1.7 - Practice Problems for Section 1
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Understanding Enthalpy Changes
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Today, weβll explore how to determine if a reaction is exothermic or endothermic using enthalpy values. Can anyone remind me what enthalpy signifies?
Isn't it the total heat content of a system?
Exactly! Enthalpy is about heat under constant pressure. What do we understand by exothermic and endothermic reactions?
Exothermic reactions release heat, and endothermic reactions absorb heat.
Great! Now remember the acronym 'HEAT' where H equals heat flow. Letβs dive into our first practice problem!
Example Problem - Exothermic vs Endothermic
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Letβs determine if the reaction of nitrogen and oxygen to form nitrogen monoxide is exothermic or endothermic. What's our enthalpy of formation for the products?
ΞH_fΒ° for NO is +90.3 kJ/mol!
Correct! Now, since nitrogen and oxygen are both in their standard states, how do we calculate the overall enthalpy change?
We use the equation ΞH_rxnΒ° = Ξ£ ΞH_fΒ°(products) β Ξ£ ΞH_fΒ°(reactants).
Exactly! Remember, the heat flow we calculate helps us determine if the reaction is exothermic or not. Can anyone summarize how we arrive at our final answer?
Hess's Law Application
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Now, letβs connect it to Hess's Lawβhow do we leverage this law for complex reactions?
We can break the overall reaction into multiple steps and sum their enthalpy changes.
Absolutely! Letβs use the combustion of methane as our example, giving us a practical en route to verifying our calculations.
By using the known values of the enthalpies of formation for the reactants and products, we can find our ΞH for the overall reaction!
Exactly! Remember, Hess's Law emphasizes that the path doesnβt affect the overall enthalpy change. Letβs summarize what we learned.
Introduction & Overview
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Quick Overview
Standard
The section presents a series of practice problems that require students to determine whether reactions are exothermic or endothermic and to compute enthalpy changes based on provided standard enthalpies of formation. The problems progress in complexity, encouraging the application of Hess's Law and the calculation of reaction enthalpies.
Detailed
In Section 1.7, we delve into practical applications of enthalpy concepts by solving practice problems aimed at determining whether specific chemical reactions are exothermic or endothermic. Each problem is structured to guide students through the necessary calculations, including the use of standard enthalpies of formation and Hess's Law. The problems require students to demonstrate their understanding of key concepts through systematic calculation and reasoning, ensuring they can apply theoretical knowledge to real chemical reactions. This section is essential for solidifying students' grasp on thermochemistry, paving the way for advanced studies in calorimetry and reaction energetics.
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Practice Problem 1: Endothermic vs. Exothermic Reaction
Chapter 1 of 3
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Chapter Content
- Problem: Determine whether each of the following reactions is exothermic or endothermic, given these enthalpies of formation:
- ΞH_fΒ°[NO(g)] = +90.3 kJ/mol
- ΞH_fΒ°[Nβ(g)] = 0
- ΞH_fΒ°[Oβ(g)] = 0
Reaction: Nβ(g) + Oβ(g) β 2 NO(g)
Detailed Explanation
To determine if a reaction is exothermic or endothermic, you can calculate the reaction's enthalpy change (ΞH_rxnΒ°) using the standard enthalpies of formation (ΞH_fΒ°) of the reactants and products.
Hereβs how to do it step-by-step:
1. Write the formula for ΞH_rxnΒ° = Ξ£ ΞH_fΒ°(products) β Ξ£ ΞH_fΒ°(reactants).
2. Calculate the total enthalpy of the products, which in this case is 2 moles of NO having ΞH_fΒ° = +90.3 kJ/mol. Therefore, products total = 2 Γ 90.3 = +180.6 kJ.
3. For the reactants Nβ(g) and Oβ(g), look up their standard enthalpy values. Both Nβ and Oβ have ΞH_fΒ° of 0, leading to a total of 0 kJ.
4. Now plug into the formula:
- ΞH_rxnΒ° = +180.6 kJ β 0 = +180.6 kJ.
5. Since ΞH is positive, this indicates the reaction is endothermic (it absorbs heat).
Examples & Analogies
Think of making a smoothie. When you blend ice (solid) and fruit (which is typically at room temperature), the ice absorbs heat from the fruit and the surrounding environment to melt. In this process, the mixture feels cold because it absorbs heat. This is similar to an endothermic reaction where energy is absorbed from the surroundings.
Practice Problem 2: Enthalpy Change Calculation
Chapter 2 of 3
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Chapter Content
- Problem: Using ΞH_fΒ° values below, calculate ΞH_rxnΒ° for:
- ΞH_fΒ°[CO(g)] = β110.5 kJ/mol
- ΞH_fΒ°[Oβ(g)] = 0
- ΞH_fΒ°[COβ(g)] = β393.5 kJ/mol
Reaction: CO(g) + Β½ Oβ(g) β COβ(g)
Detailed Explanation
To calculate the enthalpy change (ΞH_rxnΒ°) for the reaction, we follow these steps:
1. Identify the products and their ΞH_fΒ° values. For COβ(g), ΞH_fΒ° = β393.5 kJ/mol. Since we have 1 mole of COβ produced, total for products = β393.5 kJ.
