Enthalpy of physical change?

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Adriana Rangel 1A
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Joined: Fri Sep 29, 2017 7:04 am

Enthalpy of physical change?

Postby Adriana Rangel 1A » Sat Jan 13, 2018 2:53 pm

In the book chapter 8.11, there is a part:
The difference in molar enthalpy between the vapor and the liquid states is called the enthalpy of vaporization... The enthalpy of vaporization of most substances changes little with temperature.

*** For water at its boiling point, 100 C, Hvap  40.7 kJmol1; at 25 C, Hvap  44.0 kJmol1. The latter value means that to vaporize 1.00 mol H2O(l), corresponding to 18.02 g of water, at 25 C and constant pressure, we have to supply 44.0 kJ of energy as heat.

Can someone please explain how they figured out they have to supply 44.0 kJ of heat? And what each value given to me represents ? Sorry if this is a stupid question

Jason Muljadi 2C
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Joined: Thu Jul 27, 2017 3:01 am

Re: Enthalpy of physical change?

Postby Jason Muljadi 2C » Sat Jan 13, 2018 4:11 pm

I'd like to start off by saying: there is no such thing as a stupid question! Every question that you have is valid, including this one.

Standard enthalpies will be given to you; no worries. Table 8.3 lists some of the enthalpies for common substances, including water. All of the values given to you are simply numbers that you can plug in to understand the change of molar enthalpy in a phase change. Note the formula of the enthalpy of vaporization on Pg. 283.

Lily Guo 1D
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Joined: Fri Sep 29, 2017 7:03 am

Re: Enthalpy of physical change?

Postby Lily Guo 1D » Sat Jan 13, 2018 4:17 pm

I'm not entirely sure, but I think that the 44.0 kJ/mol is just a value that was experimentally determined. I think that we would be given that in a table of H values or something if we needed it on a test (don't quote me on this though). The values given -- Hvap 40.7 kJ/mol at 100 degrees Celsius and Hvap 44.0 kJ/mol at 25 degrees Celsius -- are representative of how much energy is needed to cause a physical change in the water molecules and disrupt their bonding so that 1 mol of water undergoes a phase change from liquid to gas. So you would keep adding energy in the form of heat (for example, by putting a pot of water on a stovetop and turning the flame on) until 40.7 or 44.0 kJ has been added. At that point, 1 mol of water would go from liquid to gas.

Jacob Cho 2L
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Joined: Fri Sep 29, 2017 7:05 am

Re: Enthalpy of physical change?

Postby Jacob Cho 2L » Sun Jan 14, 2018 12:07 am

I think it is good to first understand that when a substance undergoes a phase transition (physical change), its temperature does not change while its phase changes even though energy is being added into the system. The enthalpy was calculated by measuring the initial and final values and subtracting the former from the latter. This can be done because enthalpy is a state function which means that the journey isn't what matters when measuring it, only the destination. I can only assume, however, that that value is experimentally derived after many trials and measurements.

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