Bond Energy Questions

Bond Energy is also known as the average Bond Enthalpy and gives a measure of the strength of a chemical bond. The larger the bond energy the more will be the stability of the molecules.

Bond Energy Chemistry Questions with Solutions

Q1: How can we determine the strength of a bond by the radius of an atom?

Answer: Any of the radius i.e. metallic radius, ionic radius or covalent radius can be used to determine the strength of bond formed by the given atoms.

For example: Boron (B) has a covalent radius of 83 pm. It makes a B-B bond of bond length 175 pm in B₂Cl₄. This indicates that the B-B bond length is even higher than the radius of an atom of B. Thus, this is a weak bond. However, in the case of Rhenium (Re), the metallic radius is 137.5 pm. The Re-Re bond length in Re₂Cl₈ is 224 pm. This indicates that the Re-Re is a very strong quadruple bond.

Q2. What is the difference between a primary and secondary bond? Give 2 examples of each.

Answer: A Primary bond is formed by the mutual sharing or complete transfer of electrons. This type of bond is stronger than the secondary bond. For example: O₂, NaCl.

A secondary bond is formed either due to permanent dipole or due to certain imbalances in the symmetrical charge distribution within the atom- creating a dipole. These dipoles induce a charge imbalance in their immediate neighbouring atoms. For example: N₂, H₂O.

Q3. How is the melting temperature of solids related to the bond energy?

Answer: The melting temperature is associated with the breaking of bonds of solid substances. The higher the melting temperature, the stronger the bond. The stronger the bond, the higher the bond energy. Hence, the melting temperature of solids has a direct relationship with the bond energy.

Q4. What is the enthalpy change for the given reaction?

CH₃CH₃ + Cl₂ → CH₃CH₂Cl + HCl

Given the enthalpies:

Answer: The total enthalpy for a reaction is calculated by the formula:

ΔH^o = ⅀H_{bonds broken} – ⅀H_{bonds formed}

ΔH^o = (413 + 239) kJ mol^-1 – (339 + 427) kJ mol^-1

ΔH^o = -114 kJ mol^-1

Q5. Choose the correct statement.

1. A (-) ΔH_reaction indicates that energy is released.
2. A (+) ΔH_reaction indicates that energy is released.
3. A (-) ΔH_reaction indicates that energy is absorbed.
4. A (-) ΔH_reaction indicates that the reaction is endothermic.

Answer: (a.)

Explanation: In an exothermic reaction, the total energy change in reactants and products comes out to be negative because the products’ energy is lesser than the reactants energy. Hence, (-) ΔH_reaction indicates the release of energy.

Q6. Pick the false statement.

1. Single bonds are more easily broken than double bonds.
2. Double bonds are more easily broken than triple bonds.
3. Triple bonds are more easily broken than double bonds.
4. The bonds formed in the endothermic reactions are weaker than the bonds broken.

Answer: (c.)

Explanation: Triple bonds are stronger than double bonds. Hence, triple bonds cannot be more easily broken than double bonds.

Q7. What is the enthalpy change in the following decomposition reaction of HCl?

2HCl → H₂ + Cl₂

The average bond enthalpies (kj mol^-1) for the concerned bonds are:

H-Cl = 431 kj mol^-1

Cl-Cl = 242 kj mol^-1

H-H = 436 kj mol^-1

Answer: The total enthalpy for a reaction is calculated by the formula:

ΔH^o = ⅀H_{Bonds broken} – ⅀H_{Bonds formed}

ΔH^o = 2(431) kj mol^-1 – (242 + 436) kj mol^-1

ΔH^o = +185 kJ mol^-1

Hence, the reaction is endothermic.

Q8. Why does FeCl₃ have a greater covalent character than FeCl₂?

Answer: This is because the charge on Fe in FeCl₃ is greater i.e. +3 than in FeCl₂ i.e. +2. Due to this reason, the size of the Fe⁺³ ion is smaller than Fe⁺². Hence, Fe⁺³ ion creates more polarisation and hence, more neutralization of charge takes place. Therefore, the ionic character of FeCl₃ decreases and the covalent character increases.

Q9. The combustion of 1 g Graphite produces 20.7 kJ heat. Calculate its molar enthalpy change.

Answer: Molar enthalpy change is calculated as:

Molar enthalpy change for combustion of 1 g Graphite = Enthalpy of Combustion of 1 g Graphite x Molar Mass

Enthalpy of Combustion of 1 g Graphite = -20.7 kJ g^-1 (energy is released)

Since graphite (C) is made from carbon, its molar mass is 12 g mol^-1

ΔH_combustion = -20.7 kJ g^-1 x 12 g mol^-1

ΔH_combustion =-2.48 x 10² kJ mol^-1

This reaction is exothermic in nature.

Q10. Predict the spontaneity of the reaction:

N₂O₄ (g) ⇌ 2NO₂ (g); Given K_P = 0.98 and T = 298 K.

Answer: From the relationship between standard free energy change and the equilibrium constant, we have:

Δ_rG^o = -RT lnK_P

Since K_P = 0.98, Δ_rG^o comes out to be negative. This indicates that the reaction is spontaneous.

Q11. Why is the molar enthalpy of vaporisation of acetone less than that of water?

Answer: In this, the inter-molecular forces come into play. Since the water molecules have strong H-bonding, water has higher molar enthalpy of vaporisation.

Q12. Comment on the bond energies of 4 C-H bonds in CH₄.

Answer: Bond energy is the amount of energy required to break 1 mole of bonds present in between the gaseous molecules. Since the bond energies of 1^st, 2^nd, 3^rd and 4^th C-H bonds in CH₄ are not equal. An average value of the bond energies are taken.

Q13. Predict the enthalpy change for the isomerization of acetonitrile into methyl isocyanide.

Answer: The isomerization of acetonitrile into methyl isocyanide can be written into equation as:

H₃C-CN → H₃C-NC

In this, a C-C bond is broken and a new C-N bond is formed.

Bond dissociation energy for C-C = 347 kJ mol^-1

Bond dissociation energy for C-N = 305 kJ mol^-1

ΔH^o = ⅀H_{Bonds broken} – ⅀H_{Bonds formed}

ΔH^o = (347 -305) kJ mol^-1

ΔH^o = 42 kJ mol^-1

The reaction is endothermic.

Q14. Name 3 ions that are isoelectronic to Neon.

Answer: Neon has 10 electrons. The ions isoelectronic to Neon are Na⁺, Mg²⁺, and F^–.

Q15. Out of NaCl and KCl, which will have more exothermic lattice energy?

Answer: The anions in both salts are the same. Both the cations have the same charge but differ in size. Since, Na⁺ is smaller than K⁺, NaCl has more exothermic lattice energy.

Practise Questions on Bond Energy

Q1. Which of the following alkali metals has the least melting point?

1. Na
2. K
3. Cs
4. Rb

Q2. Why does Calcium have an abnormally higher boiling point than magnesium?

Q3. Name the energy associated when 1 mole of an ionic compound is formed from gaseous ions.

Q4. In Spite of the large electronegativity gap between Be and O in the BeO molecule, BeO is covalent. Why?

Q5. What are the decomposition products of Group II nitrates?

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