← Bonding, structure, properties and energy changes

Converting between mass and energy released

A calculation type in every recent paper: given ΔrH per mole and either a mass or an energy amount, find the other one — always via moles as the bridge.

The three-step bridge: mass ↔ moles ↔ energy

  • Mass to moles: n = m/M (using the molar mass of the specific substance in the equation).
  • Moles to energy: energy = n × ΔrH (per mole).
  • To go the OTHER direction (given energy, find mass): rearrange — n = energy/ΔrH, then m = n × M.
  • Always check which substance in the equation the question is actually asking about — if the equation has different mole ratios for different substances, scale accordingly before converting.
Worked example Finding a mass from a target energy release Ethanol combustion releases ΔrH = −1370 kJ mol⁻¹ (M(C₂H₅OH) = 46.0 g mol⁻¹). Find the mass of ethanol needed to release 17 500 kJ. Rearrange to find moles needed: n = energy / ΔrH = 17500 / 1370 = 12.77 mol (using the sizes, since ΔrH is negative but energy released is positive). Convert moles to mass: m = n × M = 12.77 × 46.0 = 587 g. ✓ m ≈ 587 g of ethanol. Moles are always the bridge between a mass and an energy amount — there is no direct formula connecting them without going through n. ✕

Comparing two energy releases per kilogram requires a genuine per-kg conversion, not just per-mole

A question comparing "which fuel releases more energy per kilogram" cannot be answered directly from ΔrH per mole, because different fuels have different molar masses. Convert EACH fuel's ΔrH to an energy-per-gram (or per-kg) basis using its own molar mass before comparing — comparing the raw ΔrH values per mole compares unequal masses of each substance, which is not a fair comparison. ℹ

Equal-magnitude comparisons across two substances

A recurring calculation type gives you a mass and ΔrH for ONE substance, asks you to find the energy that releases, then asks for the MASS of a DIFFERENT substance that would release that same energy using ITS OWN ΔrH and molar mass. Treat it as two separate mass-to-energy bridges chained together: first substance mass → energy, then that same energy → second substance mass, via its own M and ΔrH.

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