Calculating the heat of vaporization of chloroform [thermochemistry]

[anisotropic]

I am doing a lab writeup and am completely lost.

Summary of lab itself:

• Calculated the vapour pressure of chloroform (TCM) across a range of temperatures (using flask apparatus).
• Objective is to determine the heat of vaporization of TCM using the variation of vapour pressure with temperature.

Other info we are provided:

• Provided the Clausius-Clapeyron relation (lnP = -ΔH_v/RT + C)
• "is independent of temperature"
• "C is a constant related to the entropy of vaporization"

I'm supposed to:

• plot P_vapor vs. T, and determine the boiling temp.
• plot lnP_vapor vs. 1/T, and determine ΔH_vapor

My questions:

• How do you calculate boiling point from this data? I am assuming it involves extrapolating and determining at what temperature P_vapor = P_barometric ?
• How do you calculate ΔH_vapor from this data without knowing C? That is, how are lnP_vapor and 1/T related to C? (we are not given C and are supposed to determine it from the data)

Help would be appreciated.

 

[Borek]

Extrapolation OK, assume C is just a constant and not a function neither of P nor T.

 

[quicknote]

Hello, I am happy to help you with your lab writeup. Calculating the heat of vaporization of a substance, in this case chloroform, is an important aspect of thermochemistry. Let's break down the steps you need to take in order to determine the heat of vaporization from the data you have collected.

First, let's review the Clausius-Clapeyron relation that was provided to you: lnP = -ΔHv/RT + C. This equation relates the natural logarithm of the vapor pressure (P) to the heat of vaporization (ΔHv), temperature (T), and a constant (C) which is related to the entropy of vaporization. This means that as the temperature increases, the vapor pressure also increases, but at a decreasing rate due to the negative sign in front of ΔHv in the equation.

To determine the boiling point of chloroform from the data, you will need to plot the vapor pressure (P) versus temperature (T). The boiling point is the temperature at which the vapor pressure of the substance equals the atmospheric pressure (Pbarometric). This can be determined by extrapolating the line of best fit on your graph to the x-axis (Pvapor = Pbarometric).

Next, to calculate ΔHv, you will need to plot lnP versus 1/T. This will give you a straight line with a slope of -ΔHv/R. The slope can be determined by calculating the change in y (lnP) over the change in x (1/T) for any two points on the line. This slope will give you the value for -ΔHv/R, and since R is a known constant, you can solve for ΔHv.

Finally, you mentioned that you are supposed to determine the constant C from the data. This can be done by calculating the y-intercept of your lnP versus 1/T graph. The y-intercept will be equal to C. Alternatively, if you have calculated ΔHv from the slope of the graph, you can rearrange the Clausius-Clapeyron equation to solve for C: C = lnP + ΔHv/RT.

I hope this helps clarify the steps you need to take in order to determine the heat of vaporization of chloroform. Good luck with your lab writeup!