Intramolecular Force and Potential Energy
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AP Chemistry › Intramolecular Force and Potential Energy
A qualitative potential energy curve for a diatomic molecule shows two possible bonds: Bond X has a deeper minimum in potential energy than Bond Y. Which bond has the greater bond strength?
Both bonds have the same strength because both have a minimum on the curve
Bond X only if the molecule also has stronger intermolecular forces
Bond Y, because a shallower minimum means the atoms are closer together
Bond X, because a deeper minimum corresponds to a larger energy required to separate the atoms
Bond Y, because higher potential energy means the bond stores more energy
Explanation
This question tests the interpretation of potential energy curves for bond strength in intramolecular forces and potential energy. A deeper minimum in the potential energy curve for Bond X indicates a greater energy difference from the dissociated state, meaning more energy is required to break the bond, thus Bond X is stronger. The depth of the well corresponds to bond dissociation energy, with deeper wells signifying stronger bonds. The stimulus describes qualitative curves, linking potential energy minima to bond stability. Choice B is a tempting distractor, claiming Bond Y is stronger due to a shallower minimum meaning closer atoms, which misinterprets that shallower wells actually indicate weaker bonds. When analyzing potential energy curves, relate the depth of the minimum to bond strength, not to intermolecular forces.
Two potential energy curves are shown conceptually for diatomic molecules X and Y. Molecule X has a deeper potential energy well and a shorter equilibrium distance than molecule Y. Which molecule has the lower potential energy at equilibrium and the stronger bond?
Molecule X has higher potential energy but a stronger bond
Molecule Y has lower potential energy but a weaker bond
Both have the same bond strength because both have a potential energy minimum
Molecule Y has lower potential energy and a stronger bond
Molecule X has lower potential energy and a stronger bond
Explanation
This question evaluates potential energy curve features for bond strength and equilibrium energy, within intramolecular forces and potential energy. Molecule X has lower potential energy at equilibrium and a stronger bond, as its deeper well indicates greater stability and higher dissociation energy, with the shorter distance supporting strength. Deeper wells mean lower energy minima and stronger bonds. The stimulus compares curves conceptually, linking features to properties. Choice A is a tempting distractor, claiming Y has lower energy and stronger bond, which misinterprets shallower wells as stronger. To interpret curves, associate deeper wells and shorter distances with stronger bonds and lower equilibrium energy, not confusing with intermolecular forces.
A student compares the bond between carbon and carbon in ethane, $\mathrm{C_2H_6(g)}$ (a C–C single bond), and in ethene, $\mathrm{C_2H_4(g)}$ (a C=C double bond). Which statement is correct about the C=C bond compared with the C–C bond?
The C=C bond is weaker because ethene has fewer H atoms
The C=C bond is the same length and strength as the C–C bond
The C=C bond is stronger only because ethene has stronger intermolecular forces
The C=C bond is longer and weaker than the C–C bond
The C=C bond is shorter and stronger than the C–C bond
Explanation
This question tests understanding of how bond order affects both bond length and bond strength. The C=C double bond in ethene (C₂H₄) contains two shared electron pairs, while the C-C single bond in ethane (C₂H₆) has only one shared pair. Double bonds create stronger electrostatic attraction between nuclei, pulling them closer together (shorter bond length) and requiring more energy to break (greater bond strength). Choice A incorrectly reverses the relationship, suggesting double bonds are weaker, which contradicts the fundamental principle that more shared electrons create stronger bonds. The key concept is that bond order determines both properties: higher bond order means shorter and stronger bonds.
Two bonds are compared: an $\mathrm{O-H}$ bond in water and an $\mathrm{O-H}$ bond in methanol ($\mathrm{CH_3OH}$). Which statement best describes the bond strength of the $\mathrm{O-H}$ bond in these molecules?
