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I've been reading thermodynamics basics for chemistry here.

Consider a constant pressure process where the gas is sealed :

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Here the work done by the system is $p_{ext}(V_f-V_i)$. So far so good. Khan academy link says that a process done with out a lid is also a constant pressure process. I don't get this. Wouldn't the gas simply escape from the container since the lid is open ? Also how to calculate the change in volume ? Appreciate any help. Thanks!

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    $\begingroup$ Do you have a link to the Khan presentation? $\endgroup$
    – Bob D
    Commented Sep 2, 2018 at 15:45
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    $\begingroup$ In chemistry, we are generally not concerned with the gases. We are only concerned with the heat changes. So we say that the heat added to the system was added a constant pressure of 1 atm. Please provide a link though. $\endgroup$ Commented Sep 2, 2018 at 16:41
  • $\begingroup$ Hey @BobD I did provide the link above. Here it is again : khanacademy.org/science/chemistry/thermodynamics-chemistry/… $\endgroup$
    – AgentS
    Commented Sep 2, 2018 at 16:50
  • $\begingroup$ @rsadhvika They involve gases. But we are generally only concerned with the heat changes in the reaction and thus finding the enthalpy of the reaction. $\endgroup$ Commented Sep 2, 2018 at 16:53
  • $\begingroup$ Okay Ty, I think I get it now :) I guess we can measure the enthalpy by measuring the temperature and ignore the gas particles that get diffused into the surroundings ? @HarshitJoshi $\endgroup$
    – AgentS
    Commented Sep 2, 2018 at 17:06

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In chemistry, we are generally not concerned with the gases. We are only concerned with the heat changes. So we say that the heat added to the system was added a constant pressure of 1 atm. we are generally only concerned with the heat changes in the reaction and thus finding the enthalpy of the reaction.

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