One word of warning: when using the van ’t Hoff equation for reactions involv- ing gases, the equilibrium constants must be K, not Kc. If we want a new value for Kc for a gas-phase reaction, we convert from Kc into K at the initial temperature (by using Eq. 12), use the van ’t Hoff equation to calculate the value of K at the new tempera- ture, and then convert that K into the new Kc by using Eq. 12 at the new temperature.