Extra Notes on Thermochemistry

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)

Exothermic rxns:

• 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

Endothermic rxns:

• 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 gsibert@rvgs.k12.va.us Back to Notes Menu