Enthalpy Practice Problems

Provide the bond dissociation energy for the indicated bonds in the compound below:

Use the following BDE values:

3° C—H = 381 kJ/mol

2° C—H = 397 kJ/mol

RO—H = 426 kJ/mol

Utilizing bond dissociation energies (BDE), determine the change in enthalpy (∆H°) for the reaction given below.

In the iodination of ethane shown below, tetraiodomethane (CI₄) is used as the source of iodine in the presence of a free-radical initiator such as hydrogen peroxide (H₂O₂).
Propose a mechanism for the reaction, and determine the value of ∆H for each of the steps in your mechanism using the appropriate bond-dissociation energies.

H₃C−CH₃ + CI₄ → H₃C−CH₂I + HCI₃

Estimate the value of the standard enthalpy change, ΔH°, for the following reaction:

(CH₃)₂CHOH + HI →  (CH₃)₂CHI + H₂O

BDE values:

(CH₃)₂CH—OH bond = 95.0 kcal/mol

H—I bond = 71.0 kcal/mol

(CH₃)₂CH—I bond = 57.0 kcal/mol

HO—H bond = 119 kcal/mol

The reaction (i) is disfavored overall (ΔG° > 0) yet favored by ΔH°:

Meanwhile, reaction (ii) is favored (ΔG° < 0):

Determine the bonds formed and broken for reactions (i) and (ii), and explain why reaction (ii) is favored.

BDE values for reference: 

C—O π bond = 356 kJ/mol, RO—H bond = 426 kJ/mol, 1° C—OH bond = 381 kJ/mol

Determine the value of the standard enthalpy change, ΔH°, for the following reaction:

CH₃CH₂CH₂CH₃ + H₂ → 2 CH₃CH₃

BDE values:

CH₃CH₂—CH₂CH₃ bond = 88.0 kcal/mol

H—H bond = 104 kcal/mol

CH₃CH₂—H bond = 101 kcal/mol

Provide the value of the standard enthalpy change (ΔH°) for the following reaction:

CH₃Br + HCl → CH₃Cl + HBr

BDE values:

CH₃—Br bond = 302 kJ/mol

H—Cl bond = 432 kJ/mol

CH₃—Cl bond = 350 kJ/mol

H—Br bond = 366 kJ/mol

For the following reaction, the value of ∆H° is −42 kcal/mol. Calculate the bond dissociation enthalpy of the C—H bond when the bond dissociation enthalpies of O—Cl, C—Cl, and O—H bonds are 47, 85, and 105 Kcal/mol, respectively.

A. 101 Kcal/mol

B. 120 Kcal/mol

C. 50 Kcal/mol

D. 320 Kcal/mol

Despite being generally disfavored (∆G° > 0), the given reaction is favored according to ∆H°. Determine which bonds were broken and formed in the reaction. 

Refer to the information given in the table below. Provide the ∆H°_hydr for each step involved in the reduction of allylbenzene. 

∆H₁ + ∆H₂ + ∆H₃ + ∆H₄ = ∆H_tot = –78.2 kcal/mol (–328 kJ/mol)

Calculate the value of the standard enthalpy change (ΔH°) for each step in the chlorination of ethane. Determine the overall value of ΔH°. 

Use the following BDE values: 

CH₃CH₂—Cl bond = 355 kJ/mol

CH₃CH₂—H bond = 423 kJ/mol

Cl—Cl bond = 240 kJ/mol

H—Cl bond = 432 kJ/mol

Use the appropriate bond-dissociation enthalpies to compute ∆Hº value for the reaction below.
CH₃−CH₃ + Br₂ → CH₃CH₂Br + HBr

The following reaction illustrates an alkene addition reaction:

Provide the value of its ΔH° and predict the sign of ΔS°.

Use the following BDE values:

C—C π bond = 65 kcal/mol

I—I bond = 36 kcal/mol

2° C—I bond = 53 kcal/mol

Determine the enthalpy change (∆H°) for the given equilibrium process.

Consider the following reaction:

•CH(CH₃)₂ + HF → (CH₃)₂CH₂ + F•

The activation energy and the heat of reaction (ΔH°) of its reverse reaction are 26.1 kJ/mol (6.24 kcal/mol) and −157 kJ/mol (−37.5 kcal/mol), respectively. Determine the activation energy and the heat of reaction (ΔH°) for the forward reaction.

Based on the bond dissociation energies of the indicated bonds below, which will be easier to break?

How can this explain the following mechanism?

