Question
White gold is a common alloy of gold and palladium that is often used in jewelry. The atomic radii of the metals are given in the table above.
(a) A particular ring is made from an alloy that is 75 mole percent gold and 25 mole percent palladium. Using the box below, draw a particle-level diagram of the solid alloy consisting of 12 atoms with a representative proportion of atom types. Your diagram should clearly indicate whether the alloy is interstitial or substitutional. Use empty circles for gold and shaded circles for palladium.
White-gold jewelry is often coated with rhodium to modify the color and durability.
(b) A student hypothesizes that placing the ring in a 1.0 M solution of Rh\((NO_3)_3\)(aq) will result in a reaction in which Rh metal is deposited on the white-gold ring. Based on the information in the table below, calculate \(E^{\circ}\) for the reactions that may occur between ions in the solution and atoms in the ring, and indicate whether or not the student’s hypothesis is correct.
▶️Answer/Explanation
(a) Diagram should show 12 circles in a regular array, 9 empty and 3 shaded.
(b) \(Au + Rh^{3+} \rightarrow Au^{3+} + Rh\)
\(0.76 V -1.50 V = -0.74 V\)
\(3 Pd + 2 Rh^{3+} \rightarrow 3 Pd^{2+} + 2 Rh
0.76 V – 0.92 V = -0.16 V\)
The student’s hypothesis is incorrect. (Both voltages are negative, so the reactions are not thermodynamically favorable under these conditions.)
Question
Consider the hydrocarbon pentane, \(C_5H_{12}\) (molar mass 72.15 g).
(a) Write the balanced equation for the combustion of pentane to yield carbon dioxide and water.
(b) What volume of dry carbon dioxide, measured at 250C and 785 mm Hg, will result from the complete combustion of 2.50 g of pentane?
(c) The complete combustion of 5.00 g of pentane releases 243 kJ of heat. On the basis of this information, calculate the value of DH for the complete combustion of one mole of pentane.
(d) Under identical conditions, a sample of an unknown gas effuses into a vacuum at twice the rate that a sample of pentane gas effuses. Calculate the molar mass of the unknown gas.
(e) The structural formula of one isomer of pentane is shown below. Draw the structural formulas for the other two isomers of pentane. Be sure to include all atoms of hydrogen and carbon in your structures.
▶️Answer/Explanation
Ans:
(a)
(b)
(c)
(d)
(e)