Chapter U5.12, Problem 3E

(a)

Interpretation Introduction

Interpretation: Balanced chemical equation for the combustion of ethane needs to be written.

Concept Introduction : Combustion is a chemical reaction in which a compound or an element undergoes reaction with oxygen and releases energy in the form of light and heat. Combustion of covalent compounds comprising carbon and hydrogen produces carbon dioxide and water. The combustion of metallic substance produces metal oxide.

Answer to Problem 3E

Balanced chemical equation for the combustion of ethane is,

  ${\text{2C}}_{\text{2}}{\text{H}}_{\text{6}}+7{\text{O}}_2\to 4{\text{CO}}_2+6{\text{H}}_{\text{2}}\text{O}$

Explanation of Solution

The chemical formula of ethane is ${\text{C}}_{\text{2}}{\text{H}}_{\text{6}}$ . The reaction of a compound consisting hydrogen and carbon atoms with oxygen produces carbon dioxide and water. So, the combustion reaction of ethane is,

  ${\text{C}}_{\text{2}}{\text{H}}_{\text{6}}+{\text{O}}_2\to {\text{CO}}_2+{\text{H}}_{\text{2}}\text{O}$

The above equation is not balanced. Balanced chemical equation is the chemical equation in which number of atoms of every element is equal in reactant and product side of the chemical reaction.

To balance the equation, add coefficient 2 to ${\text{C}}_{\text{2}}{\text{H}}_{\text{6}}$ , 7 to ${\text{O}}_2$ , 4 to ${\text{CO}}_2$ and 6 to ${\text{H}}_{\text{2}}\text{O}$ .

  ${\text{2C}}_{\text{2}}{\text{H}}_{\text{6}}+7{\text{O}}_2\to 4{\text{CO}}_2+6{\text{H}}_{\text{2}}\text{O}$

(b)

Interpretation Introduction

Interpretation: The energy diagram for combustion of ethane needs to be drawn.

Concept Introduction : Combustion is a chemical reaction in which a compound or an element undergoes reaction with oxygen and releases energy in the form of light and heat. Combustion of covalent compounds comprising carbon and hydrogen produces carbon dioxide and water. So, ethane undergoes combustion reaction to produce carbon dioxide and water.

Answer to Problem 3E

The energy diagram for combustion of ethane is,

  

Explanation of Solution

From the chapter 103, the heat of combustion of ethane is $\Delta H=-1560\text{kJ /}\text{mol}$ . This heat of combustion indicates the amount of heat released to the surroundings. For exothermic reactions, the potential energy of product is lower than the reactant. However, potential energy increases on the pathway from reactants to product before it is converted to kinetic energy.

So, the heat energy diagram for combustion of ethane is,

  

(c)

Interpretation Introduction

Interpretation: The net energy exchange of the reverse reaction to form ethane is to be determined.

Concept Introduction : In a chemical process, energy is conserved. This means, net exchange of energy in a forward process is equal and opposite to exchange of net energy in the reverse process. If a forward reaction exothermic, the reverse reaction is endothermic.

Answer to Problem 3E

The net energy exchange of reverse reaction to form ethane and oxygen is $\Delta H=1560\text{kJ /}\text{mol}$ .

Explanation of Solution

The combustion reaction of ethane is as follows:

  ${\text{2C}}_{\text{2}}{\text{H}}_{\text{6}}+7{\text{O}}_2\to 4{\text{CO}}_2+6{\text{H}}_{\text{2}}\text{O}$

The net energy released in the process of combustion of ethane is $\Delta H=-1560\text{kJ /}\text{mol}$ and the process is exothermic process. So, net energy exchange of the reverse reaction should be equal and opposite to the net energy of forward process. So, the net energy exchange of reverse reaction to form ethane and oxygen is $\Delta H=1560\text{kJ /}\text{mol}$ and the process is endothermic.