Magnesium is one of the important elements in human body. Hundreds of biochemical reactions that dri — Physical Chemistry — Kinetics Chemistry Question
Magnesium compounds
Magnesium is one of the important elements in human body. Hundreds of biochemical reactions that drive energy metabolism and DNA repair are fueled by magnesium. Over 300 different enzymes rely on magnesium to facilitate their catalytic action. Magnesium maintains blood pressure and relaxes blood vessels and arteries. Magnesium deficiency leads to physiological decline in cells setting the stage for cancer. Among the numerous available magnesium dietary supplements, magnesium citrate has been reported as more bioavailable than the most commonly used magnesium oxide. Magnesium is a highly flammable metal. Once ignited, it is difficult to extinguish as it is capable of burning in water, carbon dioxide, and nitrogen.
Write a balanced equation for the formation of magnesium oxide by reaction of magnesium with
i. oxygen, O2
ii. carbon dioxide, CO2
Model Answer
i) 2 Mg(s) + O2(g) → 2 MgO(s)
ii) 2 Mg(s) + CO2(g) → 2 MgO(s) + C(s)
Magnesium hydroxide is formed by the reaction of Mg or MgO with H2O. Write a balanced equation for the formation of magnesium hydroxide by the reaction of H2O with
i. Mg
ii. MgO
Model Answer
i) Mg(s) + 2 H2O → Mg(OH)2(s) + H2(g)
ii) MgO(s) + H2O(l) → Mg(OH)2(s)
When magnesium metal is heated in N2 atmosphere the white-yellow compound A is formed. Hydrolysis of A yields the colorless gas B which has basic character when dissolved in water. The reaction of B with aqueous solution of hypochlorite ion generates chloride ion, water, and the molecular compound C which is soluble in water. The reaction of B with hydrogen peroxide also produces the compound C and water. When the colorless gas B is heated with sodium metal, a solid compound D and hydrogen gas are produced. The reaction of compound D with nitrous oxide produces gaseous ammonia, solid sodium hydroxide, and a solid compound E. When the solid E is heated it decomposes to sodium metal and nitrogen gas. Write balanced equations for the formation of each compound A, B, C, D, and E.
Model Answer
A is Mg3N2, 3 Mg(s) + N2(g) → Mg3N2(s)
B is NH3, Mg3N2(s) + 6 H2O(l) → 3 Mg(OH)2(s) + 2 NH3(g)
C is N2H4, 2 NH3(aq) + OCl– (aq) → Cl– (aq) + H2O(l) + N2H4(aq)
2 NH3(g) + H2O2(aq) → N2H4(aq) + 2 H2O(l)
D is NaNH2, 2 NH3(g) + 2 Na(s) → 2 NaNH2(s) + H2(g)
E is NaN3, 2 NaNH2(s) + N2O(g) → NH3(g) + NaOH(s) + NaN3(s)
Draw the Lewis structure of the anion present in compound E. Choose the most stable resonance structure.
Compound C was first used as rocket fuel during World War II. Today, it is used as a low-power propellant in spacecrafts. In the presence of certain catalysts such as carbon nanofibers or molybdenum nitride supported on alumina, one of the decomposition reactions of C involves production of ammonia and nitrogen gas. Write the balanced equation for the decomposition reaction of compound C generating ammonia and nitrogen gas.
Model Answer
3 N2H4(l) → 3 N2H4(g) → 4 NH3(g) + N2(g)
Estimate the energy associated with the decomposition of compound C into ammonia and nitrogen gas and standard enthalpy of formation of NH3 at 298 K. Standard enthalpy of formation of liquid and gaseous C are 50.6 and 95.4 kJ·mol-1, respectively, at 298 K. Average bond energies (BE) of N≡N, N=N, N-N and N-H are 946, 418, 163, and 389 kJ·mol-1, respectively, at 298 K.
Model Answer
∆rH 0 = – 1× BE(N≡N) – 4×3×BE(N-H) + 3×1×BE(N-N) + 3×4×BE(N-H) + 3×∆vapH 0(N2H 4)(l)
∆vapH 0(N2H4(l)) = ∆fH 0(N2H4(g)) – ∆fH 0(N2H4(l)) = 95.4 – 50.6 = 44.8 kJ mol-1
∆rH 0 = – 946 – 4×3×389 + 3×163 + 3×4×389 + 3×44.8 = –322.6 kJ
∆rH 0 = 4 × ∆fH 0(NH3(g)) – 3 × ∆fH 0(N2H4(l)) = –322.6 kJ
∆fH 0(NH3(g)) = ( –322.6 + 3×50.6 ) / 4 = – 42.7 kJ·mol-1
In an experiment, 2.00 cm3 of C is placed in a 1.00 dm3 evacuated reaction vessel containing a suitable catalyst at 298 K. After decomposition, the reaction vessel is cooled down to 298 K. Calculate the final pressure inside the vessel (density of liquid C is 1.0045 g·cm-3).
Model Answer
ρ = 1.0045 = m / V
m = 2.00 × 1.0045 = 2.00 g
n(N2H4(l)) = 2.00 g / 32.00 g mol-1 = 0.0625 mol
n(total after decompition) = 0.0625 mol × 5 / 3 = 0.1042 mol
p = nRT / V = 0.1042 × 0.082 × 298 / 1.00 = 2.54 atm
Calculate the work done if isothermal expansion of the reaction vessel discussed in part (g) occurs against the atmospheric pressure of 1 atm.
Model Answer
Vfinal = (0.1042 × 0.082 × 298) / 1.00 = 2.545 dm3
w = – p ∆V = –1.00 × (2.54 –1.00) = 1.54 atm· dm3 = 157 J