This set of Organic Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Bond Cleavage”.
1. When a bond is dissociated, what is the change in energy in surrounding?
a) Energy is absorbed
b) Energy is released
c) No change in energy
d) Firstly, energy will be absorbed and then released
Explanation: When a chemical bond is broken energy will be absorbed from surrounding and this is called as bond dissociation energy.
2. Energy released or absorbed in a chemical bond formation or dissociation is measured in which of the following units?
Explanation: Energy is measured in Joules (J). However, we often express the quantity as kilojoules (kJ) – that is, the value in Joules divided by 1000.
3. What will be the change in energy for the following reaction?
Given: To break one mole of H2, energy absorbed is 436 kJ. To break one mole of F2, energy absorbed is 158 kJ. To form one moles of HF, energy released is 568 kJ.
c) 26 KJ
d) -26 KJ
Explanation: So, applying the equation,
∆H = ∑∆H (bonds broken in reactants) − ∑∆H (bonds made in products)
ΔH reaction = (436+158)–(2X568) = −542kJ
The overall enthalpy of the reaction is negative, i.e., it’s an exothermic reaction where energy is released in the form of heat.
4. Compound undergoing homolytic bond cleavage will lead to formation of which chemical species?
c) Free radical
Explanation: During homolytic fission of a neutral molecule with an even number of electrons, two free radicals will be generated.
5. In which form energy is stored in chemical bonds?
a) Kinetic energy
b) Chemical energy
c) Potential energy
d) Thermal energy
Explanation: The first law of thermodynamics essentially states that the total energy in a chemical universe is constant but it can be converted from one form to another. The energy stored in chemical bonds is also a type of potential (stored) energy, not a type of kinetic energy.
6. Which one of the following statements best describes the enthalpy change of a reaction?
a) The energy released when chemical bonds are formed during a chemical reaction
b) The energy consumed when chemical bonds are broken during a chemical reaction
c) The difference between the energy released by bond formation and the energy consumed by bond cleavage during a chemical reaction
d) The increase in disorder of the system as a reaction proceeds
Explanation: The enthalpy change of a reaction is the overall change in enthalpy as a reaction proceeds, taking into account the energy consumed by bond cleavage and the energy liberated by bond formation. The increase in disorder of the system is represented by its entropy, not its enthalpy.
7. Which of the following terms describes a reaction in where more energy is released by bond formation than is consumed by bond cleavage?
Explanation: Exothermic is the flow of energy out of the system to its surroundings. By contrast endothermic’ describes a reaction in which there is a flow of energy into the system from its surrounding.
8. Which energy is required for homolytic cleavage?
Explanation: The triplet excitation energy of a sigma bond is the energy required for homolytic dissociation, but the actual excitation energy may be higher than the bond dissociation energy due to the repulsion between electrons in the triplet state.
9. Which energy is required for heterolytic cleavage?
Explanation: The singlet excitation energy of a sigma bond is the energy required for heterolytic dissociation, but the actual singlet excitation energy may be lower than the bond dissociation energy of heterolysis as a result of the Coulombic attraction between the two ion fragments.
10. How the bond dissociation enthalpy changes as one goes along the series of diatomics Li2, B2, C2, N2, O2 and F2?
c) Increases then decreases
d) Decreases then increases
Explanation: On going Li2, B2, C2, N2, O2, F2 the complexity of the molecular orbitals changes as the p-orbitals can interact with each other end on to give σ-bonding and σ-antibonding molecular orbitals OR they can interact side-ways on to give π-bonding and π-antibonding molecular orbitals that come in degenerate (equal energy) pairs. Electrons continue to be added according to aufbau, Pauli & Hund.
Sanfoundry Global Education & Learning Series – Organic Chemistry.
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