Ethanol (C₂H₅OH) is blended with gasoline as an automobile fuel. (b) Calculate the standard enthalpy change for the reaction, assuming H₂O(g) as a product.

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Identify the balanced chemical equation for the combustion of ethanol: \( \text{C}_2\text{H}_5\text{OH}(l) + 3\text{O}_2(g) \rightarrow 2\text{CO}_2(g) + 3\text{H}_2\text{O}(g) \).

Look up the standard enthalpy of formation (\( \Delta H_f^\circ \)) for each substance involved in the reaction: \( \text{C}_2\text{H}_5\text{OH}(l) \), \( \text{O}_2(g) \), \( \text{CO}_2(g) \), and \( \text{H}_2\text{O}(g) \).

Apply Hess's Law to calculate the standard enthalpy change of the reaction (\( \Delta H_{rxn}^\circ \)) using the formula: \( \Delta H_{rxn}^\circ = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants}) \).

Substitute the values of \( \Delta H_f^\circ \) for each product and reactant into the formula. Remember to multiply the \( \Delta H_f^\circ \) of each substance by its stoichiometric coefficient from the balanced equation.

Calculate the sum of the enthalpies of the products and subtract the sum of the enthalpies of the reactants to find the standard enthalpy change for the reaction.

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Standard Enthalpy Change

Standard enthalpy change (ΔH°) refers to the heat change that occurs at constant pressure when reactants are converted to products under standard conditions (1 atm pressure and a specified temperature, usually 25°C). It is a crucial concept in thermodynamics, allowing chemists to predict the energy changes associated with chemical reactions.

Combustion Reactions

Combustion reactions involve the reaction of a substance with oxygen, producing heat and light. In the case of ethanol, the combustion reaction typically produces carbon dioxide and water. Understanding the stoichiometry of these reactions is essential for calculating the enthalpy change associated with the combustion of ethanol.

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps, regardless of the pathway taken. This principle allows chemists to calculate the enthalpy change for complex reactions by using known enthalpy values of simpler reactions, facilitating the determination of the standard enthalpy change for the combustion of ethanol.

Ethanol (C2H5OH) is blended with gasoline as an automobile fuel. (b) Calculate the standard enthalpy change for the reaction, assuming H2O(g) as a product.