Strong and Weak Bases

The citric acid in fruits like oranges and lemons, tartaric acid in tamarind, malic acid in apples, lactic acid in milk and milk products, and hydrochloric acid in gastric juices are just a few examples of acids and bases found in nature. Many bases, such as lime water, can also be found. Many of these acids are used in our daily lives, such as vinegar or acetic acid in the kitchen, boric acid in laundry, baking soda in cooking, washing soda in cleaning, and so on.

Many acids and bases that we do not use in our daily lives are utilised in laboratories and industries, such as HCl, H₂SO₄, and NaOH, KOH, among others. The neutralisation process results in the creation of salt and water when these acids and bases are mixed in the proper quantities.

Acids: Acid comes from the Latin word ‘acidus’ or ‘acere,’ which signifies sour. Their sour flavour is the most prevalent feature. An acid is a chemical that in its aqueous solution produces the ionizable hydronium ion (H₃O+). It turns blue litmus paper red.

1. Natural acids: Natural acids are derived from natural sources like fruits and animal products. Lactic, citric, and tartaric acids, for example.
2. Mineral acids: Mineral acids are acids that have been made from minerals. Hydrochloric acid (HCl), sulphuric acid (H₂SO₄), and nitric acid (HNO₃) are among the examples.

Bases: The bitter taste and soapy texture of bases are two of their most prevalent characteristics. A base is a chemical that produces the hydroxyl ion (OH^–) in water. Bases turn the colour of red litmus paper to blue.

1. Strong bases: Strong bases completely ionise in water, resulting in a huge amount of OH^– ions. Sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), and calcium hydroxide (Ca(OH)₂) are just a few examples.
2. Weak bases: Weak bases partially ionise in water, resulting in a tiny number of OH^– ions. Magnesium hydroxide (Mg(OH)₂) and ammonium hydroxide, for example (NH₄OH).

Salts: When acids and bases react to neutralise each other, salt is created as an ionic material. Salts are made up of positively charged cations and negatively charged anions, and they can be organic or inorganic in nature. The nature of the salt is neutral because these ions are present in little concentrations.

Strong Bases

A strong base is a fully ionic substance such as sodium hydroxide or potassium hydroxide. In solution, the molecule can be thought of as being completely broken up into metal ions and hydroxide ions.

Each mole of sodium hydroxide in solution dissolves to produce a mole of hydroxide ions.

Na + OH ⇢ Na + OH

Some strong bases, such as calcium hydroxide, aren’t very water-soluble. It makes no difference; whatever does dissolve is completely ionised into calcium and hydroxide ions. Because of its 100% ionisation, calcium hydroxide is nevertheless considered a strong base.

Calculating the pH of a strong base as

pH = – log10 [H⁺]

How can a solution containing hydroxide ions have a pH since pH is a measure of hydrogen ion concentration? To understand this, you must first understand the ionic product for water. Wherever there is water, a balance is established. The following is a simplified version of this equilibrium:

H₂O (l) ⇌ H⁺ (aq) + OH

With the addition of extra hydroxide ions, such as sodium hydroxide, the equilibrium still exists, but the point of equilibrium has shifted to the left, according to Le Chatelier’s Principle. Although there will be significantly fewer hydrogen ions than in pure water, hydrogen ions will still be present. The concentration of these is measured by the pH.

A description of how to calculate the pH of a strong base:

• Work out the concentration of the hydroxide ions.
• Use Kw to work out the hydrogen ion concentration.
• Convert the hydrogen ion concentration to a pH

Examples of some common strong bases are:

1. Potassium hydroxide (KOH)
2. Sodium hydroxide (NaOH)
3. Barium hydroxide (Ba(OH)₂)
4. Caesium hydroxide (CsOH)

Weak Bases

Weak bases are basic compounds that when dissolved in liquids do not entirely break down into their constituent ions. As a result, when the weak base is dissolved in a solution, some of it dissociates into hydroxide anions and the corresponding conjugate acid, while the rest stays undissociated inside the solution.

