HUKUM HESS : Menentukan perubahan entalpi reaksi dengan Hukum Hess

Summary

This video explains the concept of determining enthalpy changes (ΔH) using Hess's Law, which states that the total enthalpy change of a reaction is the sum of the enthalpy changes of the steps involved, regardless of the reaction pathway. The speaker provides multiple examples to demonstrate how to apply Hess's Law, including balancing chemical equations, adjusting reaction coefficients, and using known enthalpy values to calculate unknown ones. The video also covers practical examples and problem-solving techniques for calculating the enthalpy of various reactions, offering viewers a clear understanding of the law's application in thermochemistry.

Takeaways

• 😀 Hess's Law states that the change in enthalpy (ΔH) for a reaction depends only on the initial and final states, not the path the reaction takes.
• 😀 The change in enthalpy for a reaction can be calculated by adding the enthalpy changes of individual steps in multi-step reactions.
• 😀 Example: Carbon (C) reacts with oxygen (O2) to form CO2, and this reaction can proceed in one step or two steps, with the total ΔH remaining the same.
• 😀 The mathematical relationship for Hess's Law is ΔH1 = ΔH2 + ΔH3, where the total enthalpy change is the sum of the enthalpy changes of the individual steps.
• 😀 When solving for ΔH in a chemical reaction, balance the coefficients of reactants and products as needed to match the desired reaction.
• 😀 In a multi-step reaction, if the desired reaction is on the opposite side of the equation, reverse the reaction and change the sign of ΔH.
• 😀 For the reaction of calcium hydroxide (CaOH2) formation, Hess’s Law is used to calculate the enthalpy change based on the given reactions.
• 😀 When dealing with the formation of compounds, identify the elemental forms of the reactants and their standard states.
• 😀 For the formation of 37g of CaOH2, the molar amount is calculated first (0.5 mol) and then multiplied by the calculated ΔH to determine the enthalpy change for that amount.
• 😀 In some cases, when the question involves a diagram or graph, Hess's Law can be applied by summing the enthalpy changes of the steps depicted.
• 😀 In the case of water evaporation, ΔH is calculated by identifying the energy required for the phase change from liquid to gas and adjusting for the number of moles involved.

Q & A

What does Hess's Law state about enthalpy changes in chemical reactions?

Hess's Law states that the total enthalpy change (ΔH) of a chemical reaction is the same regardless of the number of steps the reaction takes. The enthalpy change depends only on the initial and final states, not on the reaction pathway.

What is the significance of the relationship between ΔH1, ΔH2, and ΔH3 in Hess's Law?

According to Hess's Law, if a reaction can be broken down into multiple steps, the total enthalpy change (ΔH1) is the sum of the enthalpy changes of the individual steps (ΔH2 and ΔH3). This relationship allows for the calculation of ΔH for a complex reaction by using simpler reactions.

How does Hess's Law apply to the reaction between carbon and oxygen to form carbon dioxide?

In the reaction where carbon (C) reacts with oxygen (O2) to form carbon dioxide (CO2), Hess's Law shows that the enthalpy change is the same whether the reaction occurs in one step or through multiple steps. This is demonstrated by calculating ΔH for the one-step and two-step pathways and confirming that they yield the same result.

How can reactions be manipulated to apply Hess's Law in calculations?

Reactions can be manipulated by reversing reactions, adjusting the stoichiometric coefficients, or breaking down a complex reaction into simpler ones. Each adjustment to the reaction must also adjust the corresponding ΔH value to reflect the change in the reaction pathway.

What does the coefficient of a chemical reaction indicate in Hess's Law?

The coefficient in a chemical reaction indicates the number of moles of each substance involved. In Hess's Law calculations, when a reaction is multiplied by a coefficient, the ΔH for that reaction must also be multiplied by the same factor to maintain consistency with the stoichiometry.

How do you calculate the enthalpy change when dealing with a substance that is produced in a reaction, like in the formation of Ca(OH)2?

To calculate the enthalpy change for the formation of a substance like Ca(OH)2, first, identify the reactants in their standard states, then apply Hess's Law using the known reactions and their ΔH values. If necessary, adjust coefficients to match the reaction's stoichiometry.

What role do energy diagrams play in applying Hess's Law?

Energy diagrams visually represent the changes in enthalpy during a reaction. They show the energy levels of reactants and products, and the total enthalpy change can be determined by analyzing the difference between the initial and final energy states.

What is the importance of stoichiometry in Hess's Law calculations?

Stoichiometry is critical in Hess's Law because it ensures the correct amounts of reactants and products are used in the reaction. When adjusting reactions to apply Hess's Law, the coefficients must match the required stoichiometry, and the ΔH must be adjusted accordingly.

How do you calculate the enthalpy change when dealing with non-standard conditions, like the vaporization of water?

When dealing with non-standard conditions, such as the vaporization of water, you calculate the enthalpy change by analyzing the energy diagram and using Hess's Law to sum the enthalpy changes for the individual steps. The final result will give the enthalpy change for the overall process.

What is the significance of the numerical value for enthalpy change in a reaction?

The numerical value of enthalpy change (ΔH) indicates the amount of heat absorbed or released during a reaction. A negative ΔH indicates an exothermic reaction (heat released), while a positive ΔH indicates an endothermic reaction (heat absorbed). This value is essential for understanding the energy dynamics of a reaction.