[lukeli88]

Hi! I have a question about enthalpy. What is the difference between enthalpy and energy? Since delta(H)=delta(E)+P*delta(V) @ constant P, delta(E)=q+w, and w=P*delta(V)===>isn't delta(H)=q+P*delta(V)+P*delta(V)=q+2*P*delta(V) Why is work counted twice in enthalpy? Do you have to have constant P AND V for delta(H) to just equal q? Also, what is the difference between enthalpy and entropy; is delta(H)/T=delta(S)? If that is true, then they are almost the same thing, just separated by a constant! Thanks!!!

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H=enthalpy
S=entropy
T=temperature
w=work
q=heat
V=volume
P=pressure
E=energy
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[Yggdrasil]

w = -PΔV, so at constant pressure ΔH = q.  This is what makes enthalpy a useful quantity.  Since most of our reactions are done in open beakers at constant pressure, simply measuring the heat released by the reaction allows us to measure the change in a thermodynamic state function. 

However, for conditions that are not under constant pressure, the above relation that ΔH = q does not hold and other thermodynamic potentials become useful (for example, internal energy is useful for reactions at constant volume).

ΔS = q/T only for reversible processes at constant temperature.  If you have an irrevesible process or a process that occurs at a non-constant temperature, the relationship between ΔS and q is much more complicated.  The relationship between ΔS and other thermodynamic potentials is also somewhat complicated.  Fundamentally, energy and entropy often represent two completely different ideas.  Energy represents the stability of the system.  Because molecules seek the lowest potential energy, molecules with low energy will be more stable than molecules with high energy.  Entropy represents how ordered the system is.  Since there are many more ways for a system to be disordered than ordered, systems will tend to move toward disorder.  Often, there is a competition between a low energy, ordered state and a high energy, disordered state which is where things get interesting and you need to consider the concept of free energy.