Problem 69 - Chemistry and Chemical Reactivity

Question

Consider the chemistries of the elements sodium, magnesium, aluminum, silicon, and phosphorus. (a) Write a balanced chemical equation depicting the reaction of each element with elemental chlorine. (b) Describe the bonding in each of the products of the reactions with chlorine as ionic or covalent. (c) Draw Lewis electron dot structures for the products of the reactions of silicon and phosphorus with chlorine. What are their electron-pair and molecular geometries?

Step-by-Step Solution

Answer

(a) Na with Cl forms NaCl (ionic), Mg with Cl forms MgCl₂ (ionic), Al with Cl forms AlCl₃ (ionic), Si with Cl forms SiCl₄ (covalent), P with Cl forms PCl₃ (covalent). (b) SiCl₄ and PCl₃ are covalent. (c) SiCl₄ is tetrahedral; PCl₃ is trigonal pyramidal.

Step 1: Sodium Reaction with Chlorine

Sodium (Na) reacts with chlorine gas (Cl\(_2\)) to form sodium chloride (NaCl). The balanced equation is:\[2\text{Na}_{(s)} + \text{Cl}_2_{(g)} \rightarrow 2\text{NaCl}_{(s)}\] This reaction results in the formation of an ionic compound since sodium loses an electron to become Na\(^+\) and chlorine gains an electron to form Cl\(^-\).

Step 2: Magnesium Reaction with Chlorine

Magnesium (Mg) reacts with chlorine gas (Cl\(_2\)) to form magnesium chloride (MgCl\(_2\)). The balanced equation is:\[\text{Mg}_{(s)} + \text{Cl}_2_{(g)} \rightarrow \text{MgCl}_2_{(s)}\] The bonding in magnesium chloride is ionic. Magnesium loses two electrons to form Mg\(^{2+}\) while two chlorine atoms each gain one electron to form Cl\(^-\).

Step 3: Aluminum Reaction with Chlorine

Aluminum (Al) reacts with chlorine gas to form aluminum chloride (AlCl\(_3\)). The balanced equation is:\[2\text{Al}_{(s)} + 3\text{Cl}_2_{(g)} \rightarrow 2\text{AlCl}_3_{(s)}\] Aluminum chloride has predominantly ionic character at room temperature. Aluminum loses three electrons to form Al\(^{3+}\) and each chlorine gains one electron to form Cl\(^-\). In certain conditions, it can display covalent character as well.

Step 4: Silicon Reaction with Chlorine

Silicon (Si) reacts with chlorine gas to form silicon tetrachloride (SiCl\(_4\)). The balanced equation is:\[\text{Si}_{(s)} + 2\text{Cl}_2_{(g)} \rightarrow \text{SiCl}_4_{(l)}\] The bonding in silicon tetrachloride is covalent, with each Si-Cl bond formed by the sharing of electrons.

Step 5: Phosphorus Reaction with Chlorine

Phosphorus (P) reacts with chlorine gas to form phosphorus trichloride (PCl\(_3\)). The balanced equation is:\[\text{P}_{(s)} + \frac{3}{2}\text{Cl}_2_{(g)} \rightarrow \text{PCl}_3_{(l)}\] The bonding in phosphorus trichloride is covalent, as phosphorus shares its electrons with chlorine atoms.

Step 6: Lewis Dot Structure for Silicon Tetrachloride

Silicon tetrachloride (SiCl\(_4\)) has a central silicon atom with four chlorine atoms bonded covalently around it. Each Si-Cl bond consists of a pair of shared electrons, as represented in its Lewis structure: Si surrounded by four Cl atoms, each connected with a single line (pair of electrons). The electron-pair geometry is tetrahedral, and the molecular geometry is also tetrahedral.

Step 7: Lewis Dot Structure for Phosphorus Trichloride

Phosphorus trichloride (PCl\(_3\)) consists of a phosphorus atom bonded to three chlorine atoms. The Lewis structure shows three single bonds (pairs of electrons) between P and Cl, and a lone pair of electrons on the phosphorus, giving it a total of four electron pairs. The electron-pair geometry is tetrahedral, but the molecular geometry is trigonal pyramidal due to the lone pair.

Key Concepts

Chemical Bonding

Chemical bonding is the force that holds atoms together in compounds. There are two main types of chemical bonds: ionic and covalent. Each type of bond involves electrons and can influence the properties of the resultant compound. In ionic bonding, electrons are transferred from one atom to another, leading to the formation of charged ions. This typically occurs between metals and non-metals.
In contrast, covalent bonding involves the sharing of electron pairs between atoms, usually non-metals, allowing them to achieve stability by filling their outer electron shells. Understanding these differences is crucial in predicting the behavior and characteristics of various substances.

Lewis Structures

Lewis structures provide a visual representation of molecules, showing how atoms are bonded together and the arrangement of electrons. They are essential for predicting the shapes and reactivity of molecules. Each line in a Lewis structure represents a pair of shared electrons, while dots represent unshared electrons.

• For silicon tetrachloride (SiCl\(_4\)), the structure consists of a central Si atom bonded to four Cl atoms with lines representing shared electron pairs.
• Phosphorus trichloride (PCl\(_3\)) features a central P atom with three lines connecting to Cl atoms, plus a pair of dots indicating a lone electron pair on phosphorus.

Such representations help in understanding molecular geometries and predicting chemical behavior.

Ionic Compounds

Ionic compounds form through the transfer of electrons, resulting in a crystalline structure composed of ions. This usually involves a metal donating electrons to a non-metal, forming positive and negative ions that are held together by strong electrostatic forces.
In the given reactions, sodium chloride (NaCl), magnesium chloride (MgCl\(_2\)), and aluminum chloride (AlCl\(_3\)) (in solid form) are examples of ionic compounds. Here, metals such as sodium lose electrons to become positively charged, while non-metals like chlorine gain electrons, becoming negatively charged. This electron transfer is key to the formation of these stable ionic lattices.

Covalent Compounds

Covalent compounds result from the sharing of electrons between atoms. This sharing allows atoms to fill their valence shells, achieving greater stability. Typically formed between non-metals, covalent compounds can exist in various states—solid, liquid, or gas.
In the exercise, silicon tetrachloride (SiCl\(_4\)) and phosphorus trichloride (PCl\(_3\)) are covalent compounds. These molecules display distinct shapes due to the electron-sharing arrangement. For SiCl\(_4\), the molecule has a tetrahedral geometry, while PCl\(_3\) forms a trigonal pyramidal shape owing to the presence of a lone pair of electrons on phosphorus. The understanding of covalent bonding is vital for exploring the wide range of molecular structures and reactions in chemistry.