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Cl2 is more stable than 2Cl, since Cl appears as Cl2 in nature. In order to go from a more stable state to a less stable state, you have to put energy into the reaction, like how with radicals you have to put in energy to form them since they are unstable. Because the Cl2 needs to absorb energy from the environment to become 2Cl, this means that the energy of 2Cl is greater than that of Cl2, so delta H is positive. This also makes sense with being spontaneous at high temperatures because since the reaction is endothermic, increasing the temperature shifts the reaction towards the products.
Spontaneous process is reflected by negative Gibbs free energy. Recall in the lecture that delta G= delta H -T delta S. Producing 2Cl is an endothermic process but by doing that the system increases entropy (increase # of particles). At normal temperature, the process favors Cl2 because enthalpy dominates (called enthalpy driven). However, at high temperature, entropy plays the dominant role (entropy driven)
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