Dr.K asked two brilliant, future PhD students (Dr.A and Dr.B) taking his CHEM I
ID: 868960 • Letter: D
Question
Dr.K asked two brilliant, future PhD students (Dr.A and Dr.B) taking his CHEM I class to determine the structural formula of Hg2Cl6 (g). Both students agree that the structural formula could be determined by reacting Hg2Cl6 (g) with Al2Cl6(g) to yield HgAlCl5(g) according to the following reaction: Hg2Cl4(g) + Al2Cl6(g) rightarrow 2HgAlCl5(g). In order to prevent the gas from escaping and to be able to measure the heat released/absorbed, they designed a chamber (system) surrounded by 1 kg of water as depicted below (Fig 1). When the reaction was complete, they observed that the temperature of water surrounding the reaction chamber (system) had decreased from 25.00 degree C to 24.76 degree C. Based on their observations, the students came up with two different structural formulae for Hg2Cl4(g) as depicted in the reactions below (Fig 2 and Fig 3), but they could not seem to agree on which structure of Hg2Cl6(g) was more reasonable. (The structures of Al2Cl6(g) and HgAlCl5(g) are well known). Based on your extensive experience in thermochemistry, which structural formula makes the most sense (supported by data). Explain.Explanation / Answer
According to given data mass of water = 1 kg
temperature change, deltaT = 24.76-25.00
= -0.24 oC
Specific heat of water (s) = 4.18 J/g.oC
Heat lost by water (q) = m*s*deltaT
=1000*4.18*-0.24
=-1003.2 J
DeltaH reaction = -q
= -(-1003.2)
= 1003.2 J/mole
= 1.0032 KJ/mole
From above calculations change in enthalphy is very very small, this indicates reactants and product are almost similar bonds.
From given reactions, structure of Hg2Cl4 mostly likely to be structure shown in figure 2. Because strucure of Hg2Cl4 in figure 2 is almost similar to structure of product. This leads to very small change in enthalphy of the reaction.
In Figure 3 Hg2Cl4 contains Hg-Hg metal bond which is not appeared in product. So to break the metal metal bond we need more amount of energy, this leads to greater change in enthalphy. But experminetel values shows very small change in enthalphy. Hence structre of Hg2Cl4 in figure 2 is most preferable structure than Hg2Cl4 structure in figure 3
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