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Chemical reactions often involve energy moving between a system and its surroundings. Understanding whether a reaction absorbs energy or releases it helps explain temperature changes, reaction behavior, and why some processes need continuous heating while others give off heat on their own. The two main categories are endothermic and exothermic reactions.
In an endothermic reaction, the system takes in energy from the surroundings, so the surroundings often get cooler. In an exothermic reaction, the system releases energy to the surroundings, so the surroundings often get warmer. These energy changes are commonly described using enthalpy change, written as ΔH, and shown on energy diagrams that compare reactants, products, and activation energy.
Key Facts
- Endothermic reactions absorb energy from the surroundings and have ΔH > 0.
- Exothermic reactions release energy to the surroundings and have ΔH < 0.
- Enthalpy change is calculated by ΔH = Hproducts - Hreactants.
- If products are at higher energy than reactants, the reaction is endothermic.
- If products are at lower energy than reactants, the reaction is exothermic.
- Activation energy is the minimum energy needed to start a reaction, written Ea.
Vocabulary
- Endothermic reaction
- A chemical reaction that absorbs energy from its surroundings.
- Exothermic reaction
- A chemical reaction that releases energy to its surroundings.
- Enthalpy change
- The heat energy change of a reaction at constant pressure, written as ΔH.
- Activation energy
- The minimum energy reactant particles must have for a reaction to occur.
- Surroundings
- Everything outside the reacting system that can gain or lose energy.
Common Mistakes to Avoid
- Thinking endothermic means the reaction is hot, which is wrong because endothermic describes energy flowing into the system, often making the surroundings cooler.
- Assuming exothermic reactions need no activation energy, which is wrong because even energy-releasing reactions usually need an initial input to get started.
- Mixing up the sign of ΔH, which is wrong because ΔH is positive for endothermic reactions and negative for exothermic reactions.
- Reading an energy diagram backwards, which is wrong because you must compare the energy level of reactants to products to decide whether energy was absorbed or released.
Practice Questions
- 1 A reaction absorbs 125 kJ of energy from the surroundings. Is it endothermic or exothermic, and what is the sign of ΔH?
- 2 The enthalpy of the reactants is 210 kJ and the enthalpy of the products is 85 kJ. Calculate ΔH and identify the reaction type.
- 3 A cold pack becomes colder when chemicals inside it react. Explain whether the reaction is endothermic or exothermic and describe the direction of energy transfer.