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4.2 - Hess's Law

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Introduction to Hess's Law

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0:00
Teacher
Teacher

Today, we will explore Hess's Law, an important principle in thermochemistry. Can anyone tell me what Hess's Law states?

Student 1
Student 1

I think it relates to how we calculate heat changes in reactions, but I'm not sure how.

Teacher
Teacher

That's a great start! Hess's Law states that if a reaction can occur in a series of steps, the overall enthalpy change for the reaction is the sum of the enthalpy changes for each individual step. What do you think this means about the pathway taken by the reaction?

Student 2
Student 2

It sounds like the pathway doesn't matter as long as we know the individual steps.

Teacher
Teacher

Exactly! Since enthalpy is a state function, it only depends on the initial and final states of the system. Let's remember this with the acronym 'PATH' – it’s the *P*roducts *A*lways denotes *T*hermal *H*istory!

Student 3
Student 3

I like that! So, we can work out the total change even if we can't measure it directly?

Teacher
Teacher

Yes! That's the power of Hess's Law. Now, let’s move on to the manipulation of chemical equations.

Manipulating Equations

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0:00
Teacher
Teacher

To apply Hess's Law, we often need to manipulate our equations. Who can tell me what happens if we reverse a chemical equation?

Student 4
Student 4

The sign of the enthalpy change reverses too!

Teacher
Teacher

Right! For example, if we have A β†’ B with Ξ”H = Ξ”H₁, reversing gives B β†’ A with Ξ”H = -Ξ”H₁. Can anyone think of a scenario when we might need to reverse an equation?

Student 1
Student 1

Maybe when we know the products but not the reactants?

Teacher
Teacher

Exactly! And what about when we multiply a balanced equation by a factor?

Student 2
Student 2

The enthalpy change has to be multiplied by that same factor too.

Teacher
Teacher

Correct! Let’s remember this with the mnemonic β€˜RAMP’ - *R*everse means *A*bsolute shift, *M*ultiply enthalpy and increase its *P*ower!

Applications of Hess's Law

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0:00
Teacher
Teacher

Now, let’s talk about some applications of Hess's Law. What is one common way to apply it?

Student 3
Student 3

Using standard enthalpies of formation to find the Ξ”H for a reaction!

Teacher
Teacher

Absolutely! The formula is Ξ”H_rxnΒ° = Ξ£nΞ”H_fΒ°(products) - Ξ£mΞ”H_fΒ°(reactants). Can someone break this down for me?

Student 4
Student 4

We add up the enthalpy changes of the products and subtract those of the reactants.

Teacher
Teacher

Spot on! Now let’s take a practical example, such as the combustion of methane. How would we apply Hess's Law here?

Student 1
Student 1

We would find the enthalpy changes for COβ‚‚ and Hβ‚‚O, then subtract the one for CHβ‚„.

Teacher
Teacher

Yes! Now, let’s remember this with another acronym, β€˜FIND’, which stands for *F*ind individual Ξ”H_fΒ° values, *I*nput to the equation, *N*eglect lumps, and *D*eliver the Ξ”H!

Example Problem Solving

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0:00
Teacher
Teacher

Let’s put what we learned into practice. Here's a reaction: C + Β½Oβ‚‚ β†’ CO. We know two reactions with their Ξ”H values. How do we derive the enthalpy change?

Student 2
Student 2

We should keep the first equation and reverse the second one, changing its sign!

Teacher
Teacher

Perfect! Now, if our first reaction has Ξ”H₁ = -393.5 kJ and the second one is CO + Β½Oβ‚‚ β†’ COβ‚‚ with Ξ”Hβ‚‚ = -283.0 kJ, how do we combine them?

Student 3
Student 3

We add Ξ”H₁ + (-Ξ”Hβ‚‚)?

Teacher
Teacher

Exactly! For a net reaction, what do we get?

Student 4
Student 4

Ξ”H = -393.5 + 283.0, which gives us -110.5 kJ!

Teacher
Teacher

Great job! This application shows how powerful Hess's Law is for complex calculations.

Introduction & Overview

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Quick Overview

Hess's Law states that the overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for each step, regardless of the pathway taken.

