Hess's Law

Brightstorm

8 Sept 201005:46

Summary

TLDRThis video explains Hess's Law and its application in determining the enthalpy change (Delta H) of a reaction when it's not directly available. The instructor demonstrates how to manipulate thermochemical equations to calculate Delta H by adding and adjusting other known reactions. The example provided involves calculating the Delta H for sulfur trioxide formation from sulfur and oxygen, showing how to reverse and scale reactions. The video concludes with a discussion of formation reactions, emphasizing the importance of having one mole of the product in such calculations.

Takeaways

📉 Hess's Law helps calculate the enthalpy change (ΔH) of a reaction by adding up the ΔH values of multiple related reactions.
📘 If ΔH for a reaction is unknown, you can either measure it using a calorimeter or use Hess’s Law with known reactions.
🔁 Hess's Law states that you can reverse or multiply chemical reactions to match the desired overall reaction.
⚖️ When multiplying a reaction, the ΔH value must also be multiplied by the same factor.
↔️ Reversing a reaction changes the sign of its ΔH; an exothermic reaction becomes endothermic and vice versa.
🧪 In an example with sulfur and oxygen forming sulfur trioxide, the reactions are manipulated to match the overall reaction.
✖️ Intermediate products, like sulfur dioxide in this case, can cancel out when added across reactions.
🔢 The total ΔH for the overall reaction is the sum of the ΔH values of the individual reactions.
📊 In formation reactions, it’s common to have fractional coefficients, which is acceptable as long as there’s one mole of product.
🌡️ Hess's Law is highly useful for calculating ΔH for reactions where direct experimental data is unavailable.

Q & A

What is the main method discussed in the video to calculate Delta H when it is not known?
-The video discusses using Hess's Law to calculate Delta H by manipulating other known reactions that add up to the overall desired reaction.
What is Hess's Law?
-Hess's Law states that it is possible to add two or more thermochemical equations to produce a final equation for a reaction, and the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction.
What example reaction is used in the video to explain Hess's Law?
-The reaction used is sulfur (S) plus oxygen gas (O2) yielding sulfur trioxide (SO3), where the Delta H of the reaction is initially unknown.
How is the first reaction modified to match the desired reaction?
-The first reaction, which has one mole of sulfur (S), is multiplied by 2 to have two moles of sulfur, matching the reactant side of the target reaction.
What happens to the Delta H when the reaction is multiplied by 2?
-The Delta H is also multiplied by 2. For example, if the original Delta H was -297 kJ, it becomes -594 kJ after multiplying the reaction by 2.
Why is the second reaction flipped in the explanation?
-The second reaction is flipped because sulfur dioxide (SO2) is on the product side, but it is needed on the reactant side to match the desired reaction. Reversing the reaction also changes the sign of the Delta H.
How do the sulfur dioxide (SO2) molecules get canceled out?
-The two moles of sulfur dioxide (SO2) on the reactant side of the flipped reaction cancel out the two moles of sulfur dioxide on the product side of the first reaction.
What is the final Delta H for the overall reaction after using Hess's Law?
-The final Delta H for the reaction is -792 kJ after adding the modified Delta H values from the two reactions.
Why is the equation divided by 2 at the end?
-The equation is divided by 2 to convert it into a formation reaction, which involves forming one mole of the product, sulfur trioxide (SO3), from its elements. Formation reactions require one mole of the product.
What is the final Delta H per mole for the formation reaction?
-After dividing by 2, the final Delta H for the formation of one mole of sulfur trioxide (SO3) is -396 kJ per mole.

Outlines

00:00

🧪 Understanding Hess's Law: Thermochemical Equations

The video begins by introducing the challenge of calculating the enthalpy change (Delta H) of a reaction, especially when it's not readily available. Various methods, such as using a calorimeter or leveraging other known reactions, are introduced. The focus then shifts to Hess's Law, which allows for the summation of enthalpy changes from individual reactions to calculate the enthalpy for a final reaction. This concept is simplified by using a practical example involving sulfur, oxygen gas, and sulfur trioxide to illustrate the application of Hess's Law.

05:04

🔄 Manipulating Reactions Using Hess's Law

In this section, a specific reaction involving sulfur and oxygen is used to demonstrate how Hess's Law works. The example begins by multiplying a reaction to balance the sulfur atoms on both sides, ensuring the thermochemical equations add up correctly. The process of adjusting the enthalpy values according to the manipulation of the reactions is explained step-by-step, highlighting how reversing reactions changes the sign of Delta H. This process ultimately leads to determining the overall Delta H for the desired reaction.

