ChemistryWhy Some Reactions Heat Up and Others Get Cold
Atlas in safety goggles stands at a lab bench holding two beakers, one frosted with cold and one steaming warm, energy diagrams glowing on the wall behind.
- Explain that breaking bonds absorbs energy while forming bonds releases energy.
- Classify a reaction as endothermic or exothermic from its net energy change.
- Interpret the sign of enthalpy change (delta H) for a given reaction.
- Predict whether the surroundings feel warmer or colder during a reaction.
Key terms
- Enthalpy
- The heat content of a system measured at constant pressure, symbolized H.
- Enthalpy change
- The difference in enthalpy between products and reactants, written delta H.
- Exothermic reaction
- A reaction that releases net energy to the surroundings, giving a negative delta H.
- Endothermic reaction
- A reaction that absorbs net energy from the surroundings, giving a positive delta H.
- Bond energy
- The energy needed to break a particular chemical bond, also released when that bond forms.
Breaking and forming bonds
Energy is stored in chemical bonds, and every reaction breaks some bonds and forms others. Breaking a bond always requires an input of energy because you must pull bonded atoms apart against their mutual attraction. Forming a bond always releases energy because atoms settle into a lower-energy, more stable arrangement. A reaction does both at once: bonds in the reactants break and new bonds in the products form. The net energy change depends on which process involves more energy. If bond formation releases more than bond breaking absorbs, energy is left over and escapes as heat; if breaking absorbs more, the reaction must draw heat in.
Tracking energy with enthalpy
Chemists quantify the net heat flow of a reaction at constant pressure using enthalpy, H, and the change between products and reactants is delta H. An exothermic reaction releases energy to its surroundings, so the products hold less enthalpy than the reactants and delta H is negative; the beaker feels warm. An endothermic reaction absorbs energy from its surroundings, so the products hold more enthalpy and delta H is positive; the beaker feels cold. Burning fuel and hand warmers are exothermic, while instant cold packs and photosynthesis are endothermic. The sign of delta H is the compact record of whether energy flowed out or in.
Worked examples
A reaction's beaker feels noticeably cold to the touch. Classify it and give the sign of delta H.
- A cold beaker means the reaction pulled heat in from the surroundings.
- Absorbing heat is the definition of an endothermic reaction.
- Endothermic reactions store added energy, so the enthalpy of products exceeds reactants.
Answer: It is endothermic with a positive delta H.
In a reaction, forming new bonds releases more energy than breaking old bonds absorbs. Classify it.
- Compare the two energy amounts: formation releases more than breaking absorbs.
- The surplus energy must leave the system as heat to the surroundings.
- Net release of energy defines an exothermic reaction.
Answer: It is exothermic with a negative delta H.
Activity
Sort each chemical change into the Exothermic group or the Endothermic group.
Practice
Decide whether photosynthesis storing sunlight in glucose bonds is exothermic or endothermic and explain why.
A reaction has a positive delta H; predict whether its container feels warmer or colder and justify the answer.
Common mistakes to avoid
- Forming bonds uses up energyForming bonds always releases energy as atoms reach a more stable state; breaking bonds is what requires energy input.
- A negative delta H means the reaction loses massA negative delta H means energy is released to the surroundings, not that matter or mass is lost from the system.
Check your understanding
What happens to energy when a chemical bond is broken?
A reaction makes the beaker feel cold. What is true about it?
In a reaction, the energy released forming new bonds is greater than the energy absorbed breaking old bonds. How should this reaction be classified?
Which sign of delta H correctly describes an exothermic reaction?
Recap
Breaking bonds absorbs energy and forming bonds releases it, so a reaction's net energy change depends on which process involves more. When formation wins, energy is released and the reaction is exothermic with a negative delta H; when breaking wins, energy is absorbed and the reaction is endothermic with a positive delta H, tracked by enthalpy at constant pressure.
Reflect
How does the sign of delta H let you predict whether a reaction will warm or cool its surroundings?