Question

The decomposition of carbon dioxide, $$\mathrm{CO}_{2} \longrightarrow \mathrm{CO}+\mathrm{O}$$ has an activation energy of approximately \(460 \mathrm{~kJ} / \mathrm{mol}\). Explain why this large value is consistent with a mechanism that involves the breaking of a \(\mathrm{C}=\mathrm{O}\) bond.

Short answer

The large activation energy of 460 kJ/mol for the decomposition of carbon dioxide is consistent with the breaking of the strong \(\text{C}=\text{O}\) bond.

Step-by-step solution

Understand activation energy

Activation energy is the minimum amount of energy required for a chemical reaction to occur. It is the barrier that must be overcome for reactants to be transformed into products.

Relate activation energy to bond strength

The strength of a chemical bond is directly related to the amount of energy required to break it. A larger activation energy suggests that the bonds being broken during the reaction are very strong.

Apply the concept to the \(\text{C}=\text{O}\) bond

In the decomposition of carbon dioxide, the \(\text{C}=\text{O}\) double bond, which is known to be quite strong, must be broken. The high activation energy of approximately 460 kJ/mol indicates that a significant amount of energy is needed to break this bond, consistent with its known strength.

Key concepts

Chemical Bond Strength

The strength of a chemical bond is a fundamental concept in chemistry that determines how tightly atoms are held together in a molecule. In a substance like carbon dioxide ( CO_2), we find { CO} double bonds that are known for their significant bond strength. This means that a considerable amount of energy is required to break these bonds apart during chemical reactions.

When examining the bond dissociation energy, a high value correlates with a strong chemical bond. Essentially, it's the energy needed to break one mole of bonds in a chemical compound, under standard conditions. For the strong C=O bonds in CO_2, this value is substantial, implying that these bonds do not break easily under typical conditions and require an input of energy to overcome the inherent bond strength.

Exploring further into molecular orbital theory can provide insights into why the double bond is so robust. The overlapping of atomic orbitals results in a sigma ( σ) bond formed from head-on overlapping and a pi ( π) bond from the side-by-side overlap, forging a double bond with a heightened level of stability.

Decomposition of Carbon Dioxide

Carbon dioxide ( CO_2) decomposition is a chemical reaction where CO_2 molecules split into carbon monoxide ( CO) and oxygen ( O). This decomposition is not a spontaneous process at room temperature because it requires overcoming a substantial energy threshold known as the activation energy.

The process of decomposition indicates that the CO_2 molecule must undergo a significant structural reorganization. Because carbon dioxide is linear and the C=O bonds are equivalent, both bonds must be simultaneously or sequentially broken for decomposition to happen, which requires a high energy input.

It's crucial to note that the activation energy is not always indicative of the overall energy change in the reaction. Even though the decomposition of CO_2 has a high activation energy, this doesn't necessarily mean the reaction is endothermic. If we conduct this process in a controlled environment, with sufficient energy to break the bonds, we can observe the creation of new molecules differing in properties from the original CO_2.

Energy Barrier in Chemical Reactions

The concept of an energy barrier in chemical reactions is essential in understanding why certain reactions occur and others do not. The energy barrier we refer to is the activation energy, which is the minimum energy required to initiate a chemical reaction.

Visualize a ball at the bottom of a hill, representing the reactants. To get the ball to the other side, where products are, it must go over the hilltop, which symbolizes the energy barrier. Now, relating to our example of CO_2 decomposition, to break the strong C=O bonds, a considerable amount of energy is needed to push the ball up to the hilltop. The height of the hilltop in chemical reactions is determined by the bond strengths within the reactants, with stronger bonds corresponding to taller hills.

It is crucial to recognize that the activation energy is independent of the ΔH, the enthalpy change of the reaction. Activation energy is solely concerned with the initial energy required to start the reaction and does not predict the reaction's exothermic or endothermic nature.