Prelab Questions Copper makes beautifully colored compounds and polyatomic ions
ID: 495801 • Letter: P
Question
Prelab Questions Copper makes beautifully colored compounds and polyatomic ions one such esample is summarized (a Gray anhydrous Cus0 dissolved in water gies a blue solution. Explain. (b)The addition of Agiaris0to the mixture above causes no change. However, when NaBris) is added a noticeable color change occurs. Predict the color change and explain it intemts of LeChatelier's principle Why did the two different solids added in (b) have two different effects? 2. Cave formations like Blanchard Springs Cavems,Arkansas, are very sensitive to atmospheric CO: levels. The pertinent reaction is atmospheric co levels begin to fall, what effect is this predicted to have on the rock formations these caves? (b) Use appropriate enthalpies from your text appendices detemnine the enthalpy change for the top reaction. Under what conditions, hot or cold. are the cave formations presumed to be more soluble? Consult the Figure 14.12 Tro P683. (a Sketch a Lewis dot structure for the NO: molecule. Refer to your dot structure, and use it to explain why two No molecules dimerize make N10. (b) Bond formation always releaseseneray, lsFigure l4.12consistent with this statement? Explain.Explanation / Answer
1. (a) The water is not there as free liquid, but as a molecule in its own right and it is bound to the copper atom. These molecules fulfill a structural role in as much as they fill voids in the crystal structure and let the salt ions take up a regular and therefore crystalline, shape of minimal energy content. In the case of all transition metals, the water of hydration is linked to the d (or higher) orbitals and these distort slightly, thereby causing a band energy shift that is characterised by the colour. In the case of copper this shift is in the 600-900 nm region and this is where the blue-green colours occur.
(b) As shown in the above reaction, the addition of Br- ion leads to formation of a Green colored complex. If we remove the Br- ions from the solution, then the reaction will shift towards the left side of the equilibrium and the Green color would disappear to give blue color. If we evaporate the water molecules, then the blue color would disappear giving rise to green color. This color changes happen due to the Le Chatelier's principle according to which the equilibrium reaction moves in the direction so as to counter the effect.
(c) The addition of AgBr has no effect as it is below Copper in the electrochemical series and hence, does not have the energy to oxidise Copper to its +2 form to form CuBr2.
2.(a) If atmospheric CO2 levels fall, then the reaction would move in a direction so as to increase the concentration of CO2. Hence, the reaction will move in the reverse direction giving rise to formation of more limestone (CaCO3) product.
(b) Heat of formation of Ca2+ = -543 KJ/mol, HCO3- = -691.1 KJ/mol
CaCO3 (s) = -1207.6 KJ/mol, CO2 (g) = -393.5 KJ/mol, H2O (l) = -285.8 KJ/mol.
Heat of reaction = Heat of formation of products - reactants
= (-543 + 2×-691.1) - (-1207.6 -393.5 -285.8) KJ/mol
= -38.3 KJ/mol.
Hence, on adding more heat the reaction will move in the reverse direction so as compensate for the addition of heat as the reactants have less heat than the products. For the cave formations to be soluble the reaction needs to move in forward direction. Hence, removal of heat or cold temperature is suited for the purpose.