[Chemistry Class Notes] on Weak Bases Pdf for Exam

Bases are defined as substances that dissolve in water to produce OH- ions and are molecules or ions which are able to accept a hydrogen ion from an acid. A base is any substance that has a bitter taste, feels slippery to the touch, and causes colour changes of red litmus paper to blue. Now, what is a weak base, and what is a strong base? Weak bases can be defined as substances that do not dissociate completely into their constituent ions when dissolved in water solution. Their solutions are bad conductors of electricity. They are considered weak electrolytes. 

On the other hand, strong bases completely dissociate in an aqueous solution. There are several uses of the base in our day-to-day life. Lubricating grease, Alkaline batteries, baking soda, Even the soap we use in our daily life is also a base. Pure water acts as a weak acid and a weak base; fruits and vegetables that we eat are also basic in nature Eg: Kiwi, Watermelon, etc. The general properties of acids and bases have been known to people for more than a thousand years, but the definitions of acid and base have changed dramatically as scientists have learned more about them. If a base is dissolved in water, then it is termed Alkali. 

Classification of Bases

Bases can be broadly classified on the basis of their strength, concentration, and acidity.

On the basis of Concentration

An aqueous solution that has a relatively high percentage of the base present in the solution is termed a concentrated base. E.g., Concentrated sodium hydroxide, concentrated ammonium hydroxide, concentrated potassium hydroxide, etc.

An aqueous solution that has a relatively low percentage of the base present in the solution is a dilute base. E.g., dilute sodium hydroxide, dilute ammonium hydroxide, dilute potassium hydroxide, etc.

On the basis of Strength

That dissolves completely in water. E.g., KOH, NaOH, etc.

That doesn’t dissolve completely in water. E.g., NH₃, Zn(OH)₂

On the basis of Acidity of Bases

One hydroxyl ion and combines with only one hydrogen ion. E.g., Sodium hydroxide, Ammonium hydroxide, etc.

Two hydroxyl ions combine with two hydrogen ions. E.g., Calcium hydroxide, copper (II) hydroxide, etc.

Three hydroxyl ions combine with three hydrogen ions. E.g., Aluminium hydroxide, ferric hydroxide, etc.

In the topic, we have discussed the weak base definition, classification of the base. Now let’s discuss some weak base examples:

• Ammonia (NH₃)

• Aluminium hydroxide( Al(OH)₃)

• Lead hydroxide (Pb(OH)₂)

• Ferric hydroxide (Fe(OH)₃)

• Copper hydroxide (Cu(OH)₂)

• Zinc hydroxide (Zn(OH)₂)

• Trimethylamine (N(CH₃)₃)

• Methylamine (CH₃NH₂)

• Aniline (C₆H₅NH₂)

• Pyridine( C₅H₅N)

Ammonia

Ammonia is an inorganic compound composed of a single nitrogen atom covalently bonded with three hydrogen atoms; Its chemical formula is NH₃. It is a weak base with a pungent smell that exists as a colourless gas under standard conditions of temperature and pressure. This gas occurs naturally in the environment like air, soil, water, and in plants and animals.

• The molar mass of this base is 17.031 g/mol

• The boiling point of this base is -33.34 ℃

• The density of ammonia is 0.73 kg/m³

• The melting point of this base is -77.73 ℃

Aluminum Hydroxide

Aluminum hydroxide is an antacid with the chemical formula Al(OH)₃. It is used to treat heartburn, sour stomach, or acid indigestion. It is generally used to reduce the phosphate levels in people having certain kidney conditions.

• The molar mass of this base is 78 g/mol

• The boiling point of this base is -33.34 ℃

• The density of ammonia is 2.42 kg/m³

• The melting point of this base is 300 ℃

Lead Hydroxide

It is a hydroxide of lead with the chemical formula Pb(OH)₂; In lead oxide, lead is in the oxidation state of +2. 

• The molar mass of this base is 241.21 g/mol

• The density of lead hydroxide is 7.41 g/cm³

• The melting point of this base is 135℃

Ferric Hydroxide

It is a chemical compound made of iron, hydrogen, and oxygen with a chemical formula Fe(OH)₃.

The colour of iron III hydroxides varies from dark-brown to black. It is used in a few cosmetics, also used in aquarium water treatment as a phosphate binder.

• The molar mass of this base is 106.867 g/mol

• The density of lead hydroxide is 4.25 g/cm³

• The melting point of this base is 135 ℃

Copper Hydroxide

Copper hydroxide is also known as cupric hydroxide having the chemical formula as Cu(OH)₂, is a pale blue precipitate produced when sodium or potassium hydroxide is added in excess to a solution of a copper salt. A mixture of copper hydroxide and copper sulfate solution is use
d as insecticides or pesticides.

• The molar mass of this base is 97.561 g/mol

• The density of ammonia is 3.37 g/cm³

• The melting point of this base is 80℃

Zinc Hydroxide

It is an inorganic chemical compound with the formula Zn(OH)₂. It is an amphoteric hydroxide and is soluble in water. It is an insoluble hydroxide that reacts with strong acid and gets dissolved and is odourless. It is used for medicinal purposes as an adsorbing agent and is also used in the commercial production of pesticides and pigments.

