KEY2CHEM

Energy Changes in Reactions

Chemical or physical changes are often accompanied by a change in energy. This can be a change in potential energy (such as rearranging intermolecular forces) or a change in kinetic energy (associated with a change in temperature). A process that releases heat (decreases the energy of the system) at constant pressure is an exothermic process. A process that absorbs heat (increases the energy of the system) at constant pressure is an endothermic process.

Example 1.

Melting of solid ice to liquid water is an ________ process because it _______ heat.

A. exothermic, absorbs

B. endothermic, absorbs

C. exothermic, releases

Solution

B. endothermic, absorbs

Melting of solid ice to liquid water requires the input of energy to break up intermolecular forces holding the solid together. This requires absorbing heat and is an endothermic process.

Example 2.

Which diagram describes an exothermic process?

A.

B.

C.

Solution

A.

An exothermic process is one that decreases the energy of the system. In a chemical reaction, this means the products are lower in energy (or enthalpy) compared to the reactants.

Example 3.

Which describes an exothermic process?

A. $$\require{mhchem}\ce{A + B -> C + D + heat}$$

B. $$\require{mhchem}\ce{heat + A + B -> C + D }$$

C. $$\require{mhchem}\ce{heat + H2O(s) -> H2O(l)}$$

Solution

A. $$\require{mhchem}\ce{A + B -> C + D + heat}$$

An exothermic process is one where heat energy is released (as a product of the process).