Wednesday, March 11- Matter- Chemical Reactions
Objective: Understand that although matter is not created or destroyed, a chemical reaction can create changes in matter and energy.
We talked about changes to states of matter, which is a change to the physical property of matter. A chemical reaction changes the chemical properties of matter. In essence the atoms of the substance that existed before the reaction are rearranged and form a new substance. A forest fire is an example of a chemical reaction. The trees combine with the oxygen and produce carbon dioxide, water vapor and other compounds and the chemical reaction releases a lot of energy as heat. New substances are formed in the forest.
Atoms to not appear or disappear during a chemical reaction. They recombine. Have you ever heard matter cannot be created or destroyed? This is the Law of Conservation of Mass and it is one of the most important laws of chemistry. It says in a chemical reaction, the combined mass of all the reactants is equal to the combined mass of all the products. Basically, matter may look different, but it has not been created or destroyed. Again think of water. You have 2 Hydrogen atoms. They are a gas happily floating along. They meet up with 1 Oxygen atom, also a gas floating along. BAM! Chemical Reaction. They become a molecule of water – H2O. That molecule of water may be liquid now, but it still has 2 Hydrogen atoms and 1 Oxygen atom. Those atoms look different, but they’re still there.
Chemists write this process as a chemical equation. Yes, just like a math equation. And for the equation to be true, the number of atoms on each side has to be equal.
All chemical reactions require some sort of energy to get started. Light, heat and electricity are common sources of energy to start a chemical reaction.
Exothermic Reactions – In most chemical reactions, energy is released in the form of heat from the original bonds that are broken and the new bonds that are formed. Although all chemical reactions require energy to get started, some give off energy once they are started as a product of the reaction. (Think of burning logs.) This is a exothermic reaction.
Endothermic Reactions – Some chemical reactions require activation energy and a continuous supply of energy for the reaction to continue. In these situation, the reaction will stop if the energy source is removed. (Think baking cookies.) This is an endothermic reaction.
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