If an exothermic chemical reaction occurs, the surroundings become warmer. where does this energy come from?

It comes to from the chemical reaction itself. Exothermic reaction release heat because the energy level of the products is lower than that of the reactants.  Therefore, the net enthalpy change of the reaction is negative (-), meaning heat or light (or any other form of energy) is released into the environment. An example of an exothermic reaction is the combustion of fuel.  

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