Question

Hydrogenation reactions (e.g., the process of converting \(\mathrm{C}=\mathrm{C}\) bonds to \(\mathrm{C}-\mathrm{C}\) bonds in the food industry) are facilitated by the use of a transition metal catalyst, such as \(\mathrm{Ni}\) or \(\mathrm{Pt}\). The initial step is the adsorption, or binding, of hydrogen gas onto the metal surface. Predict the signs of \(\Delta H, \Delta S,\) and \(\Delta G\) when hydrogen gas is adsorbed onto the surface of Ni metal.

Short answer

\( \Delta H \) is negative, \( \Delta S \) is negative, and \( \Delta G \) is negative.

Step-by-step solution

Understanding the Process

The process described is adsorption, where hydrogen gas molecules are attracted to and held on the surface of a nickel catalyst. This involves the interaction of gas molecules with a solid surface.

Determining the Sign of \( \Delta H \)

Adsorption usually releases heat because hydrogen molecules form bonds with the metal surface, indicating an exothermic process. Thus, \( \Delta H \) is negative.

Determining the Sign of \( \Delta S \)

The entropy \( \Delta S \) decreases during adsorption since gas molecules go from a more disordered, high-entropy state to a more ordered, low-entropy state as they are bound to the metal surface. Therefore, \( \Delta S \) is negative.

Determining the Sign of \( \Delta G \)

For adsorption to occur spontaneously, which it does in this scenario, the Gibbs free energy change \( \Delta G \) must be negative. This is consistent with a process that is enthalpically favored (negative \( \Delta H \)) and entropically disfavored (negative \( \Delta S \)).

Key concepts

Transition Metal Catalyst

In the process of hydrogenation, transition metal catalysts like nickel (Ni) and platinum (Pt) play a vital role. These metals have the unique ability to temporarily hold molecules on their surfaces. This makes the reaction between hydrogen gas and other compounds more efficient. Transition metals are great catalysts because they have partially filled d-orbitals. This allows them to form weak bonds with atoms or molecules, facilitating easier interactions without permanent changes to the surface. Additionally, they can donate or accept electrons during the reaction, which further aids the process.
In hydrogenation, these catalysts help in converting double bonds (\( \mathrm{C} = \mathrm{C} \)) into single bonds (\( \mathrm{C} - \mathrm{C} \)). This is widely used in the food industry to convert unsaturated fats to saturated fats.

Adsorption

Adsorption is the process by which atoms, ions, or molecules from a gas, liquid, or dissolved solid adhere to a surface. In hydrogenation, hydrogen gas gets adsorbed onto the surface of a transition metal catalyst. This step is crucial as it increases the concentration of reactants at the surface, making the reaction more favorable. During adsorption, hydrogen molecules interact with the metal surface and may break into hydrogen atoms, which are ready to participate in further reactions.

• In this specific hydrogenation process, adsorption helps overcome the activation energy barrier.
• It results in the physical binding of hydrogen, not chemical bonding, keeping the catalyst unchanged after the reaction.

Understanding this basic but essential step is crucial for appreciating how catalysts facilitate chemical reactions.

Exothermic Process

An exothermic process is one that releases heat into the surrounding environment. In the context of hydrogen gas adsorbing onto a nickel surface, bonds are formed between the hydrogen and the nickel atoms. This bond formation releases energy, which is why the process is considered exothermic. The heat released during adsorption makes the enthalpy change (\( \Delta H \)) negative.
Exothermic reactions are spontaneous at room temperature, often increasing the rate of the reaction. The practical importance of exothermic processes is evident in industrial applications, where controlling heat release can improve efficiency and safety.

Entropy Change

Entropy is a measure of disorder or randomness in a system. When hydrogen gas molecules, which are highly disordered and have high entropy, adsorb onto a metal surface, they become more ordered. This decrease in disorder results in a negative entropy change (\( \Delta S \)). In simple terms, the system moves from a state of higher entropy (gas phase) to a lower entropy (adsorbed phase).

• Entropy decreases because molecules are more 'organized' or aligned on the solid surface.
• This decrease in entropy poses an energetic disadvantage, contrary to the negative enthalpy.

Even though the process is entropically unfavorable, the overall negative change in Gibbs free energy (\( \Delta G \)) ensures that adsorption is a spontaneous process.