How did they come up with this equation?

[Sammy1024]

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42. Which of the following statements is true about the heat produced from burning one gram ofethyne and one gram of ethene in the presence of excess oxygen gas under standard conditions?

A. 1.0 g C2H2(g) yields more heat than 1.0 g C2H4(g).

B. 1.0 g C2H2(g) yields less heat than 1.0 g C2H4(g).

C. 1.0 g C2H2(g) yields as much heat as 1.0 g C2H4(g).

D. The amount of heat cannot be calculated without first knowing the AHformation ofoxygen gas.

I see that in the equation says blah blah in the presence of excess Oxygen, but they don't even have excess Oxygen in the equation.

 

[NextStepTutor_1]

So in this case, the reason that they mentioned excess oxygen in the question but not in the equation is because excess oxygen is used to make sure that combustion reactions proceed to completion--that is, they result in only water and carbon dioxide. Thus, you don't have to worry about that when finding out the change in Enthalpy.

 

[Teleologist]

Enthalpy of formation of diatomic oxygen is also considered zero, so that doesn't factor into your enthalpy calculations. In other words, delta H super zero of O2 gas is zero because at a pressure of 1 atm, diatomic oxygen exists as a gas.