# Question 4416b

Dec 13, 2015

$\Delta {H}_{\text{comb" = -"17,517 kJ}}$

#### Explanation:

As it is written, the problem does not provide enough information to allow you to calculate the enthalpy change of combustion, $\Delta {H}_{\text{comb}}$, for that reaction.

More specifically, you need to know the standard enthalpy change of formation of methyl myristate, ${\text{C"_13"H"_27"CO"_2"CH}}_{3}$. I was able to find this problem here - page $801$, Alternative fuels section

The standard enthalpy change of formation for methyl myristate is said to be equal to $- \text{771.0 kJ/mol}$. The standard enthalpy change of formation for water and carbon dioxide can be found here:

http://nshs-science.net/chemistry/common/pdf/R-standard_enthalpy_of_formation.pdf

So, you can calculate the enthalpy change of reaction by using the standard enthalpy changes of formation of the products and of the reactants

color(blue)(DeltaH_"rxn"^@ = sum( n xx DeltaH_"f prod"^@) - sum( m xx DeltaH_"f react"^@))" ", where

$n$, $m$ - the stoichiometric coefficients of the products and of the reactants, respectively
$\Delta {H}_{\text{f prod}}^{\circ}$, $\Delta {H}_{\text{react}}^{\circ}$ - the standard enthalpy changes of formation for the products and for the reactants, respectively

In your case, the balanced chemical equation for this combustion reaction looks like this

$2 {\text{C"_13"H"_27"CO"_2"CH"_text(3(l]) + 43"O"_text(2(g]) -> 30"H"_2"O"_text((g]) + 30"CO}}_{\textrm{2 \left(g\right]}}$

So, plug in your values and calculate $\Delta {H}_{\text{comb}}$ - keep in mind that the standard enthalpy change of formation for oxygen is zero!

DeltaH_"comb" = [30 color(red)(cancel(color(black)("moles"))) * (-393.5 "kJ"/color(red)(cancel(color(black)("mol")))) + 30color(red)(cancel(color(black)("moles"))) * (-241.8 "kJ"/color(red)(cancel(color(black)("mol"))))] - [2 color(red)(cancel(color(black)("moles"))) * (-771.0 "kJ"/color(red)(cancel(color(black)("mol"))))]

$\Delta {H}_{\text{comb" = ( -"11,805 kJ" - "7,254 kJ") -"1542 kJ}}$

DeltaH_"comb" = color(green)(-"17,517 kJ")#