Standard

Hess's Law is a crucial concept in thermochemistry, indicating that if a chemical reaction occurs in multiple steps, the total enthalpy change can be calculated by summing the enthalpy changes for each step. This principle allows us to determine enthalpy changes for reactions that are difficult to measure directly in a calorimeter.

Detailed

Hess's Law

Hess's Law, or the Law of Constant Heat Summation, provides a method for calculating enthalpy changes in chemical reactions that occur in multiple steps or cannot be measured directly. According to Hess's Law, the overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for each individual step in the reaction, an important outcome of the fact that enthalpy is a state function. This leads to two main manipulations of equations: 1) reversing a reaction changes the sign of the enthalpy change; and 2) multiplying the coefficients in a balanced equation also requires corresponding adjustments to the enthalpy change.

Applications of Hess's Law include:
1. Calculating standard enthalpy changes of reactions using standard enthalpies of formation.
2. Using a series of known reactions to derive the enthalpy of a target reaction through algebraic manipulation of the given equations.

Overall, Hess's Law is vital for solving complex enthalpy calculations and supports the concept of enthalpy as a state function.

Youtube Videos

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Audio Book

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Introduction to Hess's Law

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Many chemical reactions are difficult or impossible to measure directly in a calorimeter. For such reactions, or for calculating enthalpy changes of reactions that proceed through multiple steps, Hess's Law of Constant Heat Summation is invaluable.

Hess's Law states that if a reaction can occur in a series of steps, the overall enthalpy change for the reaction is equal to the sum of the enthalpy changes for each individual step, regardless of the pathway taken. This is a direct consequence of enthalpy being a state function.

Detailed Explanation

Hess's Law helps us determine the heat change (enthalpy change) in chemical reactions that are hard to measure directly. It tells us that if a reaction can happen in several stages, the total heat change when going from reactants to products is the same, no matter how many steps are involved. This is because enthalpy is a property that only depends on the initial and final states of the system, not the steps taken to get there.

Examples & Analogies

Think of Hess's Law like taking a road trip. If you take a series of different routes to get to your destination, it doesn't change the total distance you’ve traveled. Whether you took one long road or several smaller roads, the total distance remains the same, just like the total heat change remains the same regardless of the pathway in a chemical reaction.

Manipulating Equations and Enthalpy Changes

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Essentially, if you can manipulate known chemical equations and their enthalpy changes to sum up to a target equation, then the sum of their enthalpy changes will give you the enthalpy change of the target equation.

  1. Reversing an equation: If a reaction is reversed, the sign of its enthalpy change must also be reversed.
  2. A β†’ B Ξ”H₁
  3. B β†’ A Ξ”H = -Ξ”H₁
  4. Multiplying an equation by a factor: If the stoichiometric coefficients in an equation are multiplied by a factor, the enthalpy change must also be multiplied by the same factor.
  5. A β†’ B Ξ”H₁
  6. 2A β†’ 2B Ξ”H = 2Ξ”H₁

Detailed Explanation

To apply Hess’s Law effectively, we often need to rearrange known chemical equations. If you reverse an equation, you not only change the direction of the reaction, but you must also change the sign of the enthalpy. For example, if a process releases energy (negative Ξ”H), reversing it means it will now absorb energy (positive Ξ”H). Similarly, if you double the amounts of substances in a chemical equation, you must also double the enthalpy change. These manipulations allow you to align known reactions to derive the enthalpy change for a new reaction.

Examples & Analogies

Imagine baking a cake by following a recipe. If you want to make a smaller cake, you can either reverse the process by removing ingredients or scale down the recipe. If you remove an ingredient, you would need to adjust all other ingredients to maintain the correct proportions. Similarly, in Hess's Law, any change to the stoichiometric coefficients must also affect the enthalpy change correspondingly.

Applications of Hess's Law

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Hess's Law is commonly used in two main ways:
1. Using Standard Enthalpies of Formation (Ξ”H_fΒ°): The standard enthalpy change of a reaction (Ξ”H_rxnΒ°) can be calculated from the standard enthalpies of formation of the products and reactants using the formula:
Ξ”H_rxnΒ° = Ξ£nΞ”H_fΒ°(products) - Ξ£mΞ”H_fΒ°(reactants)
Where 'n' and 'm' are the stoichiometric coefficients in the balanced chemical equation. Remember that the Ξ”H_fΒ° for elements in their standard states is zero.

