Ch. 6 - Thermodynamics and KineticsWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

Bond dissociation energies describe the strength of chemical bonds. They can be determined experimentally. With that said, we’ll need some basic math for this section to determine the Enthalpy (∆H˚) for reactions. 

Concept #1: How to calculate enthalpy using bond dissociation energies. 

Enthalpy (ΔH°) is the sum of bond dissociation energies for the reaction.

  • Negative values (-) indicate the making of new bonds = Exothermic
  • Positive values (+) indicate the breaking of new bonds = Endothermic

Example #1: Predict the sign and magnitude of ∆Ho in kj/mol for the following reaction. Identify the reaction as either exothermic or endothermic.

I hope that made sense! Let's try another similar problem with I, instead of Br and see how that changes things. 

Practice: Predict the sign and magnitude of ∆Ho in kj/mol for the following reaction. Identify the reaction as either exothermic or endothermic.

Additional Problems
In each of the following cases compare the bonds identified in red, and determine which bond you would expect to have the largest bond dissociation energy:
In each of the following cases compare the bonds identified in red, and determine which bond you would expect to have the largest bond dissociation energy:
Consider the following reaction: Use Table 6.1 to estimate ΔH for this reaction.
(a) Use the bond dissociation enthalpy data in Table 4.3 to calculate ΔH° for the propagation step 
The bond dissociation enthalpies of n-propyl and isopropyl chloride are the same within experimental error (Table 4.3). However, it is incorrect to conclude that the data indicate equal stabilities of n-propyl and isopropyl radical. Why? Why are the bond dissociation enthalpies of propane a better indicator of the free-radical stabilities? 
Carbon–carbon bond dissociation enthalpies have been measured for many alkanes. Without referring to Table 4.3, identify the alkane in each of the following pairs that has the lower carbon–carbon bond dissociation enthalpy, and explain the reason for your choice. (a) Ethane or propane
Carbon–carbon bond dissociation enthalpies have been measured for many alkanes. Without referring to Table 4.3, identify the alkane in each of the following pairs that has the lower carbon–carbon bond dissociation enthalpy, and explain the reason for your choice. (b) Propane or 2-methylpropane
Carbon–carbon bond dissociation enthalpies have been measured for many alkanes. Without referring to Table 4.3, identify the alkane in each of the following pairs that has the lower carbon–carbon bond dissociation enthalpy, and explain the reason for your choice. (c) 2-Methylpropane or 2,2-dimethylpropane
The iodination of ethylene at 25°C is characterized by the thermodynamic values shown. H2C = CH2(g) + I2(g) ⇌ lCH2CH2l(g)     ΔH° = - 48 kJ; ΔS° = - 0.13 kJ/K (a) Calculate ΔH° and K at 25°C.
Approximate the heat of reaction for the given halogenation reaction.   
Consider the following reaction: (c) Would you expect this process to be exothermic or endothermic? Explain.
Approximate the heat of reaction for the given halogenation reaction.  
According to the potential energy diagram, the overall reaction is:A. endothermicB. exothermic
Calculate the enthalpy of reaction for following reaction. If possible, estimate ΔG for the reaction.
Use the given bond energy values to estimate ΔH for the following gas-phase reaction. (Simple energy units required for the answer.) 
Using average bond enthalpies (linked above), estimate the enthalpy change for the following reaction: CH4(g) + 2F2(g) → CH2F2(g) + 2HF(g)
Use the given average bond energy values to estimate ΔH for the following reaction in the gas phase. Cl2 + F2 → 2FCl DCl-Cl = 239 kJ/mol DF-F = 154 kJ/mol DF-Cl = 253 kJ/mol The units expected here are simply "kJ". The implication is that the reaction coefficients refer to numbers of moles.