Science, Tech, Math › Science The Laws of Thermochemistry Understanding Enthalpy and Thermochemical Equations Print WLADIMIR BULGAR / Getty Images Science Chemistry Physical Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. Learn about our Editorial Process Updated on October 28, 2019 Thermochemical equations are just like other balanced equations except they also specify the heat flow for the reaction. The heat flow is listed to the right of the equation using the symbol ΔH. The most common units are kilojoules, kJ. Here are two thermochemical equations: H2 (g) + ½ O2 (g) → H2O (l); ΔH = -285.8 kJ HgO (s) → Hg (l) + ½ O2 (g); ΔH = +90.7 kJ Writing Thermochemical Equations When you write thermochemical equations, be sure to keep the following points in mind: Coefficients refer to the number of moles. Thus, for the first equation, -282.8 kJ is the ΔH when 1 mol of H2O (l) is formed from 1 mol H2 (g) and ½ mol O2. Enthalpy changes for a phase change, so the enthalpy of a substance depends on whether is it is a solid, liquid, or gas. Be sure to specify the phase of the reactants and products using (s), (l), or (g) and be sure to look up the correct ΔH from the heat of formation tables. The symbol (aq) is used for species in a water (aqueous) solution. The enthalpy of a substance depends upon temperature. Ideally, you should specify the temperature at which a reaction is carried out. When you look at a table of heats of formation, notice that the temperature of the ΔH is given. For homework problems, and unless otherwise specified, the temperature is assumed to be 25°C. In the real world, the temperature may be different and thermochemical calculations can be more difficult. Properties of Thermochemical Equations Certain laws or rules apply when using thermochemical equations: ΔH is directly proportional to the quantity of a substance that reacts or is produced by a reaction. Enthalpy is directly proportional to mass. Therefore, if you double the coefficients in an equation, then the value of ΔH is multiplied by two. For example: H2 (g) + ½ O2 (g) → H2O (l); ΔH = -285.8 kJ 2 H2 (g) + O2 (g) → 2 H2O (l); ΔH = -571.6 kJ ΔH for a reaction is equal in magnitude but opposite in sign to ΔH for the reverse reaction. For example: HgO (s) → Hg (l) + ½ O2 (g); ΔH = +90.7 kJ Hg (l) + ½ O2 (l) → HgO (s); ΔH = -90.7 kJ This law is commonly applied to phase changes, although it is true when you reverse any thermochemical reaction. ΔH is independent of the number of steps involved. This rule is called Hess's Law. It states that ΔH for a reaction is the same whether it occurs in one step or in a series of steps. Another way to look at it is to remember that ΔH is a state property, so it must be independent of the path of a reaction. If Reaction (1) + Reaction (2) = Reaction (3), then ΔH3 = ΔH1 + ΔH2 Cite this Article Format mla apa chicago Your Citation Helmenstine, Anne Marie, Ph.D. "The Laws of Thermochemistry." ThoughtCo, Apr. 5, 2023, thoughtco.com/laws-of-thermochemistry-608908. Helmenstine, Anne Marie, Ph.D. (2023, April 5). The Laws of Thermochemistry. Retrieved from https://www.thoughtco.com/laws-of-thermochemistry-608908 Helmenstine, Anne Marie, Ph.D. "The Laws of Thermochemistry." ThoughtCo. https://www.thoughtco.com/laws-of-thermochemistry-608908 (accessed March 19, 2024). copy citation