The $\mathrm{O-H}$ bond is stronger in water because water has hydrogen bonding
The $\mathrm{O-H}$ bond is stronger in water because oxygen is more electronegative in water
The $\mathrm{O-H}$ bond is stronger in methanol because methanol is larger
The $\mathrm{O-H}$ bond is weaker in water because water has more intermolecular forces to break
The $\mathrm{O-H}$ bond strength is expected to be similar in both because the bonded atoms are the same
Explanation
This question evaluates the concept that bond strength depends primarily on the atoms involved in the bond, within intramolecular forces and potential energy. The O-H bond in water and methanol involves the same atoms (oxygen and hydrogen), so their bond strengths are similar, as bond dissociation energy is largely determined by the atomic pair and not significantly affected by the rest of the molecule in these cases. Differences in molecular size or intermolecular forces do not alter the intramolecular O-H bond strength. The stimulus compares similar bonds in different molecules, emphasizing that bond strength is intrinsic to the bond type. Choice A is tempting, suggesting the O-H bond is stronger in water due to hydrogen bonding, which confuses intermolecular forces with intramolecular bond strength. To assess bond strengths, consider the bonded atoms and bond type, distinguishing them from intermolecular interactions.
Compare the bonds in $\mathrm{F_2(g)}$ (F–F) and $\mathrm{Cl_2(g)}$ (Cl–Cl). Which bond is expected to have the shorter bond length?
The Cl–Cl bond because chlorine has more electrons to share
The F–F bond in $\mathrm{F_2(g)}$
The Cl–Cl bond in $\mathrm{Cl_2(g)}$
They have the same bond length because both molecules are nonpolar
They have the same bond length because both are halogens
Explanation
This question tests understanding of how atomic size affects bond length. Fluorine atoms are smaller than chlorine atoms due to having fewer electron shells (F is in period 2, Cl is in period 3). When two smaller atoms bond, their nuclei can approach more closely, resulting in a shorter bond length. The F-F bond is therefore shorter than the Cl-Cl bond, despite both being single bonds between halogens. Choice A incorrectly predicts the larger Cl atoms would form a shorter bond, reversing the relationship between atomic size and bond length. The strategy is to compare atomic sizes first: smaller atoms form shorter bonds when all other factors (like bond order) are equal.
A carbon–carbon single bond in ethane ($\mathrm{C-C}$) is compared with the carbon–carbon double bond in ethene ($\mathrm{C=C}$). Which bond is expected to have the shorter bond length?
$\mathrm{C=C}$, because ethene has stronger intermolecular forces between molecules
$\mathrm{C=C}$, because double bonds have greater electron density between nuclei
$\mathrm{C-C}$, because single bonds allow atoms to get closer than double bonds
$\mathrm{C-C}$, because ethane has more H atoms that pull the C atoms together
$\mathrm{C-C}$, because the molecule with more atoms always has shorter bonds
Explanation
This question tests understanding of the relationship between bond order and bond length. Double bonds have higher electron density between nuclei than single bonds, which pulls the atoms closer together resulting in shorter bond lengths. The C=C double bond in ethene has four shared electrons creating stronger attraction and pulling the carbon atoms to approximately 134 pm apart, while the C-C single bond in ethane has only two shared electrons allowing the carbons to be about 154 pm apart. Bond length is inversely related to bond strength and bond order. Choice A incorrectly reverses the relationship, suggesting single bonds are shorter when they actually allow atoms to be farther apart due to weaker attraction. To predict relative bond lengths, use the rule that higher bond order means shorter bonds: triple < double < single bond lengths.
Two bonds are compared in similar environments: a $\mathrm{C-O}$ single bond and a $\mathrm{C=O}$ double bond. Which bond is expected to have the greater bond strength?
$\mathrm{C-O}$, because adding more bonds (like a double bond) spreads out electron density and weakens it
$\mathrm{C=O}$, because a double bond has higher bond order and a deeper potential energy well
$\mathrm{C-O}$, because oxygen can hydrogen-bond to other molecules, strengthening the bond
$\mathrm{C=O}$, because the molecule containing it usually has a higher boiling point
$\mathrm{C-O}$, because single bonds are more stable than double bonds
Explanation
This question tests understanding of how bond order affects bond strength between the same pair of atoms. The C=O double bond is stronger than the C-O single bond because it has a higher bond order with four shared electrons compared to two, creating greater attractive force between the nuclei. Double bonds have approximately 1.5-1.7 times the strength of single bonds between the same atoms, with C=O bonds typically having dissociation energies around 745 kJ/mol compared to about 350 kJ/mol for C-O single bonds. The increased electron density between nuclei in the double bond creates a deeper potential energy well. Choice E incorrectly suggests that double bonds are weaker because electron density is "spread out," when actually the opposite is true - more shared electrons create stronger attraction. When comparing bonds between the same atoms, always remember that bond strength increases with bond order: single < double < triple.
