Define : (a) Standard enthalpy of combustion. (b) Enthalpy of atomization. Draw the structure of D-ribose sugar. Write the uses of bronze alloy.

Answer:

(a) Standard enthalpy of combustion (Δ_cH°): The enthalpy change when one mole of a substance in its standard state is completely burnt (oxidised) in oxygen, with all reactants and products in their standard states at the specified temperature (usually 298 K, 1 bar). E.g. CH_4(g) + 2O_2(g) → CO_2(g) + 2H₂O_(l), Δ_cH° = −890 kJ mol⁻¹.

(b) Enthalpy of atomization (Δ_atomH): The enthalpy change when all the bonds in one mole of a substance in the gaseous state are broken to form gaseous atoms (or one mole of an element is converted into gaseous atoms). E.g. H_2(g) → 2H_(g), Δ_atomH° = 435 kJ mol⁻¹.

Structure of D-ribose (open chain, Fischer projection; C₅H₁₀O₅, an aldopentose):

All three –OH groups on C-2, C-3 and C-4 are on the right. (In RNA it is present in the cyclic furanose form, β-D-ribofuranose.)

Uses of bronze (alloy of copper and tin): making statues, medals and coins; bells and utensils; ship fittings and bearings (it resists corrosion).