Bond enthalpy and enthalpy of reaction | Chemistry | Khan Academy

Khan Academy
26 May 201611:46
EducationalLearning
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TLDRThis script introduces the concept of bond enthalpy, the energy required to break a chemical bond, and its application in calculating the enthalpy of a chemical reaction. It explains that bond enthalpy is always positive, indicating the energy input for bond breaking, and becomes negative when bonds are formed, releasing energy. The script uses the example of the hydrogenation of propyne to form propane, illustrating how to calculate the reaction's enthalpy change by summing the bond enthalpies of bonds broken and formed, resulting in an exothermic reaction.

Takeaways
  • 🔍 Bond enthalpy is the energy required to break one mole of a specific type of bond in the gas phase.
  • 🔄 Bond enthalpy values vary depending on the type of bond, with carbon-hydrogen bonds being a common example.
  • 📈 Bond enthalpy is always a positive value because breaking a bond requires the input of energy.
  • ➡️ The reverse process of bond enthalpy, which is bond formation, releases energy and thus has a negative value.
  • 🧪 Bond enthalpies can be used to estimate the enthalpy change (delta H) of a chemical reaction.
  • 📚 Bond enthalpy values can be found in textbooks or online resources, usually listed in tables with units like kilojoules per mole.
  • 🔄 To calculate the enthalpy change of a reaction, consider both the bonds broken and the bonds formed during the reaction.
  • ⚖️ A balanced chemical equation is essential for accurately determining the number of each type of bond involved in the reaction.
  • 📉 The overall enthalpy change of a reaction is calculated by summing the energy required to break bonds and subtracting the energy released in forming new bonds.
  • 📊 In the provided example, the hydrogenation of propyne to form propane is an exothermic reaction, as indicated by a negative delta H value.
  • 🛠️ The process of calculating delta H using bond enthalpies involves looking up bond enthalpy values, ensuring a balanced equation, and performing the appropriate arithmetic to find the total energy change.
Q & A

  • What is bond enthalpy?

    -Bond enthalpy is the energy required to break one mole of a specific type of chemical bond in the gas phase.

  • Why is bond enthalpy always positive?

    -Bond enthalpy is always positive because it represents the energy input needed to break a bond, which is an endothermic process.

  • What happens when a carbon-hydrogen bond is broken?

    -When a carbon-hydrogen bond is broken, the shared electrons are separated, with one electron going to the carbon and the other to the hydrogen, resulting in two separate entities.

  • How can bond enthalpy be used to estimate the enthalpy of a chemical reaction?

    -Bond enthalpy can be used to estimate the enthalpy of a reaction by calculating the total energy required to break the bonds in the reactants and subtracting the energy released when new bonds are formed in the products.

  • What is the significance of the direction of a bond enthalpy in a reaction?

    -The direction of bond enthalpy indicates whether energy is being absorbed (positive, breaking bonds) or released (negative, forming bonds) during the reaction.

  • What is the difference between bond enthalpy and enthalpy of reaction?

    -Bond enthalpy is the energy associated with breaking or forming a single type of bond, while enthalpy of reaction is the overall energy change for the entire chemical reaction, considering all bonds broken and formed.

  • How does the script define the enthalpy change in terms of bonds made and broken?

    -The enthalpy change is defined as the sum of the energy required to break all the bonds in the reactants plus the energy released when new bonds are formed in the products.

  • What is the example reaction provided in the script?

    -The example reaction is the hydrogenation of propyne (C3H4) with hydrogen gas (H2) to form propane (C3H8).

  • Why is it important to balance the chemical equation before calculating the enthalpy of reaction?

    -Balancing the chemical equation ensures that the number of atoms of each element is the same on both sides of the equation, which is necessary for an accurate calculation of the enthalpy of reaction.

  • What are the units typically used for bond enthalpy values?

    -Bond enthalpy values are typically expressed in kilojoules per mole (kJ/mol), but they can also be found in calories or kilocalories per mole.

  • How does the script describe the process of calculating the enthalpy of reaction for the given example?

    -The script describes the process by identifying the bonds broken and formed in the reaction, looking up their respective bond enthalpies, and then summing the energy required to break the bonds and the energy released when new bonds are formed to calculate the overall enthalpy change.

Outlines
00:00
🔬 Understanding Bond Enthalpy and Reaction Enthalpy

This paragraph introduces the concept of bond enthalpy, which is the energy required to break one mole of a bond in the gas phase. It emphasizes that different bond types have unique bond enthalpies, using the carbon-hydrogen bond as an example. The explanation clarifies that bond enthalpy is always positive, indicating that energy is needed to break bonds, while bond formation releases energy, making it negative. The paragraph also highlights the utility of bond enthalpy in estimating the enthalpy of a chemical reaction, which is crucial for determining whether a reaction is exothermic or endothermic. An example of propyne reacting with hydrogen to form propane is provided to illustrate the process.

