The basic premis of the experiment was to see how much energy was used by the element when it changed from a liquid to a gas, through evaporation in order to determine if molecular weight is a factor in inter molecular forces [in other words, does molecular weight play a part in the amount of energy needed by an element to induce a change of state (liquid to gas)]. We are able to use temperature to determine the amount of energy needed to change the state of matter thanks to the laws of thermodynamics. Since we know that heat flows from the warmer object (the probe) to the cooler substance (the liquid alkanes) until an equilibrium is reached, state changes are determined by the speed of molecules and that the First Law of Thermodynamics states that "Energy cannot be created or destroyed, only changed from on form to another" then we can correctly assume that the liquid alkanes (having been cooled to below zero in order to liquidize) will take the energy (in the form of heat) from the temperature probe until they evaporate (that energy is used to make the molecules move faster, which changes state), the amount of energy (in the form of heat, measured by temperature) used by the substance to change states is then visible as the amount of energy lost by the temperature probe.
In order to determine this we were to two stick temperature probes (fig. 1) into a pair test tubes each containing the liquid form of one of six different alkanes (Ethanol, Methanol, Propanol, Butanol, Pentane, and Hexane) let the probes sit in the tube for 30 seconds to determine their starting temperature, then remove the probe and tape it to the edge of the desk and measure the decrease in temperature over the course of 5 minutes. After recording the results of all six substances in "Logger Pro" we then found the change in temperature for each substance by subtracting the lowest temperature reached from the starting temperature. I then plotted the decrease in temperature (y-axis) vs. the molecular weight (x-axis) for each element on a graph in order to see if there was a correlation between the molecular weight, and the amount of energy needed to change the state of the substance.
| (Fig. 1) |
Now, above you can see my afore mentioned graph of the results of the lab. The first thing that you can see is that there is a very obvious correlation between the molecular weight, and the amount of energy used to change the state of matter. With the first four liquids (represented with boxes, I'll talk about why the circles are different in a moment) you can see that as the molecular weight gets bigger, the amount of energy needed needed to change state is lower. This is because as length of hydrocarbon chains increase (Ethanol has 1, Methanol 2, Propanol 3, and Butanol 4) the strength of the hydrogen bonds holding them together decreases, which makes it easier for the liquids to evaporate. Now, the reason that Pentane and hexane are not following the downward trend set by the other four substances is that they are not held together by a weak hydrogen bond (they can't hold more than 4 hydrocarbon chains together) they are held together by a stronger bond that requires more energy to break, but like the hydrogen bond, gets weaker with the each new set of hydrocarbon chains which would make the pattern repeat its self.
So in conclusion, this lab was very useful in demonstrating a number of things. First, it shows how molecular weight affects the bonds within the element, which affect its evaporation time. It also helped demonstrate how some of the laws of thermodynamics work, especially how heat flows from one place to another. And lastly, it proved that temperature probes hate me (spending 15 mins trying to figure out which probe is which was wa to long..)
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