Question

# Using the following equation for the combustion of octane, calculate the heat of reaction for 100.0...

Using the following equation for the combustion of octane, calculate the heat of reaction for 100.0 g of octane. The molar mass of octane is 114.33 g/mole.

2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O   ,deltaH°rxn = -11018 kJ

I got- 4820kJ but the answer is 4820kJ.

why is the answer positive when the delta h is negative?

The heat of reaction in terms of heat of formation values === Heat of formation of products - heat of formation of reactants.

whereas,

Heat of reaction in terms of combustion === Heat of combustion of reactants - heat of combustion of products

This reverse notation is due to the fact that in heat of formation the desired species is in product side ( eg: for CO2 the equation is C + O2 ---> CO2 ) whereas for heat of combustion the desired value is on reactant side (eg: here the desired value is methane since we are calling it as heat of combustion of methane CH4 + O2 ----> CO2 + water.)

That is why while calculating through combustion values we use - (deltaH products - deltaH reactants)

Thank you, So, the answer is +4820 kJ