NAME:_____________________________ CHEM 1311, Section B Third Exam 17th March WRITE YOUR NAME IN BLOCK CAPITALS ON THE TOP OF EACH SHEET

In taking this examination your are expected to adhere to the GT academic honor code. At a minimum this requires that you utilize only the materials supplied to you, and that you do not give help to, or accept help from, others. 1) a) Draw a Born-Harber cycle for the formation of MgH2 from solid magnesium

metal and hydrogen gas (MgH2 contains Mg2+ and H- ions). Label each step with the appropriate thermodynamic quantity. Calculate the enthalpy of formation for MgH2(s). (8 Points)

MgH2 lattice energy 2791 kJmol-1 Mg atomization energy 147.1 Mg 1st ionization energy 737.7 Mg 2nd ionization energy 1450.7 H2 bond enthalpy 436 H electron affinity -72.8

∆Hf = 147.1 + 436 + 737.7 + 1450.7 – (2 x 72.8) – 2791 = -165.1

Mg(s) + H2(g)

Mg(g) + H2(g)

Mg(g) + 2H(g)

Mg+(g) + e- + 2H(g)

Mg2+(g) + 2e- + 2H(g)

Mg2+(g) + 2H-

(g)

MgH2(s)

147.1 kJmol-1

436 kJmol-1

737.7 kJmol-1

1450.7 kJmol-1 2 x 72.8 kJmol-1

2791 kJmol-1

∆Hf

NAME:_____________________________ b) Assuming that the standard enthalpy of formation for MgH2 is –75.3 kJmol-1 (this is not the answer that you will get from the above calculation) and that the standard molar entropies of Mg(s), H2(g) and MgH2(s) are 33, 131 and 42 JK-1mol-1 respectively. Calculate both the standard entropy and free energy of formation for MgH2(s). (4 points)

Mg(s) + H2(g) � MgH2(s)

∆∆∆∆S for the above reaction under standard conditions is = 42 – 131 – 33 = -122 JK-1mol-1 So ∆∆∆∆G0 = ∆∆∆∆H0 – T∆∆∆∆S0 = -75.3 – (298 x –122/1000) = -75.3 + 36.3 = -38.9 kJmol-1

2) Calcium chloride is very soluble in water and heat is released as the calcium

chloride dissolves.

What is the sign of the free energy change for the dissolution process? (positive or negative) 1 point

What is the sign of the enthalpy change for the dissolution process? (positive or

negative) 1 point

Based on your answers to the previous parts, what if anything can be said about the sign of the entropy change for the dissolution process? (positive, negative, do not have enough information to draw a firm conclusion) 2points

3) Which of the following packing arrangements of atoms fills space most

efficiently? (2 points) a. Simple cubic b. Body centered cubic c. Hexagonal close packed

NAME:_____________________________

4) Which of the following four band structure diagrams is/are appropriate for a non-metal (i.e. an insulator or semiconductor). 4 points

5) How many tetrahedral holes are there per anion in a hexagonal close packed array

of anions? (1 point) 2 6) How many octahedral holes are there per anion in a hexagonal close packed array

of anions? (1 point) 1 7) Which of the following compounds has a structure that can be described as

consisting of cations occupying all the cubic holes in a primitive cubic array of anions? (1 point)

a) NaCl b) GaAs c) ZnS – zinc blende d) ZnS – wurtzite e) CaF2 – Fluorite f) CsCl

a) b)

d) p - band

s - band

c)

Non-metalNon-metal

NAME:_____________________________ 10) By examination of the diagrams determine the stoichiometry of the compounds

AXn whose structures are shown. You do not need to give the name of the compound. You only need to state the ratio of cations to anions or anions to cations. The bigger spheres in each case represent anions. (4 points) a)

1:1 or AX

b)

1:2 or AX2

11) Which of the following graphs best represents the temperature dependence of electrical conductivity that you would expect for a metal? (2 points) (b)

a)

conductivity

T/K

conductivity

T/K

b)

NAME:_____________________________ 12) Using your knowledge of the radius ratio rules and your knowledge of how ion

sizes vary with position in the periodic table. Which one of the following compounds is most likely to contain an 8 coordinate cation. (2 points)

LiI NaCl LiBr RbF KI NaI NaBr

13) Use the attached structure map to predict the coordination number of calcium in the compound CaS. Show your working. The electronegativities of Ca and S are 1.00 and 2.58 respectively. (3 points)

∆χ∆χ∆χ∆χ = 2.58 – 1.0 = 1.58 Average principle quantum number = (4+3)/2 = 3.5 This corresponds to 6 coordination of the cation.