9.4.2 Using Bond Dissociation Energies Flashcards Preview

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Flashcards in 9.4.2 Using Bond Dissociation Energies Deck (12)
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1
Q

Using Bond Dissociation Energies

A
  • 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.
2
Q

note

A
  • 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 (C 3 H 8 ) and oxygen (O 2 ) to form carbon dioxide (CO 2 ) and water (H 2 O) 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.
3
Q

Which of the following does not describe an aspect of thermochemistry?

A

Thermochemistry is a description of the bonding changes that occur when heat is used to alter molecules.

4
Q

Which of the following is not true about calculating the enthalpy of reaction, ΔHr x n?

A

The original energy level of the reactants does not affect the calculation.

5
Q

What is enthalpy?

A

Enthalpy is a numerical description of the amount of heat energy a substance has at a given temperature and pressure.

6
Q

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

A

1119 kJ

7
Q

Which statement correctly describes the approximate total bonding energy in propane, C3H8?

A

The total bonding energy is approximately equal to

8 × (bonding energy for C–H)) + (2 × (bonding energy for C–C)

8
Q

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?

A

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.

9
Q

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
A

66.4 kJ; endothermic

10
Q

What is the best term for a reaction in which heat energy is released?

A

an exothermic reaction

11
Q

Which of the following statements is not true about enthalpy?

A

Change in enthalpy is not a valuable parameter in describing the energy changes during a chemical event.

12
Q

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

A

−181 kJ

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