Calculate Delta H for The Following Reaction C2h4 H2 C2h6

Introduction

The change in enthalpy (ΔH) for a chemical reaction is a measure of the heat absorbed or released during the reaction. For the reaction C2H4 + H2 → C2H6, we can calculate ΔH using standard enthalpy values and Hess's Law.

Hess's Law states that the total enthalpy change for a reaction is the same regardless of the pathway taken, provided the initial and final states are the same. This allows us to calculate ΔH for complex reactions by combining standard enthalpy values of simpler reactions.

How to Calculate ΔH

To calculate ΔH for the reaction C2H4 + H2 → C2H6, follow these steps:

1. Identify the standard enthalpy of formation (ΔHf) for each compound involved in the reaction.
2. Calculate the total enthalpy of the reactants by summing the ΔHf values of C2H4 and H2.
3. Calculate the total enthalpy of the products by summing the ΔHf value of C2H6.
4. Determine ΔH for the reaction by subtracting the total enthalpy of the reactants from the total enthalpy of the products.

Standard enthalpy values are typically reported in units of kilojoules per mole (kJ/mol).

Standard Enthalpy Values

The standard enthalpy of formation (ΔHf) for each compound is the enthalpy change when one mole of the compound is formed from its elements in their standard states. Here are the standard enthalpy values for the compounds involved in this reaction:

Note: The standard enthalpy of formation for H2 is 0 because hydrogen gas is one of the elements in its standard state.

Example Calculation

Let's calculate ΔH for the reaction C2H4 + H2 → C2H6 using the standard enthalpy values provided.

1. Total enthalpy of reactants: ΔHf(C2H4) + ΔHf(H2) = 52.5 kJ/mol + 0 kJ/mol = 52.5 kJ/mol
2. Total enthalpy of products: ΔHf(C2H6) = -84.7 kJ/mol
3. ΔH for the reaction: ΔH_reaction = ΔHf(C2H6) - [ΔHf(C2H4) + ΔHf(H2)] = -84.7 kJ/mol - 52.5 kJ/mol = -137.2 kJ/mol

This calculation shows that the reaction C2H4 + H2 → C2H6 releases 137.2 kJ of energy per mole of ethane formed.

Interpreting Results

The calculated ΔH value provides several important insights about the reaction:

• Exothermic vs. Endothermic: A negative ΔH indicates an exothermic reaction (heat is released), while a positive ΔH indicates an endothermic reaction (heat is absorbed).
• Energy Content: The magnitude of ΔH shows how much energy is involved in the reaction. Larger absolute values indicate more energy is exchanged.
• Feasibility: Exothermic reactions are often more favorable because they release energy, which can be harnessed for useful work.

For the reaction C2H4 + H2 → C2H6, the negative ΔH value of -137.2 kJ/mol indicates that the reaction is highly exothermic, releasing a significant amount of energy.