Two bonds are compared: the $\textbf{O=O}$ bond in $\mathrm{O_2(g)}$ and the $\textbf{O\text{--}O}$ single bond in $\mathrm{H_2O_2(g)}$. Which bond corresponds to a lower minimum potential energy (a deeper potential energy well)?
Both have the same minimum potential energy because both are neutral molecules
The O–O bond in $\mathrm{H_2O_2(g)}$
The O=O bond in $\mathrm{O_2(g)}$
Both have the same minimum potential energy because both are O–O bonds
The O–O bond in $\mathrm{H_2O_2(g)}$ because hydrogen bonding makes bonds stronger
Explanation
This question tests understanding of how bond order relates to potential energy. The O=O bond in O₂(g) is a double bond, while the O-O bond in H₂O₂(g) is a single bond. Double bonds are stronger than single bonds, meaning more energy is required to break them, which corresponds to a deeper potential energy well (lower minimum potential energy). The stronger O=O double bond therefore has a lower minimum potential energy than the weaker O-O single bond. Choice D incorrectly attributes bond strength to hydrogen bonding capability of the molecule, which is an intermolecular force unrelated to the intramolecular O-O bond strength. Remember that stronger bonds have deeper potential energy wells because more energy must be added to break them.
Two diatomic species are compared: $\mathrm{F_2}$ and $\mathrm{Ne_2}$. At room temperature, $\mathrm{F_2}$ exists as a stable molecule, while $\mathrm{Ne_2}$ does not form a stable covalent bond. Which statement best explains which has the lower potential energy at an equilibrium bond distance?
$\mathrm{Ne_2}$, because its atoms are smaller so the bond must be shorter and stronger
$\mathrm{Ne_2}$, because noble gases have full valence shells that strongly attract each other
$\mathrm{F_2}$, because it has a bonding interaction that creates a potential energy minimum
$\mathrm{F_2}$, because having more atoms in the molecule always lowers potential energy
$\mathrm{Ne_2}$, because it has stronger London dispersion forces than $\mathrm{F_2}$
Explanation
This question tests understanding of bonding versus non-bonding interactions in diatomic species. F₂ has a lower potential energy at equilibrium because it forms a true covalent bond with shared electrons in bonding molecular orbitals, creating a stable molecule with a defined bond length and dissociation energy. Ne₂ cannot form a stable covalent bond because neon has a complete octet and its molecular orbital diagram would have equal numbers of bonding and antibonding electrons, resulting in zero bond order. While Ne atoms can have weak van der Waals interactions, these are intermolecular forces, not intramolecular bonds, and don't create a stable diatomic molecule. Choice A incorrectly suggests that full valence shells lead to strong bonding, when complete octets actually prevent covalent bond formation. To determine if atoms can form stable bonds, check if they have unpaired electrons available for sharing - noble gases with complete octets cannot form covalent bonds.
Two covalent bonds are compared: a C–H bond and a C–C bond, both single bonds. Which bond is expected to have the greater bond strength in general?
They are equal because both are single covalent bonds.
C–C, because carbon has more valence electrons available to strengthen the bond.
C–H, because shorter bonds to hydrogen are generally stronger.
C–H, because stronger London dispersion forces increase the bond strength.
C–C, because bonds between identical atoms are always stronger.
Explanation
This question tests understanding of bond strength trends between different atoms. The C–H bond is generally stronger than the C–C bond because hydrogen's small size allows for better orbital overlap and shorter bond length, creating a stronger bond despite carbon having more valence electrons. Typical bond dissociation energies are approximately 413 kJ/mol for C–H versus 348 kJ/mol for C–C, confirming that C–H bonds are stronger. A common misconception (choice E) is that London dispersion forces affect intramolecular bond strength, but these intermolecular forces are completely separate from the covalent bonds within molecules. When comparing single bonds, bonds to hydrogen are often stronger than bonds between heavier atoms due to hydrogen's small size enabling optimal orbital overlap.