2. Identify the reactants. For CO(g), ΞH_fΒ° = β110.5 kJ/mol and for Oβ(g), it is 0. For the reactants sum: CO = -110.5 kJ + 0 (for Oβ) = -110.5 kJ.
3. Use the formula: ΞH_rxnΒ° = Ξ£ ΞH_fΒ°(products) β Ξ£ ΞH_fΒ°(reactants).
4. Now plug in: ΞH_rxnΒ° = -393.5 kJ β (-110.5 kJ) = -393.5 + 110.5 = -283.0 kJ.
5. Hence, ΞH_rxnΒ° for this reaction shows it is exothermic since it is negative.
Examples & Analogies
Imagine a campfire. When wood burns (similar to our reaction where CO is oxidized), it releases a significant amount of heat and light into the environment, indicating an exothermic reaction. The heat released can be very comforting on a cold night, similar to how enthalpy change in a combustion reaction releases heat energy.
Practice Problem 3: Standard Enthalpy of Combustion
Chapter 3 of 3
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Chapter Content
- Problem: Given ΞH_fΒ° for CHβOH(l) = β238.7 kJ/mol, ΞH_fΒ° for COβ(g) = β393.5 kJ/mol, and ΞH_fΒ° for HβO(l) = β285.8 kJ/mol, calculate ΞH_cΒ° for methanol:
2 Oβ(g) + CHβOH(l) β COβ(g) + 2 HβO(l)
Detailed Explanation
To calculate the standard enthalpy of combustion (ΞH_cΒ°) of methanol, we need to follow similar steps:
1. Identify the products and their formation enthalpy:
- For COβ(g), we have ΞH_fΒ° = β393.5 kJ, and for 2 HβO(l), total = 2 Γ β285.8 kJ = β571.6 kJ.
2. For the reactants, CHβOH(l) = β238.7 kJ and Oβ(g) = 0.
3. So, the total for products = β393.5 + (β571.6) = β965.1 kJ.
4. The total for reactants = β238.7 kJ + 0 = β238.7 kJ.
5. Now calculate ΞH_cΒ° using the formula:
- ΞH_cΒ° = (β965.1 kJ) - (β238.7 kJ) = β965.1 + 238.7 = β726.4 kJ per mole of methanol.
6. Therefore, the enthalpy of combustion shows that burning methanol is highly exothermic.
Examples & Analogies
Think about how alcohol lamps work during camping trips. When you light the alcohol (methanol), it burns, producing heat and light, simulating a small fire. The heat produced is similar to how exothermic combustion reactions release energy, brightening your surroundings and keeping it warm.
Key Concepts
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Enthalpy Change: A measure of the heat exchanged at constant pressure during a chemical reaction.
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Exothermic vs Endothermic: Understanding these terms is crucial to interpreting reaction energetics.
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Hess's Law: A valuable tool for solving complex enthalpy problems by breaking them into simpler steps.
Examples & Applications
- The formation of nitrogen monoxide from nitrogen and oxygen illustrates how the enthalpy change is calculated with their enthalpies of formation.
- The combustion of methane demonstrates how Hess's Law can be applied to derive enthalpy changes from known values.
Memory Aids
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Rhymes
When heat flows out, an exothermic shout; Endothermic takes it in, that's how we win!
Stories
Imagine a campfire, where wood burns bright and warm. The heat spreads out, making the night delight. That's exothermic, for sure; it gives heat away!
Memory Tools
Remember 'HEAT'βH for heat, E for energy released in exothermic, A for absorbed in endothermic, and T for thermodynamics.
Acronyms
Use 'E' for Endo and 'X' for Exo
'E' absorbs (Endothermic) and 'X' exchanges heat (Exothermic).
Flash Cards
Glossary
- Exothermic Reaction
A reaction that releases heat into the surroundings, resulting in a negative enthalpy change (ΞH < 0).
- Endothermic Reaction
A reaction that absorbs heat from the surroundings, resulting in a positive enthalpy change (ΞH > 0).
- Enthalpy of Formation (ΞH_fΒ°)
The change in enthalpy when one mole of a compound is formed from its elements in their standard states.
- Hess's Law
The principle stating that the total enthalpy change for a reaction is the same, regardless of the path taken, whether it occurs in one step or several steps.
- Standard Conditions
A set of conditions, often 1 bar pressure and 298.15 K (25 Β°C), used for measuring and reporting enthalpy changes.
Reference links
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