Enthalpy: heat content of a system
DH is the change in enthalpy
q is the heat absorbed or given off
DH is equal in magnitude to the heat flow (q) for a reaction carried out directly at constant pressure. A calorimeter is usually used to determine DH of a reaction, so it is actually q for the water that is obtained, which is -DH for the reaction.
q = (m)(S.H.)(DT)
m = mass of the substance
S.H. = specific heat of that substance
DT = Tfinal - Tinitial (temperature can be in degrees Celcius or Kelvin because the size of each degree is the same)
- enthalpy of the reaction system decreases
- enthalpy of the products is < that of the reactants
- effect is to increase the temperature of the surroundings
DH = SDHf(products) - SDHf(reactants) < 0
- enthalpy of the reaction system increases
- enthalpy of the products is > that of the reactants
- effect is to decrease the temperature of the surroundings
DH = SDHf(products) - SDHf(reactants) > 0
Laws of Thermochemistry:
1. The magnitude of DH is directly proportional to the amount of reactant or product.
(the heat absorbed when a solid melts ( solid to liquid ) is referred to as the heat of fusion)
(the heat absorbed when a liquid vaporizes ( liquid to gas ) is called as the heat of vaporization)
2. DH for a reaction is equal in magnitude but opposite in sign to DH for the reverse reaction.
(in other words... the amount of energy given off in a reaction is exactly equal to the amount of heat absorbed in the reverse reaction.)
3. the value of DH for a reaction is the same whether it occurs directly or in a series of steps.
in other words...Equation 3 = Equation 1 + Equation 2 + ...
therefore: DH3 = DH1 + DH2 + ...
This is called Hess's Law
Send questions, comments or suggestions to
Gwen Sibert, at the
Roanoke Valley Governor's School
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