When a gaseous compound X containing only C, H, and Ois burned in O2, 1 volume of the unknown gas reacts with3 volumes of O2 to give 2 volumes of CO2 and 3 volumesof gaseous H2O. Assume all volumes are measured at thesame temperature and pressure.(d) Combustion of 5.000 g of X releases 144.2 kJ heat.Look up ΔH°f values for CO21g2 and H2O1g2 inAppendix B, and calculate ΔH°f for compound X.
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1
Write the balanced chemical equation for the combustion of compound X. Use the stoichiometry provided: 1 volume of X reacts with 3 volumes of O2 to produce 2 volumes of CO2 and 3 volumes of H2O. Represent the compound X as CxHyOz.
Use the stoichiometric coefficients from the balanced equation to set up the enthalpy change equation for the reaction. Use the formula: ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants).
Look up the standard enthalpy of formation (ΔH°f) values for CO2(g) and H2O(g) in the provided appendix or a reliable chemistry data source.
Substitute the ΔH°f values for CO2 and H2O into the enthalpy change equation. Include the heat released during the combustion, which is given as 144.2 kJ, to solve for the ΔH°f of compound X.
Ensure the units are consistent, typically kJ/mol, and solve for ΔH°f of compound X using the equation set up in step 3.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Stoichiometry of Combustion Reactions
Stoichiometry involves the calculation of reactants and products in chemical reactions. In combustion reactions, the stoichiometric coefficients indicate the ratio of volumes of gases involved. For the given reaction, understanding the volume ratios helps determine the moles of each substance, which is essential for calculating the enthalpy change.
The enthalpy of formation (ΔH°f) is the heat change associated with the formation of one mole of a compound from its elements in their standard states. It is crucial for calculating the overall enthalpy change of a reaction. By using the ΔH°f values of the products (CO2 and H2O), one can apply Hess's law to find the ΔH°f for the unknown compound X.
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 for the calculation of ΔH°f for compound X by using the known enthalpy changes of the products and the heat released during the combustion of X. It is a fundamental concept in thermochemistry.
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