Chemical Reactions and Equations – Solved Board Questions & Equation Balancing
Solved CBSE & ICSE board examination questions on chemical reactions, equation balancing, precipitation, thermal decomposition, and redox reactions.
Chemical Reactions – Solved Board Questions & Marking Scheme
A shiny brown-coloured element 'X' on heating in air becomes black in colour.
(a) Name the element 'X' and the black-coloured compound formed.
(b) Write the balanced chemical equation for the reaction.
(c) How can the black coating be turned back into the shiny brown metal?
(a) Identification:
- Element 'X' is Copper (Cu). [0.5 Mark]
- The black compound is Copper(II) oxide (CuO). [0.5 Mark]
(b) Balanced Chemical Equation:
2Cu (s) [Brown] + O₂ (g) ──[Heat]──> 2CuO (s) [Black]
(c) Reversal of Reaction:
Passing hydrogen gas (H₂) over the heated black Copper(II) oxide causes reduction, converting it back to brown copper metal: [1 Mark]
CuO (s) [Black] + H₂ (g) ──[Heat]──> Cu (s) [Brown] + H₂O (g)
Why do we apply paint on iron articles? Explain the chemical principle involved in prevention of rusting.
Answer: Paint forms an impermeable protective physical barrier between the iron surface and environmental moisture (H₂O) and atmospheric oxygen (O₂). In the absence of both moisture and oxygen simultaneously, the oxidation reaction 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O cannot proceed, preventing rust formation and structural weakening.
Indexed Topics & Examination Keywords
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