Final answer:
To calculate the standard enthalpy change for a reaction, we use the formula ΔH°rxn = ΣΔH°products - ΣΔH°reactants. For the given decomposition reaction, the change is -1790 kJ/mol. This is found using Hess's law and standard enthalpies of formation.
Step-by-step explanation:
Understanding ΔH°rxn for Decomposition Reactions
To determine the standard enthalpy change for a reaction (ΔH°rxn), we can use the formula ΔH°rxn = ΣΔH°products - ΣΔH°reactants. For decomposition reactions, we calculate the enthalpy change by considering the bond energies involved in breaking and forming chemical bonds. This involves using Hess's law and the standard enthalpies of formation for the substances involved in the reaction.
Using the provided data, we see that the enthalpy change for this decomposition reaction is given as:
ΔH°rxn = [ 3 (−393.5 kJ/mol) + 3 (−285.83 kJ/mol) ] - [ (−248 kJ/mol) + 4 (0 kJ/mol) ] = −1790 kJ/mol
This calculation accounts for the decomposition of reactants into their elemental forms and then the formation of products from these elements, with the appropriate adjustments made for the coefficients of each substance in the balanced chemical equation.
Example for Hess's Law Application
To estimate the enthalpy change for a different reaction, you could reverse relevant reactions, adjust coefficients, and sum the enthalpy changes. An example provided involves carbon monoxide combining with oxygen to form carbon dioxide:
- 2 CO(g) → 2 C(s) + O₂(g) ΔH = +221.0 kJ
- C(s) + O₂(g) → CO₂(g) ΔH = −393.5 kJ
By manipulating these equations, you can get the net enthalpy change for the formation of carbon dioxide from carbon monoxide.