Hukum Hess | KIMIA KELAS 11

Introduction

This tutorial explains Hess's Law, a fundamental principle in chemistry that allows us to calculate the enthalpy changes of chemical reactions. Understanding this law is essential for students in Grade 11 chemistry, as it provides a structured approach to determine the enthalpy of reactions that might be difficult to measure directly. We will also explore how to apply this law through examples and practice problems.

Step 1: Understand Hess's Law

• Hess's Law states that the total enthalpy change during a chemical reaction is the same, regardless of the number of steps in which the reaction occurs.
• This means that if a reaction can be expressed as the sum of multiple steps, the total enthalpy change will be the sum of the enthalpy changes for each step.
• Practical Tip: Visualize chemical reactions as pathways. Just like you can take various routes to reach a destination, the enthalpy change will remain constant regardless of the route taken.

Step 2: Identify the Reaction Steps

• Break down the overall reaction into simpler steps if it's complex.
• Write balanced equations for each step of the reaction:
  1. Identify reactants and products.
  2. Ensure that the number of atoms of each element is the same on both sides of the equation.
• Example:
  • If you have a reaction A → C, and you know that A → B and B → C are intermediate steps, you can use those to find the enthalpy of A → C.

Step 3: Gather Enthalpy Values

• Collect the known enthalpy changes for each step:
  • These values can often be found in thermodynamic tables or literature.
• Keep track of the sign of each enthalpy change:
  • Positive values indicate endothermic reactions (heat absorbed).
  • Negative values indicate exothermic reactions (heat released).

Step 4: Apply Hess's Law

• Use the gathered enthalpy values to calculate the total enthalpy change for the overall reaction:
  1. Add the enthalpy changes of the steps.
  2. Ensure to keep track of the signs (positive or negative) when performing the addition.
• Example Calculation:
  • If the enthalpy change for A → B is -150 kJ and for B → C is +50 kJ:
    • Total ΔH = (-150) + (+50) = -100 kJ for the reaction A → C.

Step 5: Practice with Example Problems

• Solve practice problems to reinforce understanding:
  • Write out the steps.
  • Gather enthalpy changes.
  • Apply Hess's Law to find the total enthalpy change.
• Common Pitfall: Ensure that all equations are balanced and that you correctly account for the direction of the reaction when gathering enthalpy values.

Conclusion

Hess's Law is a powerful tool for determining the enthalpy changes of reactions. By breaking down complex reactions into simpler steps, gathering the necessary enthalpy values, and applying the law correctly, you can calculate the overall enthalpy change with confidence. Practice using various examples to solidify your understanding and prepare for exams.