Question

Borazine, \((\mathrm{BH})_{3}(\mathrm{NH})_{3}\), is an analog of \(\mathrm{C}_{6} \mathrm{H}_{6}\), benzene. It can be prepared from the reaction of diborane with ammonia, with hydrogen as another product; or from lithium borohydride and ammonium chloride, with lithium chloride and hydrogen as the other products. (a) Write balanced chemical equations for the production of borazine using both synthetic methods. (b) Draw the Lewis dot structure of borazine. (c) How many grams of borazine can be prepared from \(2.00 \mathrm{~L}\) of ammonia at STP, assuming diborane is in excess?

Short answer

(a) The balanced chemical equations for the production of borazine are: 1. \(B_2H_6 + 6NH_3 \rightarrow 2(B_3N_3H_6) + 6H_2\) 2. \(6LiBH_4 + 6NH_4Cl \rightarrow 2(B_3N_3H_6) + 6LiCl + 12H_2\) (b) The Lewis dot structure of borazine is a hexagonal ring with alternating boron and nitrogen atoms, and single B-N bonds. (c) From 2.00 L of ammonia at STP, 2.39 grams of borazine can be prepared, assuming diborane is in excess.

Step-by-step solution

Synthetic method 1: Diborane reacting with ammonia

The balanced chemical equation for the reaction of diborane \((B_2H_6)\) with ammonia \((NH_3)\) to produce borazine \((B_3N_3H_6)\) and hydrogen \((H_2)\) is given by: \[B_2H_6 + 6NH_3 \rightarrow 2(B_3N_3H_6) + 6H_2\]

Synthetic method 2: Lithium borohydride reacting with ammonium chloride

The balanced chemical equation for the reaction of lithium borohydride \((LiBH_4)\) with ammonium chloride \((NH_4Cl)\) to produce borazine \((B_3N_3H_6)\), lithium chloride \((LiCl)\), and hydrogen \((H_2)\) is given by: \[6LiBH_4 + 6NH_4Cl \rightarrow 2(B_3N_3H_6) + 6LiCl + 12H_2\] For (b)

Borazine Lewis dot structure

Borazine has an alternating arrangement of boron and nitrogen atoms in a hexagonal ring with B-N bonds, similar to benzene. Each B-N bond is a single covalent bond with each boron atom having three outer electrons available and each nitrogen atom having five outer electrons available. The borazonine lewis structure can be drawn as: For (c)

Calculate the mass of borazine prepared

Given: Volume of ammonia = 2.00 L We know that at STP, (Standard Temperature and Pressure i.e., 0°C and 1 atm) 1 mole of any gas occupies 22.4 L. First, we need to calculate the number of moles of \(NH_3\) using the given volume of ammonia at STP: Number of moles of \(NH_3 = \frac{Volume \ at \ STP}{22.4 \ L/mol} = \frac{2.00 \ L}{22.4 \ L/mol} = 0.089 \ moles \ of \ NH_3\) From the balanced equation in synthetic method 1, it is observed that 6 moles of \(NH_3\) produces 2 moles of borazine \((B_3N_3H_6)\): \[6 moles \ of \ NH_3 \rightarrow 2 moles \ of \ B_3N_3H_6\] Now, we need to find the number of moles of borazine formed from 0.089 moles of \(NH_3\). We can use the ratio derived from the balanced equation. \[\frac{2 \ moles \ of \ B_3N_3H_6}{6 \ moles \ of \ NH_3} = \frac{moles \ of \ B_3N_3H_6}{0.089 \ moles \ of \ NH_3}\] Solving for moles of borazine: \[moles \ of \ B_3N_3H_6 = \frac{2 \ moles \ of \ B_3N_3H_6 \times 0.089 \ moles \ of \ NH_3}{6 \ moles \ of \ NH_3} = 0.0297 \ moles \ of \ B_3N_3H_6\] Next, we need to calculate the molar mass of borazine given by: \[Molar \ Mass \ of \ B_3N_3H_6 = 3 \times (M_B + M_N + 2 \times M_H) = 3 \times (10.8 + 14 + 2 \times 1) = 80.4 \ g/mol\] Finally, the mass of borazine that can be prepared from 2.00 L of ammonia at STP is given by: \[Mass \ of \ B_3N_3H_6 = moles \ of \ B_3N_3H_6 \times Molar \ Mass \ of \ B_3N_3H_6 = 0.0297 \ moles \times 80.4 \ g/mol = 2.39 \ grams\] Therefore, 2.39 grams of borazine can be prepared from 2.00 L of ammonia at STP, assuming diborane is in excess.

