Suppose that the gas-phase reaction 2NO(g) + O2(g) --> 2 NO2 were carried out in a constant- volume container at constant temperature. Would the measured heat change represent DE or DH? If there is a difference, which quantity is larger for this reaction? Explain.
If we work at constant volume then no P-V work can be done, and any measured heat change will correpsond to a change in internal energy (DE). In general, the difference between DE and DH is the amount of P-V work done by the system when the process occurs at constant pressure (recall that for P-V work, w = -PDV). Recall the definition of enthalpy: H = E + PV. For a reaction, then, we'd have (at constant pressure) DH = DE + PDV. Look at this equation: DH must be larger than DE by the factor PDV for the equality to hold!
Doug Chapman email@example.com 7/9/08