Chapter U5, Problem 15STP

Interpretation Introduction

Interpretation:

The correct process for the given energy diagram needs to be determined.

Concept introduction:

A chemical reaction involves the conversion of reactant to product due to making and breaking of chemical bond with certain energy change.

On the basis of energy change, the chemical reaction can be classified as exothermic and endothermic reaction. The chemical reactions which release energy in the form of heat or light are called as exothermic reactions. The chemical reactions which involves absorption of energy are called as endothermic reactions.

Answer to Problem 15STP

Option (C) is the correct option.

Explanation of Solution

Reason for correct option:

${\text{2 C}}_{\text{4}}{\text{H}}_{\text{10(g)}}{\text{+ 13 O}}_{\text{2(g)}}\stackrel{}{\to }{\text{8 CO}}_{\text{2(g)}}+10{\text{ H}}_{\text{2}}{\text{O}}_{\text{(g)}}$

The combustion of butane is an exothermic reaction as it releases a large amount of energy in the form of heat. In the given energy profile diagram, the energy of product is lesser than reactant. It means energy is released. Thus, the given energy diagram must be for the combustion of butane.

${\text{2 C}}_{\text{4}}{\text{H}}_{\text{10(g)}}{\text{+ 13 O}}_{\text{2(g)}}\stackrel{}{\to }{\text{8 CO}}_{\text{2(g)}}+10{\text{ H}}_{\text{2}}{\text{O}}_{\text{(g)}}$

Conclusion

Reasons for incorrect options:

1. An ice cube is melting at 0° C. The melting of ice must be endothermic process as it absorbs energy to convert solid ice to liquid water.
2. ${\text{H}}_{\text{2}}{\text{O}}_{\text{(g)}}\stackrel{}{\to }{\text{H}}_{\text{2(g)}}{\text{+ O}}_{\text{2(g)}}\text{ ΔH = +482kJ}$ is an endothermic reaction as the $\text{ΔH = +482kJ}$ is positive that indicates absorption of heat during chemical reaction.
3. Ammonium chloride is dissolved in water. The temperature cools by 16°C. It means temperature decreases by absorption of energy. Thus, it should be an endothermic reaction.