Question

(a) As described in Section 7.7, the alkali metals react with hydrogen to form hydrides and react with halogens to form halides. Compare the roles of hydrogen and halogens in these reactions. How are the forms of hydrogen and halogens in the products alike? (b) Write balanced equations for the reaction of fluorine with calcium and for the reaction of hydrogen with calcium. What are the similarities among the products of these reactions?

Short answer

In both hydride and halide formation reactions, alkali metals lose one electron to form cations, while hydrogen or halogens gain an electron to form anions, resulting in ionic lattice structures. The balanced equations for the reaction of fluorine with calcium and hydrogen with calcium are: \(Ca(s) + F_2(g) \rightarrow CaF_2(s)\) and \(Ca(s) + H_2(g) \rightarrow CaH_2(s)\), respectively. Both products have an ionic lattice structure, with calcium present as Ca^2+ cation and anions (F^- in CaF2 and H^- in CaH2) that have gained one electron.

Step-by-step solution

Alkali metals react with hydrogen to form metal hydrides and with halogens to form metal halides. In both these cases, alkali metals lose one electron and become positively charged ions (cations) while hydrogen or halogens gain one electron and become negatively charged ions (anions). Hydrogen becomes H^- ion (hydride ion) and halogens become X^- ion (halide ion). In the product, the alkali metal ions are held together by the negatively charged hydride or halide ions in an ionic lattice structure. #b) Writing balanced equations for the given reactions and analyzing similarities among the products#

We will first write the balanced equation for the reaction of fluorine with calcium: Reaction of fluorine with calcium: \[Ca(s) + F_2(g) \rightarrow CaF_2(s)\] Next, we will write the balanced equation for the reaction of hydrogen with calcium: Reaction of hydrogen with calcium: \[Ca(s) + H_2(g) \rightarrow CaH_2(s)\] In both reactions, calcium loses two electrons to form Ca^2+ ion while fluorine and hydrogen gain one electron each to form F^- ion and H^- ion, respectively. Similarities among the products: 1. Both products are ionic compounds and have an ionic lattice structure. 2. Calcium is present as Ca^2+ cation in both products. 3. Fluorine is present as F^- anion in CaF2 and hydrogen is present as H^- anion in CaH2, where both these anions have gained one electron.

Key concepts

Metal Hydrides Formation

When alkali metals come into contact with hydrogen, a reaction occurs that results in the formation of metal hydrides. These substances are composed of an alkali metal ion and the hydride ion, H⁻. For instance, when sodium (Na) reacts with hydrogen gas (H₂), sodium hydride (NaH) is formed through the following reaction:

\[2Na(s) + H_2(g) \rightarrow 2NaH(s)\]
Here, each sodium atom donates one electron to form a Na⁺ ion, while hydrogen molecules split into two atoms, each gaining one electron to become H⁻. The resulting product is an ionic compound with a crystal lattice structure in which each Na⁺ is surrounded by H⁻ ions.

Metal Halides Formation

Alkali metals also readily react with halogens, such as fluorine, chlorine, bromine, and iodine, to form metal halides. This is a classic example of a redox reaction where the alkali metal is oxidized and the halogen is reduced. For instance, lithium (Li) reacting with chlorine (Cl₂) would produce lithium chloride (LiCl) according to the equation:

\[2Li(s) + Cl_2(g) \rightarrow 2LiCl(s)\]
In this process, each lithium atom loses one electron to form a Li⁺ ion while each chlorine atom gains one electron to form a Cl⁻ ion, creating an ionic compound similar to metal hydrides but with halide ions.

Ionic Lattice Structure

The ionic lattice structure is foundational to understanding metal hydrides and metal halides. An ionic lattice is a highly ordered, three-dimensional arrangement of ions. It is characterized by the electrostatic attraction between positively charged metal cations and negatively charged non-metal anions. For instance, in sodium chloride (NaCl), each Na⁺ ion is surrounded by six Cl⁻ ions and vice versa, forming a cubic shape. This regular arrangement is responsible for the high melting and boiling points of ionic compounds, plus their ability to conduct electricity when melted or dissolved in water.

Chemical Reactions and Equations

The description of chemical processes using symbols and formulas is at the heart of writing chemical reactions and equations. A balanced chemical equation reflects the law of conservation of mass, indicating that the number of atoms for each element is the same on both sides of the reaction. For instance, the balanced equation for the reaction of sodium with chlorine would be:

\[2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)\]
Here, two sodium atoms (2Na) react with one molecule of chlorine gas (Cl₂) to produce two units of sodium chloride (2NaCl). Each reaction step should reflect direct changes in the composition of substances, as well as the transfer of electrons that define a redox process.