Lecture Notes: Thermochemical Equations and Hess's Law

Introduction

• Discussion on a previous example involving a reaction between NaOH and HCl.
• Key Result:
  • 50 mL of 1 M NaOH reacts with 50 mL of 1 M HCl releasing -2.87 kJ of energy.

Thermochemical Equations

• A regular balanced chemical equation cannot directly incorporate calculated ΔH values.
• Reason: Amount of heat is related to the amount of substance used.
  • Example: Larger amounts of reactants produce more energy.
• Standard ΔH (ΔH°): Used in thermochemical equations.
  • Corresponds to stoichiometric amounts as per the balanced chemical equation.

Calculation of Standard ΔH

• Example: Reaction of 0.050 mol of NaOH and HCl.
  • Equation: ΔH° = (-2.87 kJ / 0.050 mol) * 1 mol = -57.4 kJ.
• Thermochemical Equation: Represents the reaction involving one mole of each reactant.

Application of Thermochemical Equations

• Purpose: Allows calculation of energy absorbed or released during a reaction for any amount of reactants/products.
• Example: N2O formation
  • Thermochemical equation given with ΔH° = +163.14 kJ for the reaction 2 N2 + O2 -> 2 N2O.
  • Calculation needed if 200 g of N2O is produced.

Hess's Law

• Introduces Hess's Law for calculating the ΔH of reactions.
• Concept: ΔH of a reaction is the sum of ΔH of individual steps.
• State Function: ΔH depends only on the initial and final states, not the path taken.

Solving Hess's Law Problems

• Method:
  • Identify each component in the target equation from known equations.
  • Manipulate known equations to match the target equation components.
  • Adjust ΔH values as equations are manipulated.
• Example: Combination of two known reactions to find ΔH for a target reaction.
  • Reverse and multiply equations as necessary.
  • Combine to get the desired equation and calculate ΔH.

Key Strategies

• Avoid components found in multiple known equations until necessary.
• Ensure all manipulations to equations are reflected in ΔH changes.

Conclusion

• Practice Hess’s Law problems for mastery.
• Use consistent methods for success in exams.
• Future lectures will include more examples for practice.