Question

Write equations for the following (a) cracking of a hydrocarbon - use \(\mathrm{C}_{14} \mathrm{H}_{30}\), (b) incomplete combustion of acetylene, (c) addition of \(\mathrm{Br}_{2}\) to a mixture of ethane, ethylene, and ethyne.

Short answer

The short answer based on the step-by-step solution is: (a) Cracking of a hydrocarbon: \[ \mathrm{C}_{14}\mathrm{H}_{30} \rightarrow 6\mathrm{C}_{2}\mathrm{H}_{6} + \mathrm{C}_{2}\mathrm{H}_{6} \] (b) Incomplete combustion of acetylene: \[ 2\mathrm{C}_{2} \mathrm{H}_{2} + 3\mathrm{O}_{2} \rightarrow 2 \mathrm{CO} + 2 \mathrm{CO}_{2} + 2 \mathrm{H}_{2}\mathrm{O} \] (c) Addition of \(\mathrm{Br}_{2}\) to a mixture of ethane, ethylene, and ethyne: Reaction with ethane: \[ \mathrm{C}_{2}\mathrm{H}_{6} + \mathrm{Br}_{2} \rightarrow \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{Br} + \mathrm{H}\mathrm{Br} \] Reaction with ethylene: \[ \mathrm{C}_{2}\mathrm{H}_{4} + \mathrm{Br}_{2} \rightarrow \mathrm{C}_{2}\mathrm{H}_{4}\mathrm{Br}_{2} \] Reaction with ethyne: \[ \mathrm{C}_{2}\mathrm{H}_{2} + 2\mathrm{Br}_{2} \rightarrow \mathrm{C}_{2}\mathrm{H}_{2}\mathrm{Br}_{4} \]

Step-by-step solution

(a) Cracking of a hydrocarbon

Cracking is the process where large hydrocarbon molecules are broken down into smaller, more useful molecules. Since the given hydrocarbon is \(\mathrm{C}_{14}\mathrm{H}_{30}\), the goal is to break it down into smaller hydrocarbons. Here, we will break \(\mathrm{C}_{14}\mathrm{H}_{30}\) into ethane and a smaller hydrocarbon. To balance this equation, just ensure there is the same number of each element on both sides of the equation. In this case, the balanced equation is given by: \[ \mathrm{C}_{14}\mathrm{H}_{30} \rightarrow 6\mathrm{C}_{2}\mathrm{H}_{6} + \mathrm{C}_{2}\mathrm{H}_{6} \]

(b) Incomplete combustion of acetylene

Incomplete combustion occurs when a hydrocarbon reacts with insufficient oxygen, producing carbon monoxide (CO) along with carbon dioxide (CO\(_2\)) and water (H\(_2\)O). In this case, the reacting hydrocarbon is acetylene (C\(_2\)H\(_2\)). To balance this equation, adjust the coefficients in front of the molecules until the number of each atom is equal on both sides of the equation. The balanced equation for incomplete combustion of acetylene is given by: \[ 2\mathrm{C}_{2} \mathrm{H}_{2} + 3\mathrm{O}_{2} \rightarrow 2 \mathrm{CO} + 2 \mathrm{CO}_{2} + 2 \mathrm{H}_{2}\mathrm{O} \]

(c) Addition of \(\mathrm{Br}_{2}\) to a mixture of ethane, ethylene, and ethyne

In this case, we have three separate reactions happening when bromine (\(\mathrm{Br}_{2}\)) reacts with a mixture of ethane (\(\mathrm{C}_{2}\mathrm{H}_{6}\)), ethylene (\(\mathrm{C}_{2}\mathrm{H}_{4}\)), and ethyne (\(\mathrm{C}_{2}\mathrm{H}_{2}\)). We will write the balanced equations for each of the reactions separately: Reaction of \(\mathrm{Br}_{2}\) with ethane: \[ \mathrm{C}_{2}\mathrm{H}_{6} + \mathrm{Br}_{2} \rightarrow \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{Br} + \mathrm{H}\mathrm{Br} \] Reaction of \(\mathrm{Br}_{2}\) with ethylene: \[ \mathrm{C}_{2}\mathrm{H}_{4} + \mathrm{Br}_{2} \rightarrow \mathrm{C}_{2}\mathrm{H}_{4}\mathrm{Br}_{2} \] Reaction of \(\mathrm{Br}_{2}\) with ethyne: \[ \mathrm{C}_{2}\mathrm{H}_{2} + 2\mathrm{Br}_{2} \rightarrow \mathrm{C}_{2}\mathrm{H}_{2}\mathrm{Br}_{4} \]

