Mr. Pastirik's AP Chemistry Class
Chapter 6: Thermochemistry
This week the class had a question about problem 36 in Chapter 6 that I was not able to address during the class period.
Here are some hints and tips for problem 36:
- Do not let the words "excess oxygen" confuse you in a combustion reaction. Excess oxygen just means that the reaction will go to completion, or that oxygen is NOT a limiting reactant. When this is the case, the enthalpy provided in the problem statement may be applied. If oxygen is not present in either the stoichiometric amount or in excess, some of the starting reactant (in this case methane) may not be burned and then the enthalpy change provided is no longer accurate.
- Notice that part (a) of question 36 is very similar to Sample Exercise 6.4. In both cases, you are given the delta H for methane combustion, on the basis of 1 mole of methane. You are then asked to calculate the enthalpy change for a given mass (mass, not moles!) of methane. I think you can do part 1 by comparing it to example 6.4.
- Notice that part (b) is also similar to Sample Exercise 6.4. However, in this instance you are given the amount of methane in an indirect manner. Recall the gas laws! You should be able to calculate the amount of methane (moles or mass) when you are given the volume, pressure and temperature.
Good luck and be prepared to start Chapter 7 next week!