How does chemical evolution differ from natural selection?
It depends on what you mean by 'chemical evolution'.
'Chemical evolution' can mean two different things when we're talking about life science, either abiogensis or molecular evolution. Abiogenesis refers to life arising out of nonliving matter, and molecular evolution refers to the chemical structures of DNA, RNA, and proteins changes from generation to generation in response to environmental pressures. Natural selection refers to the tendency for individuals who are best fit to survive an environmental pressure and go on to reproduce, passing the genes which give them that particular fitness onto their offspring.
So, if you're talking about abiogenesis, then the difference is pretty big- one refers to the creation of life, and the other refers to biological responses to pressures that occurs across generations.
If you're talking about molecular evolution, then the difference is that whereas chemical evolution refers to the chemical processes that occur in DNA, RNA, and proteins as populations evolve from generation to generation, natural selection describes the reasoning for those chemical processes to occur at a higher rate over so long as a particular pressure persists.