Thermochemistry

Reaction. Type an equation with or without phases (CH4(g) + O2(g) → CO2(g) + H2O(l)); it is balanced with the chemistry tool and each species is looked up in the built-in table of about 130 substances and aqueous ions. Without a phase, the standard state at 25 °C is used (water is liquid, carbon is graphite). Write multiply charged ions as Ca2+(aq) or SO4^2-(aq). Any value can be edited, and species that are missing can be typed in.

Hess’s law. Enter the target reaction and the known steps with their ΔH. The tool finds the multipliers (reversals and fractions included) by least squares and says when the steps cannot make the target. Species must be written the same way in every line, phase included.

K and van 't Hoff. Convert between ΔG° and K at any temperature; predict K at a new temperature from ΔH°, or find ΔH° from two measured constants: ln(K₂/K₁) = −ΔH°/R·(1/T₂ − 1/T₁).

Calorimetry. q = m·c·ΔT with a table of specific heats; coffee-cup ΔH per mole (ΔH = −q/n, add the calorimeter constant if you know it); and the final temperature of several bodies exchanging heat.

Bond enthalpies. ΔH ≈ Σ bonds broken − Σ bonds formed, from mean bond enthalpies. Useful for estimates and for reactions with no formation data; expect errors of 5–10 %, because the values are averages and assume gases.

Data. Standard values at 298.15 K and 1 bar from the CRC Handbook of Chemistry and Physics and the NBS tables of chemical thermodynamic properties; ions use the convention that H⁺(aq) is zero. A few ΔfG° values (shown grey) are derived from ΔfH° and S° using the elements' standard entropies. Different editions of the tables differ by up to about 1 kJ/mol.