Ch.8 - Basic Concepts of Chemical Bonding

Problem 81a

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Chapter 8, Problem 81a

Consider the lattice energies of the following Group 2A compounds: BeH2, 3205 kJ/mol; MgH2, 2791 kJ/mol; CaH2, 2410 kJ/mol; SrH2, 2250 kJ/mol; BaH2, 2121 kJ/mol. (a) What is the oxidation number of H in these compounds?

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insert step 1: Understand that the oxidation number is a concept used to keep track of electrons in chemical reactions, particularly redox reactions.

insert step 2: Recognize that hydrogen typically has an oxidation number of +1 when bonded to nonmetals and -1 when bonded to metals.

insert step 3: Identify that in Group 2A metal hydrides (e.g., BeH2, MgH2), hydrogen is bonded to metals.

insert step 4: Conclude that since hydrogen is bonded to metals in these compounds, it will have an oxidation number of -1.

insert step 5: Verify that the sum of oxidation numbers in a neutral compound is zero, which supports hydrogen having an oxidation number of -1 in these metal hydrides.

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Key Concepts

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

Oxidation Number

The oxidation number, or oxidation state, is a theoretical charge assigned to an atom in a compound based on the assumption that electrons are completely transferred. In compounds, hydrogen typically has an oxidation number of +1 when bonded to nonmetals and -1 when bonded to metals. Understanding this concept is crucial for determining the oxidation states of elements in various compounds.

Group 2A Elements

Group 2A elements, also known as alkaline earth metals, include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These elements typically have an oxidation state of +2 in their compounds due to the loss of their two outermost electrons. Recognizing the behavior of these metals helps in predicting the oxidation states of hydrogen in their hydrides.

Hydrides

Hydrides are compounds formed between hydrogen and other elements, where hydrogen can exhibit different oxidation states. In the case of alkaline earth metal hydrides, hydrogen is usually in the -1 oxidation state when bonded to metals. This understanding is essential for analyzing the oxidation states in the given Group 2A hydrides.

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Related Practice

There are many Lewis structures you could draw for sulfuric acid, H2SO4 (each H is bonded to an O). (b) What Lewis structure(s) would you draw to minimize formal charge?

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Textbook Question

Some chemists believe that satisfaction of the octet rule should be the top criterion for choosing the dominant Lewis structure of a molecule or ion. Other chemists believe that achieving the best formal charges should be the top criterion. Consider the dihydrogen phosphate ion, H₂PO₄^-, in which the H atoms are bonded to O atoms. (b) What is the predicted dominant Lewis structure if achieving the best formal charges is the top criterion?

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Textbook Question

State whether each of these statements is true or false. e. Energy is stored in chemical bonds.

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Textbook Question

The ionic compound CaO crystallizes with the same structure as sodium chloride (Figure 8.3). (a) In this structure, how many O2- are in contact with each Ca2+ ion (Hint: Remember the pattern of ions shown in Figure 8.3 repeats over and over again in all three directions.)

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Textbook Question

Construct a Born–Haber cycle for the formation of the hypothetical compound NaCl2, where the sodium ion has a 2+ charge (the second ionization energy for sodium is given in Table 7.2). (a) How large would the lattice energy need to be for the formation of NaCl2 to be exothermic?

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Textbook Question

Consider the collection of nonmetallic elements O, P, Te, I, and B. (a) Which two would form the most polar single bond?

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