LEI DE HESS (TERMOQUÍMICA) | Resumo de Química para o Enem

Summary

The video is a chemistry lesson covering Hess's Law. It explains key concepts and common mistakes students make when calculating enthalpy changes, especially for the ENEM exam. The instructor emphasizes the importance of understanding global reactions and their intermediate steps, advising a review of enthalpy (ΔH) and reaction equations before proceeding. The video includes step-by-step calculations to illustrate how to correctly sum enthalpy values, adjust equations, and account for sign changes. It concludes with an ENEM exam question on benzene synthesis, demonstrating how to apply Hess's Law in a practical context.

Takeaways

• 📚 The class focuses on Hess's Law, a key concept in chemistry.
• 🧠 It's important to review enthalpy (ΔH) and global reactions before diving into Hess's Law.
• ⚙️ Hess's Law helps calculate the total ΔH (enthalpy change) of a reaction based on intermediate steps, focusing only on initial and final states.
• 🧮 The total ΔH for a global reaction is the sum of the ΔH values of each individual reaction involved.
• 💡 You can treat chemical equations like mathematical equations: addition, subtraction, multiplication, and division can be applied.
• 🔍 Common mistakes students make include forgetting to apply ΔH changes when multiplying, dividing, or reversing equations.
• 🌡️ The reaction's ΔH sign (positive for endothermic, negative for exothermic) is crucial for calculations.
• 🧪 In an example involving carbon dioxide (CO2), the ΔH is calculated by summing the ΔH values of the intermediate reactions.
• ✂️ When manipulating reactions (like inverting or multiplying equations), it's important to also adjust the ΔH accordingly.
• 📝 The ENEM question example involves calculating ΔH for the formation of benzene, demonstrating the application of Hess's Law with multiple reaction steps.

Q & A

What is the main topic covered in this chemistry lesson?

The lesson focuses on Hess's Law and how to calculate the enthalpy change (ΔH) of a global reaction based on the enthalpy changes of individual reactions.

Why is it important to review enthalpy (ΔH) and global reactions before starting the lesson?

Understanding enthalpy (ΔH) and how global reactions are calculated helps students follow the lesson on Hess's Law, which involves summing the enthalpy changes of individual reactions to find the total ΔH.

What is Hess's Law and how does it apply to chemical reactions?

Hess's Law states that the enthalpy change of a chemical reaction is independent of the path taken, meaning it depends only on the initial and final states. This allows us to calculate the ΔH of a global reaction by summing the ΔH values of individual steps.

What common mistakes do students make when applying Hess's Law?

Students often forget to apply mathematical operations (addition, subtraction, multiplication, division, or inversion) to the ΔH values when manipulating the equations, which leads to incorrect final answers.

How are chemical equations treated mathematically when using Hess's Law?

Chemical equations can be treated like mathematical equations in Hess's Law. This means that when you sum, multiply, or invert chemical equations, you must also apply the same operations to the corresponding ΔH values.

In the example involving the formation of CO2, what are the two individual reactions involved?

The two individual reactions are: 1) Carbon graphite reacts with oxygen to form CO and oxygen gas, and 2) CO reacts with oxygen to form CO2.

What happens to intermediates like CO in the overall reaction to form CO2?

Intermediates like CO are formed and then consumed in subsequent reactions, so they do not appear in the final global reaction. Only the initial reactants (carbon and oxygen) and the final product (CO2) are present.

What key detail must students pay attention to when summing ΔH values for reactions?

Students must pay attention to the signs of ΔH (positive for endothermic reactions and negative for exothermic reactions) and ensure they correctly apply any necessary operations, like inversion or multiplication, to the ΔH values.

In the provided ENEM question example, why did the ΔH value for the formation of benzene need to be multiplied and inverted?

The ΔH value needed to be multiplied because the reaction required three moles of acetylene, and it needed to be inverted because the benzene was a product in the final global reaction, but a reactant in one of the individual reactions.

What was the final ΔH value for the trimerization of acetylene to form benzene in the ENEM question?

The final ΔH value for the trimerization of acetylene to form benzene was -150 kcal, indicating that the reaction is exothermic.