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I rather grimaced at the start of this piece "Converting plastic waste into energy…" as I used to teach physics as well as chemistry. Plastic cannot be converted into energy - I get what was meant, but I would advise teachers to avoid using this form of expression in class.

In chemical changes, mass and energy are each conserved. Matter cannot be converted to energy.

Einstein rather confused matters due to the popularity of E=mc[↑]2 as cultural meme, but this refers to an equivalence between mass and energy, not an interconversion - so long as we carefully measure everything before and after a change the total energy (and mass) are unchanged. [https://science-education-research.com/science-concepts/conceptions-of-energy/mass-defect/]

In a chemical reaction there is a tiny, tiny difference in mass between reactants and products (much, much too small to measure in the school lab!) but this is balanced by a change in mass of the environment. So, an exothermic reaction will heat the surroundings, and they will also get more massive by the equivalent (tiny, tiny) amount!

It is easy to do the calculation from the total enthalpy change of the reaction and Einstein's equation, and see that the chemical 'mass defect' is miniscule.