• The molar mass of this base is 99.424 g/mol

• The boiling point of this base is -33.34℃

• The density of ammonia is 3.05 g/cm³

• The melting point of this base is 125℃

Trimethylamine 

It is a weak base with the chemical formula N(CH₃)₃. The compound can be prepared directly by the reaction of ammonia with methanol in the presence of a catalyst. This base can be produced by the reaction of paraformaldehyde and ammonium chloride.

• The molar mass of this base is 59.11 g/mol

• The boiling point of this base is 2.9 ℃

• The density of ammonia is 670 kg/m³

Aniline

Anilines are organic compounds with the chemical formula C₆H₅NH₂. It is used in the rubber industry for the processing of rubber chemicals and products such as car tires, balloons, gloves. It is a weak base, and when reacted with strong acids, it forms an anilinium ion. It is used to make agricultural chemicals, synthetic dyes, antioxidants, herbicides, varnishes, explosives and is also used in pesticides and fungicides in the agricultural industry. 

• The molar mass of this base is 59.11 g/mol

• The density of ammonia is 1.02 g/cm³

• The melting point of this base is -6℃

Pyridine

It is an organic compound with the chemical formula C5H5N. The compound has a heterocyclic structure and is a weak base. The structure of pyridine is similar to that of benzene, but one of the methine groups is replaced with a single nitrogen atom. Pyridine occurs as a colourless liquid under normal temperature and pressure; it acts as a Lewis base and has the ability to donate electron pairs to Lewis acids.

• The molar mass of this base is 79.1 g/mol

• The density of ammonia is 982 kg/m³

• The boiling point of this base is 115.4 ℃

Lithium Hydroxide

It is an inorganic compound with the chemical formula LiOH. This base is soluble in water and is slightly soluble in ethanol. It is commercially available in an anhydrous form which is the weakest base.

• The molar mass of this base is 23.95 g/mol

• The heat capacity of Lithium Hydroxide is 2.071 J/g⋅K

• The boiling point of this base is 924 ℃

These are some weak bases lists, Now let’s discuss some common uses of Bases.

Uses of Some Common Bases

Magnesium Hydroxide

It is used as an antacid. It helps to correct the excess amount of acidity in the stomach.

Calcium Hydroxide

• It is an important ingredient in whitewash and mortar.

• It is also used in the preparation of dry mixes for painting and decorating.

• Used to neutralize the acidity in soils.

• Used to make bleaching powder.

Sodium Hydroxide

• It is occasionally used to unblock drains in the home.

• It is the key ingredient in the manufacture of soaps and detergents.

• It is also used in the manufacture of paper, textiles, and detergents.

• It is used in the refining of petroleum.

Ammonium Hydroxide

• It is used to make fertilizers, rayon, plastics, and dyes.

• It is used as a reagent in a chemical laboratory.

Ionization of Weak Bases

The reaction symbol for a strong base ionizing in water is represented by an arrow facing from left to right. On the other hand, the reaction arrow for a weak base ionizing in water is a double arrow, indicating both the forward and reverse reactions occur at equilibrium. At equilibrium conditions, the weak base, its conjugate acid, and the hydroxide ion all are present in the aqueous solution. 

We can calculate the equilibrium constant K[_{b}] for a weak base.

Let’s understand this concept by taking an example of a weak base MOH and studying its ionization in water:

MOH(aq)[rightleftharpoons] M[^{+}](aq)+OH[^{-}](aq)

If the initial concentration of MOH is c mole/lit and the degree of dissociation is α, then at equilibrium, MOH is left with (c – cα) moles/lit and M+ and OH^– each has (cα) moles/lit.

So the K[_{b}] for the above equation will be:

K[_{b}] = [frac{left [ M^{+} right ]left [ OH^{-} right ]}{MOH}]

Or, K[_{b}] = [frac{Calpha ^{2}}{1-alpha }]

The above square brackets indicate the concentration of respective components. The greater the concentration of the equilibrium towards the left side, the weaker the base is. Similarly, the greater the equilibrium concentration towards the right side, the stronger the base is. Based on the above equation, Kb is directly p
roportional to the OH– concentration, which means bases that have higher Kb values are stronger bases as compared to the base with lower Kb values.

Key Points

• A weak base does not fully ionize or accept hydrogen ions in an aqueous solution because it can accept hydrogen ions (H⁺) or, more broadly, contribute a pair of valence electrons.

• Strong bases have a less dramatic effect on pH than weak bases.

• When measuring the relative H⁺/hydroxide concentration in a solution, pOH is sometimes used instead of pH.

• The base dissociation constant, K[_{b}], is related to the acid dissociation constant in that it mathematically quantifies the relative strength of the base. Weaker bases have lower K[_{b}] values.

• Weak bases, like weak acids, can be used to create buffer solutions.