Enthalpies of Reactions - Using Average Bond Enthalpies - Chemistry Tutorial
TLDRThis tutorial on Chemistry Solutions explains how to calculate enthalpies of reaction using bond enthalpies. It demonstrates the process through examples, emphasizing the importance of breaking bonds (requiring energy) and forming bonds (releasing energy). The method involves summing bond enthalpies of broken bonds and subtracting those of formed bonds, using average bond enthalpy values and considering compounds in gaseous states.
Takeaways
- π¬ Bond enthalpy is the energy required to break a bond between two atoms in a gaseous compound, measured in kilojoules per mole.
- π The process of breaking bonds requires energy, while bond formation typically releases energy.
- π To calculate the enthalpy change (ΞH) for a reaction, subtract the sum of bond enthalpies of bonds formed from the sum of bond enthalpies of bonds broken.
- π The variables N and M in the equation represent the number of moles of each type of bond involved in the reaction.
- π§ͺ Before calculating ΞH, draw out the molecules involved to identify the types of bonds present.
- π It's helpful to set up a chart to keep track of the bonds broken and formed, which aids in calculating ΞH accurately.
- π For the example given, breaking carbon-hydrogen bonds requires 413 kJ/mol, and breaking an O2 bond requires 495 kJ/mol.
- π‘ The formation of carbon-oxygen double bonds releases energy, with an average bond enthalpy of 799 kJ/mol.
- π§ The formation of oxygen-hydrogen bonds in water molecules also releases energy, with an average bond enthalpy of 460.3 kJ/mol.
- π In the provided examples, the calculated ΞH values are negative, indicating that the reactions are exothermic (release energy).
- π Understanding and applying the concept of bond enthalpies is crucial for accurately predicting the enthalpy changes in chemical reactions.
Q & A
What is a bond enthalpy?
-A bond enthalpy is the amount of energy required to break a bond between two atoms in a gaseous compound, typically measured in kilojoules per mole.
Why does the breaking of bonds require energy?
-The breaking of bonds requires energy because the process involves overcoming the attractive forces that hold the atoms together in the bond.
What is the general method to calculate the enthalpy change for a reaction using bond enthalpies?
-The enthalpy change for a reaction can be calculated by taking the sum of the bond enthalpies of the bonds broken and subtracting the sum of the bond enthalpies of the bonds formed.
What do N and M represent in the equation for calculating enthalpy change using bond enthalpies?
-In the equation, N and M stand for the number of moles of each type of bond that are broken and formed, respectively.
Why is it important to draw out the molecules when calculating enthalpy change?
-Drawing out the molecules helps to visualize and identify the different types of bonds present in the compounds, which is necessary for accurately calculating the enthalpy change.
How is the average bond enthalpy for an O2 bond represented in bond enthalpy tables?
-In bond enthalpy tables, the bond between two oxygen atoms in an O2 molecule is often not listed separately. Instead, the molecule O2 or CO2 is listed, as the structure of O2 can be represented differently in various contexts.
What is the average bond enthalpy required to break one mole of carbon-hydrogen bonds?
-The average bond enthalpy required to break one mole of carbon-hydrogen bonds is 413 kilojoules.
What is the average bond enthalpy for breaking a bond in a molecule of O2?
-The average bond enthalpy for breaking the bond in a molecule of O2 is 495 kilojoules per mole.
How many carbon-oxygen double bonds are there in the product side of the example reaction given in the script?
-In the example reaction, there are two different carbon-oxygen double bonds in the product side.
What is the average bond enthalpy for a carbon-oxygen double bond?
-The average bond enthalpy for a carbon-oxygen double bond is 799 kilojoules per mole.
How many oxygen-hydrogen bonds are there in two moles of water?
-In two moles of water, there are a total of four oxygen-hydrogen bonds, as each water molecule has two such bonds.
What is the average bond enthalpy for an oxygen-hydrogen single bond?
-The average bond enthalpy for an oxygen-hydrogen single bond is 460.3 kilojoules per mole.
How does the script suggest keeping track of the bonds broken and formed in a chemical reaction?
