Chapter 17.2, Problem 15SP

Interpretation Introduction

Interpretation: The amount of heat absorbed when $5.66\text{g}$ of ${\text{CS}}_{\text{2}}$ is formed from its elements.

Concept introduction: Endothermic reactions are those in which heat is absorbed by the reactants and enthalpy change for the reaction is positive.

The standard enthalpy of formation is the amount of energy released or consumed when one mole of a substance is formed under standard conditions from its pure elements.

Answer to Problem 15SP

The amount of heat absorbed when $5.66\text{g}$ of ${\text{CS}}_{\text{2}}$ is formed from its elements is $\text{6}\text{.61 kJ}$

Explanation of Solution

The given chemical equation is:

  $\text{C(s)+2S(s)}\to {\text{CS}}_{\text{2}}\text{(l) }\Delta \text{H=89}\text{.3 kJ}$

Thus, the standard enthalpy of formation of ${\text{CS}}_{\text{2}}$ is $\text{89}\text{.3 kJ}$

Given the amount of ${\text{CS}}_{\text{2}}$ = $5.66\text{g}$

Moles of ${\text{CS}}_{\text{2}}$ is found by dividing the given amount of ${\text{CS}}_{\text{2}}$ by the molecular mass of ${\text{CS}}_{\text{2}}$ .

Moles of ${\text{CS}}_{\text{2}}$ given = $\frac{\text{5}\text{.66g}}{\text{76}{\text{.2gmol}}^{\text{-1}}}=0.074\text{mol}$

For the formation of $1\text{ mol}$ of ${\text{CS}}_{\text{2}}$ , heat absorbed is = $\text{89}\text{.3 kJ}$

Thus, for the formation of $0.074\text{ mol}$

  ${\text{CS}}_{\text{2}}$ , heat absorbed is = $\frac{89.3\text{kJ}}{1\text{mol}}\times 0.074\text{mol=6}\text{.61 kJ}$