The industrial degreasing solvent methylene chloride, CH2Cl2, is prepared from methane by reaction with chlorine: CH4(g) + 2 Cl2(g) → CH2Cl2(g) + 2 HCl(g) Use the following data to calcualte ΔH° in kilojoules for the reaction: CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g) ΔH° = -98.3 kJ CH3Cl(g) + Cl2(g) → CH2Cl2(g) + HCl(g) ΔH° = -104 kJ
Notice that the target reaction can be seen as the sum of two given reactions: CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g) and CH3Cl(g) + Cl2(g) → CH2Cl2(g) + HCl(g).
Write the given reactions and their enthalpy changes: 1) CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g), ΔH° = -98.3 kJ; 2) CH3Cl(g) + Cl2(g) → CH2Cl2(g) + HCl(g), ΔH° = -104 kJ.
Add the two reactions together to obtain the target reaction: CH4(g) + 2 Cl2(g) → CH2Cl2(g) + 2 HCl(g).
Add the enthalpy changes of the two reactions to find the ΔH° for the target reaction: ΔH° = (-98.3 kJ) + (-104 kJ).
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Hess's Law
Hess's Law states that the total enthalpy change for a chemical reaction is the sum of the enthalpy changes for the individual steps of the reaction, regardless of the pathway taken. This principle allows us to calculate the enthalpy change for a reaction that may not be directly measurable by using known enthalpy changes of related reactions.
Enthalpy change (ΔH°) is a measure of the heat content of a system at constant pressure. It indicates whether a reaction is exothermic (releases heat, ΔH° < 0) or endothermic (absorbs heat, ΔH° > 0). Understanding ΔH° is crucial for predicting the energy changes associated with chemical reactions.
Standard state conditions refer to a set of specific conditions (usually 1 atm pressure and a specified temperature, typically 25°C) under which thermodynamic measurements are made. These conditions provide a reference point for comparing the enthalpy changes of different reactions, ensuring consistency in thermodynamic data.
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