15. which of the following is not an accurate description of chemical reactions? a. the rates of chemical reactions can vary. b. chemical reactions that take in energy never need a source of energy to start. c. during chemical reactions, energy can be released or used up. d. during chemical reactions, bonds can be broken or formed.

The correct answer is "B".
  All chemical reactions require "activation energy". Activation energy can be thought of as kickstarting a chemical reaction. If you consider lighting a match, the match does not spontaneously combust upon its removal from the matchbox. You must kickstart the reaction by heating the chemicals in the match head. This heat is generated by the friction of rubbing the match head against the side of the box. In any case, reactions that take in energy, must take that energy from somewhere, so they absolutely need a source of energy to start, and to continue!

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