How can energy be changed in a chemical reaction? Energy Changes in Chemical Reactions:All chemical substances contain energy in the form of chemical energy. During chemical reactions, the chemical energy in the reactants can be changed into other forms, more often into heat energy. The unit used in measuring energy is Joule (J). 1 kJ (kilojoules) = 1000 … [Read more...] about How can energy be changed in a chemical reaction?