Use the following BDE: 

I—I bond = 36 kcal/mol

H—I bond = 71 kcal/mol

Compute the overall ∆Hº for the iodination of methane which proceeds through the following steps:
1. I₂ + 2I•
∆Hº (per mol) = +149 kJ (+35.6 kcal)
2. CH₄ + I• → •CH₃ + HI
∆Hº (per mol) = +141 kJ (+33.7)
3. •CH₃ + I₂ → CH₃I + I•
∆Hº (per mol) = −92 kJ (−22.0 kcal) 

(i) Propose a mechanism for the reaction shown below:

(ii) Use the appropriate bond-dissociation enthalpies to find the value of ∆Hº for each step shown in the mechanism you proposed, and calculate the value of ∆Hº for the overall reaction if the bond-dissociation enthalpy for PhCH₂−Br bond is approximately 280 kJ/mol. Are these values appropriate for a rapid free-radical chain reaction?

The reaction below is said to be favorable. Determine the sign of ΔH and explain why.

(i) Calculate the heat of reaction for each step in the free-radical chlorination of methane using bond-dissociation enthalpies from the table below.

(ii) Calculate the overall heat of the reaction.

Calculate the ΔH° for the reaction shown below:

CH₃CH₂CH₂I + HCl → CH₃CH₂CH₂Cl + HI

What is the ΔH° for the reaction given below?

What is the enthalpy (∆H°) of the given reaction?

Identify the more stable radical using bond-dissociation energies. Explain the difference in stability based on their structures.

Which radical is more stable in terms of bond-dissociation energy? Rationalize the difference in stability based on their structures.

Which radical is less stable in terms of bond-dissociation energy? Explain the difference in stability based on their structures.

Cl• vs HO•

For the reaction shown below,

Encircle the broken bonds and box the bonds that formed.

For the following reaction:

Use a "/" to indicate broken bonds and a "x" to indicate formed bonds.

Calculate the heats of the reaction for the two possible sites for radical formation in the first propagation steps for the chlorination of 2,3-dimethylbutane. Label the reaction-coordinate diagram shown below to compare the activation energies of the two radical formations.

Bond dissociation energies: 1° C–H = 423 kJ/mol; 3° C–H = 403 kJ/mol; 1° C–Cl = 350 kJ/mol; 3° C–Cl = 355 kJ/mol

Iodine is a free-radical inhibitor. In a mixture containing chlorine and cyclopentane, chlorination is prevented from happening when iodine is added to the mixture. Referring to the ΔH° values for the possible reactions that might occur when iodine is present in the reaction mixture, explain why iodine inhibits chlorination. Focus on the propagation step and assume that chlorine radicals are still formed.

High temperatures will cause some polymers to oxidize and decompose. The decomposition is initiated by the free-radical abstraction of an atom by O₂. Which of the following pairs will be more prone to decomposition? Explain your answer.

Bond dissociation energies: C–H bond ≈ 400 kJ/mol; C–F bond ≈ 460 kJ/mol; HOO bond ≈ 192 kJ/mol; FOO bond ≈ 63 kJ/mol

Tributyltin hydride (Bu₃SnH) can be used to synthetically reduce alkyl halides. In this process, a halogen atom is replaced by a hydrogen atom. Suggest a mechanism for the initiation and propagation steps for the reaction below. 

Using the following bond-dissociation enthalpies, calculate the values of ΔH for the proposed steps and determine whether each step is energetically favorable.

Which of the compounds shown below is expected to have a higher flame temperature upon complete combustion? Provide a brief explanation for your answer.

Assuming no knowledge of the mechanism of the equilibrium below and given the relevant bond dissociation energies (BDE), estimate the equilibrium constant (in 3 s.f.) for the equilibrium process at 298.15 K. 

Relevant BDE values

C―C π bond = 65 kcal/mol
3° C―H bond = 91 kcal/mol
2° C―H bond = 95 kcal/mol

Given the keto-enol tautomerism below, the relevant bond dissociation energies (BDE), and no knowledge of the mechanism, estimate the equilibrium constant (in 3 s.f.) for the reaction at 298.15 K.

Relevant BDE values

C―C π bond = 65 kcal/mol
C―O π bond = 85 kcal/mol
2° C―H bond = 95 kcal/mol
RO―H bond = 102 kcal/mol

Determine the enthalpy (∆H°) of the equilibrium process given below.

What is the enthalpy (∆H°) of the following process?

What is the ∆H° for the reaction below?