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4.5. Problem 5: Bond Enthalpies Estimate
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Good morning, class! Today we're diving into bond enthalpies. Who can tell me what bond enthalpy represents?
Isn't it the energy needed to break a bond?
That's correct! Specifically, bond enthalpy is the energy required to break one mole of a bond in a gas phase. Why do you think knowing this is important?
We can use it to calculate how much energy is involved in reactions?
Exactly! By estimating the enthalpy change of a reaction, we can determine if it's exothermic or endothermic. Remember the acronym 'BREAK' for Bond Energies to Remember Approximate Change in Kinetics. In this case, breaking bonds takes energy, while forming bonds releases energy.
So using average bond enthalpies will give us an approximate value?
Right! Let’s proceed to how we estimate this change.
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Now, let’s calculate the energy change for the reaction we mentioned using acetylene and hydrogen. What is our formula for estimating ΔH?
ΔH_estimate = Σ D(bonds broken) - Σ D(bonds formed!
Exactly! For the reaction C₂H₂ + 2 H₂ -> C₂H₆, can anyone list the bonds we need to break?
We need to break one C≡C bond and two H–H bonds.
Correct! Now, could you calculate the total energy needed to break these bonds using the average bond enthalpies?
Breaking the C≡C bond costs 839 kJ and the H–H bonds cost 872 kJ total.
Great! What’s the total for bonds broken?
So that would be 1,711 kJ.
Perfect! Now, let's find out the energy for forming the bonds in C₂H₆!
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Create a free accountNow that we have our energies calculated, what bonds do we form in C₂H₆?
We form one C–C bond and four C–H bonds!
Exactly! What’s the energy for forming these bonds, using the values provided?
That's 347 kJ for the C–C bond and 1,652 kJ for the C–H bonds.
Right! Let’s sum those up. What does that bring you to?
That gives us 1,999 kJ.
Well done! Now, can someone remind me how we finalize ΔH?
ΔH_estimate = bonds broken - bonds formed = 1,711 kJ - 1,999 kJ.
Exactly! And what is our result?
ΔH_estimate is -288 kJ!
Fantastic work! This means it’s an exothermic reaction, releasing energy during hydrogenation.
Overview
Short Summary
This section focuses on using average bond enthalpies to estimate the enthalpy change of a reaction involving bond breaking and forming.
Medium Summary
In this section, students learn how to estimate the enthalpy change (ΔH) for chemical reactions by utilizing average bond enthalpies. The process involves calculating the total energy required to break bonds in the reactants and the energy released when bonds are formed in the products.
Detailed Summary
Bond Enthalpies Estimate
In this section, we explore how to estimate the enthalpy change (ΔH) of a chemical reaction by using average bond enthalpies. Bond enthalpy refers to the amount of energy required to break one mole of a specific bond in a gas-phase molecule. This principle is particularly useful when comparing the energy dynamics of reactants and products in a chemical reaction.
Key Points:
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Bonds Broken and Formed: To estimate ΔH, one must account for the total bond energies involved in the reaction:
- Total Energy to Break Bonds: The energy required to break all the bonds in the reactants.
- Total Energy Released from Forming Bonds: The energy released when new bonds are formed in the products.
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Estimating ΔH: The formula used is: This equation illustrates that the estimated enthalpy change is the difference between the energy used to break bonds (positive values) and the energy released upon bond formation (negative contributions).
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Example Reaction: A common example includes the hydrogenation of acetylene (C₂H₂) into ethane (C₂H₆).
- For acetylene, one C≡C triple bond and two C–H single bonds are broken.
- In the products, one C–C single bond and four new C–H bonds are formed.
Through this estimation method, we can understand the energy dynamics of reactions and predict whether they are exothermic (ΔH < 0) or endothermic (ΔH > 0).
Audio Book
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Create a free account- Reactant Bonds to Break: ○ C₂H₂ (acetylene) has one C≡C triple bond and two C–H bonds. But hydrogenation will break the C≡C into a C–C single bond and add H’s. More straightforward: • Break 1 C≡C bond: 839 kJ • Break 2 × H–H bonds (because 2 H₂ molecules): 2 × 436 = 872 kJ Total bonds broken = 839 + 872 = 1,711 kJ.
- Product Bonds to Form: ○ C₂H₆ (ethane) has one C–C bond and six C–H bonds. But the original C₂H₂ already has two C–H bonds; hydrogenation adds 4 new C–H bonds. So: • Form 1 C–C bond: 347 kJ • Form 4 new C–H bonds: 4 × 413 = 1,652 kJ Total bonds formed = 347 + 1,652 = 1,999 kJ.
Detailed Explanation
In this part, we break down the steps to calculate the total energy changes associated with breaking and forming bonds during the reaction:
Step 1: Identify Bonds Broken and Their Energies We take account of the specific bonds in the reactants needed to be broken (C≡C and H–H).
Step 2: Calculate Energy for Bonds Broken We aggregate the energies needed to break all bonds that are part of the reactants which equals a total of 1,711 kJ.
Step 3: Identify Bonds Formed and Their Energies Here we check the bonds in the products we are creating (C–C and four C–H bonds), totaling up to 1,999 kJ.
Step 4: Combine Values for ΔH Estimate Finally, we subtract the total energy of bonds formed from that of bonds broken: ΔH_estimate = Bonds broken - Bonds formed to get -288 kJ. This negative value indicates that the reaction is exothermic; energy is released, confirming the reaction is energetically favorable.
Examples & Analogies
This process of breaking and forming bonds can be likened to renovations in a house. You first have to ‘break down’ or remove old elements like walls (akin to bond breaking), which takes effort and energy. But as you build new walls and install fresh features (bond forming), that results in a transformed space that is both useful and appealing, often providing a net gain in value or comfort–reflecting the energy dynamics of reactions.
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Key Concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
Bond Enthalpy: Energy required to break bonds in molecules.
ΔH Estimate: Calculated using the difference between bonds broken and formed.
Exothermic vs Endothermic: Understanding energy release or absorption through ΔH.
Examples
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Flash Cards
Glossary
Bond Enthalpy
The energy required to break one mole of a specific bond in a molecule.
Exothermic Reaction
A reaction that releases heat, resulting in a negative enthalpy change (ΔH < 0).
Endothermic Reaction
A reaction that absorbs heat, leading to a positive enthalpy change (ΔH > 0).
Average Bond Enthalpy
The average energy required to break a bond in various environments.