Question

Learning Goal:

To understand how to use Hess's law to find the enthalpy of an overall reaction.

The change in enthalpy, ΔH, is the heat absorbed or produced during any reaction at constant pressure. Hess's law states that ΔH for an overall reaction is the sum of the ΔH values for the individual reactions. For example, if we wanted to know the enthalpy change for the reaction

3Mn+3O2→3MnO2

we could calculate it using the enthalpy values for the following individual steps:

Step 1: 4Al+3O2→2Al2O3

Step 2: 3Mn+2Al2O3→3MnO2+4Al

Overall: 3Mn+3O2→3MnO2

If the enthalpy change is −3352 kJ/mol for step 1 and 1792 kJ/mol for step 2, then the enthalpy change for the overall reaction is calculated as follows:

ΔH=−3352+1792=−1560 kJ/mol

It is also important to note that the change in enthalpy is a state function, meaning it is independent of path. In other words, the sum of the ΔH values for any set of reactions that produce the desired product from the starting materials gives the same overall ΔH.

Now consider the following set of reactions:

C2H5OH + 3O2→2CO2 + 3H2O ,ΔH=−1370 kJ/mol

C2H4 + 3O2→2CO2 + 2H2O ,ΔH=−1410 kJ/mol

Part A

Part complete

The equations given in the problem introduction can be added together to give the following reaction:

overall: C2H4 + H2O→C2H5OH

However, one of them must be reversed. Which one?

View Available Hint(s)

reaction 1:  C2H5OH + 3O2→2CO2 + 3H2O

reaction 2:  C2H4 + 3O2→2CO2 + 2H2O

What is the enthalpy for reaction 1 reversed?

reaction 1 reversed: 2CO2 + 3H2O→C2H5OH + 3O2

Answer 1