Question

(a) As described in Section \(7.7,\) the alkali metals react with hydrogen to form hydrides and react with halogens- for example, fluorine to form halides, Compare the roles of hydrogen and the halogen in these reactions. How are the forms of hydrogen and halogen 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 the reactions of alkali metals with hydrogen and fluorine, both hydrogen and fluorine act as electron acceptors, forming negatively charged ions (H- and F-, respectively) in their product forms. The balanced equation for the reaction of fluorine with calcium is \(Ca + F_2 → CaF_2\), and for the reaction of hydrogen with calcium, it's \(Ca + H_2 → CaH_2\). The similarities among these products are that they are both ionic compounds, calcium donates two electrons in each case, forming a \(Ca^{2+}\) ion, and both hydrogen and fluorine accept electrons, forming negatively charged ions. The products have a similar structure with one calcium ion surrounded by two ions of either hydrogen or fluorine.

Step-by-step solution

Understanding the reactions of alkali metals with hydrogen and halogens (fluorine)

Alkali metals react with hydrogen to form metal hydrides, which can be represented as \(MH\). They also react with halogens, such as fluorine, forming metal halides, which can be represented as \(MF\). In both cases, alkali metals transfer an electron to the hydrogen or halogen atom, forming an ionic compound.

Comparing the roles of hydrogen and halogen in the reaction

In both reactions, hydrogen and halogen (fluorine) act as electron acceptors. An alkali metal (M) donates an electron to either hydrogen or fluorine, forming a negatively charged ion (H- or F-, respectively) and the ionic compound (MH or MF).

Examining the similarities in the product forms of hydrogen and halogens

In both metal hydrides and metal halides, hydrogen and halogen ions are negatively charged:\(H^-\) and \(F^-\) respectively. These ions are formed by accepting one electron from an alkali metal atom. For part (b):

Writing a balanced equation for the reaction of fluorine with calcium

The balanced equation for the reaction of fluorine with calcium can be written as: \(Ca + F_2 → CaF_2\)

Writing a balanced equation for the reaction of hydrogen with calcium

The balanced equation for the reaction of hydrogen with calcium can be written as: \(Ca + H_2 → CaH_2\)

Discussing the similarities among the products of these reactions

The similarities among the products of these reactions include: 1. Both products are ionic compounds as they are formed by the combination of metal (calcium) with a non-metal (hydrogen or fluorine). 2. In both products, calcium donates two electrons, forming a positively charged ion (\(Ca^{2+}\)). 3. In both products, hydrogen or fluorine accept electrons, forming negatively charged ions (\(H^-\) and \(F^-\) respectively). 4. And finally, the structure of the products is similar, where one calcium ion is surrounded by two ions of either hydrogen or fluorine (e.g., \(CaF_2\) and \(CaH_2\)).

Key concepts

Ionic Compounds

Ionic compounds are a central feature of chemistry, especially when exploring the reactions of alkali metals with other elements. In essence, an ionic compound is formed by the electrostatic attraction between positively (cations) and negatively (anions) charged ions. This typically involves a metal transferring electrons to a non-metal, resulting in the metal becoming positively charged and the non-metal becoming negatively charged.

For instance, when an alkali metal reacts with hydrogen or halogens like fluorine, it donates an electron. The alkali metal thus becomes a cation (positively charged), while the hydrogen or halogen becomes an anion (negatively charged). The resulting compound, such as metal hydride or metal halide, showcases a crystal lattice structure. This structure comprises ions arrayed in a repeating pattern that maximizes the attractive forces between oppositely charged particles, while minimizing repulsion between like charges.

As students delving into the intricacies of chemistry, it's vital to recognize that ionic compounds have distinct properties due to their ionic bonds. They typically have high melting and boiling points, are hard and brittle in solid form, and conduct electricity when melted or dissolved in water, as their ions are free to move and carry charge.

Metal Hydrides

The formation of metal hydrides is a notable reaction of alkali metals with hydrogen. When an alkali metal reacts with hydrogen, a type of compound known as a metal hydride is formed. In metal hydrides, represented by the chemical formula \(MH\), hydrogen acts as a hydride anion (\(H^-\)), showcasing its ability to gain an electron from the alkali metal.

Alkali metals, being highly reactive due to their single valence electron, readily release this electron to the hydrogen, resulting in the creation of an ionic bond. It's essential to note that the hydride ion is a powerful reducing agent, which makes metal hydrides useful in various chemical reactions, particularly those involving reductions. Metal hydrides can also be utilized in energy storage systems, as they can absorb and release hydrogen gas. Understanding the role of hydrides in the chemistry of alkali metals and their various applications can be incredibly beneficial for students who are interested in the practical applications of chemistry.

Metal Halides

When exploring reactions of alkali metals, another important class of compounds to consider is metal halides. Metal halides are formed when alkali metals react with halogens, such as chlorine, bromine, iodine, and fluorine, which are elements from the group 17 of the periodic table. These reactions produce ionic compounds where the metal acts as the cation and the halogen as the anion, resulting in a compound represented by the formula \(MX\) (where M is the metal and X is the halogen).

For the sake of clarity, when discussing metal halides with halogens like fluorine, the formula becomes \(MF\). The halogens have seven electrons in their outermost shell and require one additional electron to achieve a stable octet configuration. This electron is willingly provided by the alkali metal, which in turn forms a halide anion (such as \(F^-\)).

Metal halides are significant because they occur extensively in nature and have a vast range of uses in industry. For example, they are key in producing various materials, from ceramics to optical lenses, and are also used in lighting and even medication. Knowledge of metal halides' properties, such as their solubility, melting and boiling points, and electrical conductivity, allows students to grasp their practicality in the real world.