Reversible Reactions | Chemistry for Grade 11 (IGCSE) PDF Download

Reversible reactions 

In reversible reactions, some key points to remember are:

• Completion of Reactions: Some reactions go to completion, where reactants are entirely used up to produce product molecules, and the reaction comes to a halt when all reactants are exhausted.
• Reversible Nature: Reversible reactions involve product molecules that can either react with each other or break down to reform the original reactant molecules.
• Bi-Directional Process: These reactions exhibit a dual nature, allowing them to proceed in two ways: the forward reaction, which results in the formation of products, and the reverse reaction, which leads to the regeneration of reactants.

Chemical Equations for Reversible Reactions

• Chemical equations for reversible reactions involve using two arrows to show forward and reverse reactions.
• Each arrow is depicted with half an arrowhead - one pointing right (→) for the forward reaction and the other pointing left (←) for the reverse reaction.

Example

• An illustration of a reversible reaction is the Haber process, where ammonia is produced from hydrogen and nitrogen.
  N₂ + 3H₂ ⇌ 2NH₃

Hydrated and Anhydrous Salts

• Hydrated salts are salts that contain water of crystallization, influencing their molecular shape and color.
• Water of crystallization is the water that is stoichiometrically included in the structure of certain salts during the crystallization process.
• An illustration of this is seen in copper(II) sulfate, which crystallizes to form copper(II) sulfate pentahydrate, CuSO₄ · 5H₂O.
• Water of crystallization is denoted by a dot positioned between the salt molecule and the surrounding water molecules.
• Anhydrous salts are those that have undergone dehydration, usually through heating, resulting in the loss of their water of crystallization.

Dehydration of Hydrated Cobalt(II) Chloride

hydrated cobalt(II) chloride ⇌ anhydrous cobalt(II) chloride + water

Hydration of Cobalt(II) Chloride 

• Anhydrous blue cobalt(II) chloride crystals change to pink when added to water, a reversible reaction.
• Heating these pink crystals in a test tube causes them to revert to their original blue color as they lose water of crystallization.
• The pink crystals represent hydrated cobalt(II) chloride, containing water molecules within their structure.
• Heating hydrated cobalt(II) chloride causes it to lose its water of crystallization and transform into anhydrous cobalt(II) chloride.
• The chemical equation for this reaction is: CoCl₂.6H₂O (s) ⇌ CoCl₂ (s) + 6H₂O (l).