12-15 Flashcards
(60 cards)
- What is bond energy?
Bond energy is the amount of energy required to break one mole of a bond in a compound in the gas state, with the bonding electrons shared equally between the atoms (homolytically).
- How can the enthalpy of a reaction be estimated using bond energies?
The enthalpy change (ΔH) of a reaction can be estimated by summing the bond energies of the bonds broken (endothermic) and subtracting the bond energies of the bonds formed (exothermic). The formula is:
ΔH = Σ (bonds broken) - Σ (bonds formed).
- What is the general trend of bond energy in covalent bonds?
Generally, the more electrons two atoms share, the stronger the bond. For example, C≡C (837 kJ/mol) is stronger than C=C (611 kJ/mol) and C−C (347 kJ/mol).
- What happens to bond strength as bond length increases?
As bond length increases, bond strength typically decreases. For example, C≡N (116 pm) is shorter and stronger than C=N (128 pm), which in turn is shorter and stronger than C−N (147 pm).
- What factors affect the actual bond energy in a molecule?
The actual bond energy depends on the surrounding atoms and other environmental factors in the molecule, making average bond energies a useful but sometimes imperfect estimation.
- Why is bond breaking endothermic and bond making exothermic?
Breaking bonds requires energy, so it is endothermic, and making bonds releases energy, so it is exothermic.
- How does the bond energy of halogens change down the group?
As you go down the halogen group, bond energy decreases. For example, F−F has a higher bond energy (144 pm) than Br−Br (228 pm).
- What is bond length and how is it measured?
Bond length is the distance between the nuclei of two bonded atoms, and it is usually measured in picometers (pm).
- How does bond length change across a period and down a group?
Across a period, bond length decreases due to increasing nuclear charge, and down a group, bond length increases due to larger atomic radii.
- What are the basic shapes predicted by VSEPR theory?
VSEPR theory predicts five basic shapes based on the arrangement of electron pairs: linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.
- What is the difference between electron group geometry and molecular geometry?
Electron group geometry describes the spatial arrangement of all electron pairs around a central atom, while molecular geometry only considers the arrangement of atoms, excluding lone pairs.
- What is the AXmEn designation in VSEPR theory?
In the AXmEn designation, ‘A’ represents the central atom, ‘X’ represents the bonded atoms, ‘E’ represents the lone pairs, and ‘m’ and ‘n’ are integers representing the number of bonded atoms and lone pairs, respectively.
- How does the presence of lone pairs affect bond angles?
Lone pairs repel more strongly than bonding pairs, causing bond angles to decrease. For example, in a tetrahedral structure with lone pairs, the bond angle is less than the ideal 109.5°.
- What is a pyramidal molecular geometry?
Pyramidal geometry occurs when there are four electron groups around a central atom, and one is a lone pair, creating a triangular base pyramid shape.
- What is a bent molecular geometry?
A bent molecular geometry occurs when there are four electron groups around a central atom, and two are lone pairs, resulting in a bent shape with a bond angle less than 109.5°.
- How do you use bond energies to estimate ΔH for the reaction: H2(g) + O2(g) → H2O2(g)?
The ΔH is estimated by summing the bond energies of the bonds broken (+936 kJ) and subtracting the bond energies of the bonds formed (-1070 kJ), resulting in ΔHrxn = -136 kJ.
- What are the key concepts of metallic bonding?
In metallic bonding, metal atoms release their valence electrons to form a “sea of electrons,” which are delocalized and free to move throughout the metal lattice. This creates strong attractions between the cations and the delocalized electrons.
- What is the significance of bond vibrations in IR spectroscopy?
Bond vibrations, such as stretching and bending, can be detected using infrared (IR) spectroscopy, as different bonds absorb IR radiation at characteristic frequencies based on their bond strength.
- What is a resonance structure and how does it affect electron geometry?
Resonance structures represent different possible arrangements of electron pairs in a molecule. However, the overall electron geometry of the central atom remains the same when considering the resonance hybrid.
- How do multiple bonds (double and triple bonds) affect molecular geometry?
In VSEPR theory, multiple bonds (double or triple) are treated as single electron groups. Therefore, the molecular geometry is determined by the total number of electron groups, not by the number of bonds within each group.
- Definition of Bond Energy
- The energy required to break one mole of a bond in a compound, typically in the gas state and homolytically, where each atom receives half of the bonding electrons.
- General Trends in Bond Energies
- The more electrons two atoms share, the stronger the covalent bond. Shorter bonds are typically stronger (e.g., C≡C > C=C > C−C).
- Using Bond Energies to Estimate ∆H°rxn
- ∆H°rxn is estimated by comparing the energy required to break bonds (endothermic) to the energy released by forming new bonds (exothermic).
- Bond Breaking and Making in Terms of ∆H
- Bond breaking is endothermic (positive ∆H), and bond making is exothermic (negative ∆H).