 Ionization of a weak base is a form of equilibrium process in which a chemical equilibrium between the concentration of the undissociated base and its constituent ions is achieved inside the solution (the conjugate acid and the hydroxide anion). It’s vital to remember that a weak base’s conjugate acid is usually always a weak acid. Similarly, the conjugate base of a weak acid will act as a weak base.

When a weak base is dissolved in water, the following type of equilibrium arises:

B + H₂O ⇌ BH⁺ + OH^–

A lone pair of electrons in the basic molecule absorbs a proton from the water molecule in this equilibrium process, resulting in the creation of a hydroxide ion. The weaker the base, the greater the concentration of the equilibrium to the left. Similarly, the stronger the base, the higher the equilibrium concentration to the right.

Examples of some common weak bases are:

1. Ammonia (NH₃): The ammonium cation (denoted by the chemical formula NH₄⁺) and the hydroxide anion are present in the solution formed when ammonia is dissolved in water (also known as ammonia water or ammonia solution). It’s worth noting, however, that only a small portion of the dissolved ammonia dissociates into these ions.
2. Trimethylamine (N(CH₃)₃): TMA stands for trimethylamine, which is a weak base with the molecular formula N(CH₃)₃. Ammonia and methanol can be combined in the presence of a catalyst to make this chemical. It can also be made by combining paraformaldehyde and ammonium chloride in a process.
3. Pyridine (C₅H₅N): Pyridine is a pyridine-like organic molecule with the formula C₅H₅N. This chemical compound is a weak base with a heterocyclic structure. Pyridine has a structure similar to benzene, with the exception that one of the methine groups is substituted with a single nitrogen atom. Pyridine is a colourless liquid that exists at typical temperatures and pressures. It’s also worth mentioning that pyridine is a Lewis base, which means it may give electron pairs to Lewis acids to produce Lewis adducts.

Sample Questions

Question 1: Explain why a sodium sulphate aqueous solution is neutral and a sodium carbonate aqueous solution is basic in nature.

Answer:

Aqueous sodium sulphate solution is hydrolyzed to create sodium hydroxide and sulphuric acid, both of which are strong bases and acids. As a result, the aqueous solution is pH neutral. When sodium carbonate is dissolved in water, it is partially hydrolyzed, yielding sodium hydroxide and carbonic acid. 

Sodium hydroxide is now a strong base that is fully ionised and produces a significant number of hydroxide ions [OH^– (aq)]. Carbonic acid, on the other hand, is a weak acid that is only minimally ionised and hence produces a modest number of hydrogen ions [H⁺ (aq)]. The combination is basic because it includes more hydroxide ions than hydrogen ions.

Question 2: Is sodium hydroxide a base that isn’t very strong? Is there any reason to believe that?

Answer:

A strong base is sodium hydroxide, a chemical substance having the formula NaOH. This is due to the fact that when sodium hydroxide is dissolved in water, it almost completely ionises. The basic chemicals that do not totally ionise in water are known as weak bases. An example of a weak base is ammonia. When NH₃ is dissolved in water, it interacts with the water molecules and dissociates into ammonium cation and hydroxide anions. However, some ammonia in the solution stays unionised.

Question 3: Is water a weak base?

Answer:

Pure water does, in fact, act as a weak base. In reality, pure water is a weak acid as well as a weak basic. This is because a little amount of water dissociates into protons and hydroxide anions, forming hydronium ions and hydroxide ions with the remaining water molecules.

Question 4: Why do acids like HCl, HNO₃, and others have acidic properties in aqueous solutions, but molecules like alcohol and glucose don’t?

Answer:

In aqueous solutions, ionised solutions such as HCl, HNO3, and others become acidic due to the presence of H+ ions. Because alcohol and glucose solutions do not generate any of these ions, they do not have acidic properties.

Question 5: Why does acid in an aqueous solution transmit electricity?

Answer:

When acid forms a solution in water, it becomes ionised, and electricity is carried through it as a result of the existence of these ions.

Question 6: Why not keep curd and sour substances in brass and copper vessels?

Answer:

Acids in curd and sour things react with copper vessels and brass to generate hazardous chemicals.

Question 7: Why does the colour of dry litmus paper not change when exposed to dry HCl gas?

Answer:

The acidic quality of dry HCl gas is not transmitted since it does not release H⁺ ions.