-The script suggests setting up a chart to keep track of the types and numbers of bonds broken and formed, which helps in accurately calculating the enthalpy change for the reaction.
What is the calculated enthalpy change (ΞH) for the first example reaction in the script?
-The calculated enthalpy change (ΞH) for the first example reaction is -808 kilojoules per mole.
What is the calculated enthalpy change (ΞH) for the second example reaction in the script?
-The calculated enthalpy change (ΞH) for the second example reaction is -85 kilojoules per mole.
Outlines
π¬ Calculating Enthalpies of Reaction Using Bond Enthalpies
This paragraph introduces the concept of bond enthalpies and how they can be used to calculate the enthalpy change (ΞH) in a chemical reaction. Bond enthalpy is defined as the energy required to break a bond between two atoms in a gaseous compound, typically measured in kilojoules per mole. The process involves breaking bonds in the reactants and forming new bonds in the products. The enthalpy change is calculated by summing the bond enthalpies of the bonds broken and subtracting the sum of the bond enthalpies of the bonds formed. The paragraph emphasizes the importance of drawing out the molecules to identify the types of bonds involved and setting up a chart to keep track of these bonds. An example reaction is provided to illustrate the calculation, with specific bond enthalpies for carbon-hydrogen, oxygen-oxygen, carbon-oxygen double, and oxygen-hydrogen bonds. The final calculated ΞH for the example reaction is -808 kilojoules per mole.
π Example Calculation of ΞH Using Average Bond Enthalpies
This paragraph provides a second example of calculating the enthalpy change (ΞH) for a chemical reaction using average bond enthalpies, assuming all compounds are in their gaseous states. The process begins with drawing the structures of the molecules involved, either from given data or using knowledge of Lewis structures. The paragraph details the steps of identifying the bonds broken and formed, multiplying these by their respective average bond enthalpies. Specific bond types and their enthalpies are discussed, such as carbon-hydrogen single bonds, carbon-carbon triple bonds, hydrogen-chlorine bonds, carbon-hydrogen bonds, carbon-carbon double bonds, and carbon-chlorine bonds. The paragraph emphasizes the importance of counting all bonds broken and formed and then subtracting the latter from the former to find ΞH. The final calculated ΞH for the example reaction is -85 kilojoules per mole.
Mindmap
Keywords
π‘Bond enthalpy
π‘Delta H
π‘Enthalpy of reaction
π‘Bond formation
π‘Bond breaking
π‘Moles
π‘Average bond enthalpies
π‘Carbon-hydrogen bond
π‘Oxygen-oxygen bond
π‘Carbon-oxygen double bond
π‘Oxygen-hydrogen bond
Highlights
Introduction to calculating enthalpies of reaction using bond enthalpies.
Definition of bond enthalpy as the energy required to break a bond between two atoms in a gaseous compound, usually given in kilojoules per mole.
Key principle: Breaking bonds always requires energy, while forming bonds releases energy.
Method to calculate enthalpy change: Sum of bond enthalpies of bonds broken minus the sum of bond enthalpies of bonds formed.
Explanation of variables N and M as the number of moles of each type of bond.
Step-by-step example provided for calculating Delta H using average bond enthalpies.
Importance of drawing out molecules to identify different types of bonds in the compounds.
Use of a chart to track bonds broken and bonds formed for clarity.
Example of calculating bond enthalpies for a carbon-hydrogen bond, requiring 413 kJ/mol.
Explanation of bond enthalpies for O2 molecule, noting an average bond enthalpy of 495 kJ/mol.
Description of bonds formed, including carbon-oxygen double bonds with an average bond enthalpy of 799 kJ/mol.
Details on calculating bond enthalpies for oxygen-hydrogen bonds, 463 kJ/mol for single bonds.
Emphasis on setting up a clear chart for easy calculation and tracking of bond enthalpies.
Second example provided for calculating Delta H with detailed steps.
Suggestion to always count all bonds broken and formed for accurate calculations.
Final Delta H calculation example showing a result of negative 85 kJ/mol.
Conclusion encouraging the use of the tutorial for understanding bond enthalpy calculations.
Transcripts
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