Energy in Chemical Reactions
Aligned to HS-PS3 — Next Generation Science Standards.
What this lesson teaches
Reactions absorb or release energy. Exothermic reactions release heat; endothermic reactions absorb it. Total energy is conserved.
Worked examples
Example 1
Burning wood is exothermic — it releases heat and light. A cold pack is endothermic — it absorbs heat, feeling cold.
Example 2
Both directions still conserve energy. An endothermic pack does not create cold; it moves heat out of your skin into the reaction.
Example 3
Whether a reaction gives out or takes in energy depends on the bonds, not on whether it feels dramatic. Rusting is exothermic and looks like nothing.
Practice questions
- Classify burning fuel as exo- or endothermic.
- Where does the energy in an exothermic reaction go?
- Why is energy conserved in a 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 en las Reacciones Químicas
Las reacciones absorben o liberan energía. Las reacciones exotérmicas liberan calor; las reacciones endotérmicas lo absorben. La energía total se conserva.
Ejemplo 1: Quemar madera es exotérmico — libera calor y luz. Una bolsa fría es endotérmica — absorbe calor y se siente fría.
Ejemplo 2: Las dos direcciones conservan la energía. Una compresa endotérmica no crea frío; saca calor de tu piel hacia la reacción.
Ejemplo 3: Que una reacción libere o absorba energía depende de los enlaces, no de si se ve espectacular. La oxidación es exotérmica y no parece nada.
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