Chapter U5.95, Problem 1E

Interpretation Introduction

Interpretation: Exothermic and endothermic chemical changes are to be defined.

Concept Introduction:Any change in matter is accompanied by a change in energy. Many chemical reactions result in products that are hotter or colder than the reactants. Chemists have categorized chemical changes according to heat transfer. Heat is a transfer of energy due to temperature differences.

Answer to Problem 1E

When heat is transferred from the surroundings to the system the process is called endothermic.

When heat is transferred out of the system to the surroundings, the process is called exothermic.

Explanation of Solution

All chemical changes are accompanied by heat transfer. Heat is either transferred to the system or out of the system.

Exothermic- Chemical changes that result in transfer of heat from the products of the reaction (the system) to the surroundings are called exothermic changes. These reactions feel hot to the observer.

Example:

Burning of methane gas is an exothermic change as heat is evolved in this process.

  ${\text{CH}}_{\text{4}}{\text{(g)+2O}}_{\text{2}}\text{(g)}\to {\text{CO}}_{\text{2}}{\text{(g)+2H}}_{\text{2}}\text{O(g)+heat}$

Endothermic- Chemical changes that result in transfer of heat from the surroundings to the products of the reaction (the system) are called exothermic changes. These reactions feel cold to the observer.

Example:

Decomposition of mercury oxide is an endothermic process as it involves heating.

  $\text{2HgO(l)+heat}\to {\text{2Hg(l)+O}}_{\text{2}}\text{(g)}$

Conclusion

Exothermic changes are felt warm by an observer while endothermic changes are felt cold by the observer.