Using Bond Dissociation Energies (9.4.2) Flashcards
• The overall enthalpy change of a reaction can be approximated using bond dissociation energies.
• The overall enthalpy change of a reaction can be approximated using bond dissociation energies.
• The calculated enthalpy change of a reaction will be slightly different when using bond dissociation energies versus using thermochemical data.
• The calculated enthalpy change of a reaction will be slightly different when using bond dissociation energies versus using thermochemical data.
The overall enthalpy change of a reaction can be
approximated by subtracting the sum of the average
bond dissociation energies of the products from the
sum of the average bond dissociation energies of
the reactants.
For example, the enthalpy change for the
combustion reaction between propane (C3H8) and
oxygen (O2) to form carbon dioxide (CO2) and water
(H2O) can be approximated using bond dissociation
energies. First, calculate the energy required to
break the bonds in the reactants. Two C-C bonds
(2 • 346 kJ/mol), eight C-H bonds (8 • 413 kJ/mol),
and five O-O bonds (5 • 498 kJ/mol) are broken, for
a total of 6486 kJ/mol. Next, calculate the energy
released when the bonds in the products are
formed. Six C=O bonds (6 • 732 kJ/mol) and eight
O-H bonds (8 • 463 kJ/mol) are formed, releasing a
total of 8096 kJ/mol. Subtracting the sum of the
average bond dissociation energies of the products
from the sum of the average bond dissociation
energies of the reactants yields –1610 kJ/mol. This
reaction is very exothermic.
The overall enthalpy change of a reaction can be
approximated by subtracting the sum of the average
bond dissociation energies of the products from the
sum of the average bond dissociation energies of
the reactants.
For example, the enthalpy change for the
combustion reaction between propane (C3H8) and
oxygen (O2) to form carbon dioxide (CO2) and water
(H2O) can be approximated using bond dissociation
energies. First, calculate the energy required to
break the bonds in the reactants. Two C-C bonds
(2 • 346 kJ/mol), eight C-H bonds (8 • 413 kJ/mol),
and five O-O bonds (5 • 498 kJ/mol) are broken, for
a total of 6486 kJ/mol. Next, calculate the energy
released when the bonds in the products are
formed. Six C=O bonds (6 • 732 kJ/mol) and eight
O-H bonds (8 • 463 kJ/mol) are formed, releasing a
total of 8096 kJ/mol. Subtracting the sum of the
average bond dissociation energies of the products
from the sum of the average bond dissociation
energies of the reactants yields –1610 kJ/mol. This
reaction is very exothermic.
The calculated enthalpy change of a reaction will be
slightly different when using bond dissociation
energies versus using thermochemical data. This
difference is because the bond dissociation
energies used are an average of several molecules
that have that type of bond.
However, bond dissociation energies are still useful
for estimating the enthalpy change of a reaction
involving molecules with unknown enthalpies of
formation.
The calculated enthalpy change of a reaction will be
slightly different when using bond dissociation
energies versus using thermochemical data. This
difference is because the bond dissociation
energies used are an average of several molecules
that have that type of bond.
However, bond dissociation energies are still useful
for estimating the enthalpy change of a reaction
involving molecules with unknown enthalpies of
formation.
What is ΔH of the reaction below?
C(graphite) + Cl2(g) + F2(g) →
C(g) + 2Cl(g) + 2F(g),
Given the following ΔH f values
C(g) = 717 kJ / mol
Cl(g) = 122 kJ / mol
F(g) = 79 kJ / mol
1119 kJ (C)
ΔH = 717 kJ / mol + 2 (122 kJ / mol) + 2 (79 kJ / mol) ΔH = 1119 kJ.
Calculate the ΔH, of the reaction below and identify whether the reaction is endothermic or exothermic.
N2(g) + 2O2(g) → 2NO2(g)
ΔH°f (NO2 ) = 33.2 kJ / mol NO2 ΔH°f O2 = ΔH°f N2 = 0
66.4 kJ; endothermic (A)
N2(g) + 2O2(g) → 2 NO2(g)
ΔH° = 2 (ΔH°f(NO2 ) ΔH° = 2 (33.2 / mol NO2 ). ΔH° = 66.4 kJ.
The reaction is endothermic because the value of ΔH° is positive.
For the reaction: C(graphite) + Cl(g) + F2(g)→ CCl2F2(g)
The intermediate equation for this event is C(g) + 2Cl(g) + 2F(g)→ CCl2F2(g). This last step occurs through an exothermic event. In addition, the final energy level for the product, CCl2F2(g), is lower than the starting energy level for the reactants, C(graphite), Cl2(g), and F2(g). Which of the following statements is correct?
The enthalpy change for this step will be negative because heat is released in an exothermic process. The overall enthalpy change for the entire event will have a negative value as well. (D)
In the first step of this process you are releasing heat energy in order to form the final product. Therefore, an exothermic event is occurring. Because heat is being released, there will be a negative enthalpy change for this step. In addition, you know that the final energy level for the product, CCl2F2(g), is lower than the starting energy level for the reactants, C(graphite), Cl2(g), and F2(g). This means that the energy (and enthalpy) change is negative.
What is enthalpy?
Enthalpy is a numerical description of the amount of heat energy a substance has at a given temperature and pressure. (D)
Enthalpy specifically describes the amount of heat in a substance. When you speak of enthalpy change for a reaction, you are focusing on the difference between the sum of the heats for the reactants and the sum of the heats for the products.
NO gas and O2 gas interact to form NO2 gas in the following manner:
2NO(g) + O2(g) → NO2(g)
In order for this reaction to occur, the bonds in 2 moles of NO(g) must first be broken into a standard state of N2(g) + O2(g).
2NO(g) → N2(g) + O2(g)
What is the ΔH for this step of the process?
ΔH°f (NO) = 90.3 kJ / mol NO
-181 kJ (A)
ΔH f for NO is 90.3 kJ / mole This is the amount of energy from the reaction:
½N2(g) + ½O2(g)→ NO(g)
The reaction in question is the reverse of the above reaction multiplied by 2, therefore the ΔH of the reaction in question = −90.3 kJ × 2 = −181 kJ.
Which of the following does not describe an aspect of thermochemistry?
Thermochemistry is a description of the bonding changes that occur when heat is used to alter molecules. (C)
This statement is not true. Although bond changes do occur, thermochemistry focuses (through measurements and predictions) on the role of heat during these events.
Which of the following is not true about calculating the enthalpy of reaction, ΔHr x n?
The original energy level of the reactants does not affect the calculation. (A)
The starting (energy) level of the reactants is very important. If the reactants are in a highly agitated (i.e., unstable) state, the bonds in the reactant are weaker. This results in a potentially greater enthalpy change.
What is the best term for a reaction in which heat energy is released?
an exothermic reaction (D)
This is the best choice. In an exothermic reaction, a chemical change causes heat to be released to the surroundings. An exothermic reaction is a specific type of thermochemical reaction. That is why this answer is better than simply referring to a thermochemical reaction; it gives you more information.
Which of the following statements is not true about enthalpy?
Change in enthalpy is not a valuable parameter in describing the energy changes during a chemical event. (B)
This statement is not true. Change in enthalpy is used to describe energy changes in chemical reactions.
Which statement correctly describes the approximate total bonding energy in propane, C3H8?
The total bonding energy is approximately equal to
(8 × (bonding energy for C–H)) + (2 × (bonding energy for C–C)). (B)
(8 × (bonding energy for C–H)) + (2 × (bonding energy for C–C)) is correct. It is the sum of each of the C–C bond energies and the C–H bond energies.
