Constant Pressure Calorimetry with Piston

In summary, the book gives contradictory answers as to whether the volume of the system will increase or decrease after burning 1.250 octane.
  • #1
twotwntytwo
2
0
This is a conceptual question...hope you guys can explain the answers that the book gives! Its frustrating me to no end. This is actually a problem in the Enthalpy chapter, calorimetry section of section exercises of the Olmsted and Williams chemistry textbook, 3rd ed.

Any help would be GREATLY appreciated! Is this just a poorly worded question, or do I have the incorrect concept?

Q: Imagine a calorimeter with a sliding piston that makes it able to perform constant pressure calorimetry experiments on a mixture of gasses and liquids

Consider burning 1.250 octane in this calorimeter, which is initially at 25 degrees celsius

Q1: Will the calorimeter temperature rise or fall?
A: Rise

[note: I understand this, since burning octane is an exothermic reaction, leading to the loss of heat from the piston to the calorimeter]

Q2: Use the ideal gas equation to determine whether the volume of the system will increase or decrease. (The temperature after burning rises about 6 degrees celcius)
A: Decrease: The volume will decrease. Although the temp rises slightly, the moles of gas decrease

[note: I don't understand this! Since it is exothermic, doesn't the reaction cause work to be performed on the piston, increasing the volume of the piston?]

Q3: Is the work, negative, positive, or zero?
A: Positive

[note: i agree with this answer. however, it seems to contradict the last answer. also, the question is not specific as to what is experiencing the work, the piston or calorimeter]
 
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  • #2
Reaction equation will help, although I feel like second part is a little bit ambiguous.

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methods
 
  • #3
Hi Borek-

No equation is provided with the problem, although I think the reaction is combustion and the liquid gas mixture they are referring to is the mixture of octane, water, and co2

2C8H18 +25O2-----16CO2+H20

Thanks for responding! And I agree that the question is vague, the answers given seem to be contradictory :(
 
  • #4
18H2O, but I suppose it is a just a typo.

Trick is - we don't know what is the final temperature. If it is below 100 deg C, water is liquid - you have started with 25 volumes of gas, you are left with 16 volumes. Does it mean increase or decrease of volume?

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Related to Constant Pressure Calorimetry with Piston

1. How does constant pressure calorimetry with piston work?

Constant pressure calorimetry with piston is a technique used to measure the heat capacity of a substance. It involves placing a substance in a sealed container with a piston that allows for changes in volume. The container is then heated or cooled at a constant pressure, and the temperature change is measured. By knowing the heat capacity of the container and the change in temperature, the heat capacity of the substance can be calculated.

2. What is the purpose of using a piston in constant pressure calorimetry?

The piston in constant pressure calorimetry allows for changes in volume, which is necessary to accurately measure the heat capacity of a substance. Without the ability to change volume, the pressure inside the container could increase or decrease, affecting the temperature change and leading to inaccurate results.

3. What are the advantages of using constant pressure calorimetry with piston?

One advantage of using constant pressure calorimetry with piston is that it allows for the measurement of heat capacity at a constant pressure, which is often more relevant to real-world conditions. Additionally, the use of a piston allows for a more accurate measurement of heat capacity, as it prevents any changes in pressure that could affect the results.

4. Can constant pressure calorimetry with piston be used to measure the heat capacity of any substance?

Constant pressure calorimetry with piston can be used to measure the heat capacity of most substances, as long as they can be contained in the calorimeter and do not react with the container or piston. It is commonly used for measuring the heat capacity of liquids and solids.

5. Are there any limitations to using constant pressure calorimetry with piston?

One limitation of constant pressure calorimetry with piston is that it only measures the heat capacity at a constant pressure, which may not accurately represent the conditions in certain situations. Additionally, the accuracy of the results can be affected by any heat loss or gain to the surroundings, which must be carefully controlled and accounted for.

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