Silver Copper Replacement Lab? Wait...what?

Fact: Copper wire reacts with aqueous silver nitrate.
 Fact: We did a lab about this 5 weeks ago.
 Fact: I should not slack on my blog posts.


 So obviously we did another lab, the purpose of this one was to demonstrate a replacement reaction, and show us mole ratios in action, in our case we were taking silver nitrate and copper, trading out the silver for copper, and creating copper nitrate and pure silver. We did this by suspending a copper wire in an aqueous solution of silver nitrate and letting it sit for a day, and then observing the reaction. To make the aqueous solution we mixed silver nitrate (AgNO3) with distilled water until it had dissolved. We then placed a coil of copper wire in the solution, covered the tube with wax, and let it sit in the fume hood until day two. During the time we were waiting for the reaction to take place we sat down and did some math. Using the equation 2AgNO3 + Cu ----> Cu(NO3)2 + 2Ag and we formed predictions for how much silver should be formed and how much copper became Cu(NO3)2 in the reaction.  I won't bore you with the details but, our predictions said that we should expect to form about .4513g (.0071 mol) of Ag and loose about .2269g (.0036 mol) of Cu in the reaction otherwise known as a 1:2 ratio of replacement. To explain how the experiment works in a more descriptive manner I will refer to one of my lab partners description:  <In the experiment, copper changed from its elemental form, Cu, to its blue aqueous ion form, Cu2+(aq). At the same time, silver ions (Ag+(aq)) were removed from solution and deposited on the wire in the elemental Ag metallic form.> On day two we observed that a crystal structure had formed on the copper wire in the tube (fig. 1). Sadly, we had to remove the structure to measure our results. We then emptied the test tube, and filtered the silver particles out of the copper nitrate solution, set the particles and the wire in the fume hood to dry overnight, and disposed of the copper nitrate solution. We are able to tell how much copper was used by measuring the new mass of the wire and comparing it to the mass from before the experiment. On day three we weighed the copper coil and silver nitrate, and then got to work figuring out how much Ag was formed and how much Cu was lost. In other words, more math...

(fig.1)

   

     Our predictions said that we should expect to form about .7703g of Ag and loose about .2269g of copper. The first step was changing the predictions we had from grams to mols, when we did this we got .0103 mol.of Ag, and .0036 mol of copper. In other words a 1 to 3 ratio of Cu to Ag. When this information was compared to our predictions (which said we would have a 1 to 2 ratio of Cu to Ag) we got 144% yield of silver and a 100% yield of copper. The inconstancy most likely stems from the paper we weighed our Ag on was still slightly wet and added weight to our readings. All in all, this was a pretty fun lab.

Intermolecular Forces Lab (With a mix of Data, and Words)

It's time for another blog post!!! It's only been about a month since my last one, so I'm a little ahead of my normal schedule, but this post is a rather good one on Intermolecular Forces (the bonds that hold the elements together). Recently, we did a lab in chemistry where we measured how different chemicals (in this case alkanes) can affect the rate of cooling of a temperature probe in order to understand how molecular weight plays a part in intermolecular forces. Needless to say, the temperature probes and I did not get along very well.

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..)