Which of the following reactions is an oxidation-reduction reaction?
A. H₂ + Cl₂ → 2HCl
B. CaCO₃ → CaO + CO₂
C. NaOH + HCl → NaCl + H₂O
D. NaCl + AgNO₃ → AgCl + NaNO₃
Answer: Option A
Solution (By JKSSB Mock Tests)
In H₂ + Cl₂ → 2HCl, hydrogen is oxidized (0 to +1) and chlorine is reduced (0 to -1). Thus it's a redox reaction. The others: A and C are double displacement with no oxidation state change; D is decomposition with no change (Ca +2, C +4, O -2 throughout).
Explanation:
Oxygen (atomic number 8) has electronic configuration 2,6. It needs 2 more electrons to complete its octet. In water (H₂O), it shares two electrons (one each with two hydrogen atoms), forming two covalent bonds. Hence valency = 2. Oxidation number is -2.
Explanation:
CaO is an ionic compound: Ca loses 2 electrons to form Ca²⁺, O gains 2 electrons to form O²⁻. CH₄, CCl₄, NH₃ are covalent molecules where electrons are shared. Electrovalent compounds are formed between metals and non-metals with a large electronegativity difference.
Explanation:
Hydrochloric acid (HCl) is a strong acid because it completely dissociates into H⁺ and Cl⁻ ions in aqueous solution. Acetic, citric, and carbonic acids are weak acids as they only partially dissociate in water, establishing an equilibrium between the unionized molecules and their ions.
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