Problem 5: Bond Enthalpies Estimate
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Introduction to Bond Enthalpies
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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! Letβs proceed to how we estimate this change.
Calculating Energy Changes
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Now, letβs calculate the energy change for the reaction we mentioned using acetylene and hydrogen. What is our formula for estimating ΞH?
ΞH_estimate = Ξ£ D(bonds broken) - Ξ£ D(bonds formed!
Exactly! For the reaction CβHβ + 2 Hβ -> CβHβ, can anyone list the bonds we need to break?
We need to break one Cβ‘C bond and two HβH bonds.
Correct! Now, could you calculate the total energy needed to break these bonds using the average bond enthalpies?
Breaking the Cβ‘C bond costs 839 kJ and the HβH bonds cost 872 kJ total.
Great! Whatβs 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 CβHβ!
Putting it Together
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Now that we have our energies calculated, what bonds do we form in CβHβ?
We form one CβC bond and four CβH bonds!
Exactly! Whatβs the energy for forming these bonds, using the values provided?
That's 347 kJ for the CβC bond and 1,652 kJ for the CβH bonds.
Right! Letβs 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 ΞH?
ΞH_estimate = bonds broken - bonds formed = 1,711 kJ - 1,999 kJ.
Exactly! And what is our result?
ΞH_estimate is -288 kJ!
Fantastic work! This means itβs an exothermic reaction, releasing energy during hydrogenation.
Introduction & Overview
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Quick Overview
Standard
In this section, students learn how to estimate the enthalpy change (ΞH) 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
Bond Enthalpies Estimate
In this section, we explore how to estimate the enthalpy change (ΞH) 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:
- Bonds Broken and Formed: To estimate ΞH, 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 ΞH: The formula used is:
$$\Delta H_{\text{estimate}} = \sum D_{\text{bonds broken}} - \sum D_{\text{bonds formed}}$$
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). - Example Reaction: A common example includes the hydrogenation of acetylene (CβHβ) into ethane (CβHβ).
- For acetylene, one Cβ‘C triple bond and two CβH single bonds are broken.
- In the products, one CβC single bond and four new CβH bonds are formed.
Through this estimation method, we can understand the energy dynamics of reactions and predict whether they are exothermic (ΞH < 0) or endothermic (ΞH > 0).
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Calculating Total Energy Changes
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Chapter Content
- Reactant Bonds to Break:
β CβHβ (acetylene) has one Cβ‘C triple bond and two CβH bonds. But hydrogenation will break the Cβ‘C into a CβC single bond and add Hβs.
More straightforward:
β’ Break 1 Cβ‘C bond: 839 kJ
β’ Break 2 Γ HβH bonds (because 2 Hβ molecules): 2 Γ 436 = 872 kJ
Total bonds broken = 839 + 872 = 1,711 kJ. - Product Bonds to Form:
β CβHβ (ethane) has one CβC bond and six CβH bonds. But the original CβHβ already has two CβH bonds; hydrogenation adds 4 new CβH bonds.
So:
β’ Form 1 CβC bond: 347 kJ
β’ Form 4 new CβH bonds: 4 Γ 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 (Cβ‘C and HβH).
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 (CβC and four CβH bonds), totaling up to 1,999 kJ.
Step 4: Combine Values for ΞH Estimate
Finally, we subtract the total energy of bonds formed from that of bonds broken: ΞH_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 βbreak downβ 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 comfortβreflecting the energy dynamics of reactions.
Key Concepts
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Bond Enthalpy: Energy required to break bonds in molecules.
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ΞH Estimate: Calculated using the difference between bonds broken and formed.
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Exothermic vs Endothermic: Understanding energy release or absorption through ΞH.
Examples & Applications
Calculating the enthalpy change for the reaction: CβHβ + 2 Hβ β CβHβ using bond enthalpies.
Comparing the estimated ΞH to known values to assess accuracy.
Memory Aids
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Rhymes
To break a bond, think of the cost, Energy required, without it, weβre lost!
Stories
Imagine a superhero trying to break a 'stronghold' (a bond); he uses energy to break in, but when he makes friends (forms bonds), they share power.
Memory Tools
Remember 'Broke-Five-Formed' to calculate ΞH: Energy to break bonds minus energy released when bonds form.
Acronyms
B.R.A.K.U. (Bonds, Required Energy, Added Energy, Kinetics Understood) helps you remember the process of estimating ΞH using bond enthalpies!
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 (ΞH < 0).
- Endothermic Reaction
A reaction that absorbs heat, leading to a positive enthalpy change (ΞH > 0).
- Average Bond Enthalpy
The average energy required to break a bond in various environments.
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