Chemical reactions involve energy changes because bonds must be broken and new bonds are formed. Endothermic processes absorb energy from the surroundings, while exothermic processes release energy to the surroundings. This difference explains why some reactions feel cold, such as instant cold packs, and others feel hot, such as combustion.
Understanding the direction of energy flow helps predict temperature changes, reaction diagrams, and the sign of enthalpy change.
Key Facts
- Endothermic process: heat flows into the system from the surroundings, so ΔH > 0.
- Exothermic process: heat flows out of the system to the surroundings, so ΔH < 0.
- Enthalpy change: ΔH = Hproducts - Hreactants.
- Approximate bond energy method: ΔH = energy to break bonds - energy released forming bonds.
- Endothermic energy diagrams have products at higher energy than reactants.
- Exothermic energy diagrams have products at lower energy than reactants.
Vocabulary
- System
- The part of the universe being studied, such as the chemicals reacting in a beaker.
- Surroundings
- Everything outside the system that can exchange energy with it, such as the beaker, air, or your hand.
- Endothermic
- A process that absorbs heat energy from the surroundings and has a positive enthalpy change.
- Exothermic
- A process that releases heat energy to the surroundings and has a negative enthalpy change.
- Activation Energy
- The minimum energy needed for reactants to reach the transition state and begin forming products.
Common Mistakes to Avoid
- Calling a reaction endothermic just because it needs a spark is wrong because the spark may only provide activation energy, while the overall reaction can still release heat.
- Forgetting the sign of ΔH is wrong because endothermic processes have positive ΔH and exothermic processes have negative ΔH.
- Comparing only activation energy instead of reactant and product energy is wrong because ΔH depends on the difference between products and reactants, not the height of the energy barrier alone.
- Saying bonds release energy when they break is wrong because breaking bonds requires energy, while forming bonds releases energy.
Practice Questions
- 1 A reaction has Hreactants = 150 kJ/mol and Hproducts = 230 kJ/mol. Calculate ΔH and state whether the reaction is endothermic or exothermic.
- 2 Breaking bonds in the reactants requires 820 kJ/mol, and forming bonds in the products releases 1040 kJ/mol. Estimate ΔH and classify the process.
- 3 A disposable cold pack becomes cold when the inner pouch is broken and a salt dissolves in water. Explain whether the dissolving process is endothermic or exothermic, and describe the direction of heat flow.