➗ Simplifying to Formation Reactions

The video further refines the example by introducing the concept of formation reactions, where sulfur trioxide is formed from its elemental components. To simplify the reaction into a formation reaction, everything is divided by two, including the enthalpy, yielding a more standard format with one mole of product. It also explains that in formation reactions, fractional coefficients are acceptable as long as there's one mole of the product. The section concludes by emphasizing the usefulness of Hess's Law in finding Delta H for complex reactions.

Mindmap

Keywords

💡Thermochemical equations

Thermochemical equations describe chemical reactions along with the associated enthalpy changes (ΔH), indicating whether energy is absorbed or released. In the video, the speaker refers to how these equations can be used to determine the heat change of reactions, which is crucial when calculating the ΔH of a reaction.

💡Delta H (ΔH)

Delta H represents the change in enthalpy, or heat content, during a reaction. It's a key concept in the video, as the speaker is trying to find the ΔH for a reaction using Hess's law. ΔH can indicate whether a reaction is exothermic (negative ΔH) or endothermic (positive ΔH).

💡Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the path taken, as long as the initial and final conditions are the same. This law allows chemists to calculate the enthalpy change of a reaction by adding the enthalpy changes of known reactions, which is demonstrated when the speaker manipulates and adds reactions to find the unknown ΔH.

💡Calorimeter

A calorimeter is an instrument used to measure the amount of heat released or absorbed during a chemical reaction. The speaker mentions it as one method to determine ΔH for a reaction, but emphasizes that using Hess's law is often more practical when a calorimeter is unavailable.

💡Sulfur trioxide (SO3)

Sulfur trioxide is a chemical compound formed from sulfur and oxygen. In the video, the speaker is trying to find the ΔH for the formation of sulfur trioxide using Hess’s Law. It serves as the final product of the reactions being manipulated.

💡Formation reaction

A formation reaction involves creating one mole of a compound from its elements in their standard states. The speaker concludes by discussing how sulfur trioxide is formed from sulfur and oxygen, and explains that in formation reactions, fractional coefficients are allowed as long as one mole of the product is produced.

💡Reverse reaction

A reverse reaction is when the products of a reaction are turned back into the reactants. The speaker flips one of the reactions in the video to balance sulfur dioxide and sulfur trioxide, explaining that reversing a reaction changes the sign of its ΔH.

💡Exothermic reaction

An exothermic reaction releases energy in the form of heat, resulting in a negative ΔH. In the video, the speaker reverses a reaction, turning it into an exothermic one, and explains how this affects the overall enthalpy.

💡Endothermic reaction

An endothermic reaction absorbs energy, which results in a positive ΔH. The speaker initially discusses the forward reaction, which is endothermic, but when reversed, it becomes exothermic.

💡Fractional coefficients

Fractional coefficients are used in balanced chemical equations when partial moles of a substance are involved, particularly in formation reactions. The speaker demonstrates this when adjusting the equation to ensure that only one mole of sulfur trioxide is formed, leading to fractional oxygen in the equation.

Highlights

Hess's law allows you to calculate the Delta H of a reaction by adding up the enthalpy changes of multiple reactions.

If you don't know the Delta H of a reaction, you can measure it with a calorimeter or use Hess's law to calculate it using other reactions.

Hess's law states that the sum of the enthalpy changes of individual reactions equals the enthalpy change of the final reaction.

When applying Hess's law, you can manipulate reactions by multiplying them to match the desired reaction.

If you flip a reaction, you also need to flip the sign of the Delta H.

To match the desired equation, sulfur needed to be multiplied by two, and the corresponding Delta H was doubled.

Reversing the reaction to swap the sides of sulfur dioxide and sulfur trioxide requires reversing the sign of Delta H.

The sulfur dioxide molecules cancel out when combining two reactions to form sulfur trioxide, leaving the final reaction balanced.

Adding the Delta H values of both reactions gives the final Delta H for the reaction, which is -792 kilojoules.

For formation reactions, it’s important to reduce the reaction to produce one mole of the product, which involves dividing the entire equation by two.

Formation reactions often involve fractional coefficients to ensure that the product has one mole.

When dividing the reaction for a formation process, the enthalpy change also needs to be halved.

Hess's law is useful when the Delta H of a reaction is unknown and allows you to calculate it easily by using related reactions.

Formation reactions focus on creating one mole of a product from its elements, and fractional coefficients are acceptable in these equations.

Using Hess's law simplifies finding the enthalpy changes without performing direct experimental measurements.