Example: Calculate Ξ”H_rxnΒ° for the combustion of methane: CHβ‚„(g) + 2Oβ‚‚(g) β†’ COβ‚‚(g) + 2Hβ‚‚O(l)
Given: Ξ”H_fΒ°(CHβ‚„(g)) = -74.8 kJ mol⁻¹ Ξ”H_fΒ°(COβ‚‚(g)) = -393.5 kJ mol⁻¹ Ξ”H_fΒ°(Hβ‚‚O(l)) = -285.8 kJ mol⁻¹ Ξ”H_fΒ°(Oβ‚‚(g)) = 0 kJ mol⁻¹
Ξ”H_rxnΒ° = [1 Γ— Ξ”H_fΒ°(COβ‚‚(g)) + 2 Γ— Ξ”H_fΒ°(Hβ‚‚O(l))] - [1 Γ— Ξ”H_fΒ°(CHβ‚„(g)) + 2 Γ— Ξ”H_fΒ°(Oβ‚‚(g))]
Ξ”H_rxnΒ° = [1 Γ— (-393.5) + 2 Γ— (-285.8)] - [1 Γ— (-74.8) + 2 Γ— (0)] Ξ”H_rxnΒ° = [-393.5 - 571.6] - [-74.8] Ξ”H_rxnΒ° = -965.1 + 74.8 = -890.3 kJ mol⁻¹

  1. Using a series of known reactions: This involves algebraically manipulating given equations to match the target equation.

Detailed Explanation

Hess's Law is applied mainly in two ways. First, to calculate the enthalpy change of a reaction using standard enthalpies of formation. The formula takes into account the enthalpy changes of products and reactants, which allows us to determine the overall change for the reaction. Second, it enables us to use known reactions and manipulate their equations to derive the enthalpy change for a new target reaction. This algebraic manipulation is key to calculating enthalpy changes for reactions that are difficult to measure directly.

Examples & Analogies

Think of solving a puzzle where you know some pieces (reactions) can be combined to create a larger picture (target reaction). By carefully selecting and connecting the right pieces together, just like adding the enthalpies from each piece, you can discover the total enthalpy change for the complete picture. The process is similar to following a known recipe, where known ingredients lead to desired outcomes.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Hess's Law: The overall enthalpy change is the sum of the individual steps.

  • Enthalpy Change (Ξ”H): Represents heat absorbed or released in a reaction.

  • Reversing Equations: Changing the sign of Ξ”H when a reaction is reversed.

  • Multiplying Equations: Multiplying the reaction coefficients also scales Ξ”H.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • Calculating the Ξ”H for combustion reactions using standard enthalpy of formation.

  • Using known reactions to derive Ξ”H for a target reaction via manipulation of equations.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎡 Rhymes Time

  • Hess's law, oh what a joy, sum the steps, don’t be coy!

πŸ“– Fascinating Stories

  • Imagine you're on a journey with different paths. No matter how you choose to go, the distance doesn't change!

🧠 Other Memory Gems

  • Remember β€˜RAMP’ for manipulating: Reverse, Absolute, Multiply, Power!

🎯 Super Acronyms

Use β€˜FIND’ for enthalpy changes

  • *F*ind values
  • *I*nput them
  • *N*eglect lumps
  • *D*eliver Ξ”H.

Flash Cards

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Glossary of Terms

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  • Term: Hess's Law

    Definition:

    The principle stating that the total enthalpy change of a reaction is the sum of the enthalpy changes for each step.

  • Term: Enthalpy Change (Ξ”H)

    Definition:

    The heat absorbed or released during a chemical reaction at constant pressure.

  • Term: Standard Enthalpy of Formation (Ξ”H_fΒ°)

    Definition:

    The enthalpy change when one mole of a compound is formed from its elements in their standard states.

  • Term: Exothermic Reaction

    Definition:

    A reaction that releases heat, resulting in a negative Ξ”H.

  • Term: Endothermic Reaction

    Definition:

    A reaction that absorbs heat, resulting in a positive Ξ”H.