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Topic: Thermodynamics Question  (Read 1700 times)

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Offline loganmcentire

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Thermodynamics Question
« on: September 30, 2015, 12:19:24 AM »
Natural gas is primarily methane, CH4, and is used to heat homes. A typical home is approximately 2000 ft2 and the ceilings are 8 ft high. The following data may be helpful: The heat capacity of air is 1.01 J/g·K and the and the enthalpy of combustion of methane is -890.8 kJ/mol. Assume that the molecular weight of air is the same as nitrogen, its major component.
How many grams of methane are required to raise the temperature in the home from 40.°F to 69°F?

 How many grams of CO2 does this reaction produce?

For the first part, i have: Calculated the volume of the house. Calculated the density of air and therefore mass of air.

energy to raise the house's temperature = mass of air * specific heat * temperature change

Divided the energy required by the heat of combustion for methane, that gives you moles of methane needed. mass of methane = moles of methane * 16.044 g/mole.
Which i ended up getting 4689.46g but it is wrong, Any help?

Offline Borek

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Re: Thermodynamics Question
« Reply #1 on: September 30, 2015, 02:31:41 AM »
Approach seems OK, hard to say what went wrong not seeing the numbers. Please show your work.
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Offline Enthalpy

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Re: Thermodynamics Question
« Reply #2 on: September 30, 2015, 01:03:59 PM »
I get 0.16kg of methane if not heat is lost at the chimney.

Did you find 555kg air and 9MJ heat?

As a sidenote: there are two different heats of combustion, depending on if the vapour condenses or not, and this distinguishes two types of boilers with different efficiency, so one should always tell which one he takes.

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