Problem 119 - Chemistry: An Atoms-Focused Approach

Question

Use appropriate bond energies from Table A4.1 of Appendix 4 to predict whether the reaction in which ethylene forms polyethylene plastic is exothermic, endothermic, or involves no change in enthalpy. The reaction can be written: $$n \mathrm{CH}_{2}=\mathrm{CH}_{2} \rightarrow\left[-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\right]_{n}$$ where the structure in the brackets is the repeating unit of polyethylene and the value of \(n\) is typically in the thousands.

Step-by-Step Solution

Answer

Answer: The reaction is endothermic.

Step 1: Identify the bonds broken and formed in the reaction

In the reaction, ethylene molecules bond together to form polyethylene plastic. We can represent the reaction as follows, with the bond energies included for each bond: $$\mathrm{CH}_{2}=\mathrm{CH}_{2} \xrightarrow{reaction} [-\mathrm{CH}_{2}-\mathrm{CH}_{2}-]$$ In this reaction, the following bonds are broken: - 1 C=C bond The following bonds are formed: - 1 C-C bond - 2 C-H bonds

Step 2: Calculate bond energy changes during the reaction

Using the bond energies provided in the table, we can calculate the energy change for each bond broken and formed. Breaking bond energy: - C=C: 612 kJ/mol Forming bond energy: - C-C: 348 kJ/mol - C-H: 440 kJ/mol (note that 2 C-H bonds are formed) Total energy required to break the bond is: $$\Delta H_{break} = 612 \, kJ/mol$$ Total energy released in forming new bonds is: $$\Delta H_{form} = 348 + 2\times{440} \, kJ/mol = 348 + 880 \, kJ/mol = 1228 \, kJ/mol$$

Step 3: Calculate the overall enthalpy change for the reaction

Now, we can calculate the overall enthalpy change for the reaction: $$\Delta H = \Delta H_{form} - \Delta H_{break}$$ $$\Delta H = 1228 - 612 \, kJ/mol = 616 \, kJ/mol$$ Since the overall enthalpy change is positive, the reaction is endothermic, meaning it requires heat to proceed.

Key Concepts

enthalpy change

Enthalpy change is a critical concept in understanding the energy aspects of chemical reactions. Specifically, it refers to the difference in energy between the reactants and the products of a reaction. In simpler terms, it tells us how much heat is absorbed or released during a chemical reaction. This change is denoted by \( \Delta H \). Positive \( \Delta H \) values indicate that the reaction absorbs energy, while negative values mean energy is released. In the case of polyethylene formation, bonds between ethylene molecules are rearranged. We break one carbon-carbon double bond (C=C), which requires energy, and form a combination of carbon-carbon single bonds (C-C) and carbon-hydrogen bonds (C-H), which release energy. Calculating the total energies involved in breaking and forming these bonds allows us to determine the enthalpy change for the reaction. The calculations showed that the formation of polyethylene is endothermic, with a net \( \Delta H \) of +616 kJ/mol, pointing out the absorption of heat.

polyethylene formation

Polyethylene is a common plastic, used in everything from packaging to bottles. Its formation involves polymerizing ethylene, a simple hydrocarbon with a double bond between carbon atoms. This process can be understood as converting the double bonds in ethylene into long chains of single-bonded carbon atoms, forming the backbone of polyethylene.The reaction involves the breaking of a C=C bond in ethylene, so the ethylene molecules can link up, forming a long chain with repeating units \([-CH_2-CH_2-]_n\). The term \(n\) denotes the number of repeating units, which can be in the thousands, leading to very large molecules. This transformation from small molecules to long chain polymers is crucial as it gives polyethylene its strength and flexibility properties.Understanding the reaction from a chemical perspective involves recognizing the change in bond types, moving from a reactive double bond to stable single bonds in a polymer. By assessing the bond energies involved, we can better understand why polyethylene formation absorbs energy, making it an endothermic process.

endothermic reaction

Endothermic reactions are processes that absorb energy from their surroundings as they occur. This energy uptake is required for breaking chemical bonds in the reactants during the first stage of a reaction, before new bonds can form in the products. As a result, these reactions have a positive enthalpy change, meaning more energy is consumed in breaking bonds than is released in forming new ones. In the context of polyethylene formation from ethylene, the reaction is endothermic. During this process, the energy necessary to break the original C=C double bonds of ethylene exceeds the energy released by the formation of the new C-C and C-H single bonds in polyethylene. This is why the process requires heat to proceed; the surrounding environment needs to supply that extra energy, which is evidence of an endothermic reaction. Recognizing the nature of a reaction as endothermic or exothermic is important for anticipating the conditions required to drive the reaction forward, particularly in industrial applications like polymer production.