We've all been there, someone comes into the classroom, farts through the door, and leaves.
What happenes to the room? How comes the fart can eventually even be smelt at the back of the room? Does the fart warm or cool the farter? These are all important questions that can be dealt with using thermodynamics. Today I will deal with the first part. What happens to the room.
Lets consider the room to be our system. The fart (composed of fartrogen dixoide molecules) is a fluid. Remember a fluid is either a gas or a liquid. So the fluid enters the room and with it comes the energy it contains. Energy is contained by all material (the particles whizz around at room temperature) and this is called the internal energy, U of the material, or in this case of the fart. In addition some work has been done by the farter to PUSH the fart into the room (our system). This work is the work of gas expansion, which is just the pressure times the volume of the gas, PV.
Thus the total energy added to the room is U + PV.
It turns out that this is how we define Enthalpy H so that
H = U + PV
In engineering we use enthalpy to follow the energy when fluids enter or leave systems. If there is a vent in the room, and there hopefuly is, fluids will be leaving the room and we would have to account for this when summing the energy of the system.
It turns out that under constant pressure conditions changes in enthalpy are equal to the heat (given off or supplied to) the system, q, i.e
ΔH = Δq
where Δ signifies a measurable change in the quantity.
This is why chemists use the term enthalpy to describe the heat of reaction, they are equivalent. We can use enthalpy to describe heat only under constant pressure conditions. Conveniently when looking at reactions in your test tube, pressure is pretty constant, it is atmospheric pressure!!