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As we learned earlier in the course, enthalpy is the heat transferred at constant pressure. We’ve encountered enthalpy several times throughout the course, and we discussed it thoroughly in the context of physical processes such as phase transitions. Chemical processes also have enthalpy changes associated with them. The enthalpy change associated with a chemical reaction is known as the heat of reaction (ΔHrxn). Like other values of ΔH that we have encountered, ΔHrxn can be positive or negative (endothermic or exothermic). In an endothermic reaction the bonds that are broken during the reaction are stronger on average than the bonds that are formed. Conversely, in an exothermic reaction the bonds that are broken during the reaction are weaker on average than the bonds that are formed. As we have seen repeatedly in this course, the “Δ” in front of ΔH signifies a difference of final minus initial: Hfinal-Hinitial. In the context of a chemical reaction, this means that ΔHrxn equals Hproducts – Hreactants. Positive values of ΔHrxn signify endothermic reactions and negative values signify exothermic reactions.

Enthalpy is an extensive property, meaning that it depends on the amount of material that is present. Thus, an exothermic reaction will release more heat when more reactants are consumed. Just like for opposing phase transitions (e.g., condensation/vaporization), ΔHrxn for a reaction is equal in magnitude but opposite in sign from the reverse reaction. ΔHrxn values for reactions are also depended on the phases of matter of the reactants and products. This should make sense since we already know that phase transitions are accompanied by changes in enthalpy. Thermochemistry is the study of energy changes that take place during chemical processes. The potential energy that is associated with bonding (i.e., chemical energy) changes during chemical processes and bonds break and form. If the potential energy associated with bonding (i.e., enthalpy) decreases, then that potential energy is converted to heat and lost to the surroundings. Conversely, chemical reactions in which enthalpy increases absorb heat as they take place.

An example of a thermochemical equation is the combustion of hydrogen to produce water. A thermochemical equation includes the balanced chemical equation along with the enthalpy change. This particular process is exothermic, releasing 483.6 kJ of heat when two moles of H_2 react with one mole of O_2 . The bonds that are formed during the reaction (the four O-H bonds) are stronger than the bonds that are broken during the reaction (two H-H bonds and one O=O bond), which is partly because the O-H bonds have a polar component, and partly because charge-charge repulsion in O=O is relieved. Thus, more energy is released through the formation of bonds than is absorbed during the breaking of bonds, leading to a net decrease in energy. The potential energy that is lost is converted to heat and lost to the surroundings. (See this explained in the video!)