Ch.7 - Periodic Properties of the Elements

Problem 113c

Chapter 7, Problem 113c

When magnesium metal is burned in air (Figure 3.6), two products are produced. One is magnesium oxide, MgO. The other is the product of the reaction of Mg with molecular nitrogen, magnesium nitride. When water is added to magnesium nitride, it reacts to form magnesium oxide and ammonia gas. (c) In an experiment, a piece of magnesium ribbon is burned in air in a crucible. The mass of the mixture of MgO and magnesium nitride after burning is 0.470 g. Water is added to the crucible, further reaction occurs, and the crucible is heated to dryness until the final product is 0.486 g of MgO. What was the mass percentage of magnesium nitride in the mixture obtained after the initial burning?

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Determine the mass of magnesium nitride in the initial mixture by subtracting the final mass of MgO from the mass of the mixture after burning. This gives the mass of magnesium nitride that reacted with water to form additional MgO.

Calculate the mass of MgO initially present in the mixture after burning by subtracting the mass of magnesium nitride (calculated in step 1) from the total mass of the mixture after burning.

Convert the mass of magnesium nitride to moles using its molar mass (Mg3N2, where Mg = 24.305 g/mol and N = 14.007 g/mol).

Calculate the mass percentage of magnesium nitride in the initial mixture by dividing the mass of magnesium nitride by the total mass of the mixture after burning and then multiplying by 100 to get the percentage.

Verify the calculations by ensuring the sum of the mass percentages of magnesium nitride and MgO equals 100%.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Stoichiometry

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction. It allows chemists to calculate the amounts of substances consumed and produced in a reaction based on balanced chemical equations. Understanding stoichiometry is essential for determining the mass percentages of components in a mixture, as it provides the necessary framework for relating the masses of reactants to the masses of products.

Mass Conservation

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. This principle is fundamental in chemistry, as it implies that the total mass of the reactants must equal the total mass of the products. In the context of the given question, this concept is crucial for calculating the mass percentage of magnesium nitride, as it allows for the determination of how much mass is accounted for by the magnesium oxide and magnesium nitride after the reaction.

Chemical Reactions of Magnesium

When magnesium burns in air, it reacts with oxygen to form magnesium oxide (MgO) and can also react with nitrogen to form magnesium nitride (Mg3N2). The subsequent reaction of magnesium nitride with water produces magnesium oxide and ammonia gas. Understanding these reactions is vital for analyzing the products formed during the burning of magnesium and for calculating the mass contributions of each product in the final mixture.

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Textbook Question

One way to measure ionization energies is ultraviolet photoelectron spectroscopy (PES), a technique based on the photoelectric effect. (Section 6.2) In PES, monochromatic light is directed onto a sample, causing electrons to be emitted. The kinetic energy of the emitted electrons is measured. The difference between the energy of the photons and the kinetic energy of the electrons corresponds to the energy needed to remove the electrons (that is, the ionization energy). Suppose that a PES experiment is performed in which mercury vapor is irradiated with ultraviolet light of wavelength 58.4 nm. (c) The kinetic energy of the emitted electrons is measured to be 1.72 * 10-18 J. What is the first ionization energy of Hg, in kJ>mol?

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Textbook Question

One way to measure ionization energies is ultraviolet photoelectron spectroscopy (PES), a technique based on the photoelectric effect. (Section 6.2) In PES, monochromatic light is directed onto a sample, causing electrons to be emitted. The kinetic energy of the emitted electrons is measured. The difference between the energy of the photons and the kinetic energy of the electrons corresponds to the energy needed to remove the electrons (that is, the ionization energy). Suppose that a PES experiment is performed in which mercury vapor is irradiated with ultraviolet light of wavelength 58.4 nm. (d) Using Figure 7.10, determine which of the halogen elements has a first ionization energy closest to that of mercury.

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Textbook Question

Mercury in the environment can exist in oxidation states 0, +1, and +2. One major question in environmental chemistry research is how to best measure the oxidation state of mercury in natural systems; this is made more complicated by the fact that mercury can be reduced or oxidized on surfaces differently than it would be if it were free in solution. XPS, X-ray photoelectron spectroscopy, is a technique related to PES (see Exercise 7.111), but instead of using ultraviolet light to eject valence electrons, X rays are used to eject core electrons. The energies of the core electrons are different for different oxidation states of the element. In one set of experiments, researchers examined mercury contamination of minerals in water. They measured the XPS signals that corresponded to electrons ejected from mercury's 4f orbitals at 105 eV, from an X-ray source that provided 1253.6 eV of energy 11 ev = 1.602 * 10-19J2. The oxygen on the mineral surface gave emitted electron energies at 531 eV, corresponding to the 1s orbital of oxygen. Overall the researchers concluded that oxidation states were +2 for Hg and -2 for O. (b) Compare the energies of the 4f electrons in mercury and the 1s electrons in oxygen from these data to the first ionization energies of mercury and oxygen from the data in this chapter.

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Textbook Question

When magnesium metal is burned in air (Figure 3.6), two products are produced. One is magnesium oxide, MgO. The other is the product of the reaction of Mg with molecular nitrogen, magnesium nitride. When water is added to magnesium nitride, it reacts to form magnesium oxide and ammonia gas. (d) Magnesium nitride can also be formed by reaction of the metal with ammonia at high temperature. Write a balanced equation for this reaction. If a 6.3-g Mg ribbon reacts with 2.57 g NH31g2 and the reaction goes to completion, which component is the limiting reactant? What mass of H21g2 is formed in the reaction?

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Open Question

(e) While it is possible to form BiF5 in the manner just described, pentahalides of bismuth are not known for the other halogens. Explain why the pentahalide might form with fluorine but not with the other halogens. How does the behavior of bismuth relate to the fact that xenon reacts with fluorine to form compounds but not with the other halogens?

Textbook Question

Potassium superoxide, KO₂, is often used in oxygen masks (such as those used by firefighters) because KO₂ reacts with CO₂ to release molecular oxygen. Experiments indicate that 2 mol of KO₂(s) react with each mole of CO₂(g). (a) The products of the reaction are K₂CO₃(s) and O₂(g). Write a balanced equation for the reaction between KO₂(s) and CO₂(g).