Key concepts

Cracking of Hydrocarbons

Understanding the cracking of hydrocarbons is essential in the study of organic chemistry and for processes in the petrochemical industry. Cracking is the process of breaking down large hydrocarbon molecules, such as C₁₄H₃₀, into smaller, more manageable and useful molecules like ethane. During the cracking process, high temperatures and sometimes catalysts are used to facilitate the breaking of chemical bonds.

Imagine you have a long chain of paperclips linked together; cracking hydrocarbons is somewhat like separating these paperclips into smaller chains or individual clips. This is achieved by ensuring that the number of carbon (C) and hydrogen (H) atoms on the reactant side is equal to the number on the product side, which is the essence of balancing chemical equations. When balancing the equation for cracking, you end up with multiple smaller hydrocarbon molecules from the original large molecule, as seen in the example with C₁₄H₃₀.

Incomplete Combustion of Hydrocarbons

When hydrocarbons such as acetylene (C₂H₂) burn in an environment with limited oxygen, we observe incomplete combustion. This is different from complete combustion, which uses ample oxygen, leading mostly to carbon dioxide (CO₂) and water (H₂O) as products.

Incomplete combustion results in a mix of carbon monoxide (CO), carbon dioxide, and water. This is because the oxygen available can't fully oxidize the carbon atoms to carbon dioxide. As a visual aid, consider a scenario where you are trying to light a large bonfire with just a few matches; the fire won't have enough to burn brightly and completely. This concept highlights the importance of having a balanced chemical equation to represent the reaction properly. If the equation isn't balanced, it's akin to not accounting for all the wood or matches in our bonfire analogy.

Bromination of Hydrocarbons

Bromination is an example of an addition reaction where bromine (Br₂) is added across the double or triple bonds in unsaturated hydrocarbons such as ethylene or ethyne. This reaction is also used as a test to differentiate between saturated and unsaturated hydrocarbons, as bromine water undergoes a color change when it reacts with unsaturated hydrocarbons.

The nature of bromination reactions varies with the type of hydrocarbon. With ethane (C₂H₆), a substitution reaction occurs; in ethylene (C₂H₄) and ethyne (C₂H₂), bromine adds on to the double or triple bonds to form dibromo or tetrabromo compounds, respectively. These reactions are also a beautiful illustration of the versatility and variability in organic chemistry, where different types of bonds and molecular structures lead to different reactions and products.

Balancing Chemical Equations

Balancing chemical equations is a fundamental skill in chemistry, akin to ensuring that both sides of a scale are equal. It involves adjusting the coefficients, the numbers before the chemical formulas, so that the number of atoms of each element is the same on both the reactant and product sides of the equation.

Consider that each chemical symbol or formula in an equation is like a puzzle piece; balancing the equation is about arranging these pieces so that they fit perfectly on either side of the reaction arrow. It's crucial for students to understand this principle as it applies to all kinds of chemical reactions, whether it's the cracking of hydrocarbons, incomplete combustion, or addition reactions like bromination.

Organic Chemistry Reactions

Organic chemistry is the study of carbon-containing compounds, and it comprises a multitude of reactions, each with its own unique set of rules and outcomes. There are reactions for creating new bonds, breaking others, and changing the structures of molecules.

Some reactions, such as the cracking of hydrocarbons, involve the breaking of large molecules into smaller ones. Incomplete combustion of hydrocarbons reveals what happens when there isn't enough oxygen; only partly oxidized products are formed. Bromination showcases the addition of elements across unsaturated bonds. The capacity to understand organic chemistry hinges on grasping the behavior of diverse molecules and how to depict those behaviors through the language of chemical equations. Think of each type of reaction as a different form of molecular choreography, where atoms and groups of atoms move and combine in various patterns to create a spectacular array of chemical compounds.