Gas A2 reacts with gas B2 to form gas AB at a constant temperature. The bond energy of AB is muc

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Gas A2 reacts with gas B2 to form gas AB at a constant...

Gas $\mathrm{A}_{2}$ reacts with gas $\mathrm{B}_{2}$ to form gas $\mathrm{AB}$ at a constant temperature. The bond energy of AB is much greater than that of either reactant. What can be said about the sign of $\Delta H ? \Delta S_{\text {surr }}$ ? $\Delta S ?$ Explain how potential energy changes for this process. Explain how random kinetic energy changes during the process.

Key Concepts

Bond Energy and Potential Energy Changes

Bond energy is the energy required to break a bond, and stronger bonds correspond to lower potential energy states. When a stronger bond forms in the product, the system’s potential energy decreases, as the energy stored in the new bond is lower than that in the reactants’ bonds. This lowering of potential energy is a key driver for the exothermic nature of the reaction.

Random Kinetic Energy Changes

Random kinetic energy is associated with the motion of particles and is directly related to temperature. In an exothermic reaction, the decrease in potential energy is accompanied by an increase in random kinetic energy, as the energy released is transferred to the surrounding particles. Even if the temperature is maintained constant by an external reservoir, the immediate effect is that excess energy enhances molecular motion before equilibrium is re-established.

Exothermic Reaction and Enthalpy Change

This concept refers to the overall energy change (?H) during a chemical reaction. In the scenario where the product's bond energy is much greater than that of the reactants, forming the product releases more energy than is consumed in breaking the reactants’ bonds. This net release of energy makes the reaction exothermic, meaning ?H is negative.

Entropy Change of the System and Surroundings

Entropy (?S) measures the degree of disorder or randomness. In reactions where two gas molecules combine to form one, the system experiences a decrease in the number of particles and a reduction in freedom of movement, leading to a negative ?S for the system. However, because the reaction is exothermic, heat is released into the environment, increasing the randomness there and resulting in a positive ?S for the surroundings.

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