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Chemistry · Glossary

What is Reaction quantity, reaction enthalpy?

Also known as: reaction quantity · reaction enthalpy

Definition 1.1 University Chemistry — Year 2 · Chapter 1 — Enthalpies of Reaction

For an extensive state function XX of a closed system in which the reaction 0=∑iνiAi0 = \sum_i\nu_i\mathrm A_i takes place, the reaction quantity is the partial derivative

ΔrX=(∂X∂ξ)T,p,\Delta_r X = \left(\frac{\partial X}{\partial \xi}\right)_{T,p} ,

the change of XX per unit extent at fixed temperature and pressure, in units of XX per mole. For X=HX = H it is the reaction enthalpy ΔrH\Delta_r H, in kJ/mol\mathrm{kJ}/\mathrm{mol}.

Examples

Example 1.21 (Ammonia synthesis when hot)

For NX2(g)+3 HX2(g)→2 NHX3(g)\ce{N2(g) + 3H2(g) -> 2NH3(g)}, ΔrH∘(298)=−91.9 kJ/mol\Delta_r H^\circ(298) = -91.9\,\mathrm{kJ}/\mathrm{mol} and, with the heat capacities at 298 K298\,\mathrm{K}, ΔrCp∘=2(35.65)−29.12−3(28.84)=−44.3 J/(K mol)\Delta_r C_p^\circ = 2(35.65) - 29.12 - 3(28.84) = -44.3\,\mathrm{J}/(\mathrm{K}\,\mathrm{mol}). Taken as constant, it gives ΔrH∘(700)=−91.9−0.0443×402=−109.7 kJ/mol\Delta_r H^\circ(700) = -91.9 - 0.0443 \times 402 = -109.7\,\mathrm{kJ}/\mathrm{mol}. The tables, which follow the heat capacities as they grow with temperature, give −105.2 kJ/mol-105.2\,\mathrm{kJ}/\mathrm{mol}: the constant-CpC_p estimate is off by 4 %, a fair price for a one-line computation. Over 400 K400\,\mathrm{K}, the reaction enthalpy changes by about a seventh.

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