   Chapter 5.8, Problem 1.1ACP

Chapter
Section
Textbook Problem

The standard enthalpies of formation of KNO3(s) and K2S(s) are −494.6 kJ/mol and −376.6 kJ/mol, respectively. a. Determine the standard enthalpy change for the reaction of black powder according to the balanced equation on the previous page. b. Determine the enthalpy change that occurs when 1.00 g of black powder decomposes according to the stoichiometry of the balanced equation above. (Even though black powder is a mixture, assume that we can designate 1 mol of black powder as consisting of exactly 2 mol of KNO3, 3 mol of C, and 1 mol of S.)

(a)

Interpretation Introduction

Interpretation:

The standard enthalpy change for the reaction of black powder has to be determined.

Concept Introduction:

The standard enthalpy of formation is the enthalpy change for the formation of 1mol of the compound directly from its component elements in their standard states.

Enthalpy change for the reaction ΔrH0=ΣnΔfH0(products)-ΣnΔfH0(reactants)

Explanation

Given balanced reaction for the gunpowder reaction is:

2KNO3(s)+3C(s)+S(s)K2S(s)+N2(g)+3CO2(s)

The standard formation enthalpies are:

ΔfH0(K2S)=-376.6kJ/mol

ΔfH0(N2)=0kJ/mol

ΔfH0(CO2)=-494.6kJ/mol

ΔfH0(C)=0kJ/mol

ΔfH0(S)=0kJ/mol

The enthalpy change for the reaction is:

ΔrH0=ΣnΔfH0(products)-ΣnΔfH0(reactants)

ΔrH° =((1molK2S/mol)[ΔfH0(K2S)]+(1molN2/mol)[ΔfH0(N2)]+(3molCO2/mol)[Δf

(b)

Interpretation Introduction

Interpretation:

The enthalpy change that occurs when 1.00g of black powder decomposes has to be calculated.

Concept Introduction:

Standard enthalpy of the reaction,ΔrHo, is the change in enthalpy that happens when matter is transformed by a given chemical reaction, when all reactants and products are in their standard states.

Enthalpy change for the reaction ΔrH0=ΣnΔfH0(products)-ΣnΔfH0(reactants)

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