Examen de Complements de química URV 2023 · Juny

Question 6 (2.5 points). Short questions on organic structure and reactivity

Answer the following short questions (0.5 points each):

a.

For the acid/base reaction between but-1-yne ($\mathrm{HC \equiv C - CH_2 - CH_3}$) and potassium phenolate ($\mathrm{Ph\text{-}O^- K^+}$):

1. Predict and draw the structures of the expected products.

2. Label the acid (A), the base (B), the conjugate base (CB), and the conjugate acid (CA).

3. Indicate the direction of the chemical equilibrium by choosing the appropriate set of arrows, and provide a brief justification based on relative acidities.

b.

Identify all the functional groups present in each of the three molecules shown below:

c.

Explain in detail why the 4-hydroxy-thiopyrylium cation shown below displays aromatic character:

d.

For each of the structures shown below, identify the most acidic proton and draw its corresponding conjugate base. Provide a brief explanation for your selection in each case:

e.

Which of the following molecules/species possess delocalized electrons? Select the appropriate compound(s) and draw at least one significant resonance contributor to justify your choice:

Question 7 (0.5 points). Crystal structure and conductivity of calcium

Calcium crystallizes in a cubic close-packed (ccp) structure.

a)

[0.4 points] Calculate the density of calcium in $\text{g/cm}^3$.

Data: Atomic weight = $40.1\text{ g/mol}$; unit cell edge $a = 5.56\text{ \AA}$.

g/cm³

b)

[0.1 points] Indicate the electrical conductivity behavior of calcium and justify it based on band theory.

Question 8 (1.25 points). Non-ionic contribution in potassium bromide

Potassium bromide (KBr) adopts the crystal structure illustrated in the unit cell diagram below. By comparing the theoretical and experimental lattice energies, determine the non-ionic (covalent) character percentage of KBr.

Thermodynamic Data:

• Standard enthalpy of formation: $\Delta H_f^\circ(\text{KBr}) = -391\text{ kJ/mol}$

• Enthalpy of sublimation of potassium: $\Delta H_{\text{sub}}(\text{K}) = 88\text{ kJ/mol}$

• First ionization energy of potassium: $\text{IE}(\text{K}) = 418\text{ kJ/mol}$

• Enthalpy of vaporization of bromine: $\Delta H_{\text{vap}}(\text{Br}_2) = 30\text{ kJ/mol}$

• Dissociation enthalpy of bromine: $\Delta H_{\text{diss}}(\text{Br}_2) = 193\text{ kJ/mol}$

• Electron affinity of bromine: $\text{EA}(\text{Br}) = -324\text{ kJ/mol}$

Structural & Physical Constants:

• Ionic radii: $r(\text{K}^+) = 1.33\text{ Å}$, $r(\text{Br}^-) = 1.95\text{ Å}$

• Atomic numbers: $Z_{\text{K}} = 19$, $Z_{\text{Br}} = 35$

• $N_A = 6.023 \times 10^{23}\text{ mol}^{-1}$; $e = 4.8 \times 10^{-10}\text{ esu}$; $1\text{ Å} = 10^{-8}\text{ cm}$; $1\text{ erg} = 10^{-7}\text{ J} = 10^{-10}\text{ kJ}$

Structure · Madelung Constant ($A$)

Cesium chloride · 1.76267

Sodium chloride · 1.74756

Zinc blende · 1.63806

Wurtzite · 1.64132

Fluorite · 2.51939

Antifluorite · 2.51900

Rutile · 2.40800

Configuration / Ion · Born Exponent ($n$)

He · 5

Ne · 7

Ar, Fe$^{2+}$ · 9

Kr, Ag$^+$ · 10

Xe, Au$^+$ · 12

1.

2.

3.

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Question 9 (0.75 points). Ranking and justification of melting points

Classify the following elements and compounds as molecular or non-molecular, specify the primary type of bonding present in each, and arrange them in descending order of their expected melting points:

$$\mathrm{Na},\ \mathrm{NaCl},\ \mathrm{MgO},\ \mathrm{HF},\ \mathrm{Co}$$

a)

Classification and bonding:

Na:

NaCl:

MgO:

HF:

Co:

b)