Applications of Hess's Law

Applications of Hess's Law

Introduction

• Hess's Law allows us to calculate the enthalpy change of a reaction using known enthalpy changes of related reactions.
• In this section, we will explore two examples of how Hess's Law is applied to calculate enthalpy changes.

Example 1: Formation of Ethanoic Acid

Target Equation: The formation of ethanoic acid (CH₃COOH) from its elements:

• C(s) + 2H₂(g) + 2O₂(g) → CH₃COOH(l)

We'll use three equations:

• The combustion of carbon:
  • C(s) + O₂(g) → CO₂(g)
• The combustion of hydrogen:
  • H₂(g) + 1/2O₂(g) → HO(l)
• The combustion of ethanoic acid:
  • CH₃COOH(l) + 3/2O₂(g) → 2H₂O(l) + 2CO₂(g)

Steps:

• Adjust equations as needed:
  • Reverse equation 2 and multiply equation 1 by 2.
  • Check if equations cancel out to reach the target equation.
  • Calculate ΔH for the target equation.
• The enthalpy change for the formation of ethanoic acid is calculated using the adjusted equations.

Example 2: Formation of a Compound

Target Equation: Formation of a compound (not specified in the question):

• A(s) + B(g) → AB(s)

We'll use three equations:

• Equation 1: Given enthalpy change for A(s) → A(g)
• Equation 2: Given enthalpy change for B(g) → B(s)
• Equation 3: Enthalpy change for the target equation AB(s) → A(g) + B(g)

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Applications of Hess's Law

Steps:

• Write out the equations and number them.
• Modify equations to fit the target equation (reverse or multiply as needed).
• Check if equations can be combined to reach the target equation.
• Calculate ΔH for the target equation.
• The enthalpy change for the formation of the compound AB(s) is calculated using the adjusted equations.

Conclusion

• Hess's Law allows us to calculate enthalpy changes for reactions by manipulating known enthalpy change equations.
• We can adjust and combine equations to reach the target equation.
• This approach is a systematic way to calculate enthalpy changes and is widely used in thermochemistry.