Question

How is enthalpy used to predict whether a reaction is endothermic or exothermic?

Answer 1

Answer:

$329.48 KJ/mol$ heat is released by the combustion of $2 mol$of methane

Explanation:

The value of enthalpy determines whether the reaction is exothermic or endothermic. If the enthalpy change is positive, then the reaction is endothermic (heat or energy released) and if the enthalpy change is negative then the reaction is exothermic (heat or energy absorbed).

$2CH_4(g)+4O_2(g) => 2CO_2(g)+4H_2O(g)$

$\Delta H \ reaction=[ 2(\Delta HfCO_2)+ 4(\Delta HfH_2O)]-[2(\Delta HfCH_4)+4(\Delta HfO_2)]$

                            =$2 ( -(393.5 KJ)/mol)-[2( -74.6 KJ/mol)+4(-241.82 KJ/mol)]$

                            $= -787 KJ/mol-[ -149.2 KJ/mol-967.28 KJ/mol]$

                           $= -787 KJ/mol+1116.48 KJ/mol$

                                          $=329.48 KJ/mol$

In this question, the enthalpy of formation has positive value and hence the reaction is endothermic in which the heat is released.