Endothermic and Exothermic Processes
Introduced| English |
|---|
| exothermic/eɡzəˈðɜːmɪk/ |
| endothermic/ˌendəʊˈθɜːmɪk/ |
Heat in, heat out
- A hand warmer gets hot; an instant cold pack turns icy.
- Both are chemistry moving heat around.
- Some changes pour heat out; others soak it up.
- The direction of that heat flow has a name.
Releasing heat
- An exothermic 放热 process releases heat to the surroundings.
- It feels hot, and its $\Delta H$ is negative.
- Burning and freezing are exothermic.
An exothermic reaction has a $\Delta H$ that is...
Releasing heat makes $\Delta H$ negative.
An exothermic process releases heat to the ____.
Heat flows from the system out to the surroundings.
Absorbing heat
- An endothermic 吸热 process absorbs heat from the surroundings.
- It feels cold, and its $\Delta H$ is positive.
- Melting and evaporating are endothermic.
Endothermic or exothermic?
Sort each process by whether it releases or absorbs heat.
A reaction that makes its container feel cold is...
Absorbing heat from your hand feels cold -- endothermic.
Select all exothermic processes.
Burning and freezing release heat; melting absorbs it.
Breaking and making bonds
- Breaking bonds absorbs energy; making bonds releases it.
- If more is released than absorbed, the reaction is exothermic overall.
- The balance of the two sets the sign of $\Delta H$.
Breaking a chemical bond...
Breaking bonds always requires (absorbs) energy.
Forming a chemical bond releases energy.
Bond formation is exothermic -- the opposite of breaking.
Is burning methane exothermic or endothermic?
- It releases a lot of heat, since the flame is hot.
- So it is exothermic, with a negative $\Delta H$.
Mind the sign: exothermic $\Delta H$ is negative (heat leaves the system), endothermic is positive. "Feels hot" means exothermic, because heat flowed out to your hand. Breaking bonds always costs energy and forming bonds always releases it -- never reverse this.
An exothermic process releases heat (negative $\Delta H$, feels hot) and an endothermic one absorbs heat (positive $\Delta H$, feels cold). Breaking bonds costs energy while forming bonds releases it, and the balance of the two decides the sign of $\Delta H$.