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What are the necessary conditions for any system to be aromatic?
List the structural and electron-count requirements together.
An aromatic system must be cyclic, planar, fully conjugated with a p orbital at every ring atom, and contain (4n+2) pi electrons.
Show important resonance contributors and electron movement for phenol, nitrobenzene, CH₃CH=CHCHO, benzaldehyde, benzyl cation and allyl cation.
Move only pi electrons or lone pairs with curved arrows; keep atom positions fixed and preserve total charge.
Whenever a reaction between an oxidising agent and a reducing agent is carried out, a compound of lower oxidation state is formed if the reducing agent is in excess and a compound of higher oxidation state is formed if the oxidising agent is in excess. Justify this statement giving three illustrations.
Use the same element with accessible adjacent oxidation states. Limiting reagent controls whether oxidation stops at a lower state or continues to a higher state.
A sample of HI(g) is placed in flask at a pressure of 0.2 atm. At equilibrium the partial pressure of HI(g) is 0.04 atm. Determine Kp for the given equilibrium ? 2HI (g) H2 (g) + I2 (g)
The fall in HI pressure is 0.16 atm=2x, so each product pressure is 0.08 atm.
Kp=4.00.
Carbon combustion releases 393.5 kJ per mole of CO₂. Find heat released when \(35.2\,\mathrm{g}\) CO₂ forms.
Convert carbon dioxide mass to amount and multiply by molar combustion enthalpy.
314.8 kJ is released (ΔH=−314.8 kJ).
Explain resonance in the carbonate ion.
No single Lewis contributor accounts for three identical C–O bonds. Three contributors differ only in electron placement; the real ion is their lower-energy hybrid with delocalised π electrons.
All C–O bonds are equivalent with average bond order \(\frac{4}{3}\).
Define isoelectronic species and give one species isoelectronic with each of F⁻, Ar, Mg²⁺ and Rb⁺.
Isoelectronic species contain the same number of electrons. F⁻ and Mg²⁺ each have 10 electrons; Ar has 18; Rb⁺ has 36.
Examples: F⁻–Ne, Ar–K⁺, Mg²⁺–Na⁺, Rb⁺–Kr.
A 25 W bulb emits monochromatic light of wavelength 0.57 μm. Calculate the photons emitted each second.
Power is energy per second. Divide 25 J s⁻¹ by the energy of one photon.
A solution is made by dissolving \(20.0\,\mathrm{g}\) sucrose, C₁₂H₂₂O₁₁, and making the final volume 2.00 L. Find its molarity.
Find the sucrose amount using its molar mass and divide by the final solution volume.
Glycerine has volume-expansion coefficient 49×10⁻⁵ K⁻¹. Estimate its fractional density change for a 30°C rise.
Principle and reasoning. Work from the governing physical relation and apply it to every stated part.
Mass remains constant, so density varies inversely with volume. For a small expansion, Δρ/ρ≈−ΔV/V.