Problem 140 - Chemistry The Science in Context

Question

Industrial Use of Cellulose Research is being carried out on cellulose as a source of chemicals for the production of fibers, coatings, and plastics. Cellulose consists of long chains of glucose molecules \(\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right),\) so for the purposes of modeling the reaction, we can consider the conversion of glucose to formaldehyde (CH \(_{2} \mathrm{O}\) ). a. Is the reaction to convert glucose into formaldehyde an oxidation or a reduction? b. Calculate the heat of reaction for the conversion of 1 mole of glucose into formaldehyde, given the following thermochemical data: \(\Delta H_{\text {comb }}^{\circ}\) of formaldehyde gas \(\quad=-572.9 \mathrm{kJ} / / \mathrm{mol}\) \(\Delta H_{\mathrm{f}}^{\circ}\) of solid glucose \(\quad=-1274.4 \mathrm{kJ} / \mathrm{mol}\) \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(s) \rightarrow 6 \mathrm{CH}_{2} \mathrm{O}(g)\) Glucose Formaldehyde

Step-by-Step Solution

Answer

Answer: The conversion of glucose into formaldehyde is an oxidation reaction. The heat of reaction for this process is -2163 kJ/mol.

Step 1: Determine the type of reaction

In order to determine if the reaction is an oxidation or reduction, we will analyze the change in oxidation numbers of glucose and formaldehyde. To do this, let us first find the oxidation number of the carbon atom in glucose and formaldehyde. Oxidation numbers: Oxygen: -2 Hydrogen: +1 The glucose molecule consists of C, H, and O atoms. The oxidation number for carbon (C) in the molecule is unknown. Let x be the oxidation number of C. In glucose, C6H12O6: 6x + 12(+1) + 6(-2) = 0 6x = 0 x = 0 The oxidation number of carbon in glucose is 0. In formaldehyde, CH2O: x + 2(+1) + 1(-2) = 0 x - 2 = 0 x = +2 The oxidation number of carbon in formaldehyde is +2. Now, we will compare the changes in the oxidation numbers to determine if the reaction is oxidation or reduction. For glucose, C6H12O6: 6C(0) → 6C(+2) The conversion of glucose to formaldehyde involves an increase in the oxidation number of carbon from 0 to +2. Because the oxidation number of carbon in glucose increases, this is an oxidation reaction.

Step 2: Calculate the heat of reaction

To calculate the heat of reaction, we need to find the difference in enthalpy between the products and reactants. We are given the heat of combustion of formaldehyde gas and the heat of formation of solid glucose. The balanced equation for the conversion of glucose to formaldehyde is: \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(s) \rightarrow 6 \mathrm{CH}_{2} \mathrm{O}(g)\) Let's use the given thermochemical data to calculate the heat of reaction: ΔH(reactants) = -1274.4 kJ/mol (Heat of formation of solid glucose) ΔH(products) = 6 × (-572.9) kJ/mol (Heat of combustion of formaldehyde gas, multiplied by 6 since there are 6 moles of formaldehyde in the reaction) Now, subtract ΔH(reactants) from ΔH(products) to find the heat of reaction: ΔH(reaction) = ΔH(products) - ΔH(reactants) ΔH(reaction) = 6 × (-572.9) - (-1274.4) ΔH(reaction) = -3437.4 + 1274.4 ΔH(reaction) = -2163 kJ/mol The heat of reaction for the conversion of 1 mole of glucose into formaldehyde is -2163 kJ/mol.

Key Concepts

Cellulose Research

Research into cellulose is crucial in industrial chemistry for its potential to produce diverse materials. Cellulose is a natural polymer found in plant cell walls and consists of long chains of glucose units. It is a renewable resource, making it a sustainable choice for manufacturing various industrial products such as fibers, coatings, and plastics. Understanding the chemistry of cellulose allows scientists to convert it into valuable derivatives through chemical reactions. This includes breaking down cellulose into its glucose components and further converting them into different chemicals, like formaldehyde, used in numerous industrial applications. This conversion requires understanding the chemical processes involved, such as oxidation and thermochemical calculations.

Oxidation Reaction

An oxidation reaction involves the increase in the oxidation state of a molecule, atom, or ion. In the conversion of glucose to formaldehyde, an oxidation reaction occurs. Each carbon atom's oxidation state changes from 0 in glucose to +2 in formaldehyde.
The oxidation number acts as an indicator of the electron exchange in a chemical reaction:

• For glucose, the carbon atoms initially have an oxidation state of 0.
• In formaldehyde, the oxidation state of carbon becomes +2.

This increase indicates that the carbon atoms have lost electrons, confirming the process as oxidation. Understanding these changes allows us to properly categorize the type of reaction taking place, which is fundamental in industrial chemistry applications.

Heat of Reaction Calculation

The heat of reaction, also known as the enthalpy change, is calculated by determining the difference in energy between the reactants and the products of a chemical reaction. This is essential to measure for understanding the energy efficiency and feasibility of industrial processes.
In the glucose to formaldehyde conversion, the heat of reaction is determined using these steps:

• Record the heat of formation for the reactant: glucose (\( \Delta H_{\mathrm{f}}^{\circ} = \text{-1274.4 kJ/mol} \)
• Record the heat of combustion for the product: formaldehyde (\( \Delta H_{\text{comb}}^{\circ} = \text{-572.9 kJ/mol} \)
• Multiply this value by 6, because six formaldehyde molecules are produced.
• Calculate the enthalpy of products and subtract the enthalpy of reactants to find the heat of reaction:\[ \Delta H(\text{reaction}) = (6 \times -572.9) - (-1274.4) = -2163 \text{ kJ/mol} \]

Calculating this heat helps scientists and engineers evaluate the practicality and sustainability of chemical processes.

Thermochemistry

Thermochemistry studies the heat involved in chemical reactions. It helps measure energy changes so that we can predict product yield and reaction conditions.
In industrial chemistry, concepts like enthalpy, exothermic, and endothermic reactions are important:

• **Enthalpy (\( \Delta H \))** is the heat content of a system at constant pressure. Calculating the enthalpy change in reactions helps determine how heat is absorbed or released.
• **Exothermic reactions** release energy, as seen in the conversion of glucose to formaldehyde, which has a negative heat of reaction.
• **Endothermic reactions** absorb energy from their surroundings, indicated by a positive heat of reaction.

Understanding thermochemistry allows us to optimize reactions for energy consumption, making chemical processes more efficient and sustainable.