Chapter 5, Problem 5.70QE

Interpretation Introduction

Interpretation:

$\Delta H$ has to be determined for the reaction, $\text{Mg}\left(s\right)\text{ + }\frac{1}{2}{\text{O}}_{\text{2}}\left(g\right)\underset{}{\to }\text{ MgO}\left(s\right).$

Concept Introduction:

According to Hess’s Laws, change in enthalpy in an overall reaction can be calculated from the change in enthalpy of other reactions.

• If an equation is obtained by the addition of more than one thermochemical equations, then the enthalpy change of that equation is the sum of change in enthalpy of all the equations added.
• If an equation is the reverse direction of a thermochemical equation, then the change in enthalpy has same numerical value but opposite sign.
• The enthalpy change depends on the mass of reacting substance. If the coefficients present in an equation is multiplied with a factor, then change in enthalpy also should be multiplied with that same factor.