Lesson Notes By Weeks and Term v4 - SHS 3
EQUILIBRIA
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EQUILIBRIA
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Subject: Chemistry
Class: SHS 3
Term: 2nd Term
Week: 16
Grade code: 3.1.2.LI.3
Strand code: 1
Sub-strand code: 2
Content standard code: 3.1.2.CS.4
Indicator code: 3.1.2.LI.3
Theme: PHYSICAL CHEMISTRY
Subtheme: EQUILIBRIA
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This lesson explores the fascinating world of oxidation-reduction (redox) reactions, a fundamental concept within chemical equilibria. While the sub-strand is "Equilibria," a deep understanding of redox reactions is essential, as they are the basis for electrochemical cells (like batteries) which operate until they reach equilibrium (i.e., they "go flat"). In our daily lives in Ghana, we see redox reactions everywhere: the rusting of iron gates and roofing sheets, the browning of a cut yam or plantain left in the air, the way 'Parazone' bleach whitens our clothes, and the very process that powers our phones and car batteries.
A. What are Redox Reactions? A redox reaction is any chemical reaction in which the oxidation states (or numbers) of atoms are changed. It involves two simultaneous processes: Oxidation and Reduction.
A useful mnemonic is OIL RIG: Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons)
While the OIL RIG definition is helpful, the most powerful and universal way to understand redox is through Oxidation Numbers. Oxidation: An increase in the oxidation number of an element. Reduction: A decrease in the oxidation number of an element. B. Rules for Assigning Oxidation Numbers The oxidation number is the hypothetical charge an atom would have if all bonds were 100% ionic. We use the following rules to assign them: Free Elements: The oxidation number of an atom in its elemental form (e.g., Na, O₂, P₄, S₈) is always 0. Simple Ions: The oxidation number of a monatomic ion is equal to its charge (e.g., in NaCl, Na is +1; in MgS, S is -2). Fluorine: The oxidation number of fluorine in a compound is always -1. Oxygen: The oxidation number of oxygen in most compounds is -2. *Exceptions:* In peroxides (like H₂O₂, Na₂O₂), it is -1. In compounds with fluorine (like OF₂), it is +2. Hydrogen: The oxidation number of hydrogen is usually +1 when bonded to non-metals. *Exception:* In metal hydrides (like NaH, CaH₂), it is -1. Sum in a Neutral Compound: The sum of the oxidation numbers of all atoms in a neutral compound is 0. Sum in a Polyatomic Ion: The sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.