Since the second equation represents the formation of 2 moles of H 2 O(g), the
quantity is twice −241.8, or −483.6 kJ. It is assumed that the initial and final
states are measured at 25°C and 1 atmosphere, although the reaction occurs at a
higher temperature.
Problem
How much heat is liberated when 40.0 grams of H 2 (g) reacts with excess O(g)?
The reaction equation is:
This represents 1 mol or 2 g of H(g) forming 1 mol of H 2 O(g):
Since each mole gives off −241.8 kJ, then
Notice that the physical state of each participant must be given since the
phase changes involve energy changes. Combustion reactions produce a
considerable amount of energy in the form of light and heat when a substance is
combined with oxygen. The heat released by the complete combustion of 1 mole
of a substance is called the heat of combustion of that substance. Heat of
combustion is defined in terms of 1 mole of reactant, whereas the heat of
formation is defined in terms of 1 mole of product. All substances are in their
standard state. The general enthalpy notation, ΔH, applies to heats of reaction, but
the addition of a subscript c, ΔHc, specifically indicates heat of combustion.
ADDITIVITY OF REACTION HEATS AND HESS’S LAW
Chemical equations and ΔH^0 values may be manipulated algebraically. Finding
the ΔH for the formation of vapor from liquid water shows how this can be done.
Since we want the equation for H 2 O(l) → H 2 O(g), we can reverse thesecond equation. This changes the sign of ΔH.