The following is known as the thermite reaction: 2 Al(s) + Fe2O3(s) → Al2O3(s) + 2 Fe(s). This highly exothermic reaction is used for welding massive units, such as propellers for large ships. Using standard enthalpies of formation in Appendix C, calculate _x001F_H ° for this reaction.

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Identify the standard enthalpy of formation (ΔH_f°) for each compound involved in the reaction from Appendix C. Note that elements in their standard state, such as Al(s) and Fe(s), have a ΔH_f° of 0.

Write the balanced chemical equation for the thermite reaction: 2 Al(s) + Fe2O3(s) → Al2O3(s) + 2 Fe(s).

Apply Hess's Law to calculate the standard enthalpy change (ΔH°) for the reaction using the formula: ΔH° = Σ(ΔH_f° of products) - Σ(ΔH_f° of reactants).

Substitute the ΔH_f° values for each compound into the formula. For Al2O3(s), use its ΔH_f° value, and for Fe2O3(s), use its ΔH_f° value. Remember that the ΔH_f° for Al(s) and Fe(s) is 0.

Calculate the sum of the ΔH_f° values for the products and subtract the sum of the ΔH_f° values for the reactants to find the ΔH° for the reaction.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Thermodynamics and Enthalpy

Thermodynamics is the study of energy transformations, and enthalpy (ΔH) is a key concept within it, representing the heat content of a system at constant pressure. The standard enthalpy of formation (ΔH°f) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. Understanding these principles is essential for calculating the overall enthalpy change for a chemical reaction.

Standard Enthalpy of Formation

The standard enthalpy of formation (ΔH°f) is a specific value that indicates the energy change associated with the formation of one mole of a compound from its constituent elements in their standard states. Each substance has a unique ΔH°f, which can be found in thermodynamic tables. To calculate the enthalpy change for a reaction, the ΔH°f values of the products and reactants are used in the formula: ΔH° = ΣΔH°f(products) - ΣΔH°f(reactants).

Exothermic Reactions

Exothermic reactions are chemical reactions that release energy, usually in the form of heat, to the surroundings. In the thermite reaction, the combination of aluminum and iron(III) oxide produces aluminum oxide and iron, releasing a significant amount of heat. Recognizing that the reaction is exothermic helps in understanding the energy dynamics involved and the practical applications, such as welding, where high temperatures are required.

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(a) Why does the standard enthalpy of formation of both the very reactive fluorine (F₂) and the almost inert gas nitrogen (N₂) both read zero?

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For each of the following compounds, write a balanced thermochemical equation depicting the formation of one mole of the compound from its elements in their standard states and then look up ΔH°_f for each substance in Appendix C. (a) NO₂(g) (b) SO₃(g) (c) NaBr(s) (d) Pb(NO₃)₂(s).

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Textbook Question

Using values from Appendix C, calculate the value of H for each of the following reactions: (a) CaO(s) + 2 HF(g) → CaF₂(s) + H₂O(g) (b) Fe₂O₃(s) + 3 C(s) → 2 Fe(s) + 3CO(g) (c) 2 CO(g) + 2 NO(g) → N₂(s) + 2 CO₂(g) (d) 4 NH₃(g) + 5 O₂(g) → 4 NO(g) + 6 H₂Og)