05:03
📚 Calculating Reaction Enthalpy Using Bond Enthalpies

The second paragraph delves into the process of calculating the enthalpy of a reaction using bond enthalpies. It begins by identifying the bonds that are broken and formed during the reaction, using Lewis structures for clarity. The paragraph stresses the importance of a balanced chemical equation and accurately tracking the number of each type of bond involved. Bond enthalpy values for carbon-carbon triple bonds, hydrogen-hydrogen bonds, carbon-carbon single bonds, and carbon-hydrogen bonds are provided, with the understanding that these values are essential for calculating the overall energy change during the reaction. The summary explains that the total enthalpy change is the sum of the energy required to break bonds (positive values) and the energy released from forming new bonds (negative values).

10:05
🔢实操：利用键能计算反应焓变

第三段详细展示了如何将键能值代入计算中以得到反应焓变。首先，计算了断裂碳碳单键和两个氢氢键所需的能量。随后，计算了形成新的碳碳单键和四个碳氢键时释放的能量。通过将这些值输入计算器，得出了丙炔与氢气反应生成丙烷的反应焓变为负数，即-291千焦耳/摩尔，表明这是一个放能反应（exothermic）。本段通过具体的数值和计算步骤，展示了如何使用键能来计算并理解化学反应的热效应。

Mindmap
Keywords
💡Bond Enthalpy
Bond enthalpy refers to the energy required to break one mole of a specific type of bond in the gas phase. It is a fundamental concept in the video, as it is used to calculate the enthalpy of a chemical reaction. The script explains that different bonds have different bond enthalpies, and it uses the carbon-hydrogen bond as an example to illustrate this concept.
💡Enthalpy of Reaction
The enthalpy of reaction, denoted as delta H, is the heat effect of a chemical reaction. It is a central theme of the video, as the script discusses how to calculate it using bond enthalpies. The script clarifies that a negative delta H indicates an exothermic reaction, which releases energy, while a positive value indicates an endothermic reaction, absorbing energy.
💡Carbon Hydrogen Bond
A carbon hydrogen bond is a type of covalent bond formed between a carbon and a hydrogen atom. The script uses this bond as an example to explain the concept of bond enthalpy, emphasizing that breaking this bond requires energy, which is why bond enthalpies are always positive.
💡Lewis Structures
Lewis structures are diagrams that represent the valence electrons of atoms within a molecule and how they are paired in covalent bonds. The script refers to Lewis structures when describing how electrons are represented after a bond is broken, using single dots to indicate lone electrons.
💡Gas Phase
The gas phase is one of the states of matter in which a substance exists as a gas. The script specifies that bond enthalpies are defined for bonds in the gas phase, which is an important distinction because the energy required to break a bond can vary with the state of matter.
💡Propyne
Propyne is an alkyne with the molecular formula C3H4. The script uses propyne in an example reaction, where it reacts with hydrogen gas to form propane. The Lewis structure of propyne is drawn to illustrate the breaking and forming of bonds during the reaction.
💡Hydrogenation
Hydrogenation is a chemical reaction that adds hydrogen to a molecule. In the script, the hydrogenation of propyne is the example reaction used to demonstrate the calculation of the enthalpy of reaction using bond enthalpies.
💡Balanced Reaction
A balanced reaction is a chemical equation where the number of atoms of each element on the reactant side is equal to the number on the product side. The script emphasizes the importance of having a balanced reaction before calculating the enthalpy of reaction.
💡Triple Bond
A triple bond is a type of chemical bond where three pairs of valence electrons are shared between two atoms. The script mentions the carbon-carbon triple bond in propyne as one of the bonds that needs to be broken during the hydrogenation reaction.
💡Exothermic Reaction
An exothermic reaction is a chemical reaction that releases energy, usually in the form of heat. The script concludes that the hydrogenation reaction of propyne is exothermic because the calculated delta H is negative.
💡Endothermic Reaction
An endothermic reaction is a chemical reaction that absorbs energy from its surroundings. The script contrasts this with exothermic reactions and explains that a positive delta H value would indicate an endothermic process.
Highlights

Introduction to bond enthalpy as the energy required to break one mole of a bond.

Different types of bonds have different bond enthalpies.

Illustration of a carbon-hydrogen bond as an example of bond enthalpy.

Explanation of bond enthalpy in the context of the gas phase.

The process of breaking a bond and the resulting energy state of the atoms.

Bond enthalpy is always positive, indicating energy required to break bonds.

Formation of bonds releases energy, hence the negative sign for bond formation enthalpy.

Using bond enthalpy to estimate the enthalpy of a chemical reaction.

The importance of Hess's law and other methods for calculating enthalpy of reaction.

Example reaction: Hydrogenation of propyne to form propane.

Lewis structures used to visualize the bonds in chemical reactions.

Balancing the chemical equation to ensure accurate bond enthalpy calculations.

Calculating the energy required to break specific types of bonds in the reaction.

Determining the bond enthalpy values from a reference table.

Summing the energy changes from bond breaking and formation to find the overall enthalpy change.

The calculation of delta H for the hydrogenation reaction, resulting in an exothermic process.

Demonstration of how bond enthalpies can be applied to determine if a reaction is exothermic or endothermic.

Transcripts

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Bond EnthalpyReaction EnergyChemical BondsEnthalpy CalculationChemistry EducationEnergy ReleaseExothermic ReactionsLewis StructuresMolecular BreakdownThermochemistry