The first reaction is endothermic. It consumes or absorbs energy to form products. So, NO has more energy than its reactants. Hence, NO is unstable.
In the second reaction, it reacts with oxygen to form NO2, and gives off energy. It’s an exothermic reaction. So, NO2 has less energy than its reactants, and is stable. It is stabilised with minimum energy.
Hence unstable NO gets converted into stable NO2.
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