solution, [H+] = 1 x 10-7 mol/L, pH = 7
▪ Acidic solution, [H+] > 1 X 10-7 mol/L, pH < 7
▪ Basic solution, [H+] < 1 X 10-7 mol/L, pH > 7
▪ To find the hydrogen ion concentration
[H+] = 10-pH
• For bases pOH = -log10[OH-]
[OH-] = 10-pOH
▪ Another useful equation
pH + pOH = 14
Arrhenius Acid-Base
Theory
▪ The acidic nature of acidic solutions was
due to the presence of H+ ions in solution
▪ Any compound that could donate a H+
ion to a solutions was a possible acid
▪ The basic nature of basic solutions was
due to the presence of OH- ions
▪ Any compound that could donate a OHion to a solutions was a possible base
▪ Arrhenius Acids
▪ Hydrochloric acid - HCl (aq)
▪ Sulphuric acid
- H2SO4 (aq)
▪ Arrhenius Bases
▪ Sodium hydroxide - NaOH (aq)
▪ Calcium hydroxide - Ca(OH)2 (aq)
Problems with Arrhenius Theory
• Arrhenius did not explain the basic properties of “basic
salts”
• A solution of sodium carbonate is basic
• Formula Na2CO3 does not contain any OH- groups
so how could it produce a basic solution?
• Na2CO3 (s)
2 Na1+(aq) + CO32-(aq)
• CO32-(aq) + H2O(l)
HCO31-(aq) + OH- (aq)
• The carbonate ion reacts with water to produce the
OH- ions (this is called hydrolysis)
Problems with Arrhenius Theory
• The solvent problem
•
•
•
•
• For a base to dissociate, and an acid to ionize then
dissociate, it must be in the proper solvent
Example: HCl(g) water H+ (aq) + Cl- (aq) acidic
hexane
HCl(g)
neutral
Water is polar and is a good environment for ions so
ionization and dissociation can take place
Hexane is non-polar and is NOT a good environment
for ions so ionization and dissociation cannot take
place
Bronsted-Lowry Theory of Acids
and Bases
▪ An acid is any molecule or ions that can give up a
▪
▪
▪
▪
hydrogen ion.
A base is any molecule or ion that can accept a
hydrogen ion.
Therefore;
An acid is a hydrogen-ion donor
▪ Any molecule or ion that contains hydrogen ions
is a potential acid
A base is a hydrogen-ion acceptor
▪ Any molecule or ion that contains a lone pair of
electrons available for bonding with a hydrogen
ion is a potential base
Another way to look at acid-base reactions
• They are hydrogen-ion exchange reactions or
proton transfer reactions
• An acid will only behave as an acid only in
the presence of a base willing and able to
accept the hydrogen ion
Bronsted-Lowry Acids
• When HCl (g) is added to water, an acid-base
reaction occurs between the HCl (g) and
water molecules
• Covalent bonds in the HCl (g) break and
the free H+ ion binds to the oxygen in the
H2O molecule
• Formation of HYDRONIUM ION (H3O+)
Formation of ions
▪ Process called IONIZATION
▪ HCl donates a hydrogen ion
▪ (Bronsted-Lowry Acid)
▪ H2O accepts a hydrogen ion
▪ (Bronsted-Lowry base)
Bronsted-Lowry Bases
▪ Common Bronsted-Lowry bases are
soluble metal hydroxides
▪ NaOH, KOH etc.
▪ Solid ionic compounds
▪ When solid mixed with water
▪ Ions dissociate
▪ OH- able to accept H+ (Bronsted-Lowry base)
Common base ammonia (NH3) does not meet the
requirement of an Arrhenius base (no OH-)
▪ Must use Bronsted-Lowry theory
▪ NH3 has lone pair of electrons that are able to
accept a H+ ion from H2O
▪ This will create a solution that has free OHions
▪ In this case the ammonia was acting as a base
and the water was acting as an acid
NH3(aq) + H2O(l)
NH4+(aq) + OH-(aq)
Conjugate Acid-Base Pairs
▪ In the Bronsted-Lowry Theory every acid
has a corresponding base and every base
has a corresponding acid
▪ These are called a conjugate acid base
pair
▪ They differ in formula by one H+ ion
HCl(g) + H2O(l)
Acid
base
H3O+(aq)
conjugate
acid
+
Cl-(aq)
conjugate
base
NH3(g)
Base
+
H2O(l)
acid
OH-(aq)
conjugate
base
+
NH4+(aq)
conjugate
acid
In the preceding examples, water acts as a base and as an acid.
Substances that can act as a base or as an acid, depending on
the circumstances, are called AMPHOTERIC substances.