Chapter 15.3, Problem 27SSC

Interpretation Introduction

(a)

Interpretation: 

Whether the given reaction of ethanol is an exothermic or endothermic reaction needs to be identified.

Concept introduction :

Endothermic changes: Heat is absorbed by the substance to undergo chemical reaction or phase change.

Exothermic changes: Heat is released by the substance to undergo chemical reaction or phase change.

Answer to Problem 27SSC

Ethanol undergoes a phase change from liquid to gas. This requires ethanol to absorb energy and turn to vapour. Hence, this change is endothermic in nature.

Explanation of Solution

The phase change from liquid to gas requires energy to overcome the attraction between the molecules of ethanol. This is energy is absorbed by the ethanol from its surroundings to undergo the phase change to gas. As a result, this change is endothermic in nature.

(b)

Interpretation Introduction

Interpretation: 

Whether the given reaction of bromine is an exothermic or endothermic reaction needs to be identified.

Concept introduction : 

Endothermic changes: Heat is absorbed by the substance to undergo chemical reaction or phase change.

Exothermic changes: Heat is released by the substance to undergo chemical reaction or phase change.

Answer to Problem 27SSC

Bromine undergoes a phase change from liquid to solid. This requires bromine to release energy and turn to solid. Hence, this change is exothermic in nature.

Explanation of Solution

The phase change from liquid to solid releases energy to facilitate the attraction between the molecules of bromine. This is energy is released by the bromine to its surroundings to undergo the phase change to solid. As a result, this change is exothermic in nature.

(c)

Interpretation Introduction

Interpretation: 

Whether the given reaction of pentane is an exothermic or endothermic reaction needs to be identified.

Concept introduction : 

Endothermic changes: Heat is absorbed by the substance to undergo chemical reaction or phase change.

Exothermic changes: Heat is released by the substance to undergo chemical reaction or phase change.

Answer to Problem 27SSC

The combustion of pentane produces carbon dioxide and water. These compounds are stable and do not undergo combustion. Based on this fact, the combustion of pentane can be classified as an exothermic reaction.

Explanation of Solution

The products formed in an exothermic reaction are stable and have high bond enthalpies. These products do not undergo combustion to further form any simpler compounds. As a result, the overall change in heat enthalpy of the reaction is negative in value.

(d)

Interpretation Introduction

Interpretation: 

Whether the given reaction of ammonia is an exothermic or endothermic reaction needs to be identified.

Concept introduction : 

Endothermic changes: Heat is absorbed by the substance to undergo chemical reaction or phase change.

Exothermic changes: Heat is released by the substance to undergo chemical reaction or phase change.

Answer to Problem 27SSC

Ammonia undergoes a phase change from gas to liquid. This requires ammonia to release energy and turn to liquid. Hence, this change is exothermic in nature.

Explanation of Solution

The phase change from gas to liquid releases energy to facilitate the attraction between the molecules of ammonia. This is energy is released by the ammonia gas to its surroundings to undergo the phase change to liquid. As a result, this change is exothermic in nature.

(e)

Interpretation Introduction

Interpretation: 

Whether the given reaction of salt is an exothermic or endothermic reaction needs to be identified.

Concept introduction : 

Endothermic changes: Heat is absorbed by the substance to undergo chemical reaction or phase change.

Exothermic changes: Heat is released by the substance to undergo chemical reaction or phase change.

Answer to Problem 27SSC

Salt undergoes a phase change from solid to liquid. This requires salt to absorb energy and turn to liquid. Hence, this change is endothermic in nature.

Explanation of Solution

The phase change from solid to liquid requires energy to overcome the attraction between the molecules of ethanol. This is energy is absorbed by the solid ethanol from its surroundings to undergo the phase change to liquid. As a result, this change is endothermic in nature.