Chemistry Practice Problems Hess's Law Practice Problems Solution: Consider the following set of reactions:N2 + 2O2 →...

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Solution: Consider the following set of reactions:N2 + 2O2 → N2O4 ,ΔH=−8 kJ/molN2 + O2 → 2NO ,ΔH=180 kJ/molThe equations given in the problem introduction can be added together to give the following reaction:overall: N2O4 → 2NO + O2However, one of them must be reversed, reaction 1: N2 + 2O2 → N2O4What is the enthalpy for reaction 1 reversed?reaction 1 reversed: N2O4 → N2 + 2O2Express your answer numerically in kilojoules per mole.What is the enthalpy for reaction 2?

Problem

Consider the following set of reactions:

N2 + 2O→ N2O4H=−8 kJ/mol

N2 + O→ 2NO ,ΔH=180 kJ/mol


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

overall: N2O→ 2NO + O2

However, one of them must be reversed, reaction 1: N2 + 2O→ N2O4

What is the enthalpy for reaction 1 reversed?

reaction 1 reversed: N2O→ N2 + 2O2

Express your answer numerically in kilojoules per mole.

What is the enthalpy for reaction 2?

Solution

We’re being asked to determine the enthalpy change (ΔHrxn) for the chemical reaction:


N2O→ 2 NO + O2


We can use Hess’s Law to determine the enthalpy change of the overall reaction from the given reactions:


1. N2 + 2 O2  →  N2O4      ΔH= −8 kJ/mol

2. N2 + O2  →  2 NO         ΔH= 180 kJ/mol


We now need to find a combination of reactions that when added up, gives us the overall reaction.


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