You can think of enthalpy as the total potential energy of a system given by this equation:
ΔH = ΔU + Δ(pV) (U = internal energy, p = pressure, V = volume)
Since for most experiments, pressure is constant, that equation becomes:
ΔH = ΔU + pΔV
The heat transfer of a system is given by q and it can be positive or negative. A hot cup of coffee on a cold morning is warmer than its environment, so heat will flow from the coffee to the cooler surrounding air, since heat always flows from hot to cold, so q is negative. If you have ice-cold lemonade on a hot day, heat flows from the environment to the lemonade, so q is positive. The mathematical equation for heat is:
q = ΔU − W (W = work)
When you combine the equations to find the relationship between heat and enthalpy, you find that:
ΔH = q when pressure is constant. Now let’s learn how to use this equation in chemistry to find the energy in a chemical reaction.
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