Key concepts

Balanced Chemical Equations

Understanding balanced chemical equations is crucial for predicting the outcome of chemical reactions and conserving matter. A balanced equation successfully follows the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction. Therefore, the number of atoms for each element must be the same on both sides of the equation.

For example, the synthesis of borazine can occur via two methods. In the first method, diborane reacts with ammonia. The balanced equation is
\[B_2H_6 + 6NH_3 \rightarrow 2(B_3N_3H_6) + 6H_2\]
This equation clearly shows that all atoms on the reactant side are accounted for in the products. Each element's number of atoms before and after the reaction is equal, resulting in a balanced state.
For the second method, involving lithium borohydride and ammonium chloride, the equation is
\[6LiBH_4 + 6NH_4Cl \rightarrow 2(B_3N_3H_6) + 6LiCl + 12H_2\]
This balanced reaction demonstrates similar principles of conservation with all participating atoms balanced.

Lewis Dot Structure

The Lewis dot structure is an illustrative method for visualizing the valence electrons of atoms within a molecule and how they are bonded. It is essential for predicting molecular geometry, bond formations, and reactivity.

The Lewis dot structure for borazine, \(B_3N_3H_6\), consists of an alternating sequence of boron and nitrogen atoms forming a ring, reminiscent of the structure of benzene. Each boron has three valence electrons, while each nitrogen has five. Bonding leads to a stable structure with a single covalent bond between adjacent boron and nitrogen atoms, and additional hydrogen atoms bonded to each nitrogen and boron. This arrangement portrays the 'three-center two-electron' bonding typical for electron-deficient molecules like borazine and diborane.

Molar Mass Calculation

Molar mass is the weight of one mole (6.022 x 10²³ atoms or molecules) of any chemical compounds, usually expressed in grams per mole. It is a compound-specific value calculated as the sum of the atomic masses of all atoms present in the molecule.

For borazine, \(B_3N_3H_6\), the molar mass calculation would consider the atomic masses of boron (B), nitrogen (N), and hydrogen (H). The equation for this calculation is
\[Molar \/ Mass \of \ B_3N_3H_6 = 3 \times (M_B + M_N + 2 \times M_H) = 3 \times (10.8 + 14 + 2 \times 1) = 80.4 \/ g/mol\]
In this example, we see the individual atomic masses multiplied by the number of atoms for each type in the compound, followed by addition to reach the total molar mass.

Stoichiometry

Stoichiometry is the section of chemistry that relates to calculating the quantities of reactants and products in chemical reactions. It is a fundamental concept that enables chemists to predict product yields and ensure material balance in reactions.

In the context of borazine synthesis, stoichiometry allows us to determine how much borazine can be created from a given amount of ammonia at standard temperature and pressure (STP). Given the balanced chemical equation, we can set up a proportion that correlates the moles of reactants to the moles of products. With the known molar volume of a gas at STP (22.4 L/mol), and the calculated molar mass of borazine, we can find the expected yield of borazine from 2.00 L of ammonia.
This understanding of stoichiometry not only aids in predicting product amounts but also plays a vital role in scaling up chemical processes for industrial production, ensuring that chemical reactions are efficient and sustainable.