Can someone please explain how and why Delta H can equal q at constant pressure? The forumlas are as follow: #1 U=q+w #2 Delta H=Delta U+ p(Delta V) If in #2 you hold pressure constant, wouldn't H=U+pV? But on page 56 of EK Chemistry, they say "For reactions involving no change in pressure, the change in enthalpy is equal to the heat" I understand that if the volume is held constant, then there is no PV work done, so U=q (Forumla 1 above). But it doesn't make sense that if pressure is held constant then H=q. How is this possible? A brief explanation would be greatly appreciated! God, I hate thermodynamics!