Problem 71 - Chemistry: The Central Science in SI Units, Global Edition

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. Write balanced equations for the reaction of fluorine with calcium and for the reaction of hydrogen with calcium. (b) What is the oxidation number and electron configuration of calcium in each product?

Step-by-Step Solution

Answer

In reactions with alkali metals, hydrogen and halogens both act as electron acceptors. For the reaction between calcium (Ca) and fluorine (F₂): \(Ca + F_2 \rightarrow CaF_2\). For the reaction between calcium (Ca) and hydrogen (H₂) gas: \(Ca + H_2 \rightarrow CaH_2\). In both products, the oxidation number of calcium is +2, and its electron configuration is [Ar], as it loses two valence electrons from the 4s orbital.

Step 1: Comparing the Roles of Hydrogen and Halogens in Reactions with Alkali Metals

When alkali metals react with hydrogen, they form metal hydrides. And when they react with halogens, they form metal halides. In both these reactions, alkali metals lose electrons and get oxidized. Hydrogen and halogens act as electron acceptors; hydrogen gains an electron to form a hydride ion (H⁻), while halogens, like fluorine, gain an electron to form a halide ion (e.g., F⁻).

Step 2: Writing Balanced Equations for the Reaction of Calcium with Fluorine and Hydrogen

For the reaction between calcium (Ca) and fluorine (F₂): Ca + F₂ → CaF₂ For the reaction between calcium (Ca) and hydrogen (H₂) gas: Ca + H₂ → CaH₂

Step 3: Finding Oxidation Number and Electron Configuration of Calcium in Both Products

(a) In CaF₂: The oxidation number of calcium is +2 because it loses two electrons to form a Ca²⁺ ion, and two fluorine atoms each gain one electron to form F⁻ ions. Thus, the electron configuration of calcium in CaF₂ is [Ar], where it has lost the two valence electrons from the 4s orbital. (b) In CaH₂: The oxidation number of calcium is also +2 because it loses two electrons to form a Ca²⁺ ion, and two hydrogen atoms each gain one electron to form H⁻ ions (hydride ions). The electron configuration of calcium in CaH₂ is also [Ar], where it has lost its two valence electrons from the 4s orbital.

Key Concepts

Alkali Metals

Alkali metals are a fascinating group of elements found in Group 1 of the periodic table. These metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). They share some unique characteristics that define their reactions with other elements.

• Alkali metals are highly reactive, especially with water and air, due to their single electron in the outermost shell.
• This reactivity increases as you move down the group, with francium being the most reactive.
• Their low ionization energy makes it easy for them to lose their outer electron and form positive ions, or cations (e.g., Na⁺).

These metals react with hydrogen to form metal hydrides and with halogens to form metal halides. When alkali metals encounter hydrogen, they donate their single outer electron to form hydrides; similarly, they donate an electron to halogens, forming halides. Both reactions highlight the readiness of alkali metals to achieve a stable electronic configuration by shedding their lone valence electron.

Oxidation Number

Understanding oxidation numbers is crucial in identifying how atoms change during chemical reactions. An oxidation number signifies the total number of electrons that an atom either gains or loses to form a chemical bond.

In chemical reactions involving calcium:

• When calcium reacts to form CaF₂, it loses two electrons becoming a Ca²⁺ ion. The oxidation number here is +2.
• Similarly, in the formation of CaH₂, calcium loses two electrons to become a Ca²⁺ ion, maintaining the oxidation number of +2.

The oxidation number helps to balance equations and predict the outcome of chemical reactions, showcasing the electron transfer that occurs. In both CaF₂ and CaH₂, calcium's consistent oxidation number reflects its permanence in these reactions, making it easier to identify the products formed.

Balanced Equations

Balanced chemical equations ensure that the same number of each type of atom appears on both sides of the equation. This balance aligns with the law of conservation of mass, ensuring that mass is neither created nor destroyed during a chemical reaction.

For reactions involving calcium, we have:

• Calcium reacts with fluorine (F₂): \[\text{Ca} + \text{F}_2 \rightarrow \text{CaF}_2\]
• Calcium reacts with hydrogen (H₂): \[\text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2\]

These equations show a 1:1 ratio for reactants and products, meaning calcium combines with a fixed amount of fluorine or hydrogen, resulting in even exchanges of electrons. Ensuring equations are balanced is key to accurately representing chemical changes and predicting how chemicals will interact and form new substances.

Electron Configuration

Electron configuration is a representation of the arrangement of electrons in an atom's orbitals, crucial for understanding chemical properties and reactivity.

Calcium, with an atomic number of 20, has the initial electron configuration of [\(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2\)]. This indicates that its outermost shell is the 4s orbital with 2 electrons.

• In CaF₂ and CaH₂, calcium loses these 4s electrons, resulting in the electron configuration of [Ar].
• This shift reflects calcium's ability to form cations (Ca²⁺) by releasing its valence electrons, achieving a more stable electron configuration like that of a noble gas.

Understanding electron configurations not only provides insight into why elements undergo certain reactions but also explains how they bond and what compounds they will form. By knowing an element's configuration, one can predict its behavior and interactions in various chemical contexts.