Chapter 20: Problem 48

Lewis structures can be used to understand why some molecules react in certain ways. Write the Lewis structure for the reactants and products in the reactions described below. a. Nitrogen dioxide dimerizes to produce dinitrogen tetroxide. b. Boron trihydride accepts a pair of electrons from ammonia, forming \(\mathrm{BH}_{3} \mathrm{NH}_{3}\). Give a possible explanation for why these two reactions occur.

Short Answer

Expert verified

In reaction (a), two NO₂ molecules dimerize to form N₂O₄, resulting in a more stable molecule without any unpaired electrons. This occurs due to nitrogen atoms sharing their unpaired electrons. In reaction (b), the electron-deficient boron atom in BH₃ achieves a complete octet by accepting a lone pair of electrons from the nitrogen atom in NH₃, forming a more stable complex molecule. This is an example of a Lewis acid-base reaction.

Step by step solution

To draw the Lewis structure for NO₂, start by counting the valence electrons. Nitrogen (N) has 5 valence electrons and each oxygen (O) has 6 valence electrons. Thus, we have a total of 5 + 2(6) = 17 valence electrons. Place the nitrogen atom in the center and draw single bonds to the two oxygen atoms. This uses up 4 valence electrons (2 for each bond). Distribute the remaining 13 valence electrons as lone pairs on the oxygen atoms (each oxygen atom should have 6 valence electrons). At this point, we still have one valence electron left, which belongs to the nitrogen atom. Consequently, nitrogen has an unpaired electron, making NO₂ a free radical. Lewis structure for NO₂: O = N - O #Step 2: Draw the Lewis structure for dinitrogen tetroxide (N₂O₄)#

To draw the Lewis structure for N₂O₄, start by counting the valence electrons. We have two nitrogen atoms and four oxygen atoms. Thus, we have a total of 2(5) + 4(6) = 34 valence electrons. Place the two nitrogen atoms in the center and draw single bonds to the four oxygen atoms. This uses up 8 valence electrons (2 for each bond). Distribute the remaining 26 valence electrons as lone pairs on the oxygen atoms and the nitrogen atoms (each oxygen atom and nitrogen atom should have 6 valence electrons). Finally, dimerize the nitrogen dioxide molecules by forming a bond between the unpaired electrons on the nitrogen atoms. Lewis structure for N₂O₄: O = N - O - N = O #Step 3: Draw the Lewis structure for boron trihydride (BH₃)#

To draw the Lewis structure for BH₃, start by counting the valence electrons. Boron (B) has 3 valence electrons and each hydrogen (H) has 1 valence electron. Thus, we have a total of 3 + 3(1) = 6 valence electrons. Place the boron atom in the center and draw single bonds to the three hydrogen atoms. This uses up 6 valence electrons (2 for each bond). Lewis structure for BH₃: H - B - H | H #Step 4: Draw the Lewis structure for ammonia (NH₃)#

To draw the Lewis structure for NH₃, we start by counting the valence electrons. Nitrogen (N) has 5 valence electrons and each hydrogen (H) has 1 valence electron. Thus, we have a total of 5 + 3(1) = 8 valence electrons. Place the nitrogen atom in the center and draw single bonds to the three hydrogen atoms. This uses up 6 valence electrons (2 for each bond). Distribute the remaining 2 valence electrons as a lone pair on the nitrogen atom. Lewis structure for NH₃: H - N - H | H #Step 5: Draw the Lewis structure for the complex \(\mathrm{BH}_{3} \mathrm{NH}_{3}\) and provide a possible explanation for the reaction#

According to the reaction, BH₃ accepts a pair of electrons from the lone pair of NH₃. Hence, we form a coordinate bond between the nitrogen atom of NH₃ and the boron atom of BH₃. Lewis structure for \(\mathrm{BH}_{3} \mathrm{NH}_{3}\): H - N - H | | B - H | H Explanation: The reaction occurs because boron in BH₃ has an incomplete octet. By forming a coordinate bond with nitrogen in NH₃, boron can achieve a complete octet, making the complex more stable. #Step 6: Provide a possible explanation for reactions (a) and (b)#

Possible explanation for reaction (a): In the case of NO₂, it exists as a free radical due to its unpaired electron in the nitrogen atom. When two NO₂ molecules dimerize to form N₂O₄, they form a more stable molecule without any unpaired electrons by sharing the unpaired electron of each nitrogen atom. Possible explanation for reaction (b): The reaction between BH₃ and NH₃ is an example of a Lewis acid-base reaction. The boron atom in BH₃ has only six valence electrons, making it electron-deficient. The nitrogen atom in NH₃ has a lone pair of electrons, making it an electron donor. Thus, the reaction occurs because boron in BH₃ achieves a complete octet by accepting a lone pair of electrons from the nitrogen atom in NH₃, leading to the formation of a more stable complex molecule.