The first law of thermodynamics is again noted.
→ Considering the minute changes, dQ = dU + PdV （１）
The enthalpy (H) is defined as H = U + PV.
The enthalpy of a substance means the total energy it possesses.
【Enthalpy (total energy)】＝【Internal energy (total kinetic energy of molecules)】
＋【Energy (pressure x volume) that can be extracted by work to the outside world】
Consider a small change in the enthalpy of the entire system.
A minute change in enthalpy is expressed as follows
dH = d(U + PV) = dU + PdV + VdP
Since chemical experiments are performed at constant total pressure (atmospheric pressure), dP = 0, i.e.,
dH = dU + PdV (2).
Thus, from (1) and (2),
Therefore, the enthalpy change of the entire system before and after the chemical reaction (∆H) is equal to the heat in and out (∆Q) from the chemical reaction.
When describing heat transfer, is the system losing heat and giving a "minus sign" or is the outside world receiving heat from the system and giving a "plus sign"? These are the same phenomena, but it is easy to get confused about whether to put a positive or negative value on them. When describing the first law of thermodynamics (ΔQ = ΔU + PΔV), +ΔQ is the case where the system receives heat.
