Hess's law example | Thermodynamics | Chemistry | Khan Academy

Summary

The video script explains Hess's Law, using it to calculate the enthalpy change for the formation of methane from graphite and hydrogen gas. It details how to reverse and combine combustion reactions of carbon, hydrogen, and methane to derive the desired reaction, emphasizing the importance of balancing reactants and products. The calculated enthalpy change for methane formation is found to be exothermic, releasing energy.

Takeaways

• 🔍 The problem involves calculating the enthalpy change for the formation of methane from solid carbon (graphite) and hydrogen gas.
• 🚫 Direct measurement of the enthalpy change for this reaction is not feasible in the lab due to its slow rate.
• 🔥 Enthalpy changes for the combustion of carbon, hydrogen, and methane are provided and can be used to calculate the desired enthalpy change.
• 📚 Hess's Law is introduced as a method to calculate the enthalpy change of a reaction when direct measurement is not possible.
• 🔄 Hess's Law states that the enthalpy change of a reaction is the sum of the enthalpy changes of the reactions it is composed of.
• 📈 The script demonstrates how to reverse and manipulate reaction equations to construct the desired reaction for enthalpy change calculation.
• ➕ The reactions for the combustion of carbon and hydrogen are used, and the combustion of methane is reversed to align with the target reaction.
• 🔢 The script includes a step-by-step calculation process, emphasizing the importance of balancing the reactions and their enthalpy changes.
• 📉 The final calculation shows that the formation of methane from graphite and hydrogen gas is an exothermic reaction, releasing energy.
• 📝 The process concludes with a clear explanation of how the sum of the manipulated reactions equals the desired reaction, and the enthalpy change is calculated as -74.8 kJ/mol.

Q & A

What is the main topic discussed in the transcript?

The main topic discussed in the transcript is the calculation of the enthalpy change for the formation of methane (CH4) from solid carbon (graphite) and hydrogen gas using Hess's Law.

Why can't the enthalpy change for the formation of methane be measured directly in the laboratory?

The enthalpy change for the formation of methane cannot be measured directly in the laboratory because the reaction is very slow, making it difficult to measure the temperature change or any meaningful heat exchange.

What is Hess's Law and how does it relate to this problem?

Hess's Law states that the enthalpy change of a reaction is the same whether it occurs in one step or several steps. If a reaction is the sum of two or more other reactions, the enthalpy change of this reaction is the sum of the enthalpy changes of those reactions. This law is used in the transcript to calculate the enthalpy change for the formation of methane from its elements.

What are the combustion reactions given in the transcript for calculating the enthalpy change?

The combustion reactions given in the transcript are the combustion of carbon (graphite), the combustion of hydrogen gas, and the combustion of methane.

How is the reverse reaction of methane combustion used in the calculation?

The reverse reaction of methane combustion is used by flipping the reaction arrow and considering methane as a product, which helps in constructing the desired reaction for the formation of methane from carbon and hydrogen.

What is the significance of reversing the reaction of methane combustion?

Reversing the reaction of methane combustion changes the sign of the enthalpy change from exothermic (releases heat) to endothermic (absorbs heat), which is necessary for calculating the enthalpy change for the formation of methane.

Why is the combustion of hydrogen reaction multiplied by 2 in the calculation?

The combustion of hydrogen reaction is multiplied by 2 to ensure that the number of water molecules produced matches the requirement of the reaction that forms methane, which needs two molecules of water.

How are the reactants and products balanced in the sum of reactions to get the desired reaction?

The reactants and products are balanced by ensuring that each molecule or atom on the reactant side of the individual reactions is accounted for on the product side of the overall reaction. This includes canceling out common molecules like carbon dioxide and water that appear on both sides.

What is the final enthalpy change calculated for the formation of methane from graphite and hydrogen gas?

The final enthalpy change calculated for the formation of methane from graphite and hydrogen gas is -74.8 kilojoules per mole of the reaction, indicating that the reaction is exothermic.

Why is it important to ensure that the sum of the individual reactions equals the desired reaction?

Ensuring that the sum of the individual reactions equals the desired reaction is important because it confirms that the correct enthalpy change can be calculated using Hess's Law. It verifies that the reactions are correctly balanced and that the overall process is accurately represented.