Science, Tech, Math › Science Heat of Formation Worked Problem Learn How to Calculate the Enthalpy Change Print Heat of formation is a measure of energy of a reaction. PM Images / Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry 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 November 04, 2019 Heat of formation is the enthalpy change that occurs when a pure substance forms from its elements under conditions of constant pressure. These are worked example problems calculating the heat of formation. Review The symbol for the standard heat of formation (also known as the standard enthalpy of formation) is ΔHf or ΔHf° where: Δ indicates a change H indicates enthalpy, which is only measured as a change, not as an instantaneous value ° indicates thermal energy (heat or temperature) f means "formed" or that a compound is being formed from its component elements You may wish to review the Laws of Thermochemistry and endothermic and exothermic reactions before you begin. Tables are available for heats of formation of common compounds and ions in aqueous solution. Remember, heat of formation will tell you whether heat was absorbed or released and the quantity of heat. Problem 1 Calculate ΔH for the following reaction: 8 Al(s) + 3 Fe3O4(s) → 4 Al2O3(s) + 9 Fe(s) Read More Heat of Formation Table for Common Compounds By Anne Marie Helmenstine, Ph.D. Solution ΔH for a reaction is equal to the sum of the heats of formation of the product compounds minus the sum of the heats of formation of the reactant compounds: ΔH = Σ ΔHf products - Σ ΔHf reactants Omitting terms for the elements, the equation becomes: ΔH = 4 ΔHf Al2O3(s) - 3 ΔHf Fe3O4(s) The values for ΔHf may be found in the Heats of Formation of Compounds table. Plugging in these numbers: ΔH = 4(-1669.8 kJ) - 3(-1120.9 kJ) ΔH = -3316.5 kJ Answer ΔH = -3316.5 kJ Problem 2 Calculate ΔH for the ionization of hydrogen bromide: HBr(g) → H+(aq) + Br-(aq) Solution ΔH for a reaction is equal to the sum of the heats of formation of the product compounds minus the sum of the heats of formation of the reactant compounds: ΔH = Σ ΔHf products - Σ ΔHf reactants Remember, the heat of formation of H+ is zero. The equation becomes: ΔH = ΔHf Br-(aq) - ΔHf HBr(g) The values for ΔHf may be found in the Heats of Formation of Compounds of Ions table. Plugging in these numbers: ΔH = -120.9 kJ - (-36.2 kJ) ΔH = -120.9 kJ + 36.2 kJ ΔH = -84.7 kJ Answer ΔH = -84.7 kJ Cite this Article Format mla apa chicago Your Citation Helmenstine, Anne Marie, Ph.D. "Heat of Formation Worked Problem." ThoughtCo, Aug. 28, 2020, thoughtco.com/heat-of-formation-example-problem-609556. Helmenstine, Anne Marie, Ph.D. (2020, August 28). Heat of Formation Worked Problem. Retrieved from https://www.thoughtco.com/heat-of-formation-example-problem-609556 Helmenstine, Anne Marie, Ph.D. "Heat of Formation Worked Problem." ThoughtCo. https://www.thoughtco.com/heat-of-formation-example-problem-609556 (accessed April 26, 2024). copy citation