Intro to Chemical Reactions

 What is the difference between physical change and chemical change?  How can you be relatively sure that a chemical change has occurred?  Explain Dalton’s Atomic Theory.  Explain the Law of Conservation of Mass.  What are reactants? What are products? Framing Questions

Chemical Changes vs. Physical Changes Physical change = new properties may occur, but no new substance is formed; may be easily reversed. –Ex: Ice melting Chemical change = always causes at least one new substance with new properties; very difficult or impossible to reverse. –Ex: Precipitate demo

Indicators of Chemical Change If you make two or more of the following observations, a chemical change has probably occurred: –Heat is produced or absorbed –The starting material is used up –A new colour appears –A starting colour disappears –A material with new properties forms –Gas bubbles form in a liquid –Grains of solid precipitate form in a liquid

 Every time we have a chemical reaction, an irreversible change occurs. Does that mean that matter is lost?  ABSOLUTELY NOT!  Matter cannot be created or destroyed, so it has to still exist in some respect.  Dalton’s Atomic Theory states that atoms (and therefore, matter) cannot be created or destroyed.  This leads us to another very important law in the world of chemistry….. Chemical Reactions

Law of Conservation of Mass (Lavoisier’s Law) The Law of Conservation of Mass states that the total mass of reactants is always equal to the total mass of the products.The Law of Conservation of Mass states that the total mass of reactants is always equal to the total mass of the products. Reactants = the substances that you start with before the chemical reaction occurs. Products = the substances you end up with after the chemical reaction occurs. Does this mean that things stay the same? –No, it just means that the same atoms that enter must also exit in some respect. –They can recombine into different compounds, but they all still exist.

SHOW DEMO: COMPARING MASS OF REACTANTS AND MASS OF PRODUCTS

Questions When the solutions were mixed, did a chemical change occur? How did the mass of the reactants compare with the mass of the products after the reaction? If there was a change in mass, offer an explanation to account for it. Do the results of this demonstration support the Law of Conservation of Mass? Explain.