Bond Energy and Reaction Energy
Aligned to HS-PS1-4 — Next Generation Science Standards.
What this lesson teaches
Breaking chemical bonds absorbs energy, and forming new bonds releases energy. Whether a reaction overall releases energy (exothermic) or absorbs it (endothermic) depends on comparing the total energy needed to break the reactant bonds with the total energy released forming the product bonds.
Worked example
For H2 + Cl2 -> 2 HCl, breaking one H-H bond (436 kJ) and one Cl-Cl bond (243 kJ) absorbs 679 kJ. Forming two H-Cl bonds releases 2 x 431 = 862 kJ. Since more energy comes out than goes in (862 - 679 = 183 kJ released), the reaction is exothermic.
Practice questions
- Does breaking a bond release or absorb energy?
- A reaction absorbs 500 kJ breaking bonds and releases 620 kJ forming bonds. Is it exothermic or endothermic, and by how much?
- Draw an energy diagram showing reactants, products, and the net energy change for an exothermic reaction.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Energía de enlace y energía de reacción
Romper enlaces químicos absorbe energía, y formar enlaces nuevos libera energía. Que una reacción libere energía en total (exotérmica) o la absorba (endotérmica) depende de comparar la energía total necesaria para romper los enlaces de los reactivos con la energía total liberada al formar los enlaces de los productos.
Ejemplo: Para H2 + Cl2 -> 2 HCl, romper un enlace H-H (436 kJ) y un enlace Cl-Cl (243 kJ) absorbe 679 kJ. Formar dos enlaces H-Cl libera 2 x 431 = 862 kJ. Como sale más energía de la que entra (862 - 679 = 183 kJ liberados), la reacción es exotérmica.
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