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Mr. Pastirik's AP Chemistry Class Discussion Notes

NOTE TO ALL: There is a test on Monday covering Chapter 7. Come prepared! I will check notebooks for Chapter 7 homework while you work on the exam.

Chapter 6: Thermochemistry

Hints and Tips for Problem 36
  • Do not be confused by "excess oxygen" in the problem statement. This just means that the combustion reaction goes to completion and the enthalpy given may be used. If the combustion did NOT happen with the stoichiometric amount or excess of oxygen, all of the methane would not be burned. In this case, the given enthalpy could not be directly applied.
  • Notice the similarity of this problem to Sample Exercise 6.4. Part (a) is nearly identical. An enthalpy change for the reaction is given on the basis of one mole of methane. Part a, like Sample 6.4, then asks for the enthalpy change if a given mass of methane is burned. Part b, is also very similar. However, here the amount of methane is not given directly. However, recall the gas laws! You should be able to calculate the amount of methane (mass or moles) when you are given a volume, pressure and temperature. Once you have done this, the problem should look similar to part a and example 6.4.

Hints & Tips for Problem 60
  • This question is asking you to go back and write chemical equations from words.
  • Part (a) is looking for you to recall how to write the chemical formula for alumnium oxide. Then, you must realize what elemnets must come together to form soild aluminum oxide. From the definition of standard enthalpy (p. 261) "note that the reaction is written so that both elemnts are in their standard states, and 1 mole of proudct is formed". Of course, check to make sure your reaction is balanced. If you are unsure what the standard state should be (gas, solid or liquid) review the yellow box on the bottom of page 260.
  • Part (d) is similar to part (a).
  • Part (b) & (c) ask you to write a reaction that would represent the standard enthlpies of two different reactions. For combustion of liquid ethanol, it will be similar to sample exercise 6.11. Keep in mind fo that for standard enthalpies, the book may require that you put it on a 1 mole basis of ethanol (I am not sure on this, so I will have to check the solution guide). You should be familiar with neutralization reactions from the acid/base problems we have had previously in lab (hint: water will be formed!).
  • Part (e) is very similar to Part (b). However, notice that it does not ask for standard enthalpy.
  • For Part (f), think about the reactions you have studied in solutions (remember the solutions lab?). This should help you write the equation for what happens when solid ammonium bromide is put in water.

Also, check out this page from UC Riverside's Department of Chemistry for some extra problems (with solutions!) on enthalpy.
ch6_xtra.pdf

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