3.16: Enthalpy

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Enthalpy

Heat changes in chemical reactions are often measured in the laboratory under conditions in which the reacting system is open to the atmosphere. In that case, the system is at a constant pressure so the energy change is the enthalpy of the reaction, Enthalpy (ΔH). The change in enthalpy of a reaction is a measure of the differences in enthalpy of the reactants and products.

Several factors influence the enthalpy of a reaction. The state of reactants and products (solid, liquid, or gas) influences the enthalpy value for a system. The direction of the reaction affects the enthalpy value. A reaction that takes place in the opposite direction has the same numerical enthalpy value, but the opposite sign. Enthalpy is an extensive property, which means it depends on the quantity of substances involved in the reaction.

Example: The combustion of hydrocarbons are exothermic reactions. For example, for every one mole of methane gas that burns, 890.3 kJ of energy are released. The energy in this case, can be treated as a product in the reaction since it is released from the system.

CH₄(g) + 2 O₂(g) -> CO₂(g) + 2H₂O(l) + 890.3 kJ

The enthalpy of the reaction can also be written separately from the equation for the reaction:

CH₄(g) + 2 O₂(g) -> CO₂(g) + 2H₂O(l) ΔH = -890.3 kJ

The negative sign on the enthalpy indicates the reaction is exothermic.

If 5.00 mol of methane are combusted, then five times the amount of energy will be released:

5.00 mol CH₄ x -890.4kJ/1 mol CH₄ = -4450 kJ

On a fundamental level, chemical reactions involve breaking the bonds of the reactants, rearranging the atoms and forming the bonds of the products. As a result, the enthalpy of a reaction can be estimated using average bond energies. Energy is required to break bonds and energy is released when bonds form. The quantity of the energy involved in breaking and forming a particular bond is the same, but the sign is different to indicate whether the process is exothermic or endothermic. By convention, when energy is released by a system, the sign is negative and when energy is added to a system the sign is positive. The enthalpy of reaction can be estimated by comparing the energy needed to break all of the bonds of the reactants to the energy released when all of the bonds in the products form. If more energy is released when bonds form than was needed to break the bonds of the reactants, the overall reaction is exothermic. If more energy is needed to break bonds than is released when the bonds form, then the reaction is endothermic.

Table 1: Average Bond Energies (kJ/mol) (Bond Energies)
Single Bonds						Multiple Bonds
H—H	432	N—H	391	I—I	149	C = C	614
H—F	565	N—N	160	I—Cl	208	C ≡ C	839
H—Cl	427	N—F	272	I—Br	175	O = O	495
H—Br	363	N—Cl	200			C = O*	745
H—I	295	N—Br	243	S—H	347	C ≡ O	1072
		N—O	201	S—F	327	N = O	607
C—H	413	O—H	467	S—Cl	253	N = N	418
C—C	347	O—O	146	S—Br	218	N ≡ N	941
C—N	305	O—F	190	S—S	266	C ≡ N	891
C—O	358	O—Cl	203			C = N	615
C—F	485	O—I	234	Si—Si	340
C—Cl	339			Si—H	393
C—Br	276	F—F	154	Si—C	360
C—I	240	F—Cl	253	Si—O	452
C—S	259	F—Br	237
		Cl—Cl	239
		Cl—Br	218
		Br—Br	193
*C == O(CO₂) = 799

Summary

Enthalpy changes are a measure of the energy changes in chemical reactions. The energy involved in a particular reaction depends on the quantity of the substances involved. Enthalpy changes for reactions can be estimated using average bond energies.

Enthalpy

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