Question

Determine the molar enthalpy of the balanced reaction if 4441.9 kJ of energy is released when 100 g of C₈H₁₈ combusts with 400 g of O₂.

Answer 1

First write the equation,

C₈H₁₈$+\frac{25}{2}$ O₂$\longrightarrow$ 8CO₂+9H₂O

From this equation we realize that 114g of C₈H₁₈ needs 400g of O₂. There is 400g of oxygen but only 100g of Octane, implying that octane is the limiting reagent.

The following relation will be therefore applied.

100g Octane $\equiv$ 4441.9KJ

Use the above relation find the joules that will be produced for one molar octane which is 114g.

Molar enthalpy= $\frac{114g/mol}{100g}×-4441.9KJ=-5063.766KJ/mol$

Note that the negative sign is used to denote that it's an exothermic reaction.