By: Jenny Kocan

 A chemical reaction is a change in which two or more substances combine to form a new substance or substances.  The two or more products you begin with are called reactants.  The new substance(s) formed are called products.  For example, when the acetic acid in vinegar and baking soda (reactants) are combined, the products are bubbles of carbon dioxide gas, water, and sodium acetate. B1%5D.gif

 The chemical properties of the reactants determine what happens during the reaction—and how quickly it happens.  Generally for a chemical reaction to occur, one or more bonds between atoms and molecules are either broken, or a new bond is formed.  These bonds are broken and formed by the interaction of electrons with each other, and the nuclei of the atoms involved in the bond.  Atoms of all elements remain essentially intact during all chemical reactions. It’s only the outermost electrons that are affected during chemical reactions. When atoms approach each other, the outer electrons determine whether the atoms will repel each other, or bond.

 Some common chemical reaction signs include:  Burning, rusting, change of color, change of temperature, formation of gas (ex. Bubbles), formation of precipitate (a solid formed in a chemical reaction that is different from either of the reactants)

 The law of conservation of mass and reactions states that matter is neither lost nor gained in traditional chemical reactions; it simply changes form.  This relates to chemical reactions because even though a new substance is formed, it has only changed form. Even though it may look completely different, it’s mass and number of atoms is still the same.  Therefore, if we have a certain number of atoms of an element on the left side of an equation, we have to have the same number on the right side.  This implies that mass is also conserved during a chemical reaction.  Example: The Water Reaction

 In chemicals, the potential energy is contained within the chemical bonds.  When a reaction is taking place, there is an exchange of energy.  Whenever chemical bonds are broken, energy is released. When new bonds are formed, energy is stored. Sometimes energy is released during a reaction. This energy may be given off as heat, or it may be used to do work. Sometimes, the products of the reaction store more energy in their chemical bonds, than the reactants had stored. This stored energy can be held for later use.

 Activation Energy: The minimum amount of energy needed for a chemical reaction to take place. For some reactions this is very small (it only takes a spark to make gasoline burn). For others, it's very high (when you burn magnesium, you need to hold it over a Bunsen burner for a minute or so).  Chemical Potential Energy: The energy in chemical bonds holding the atoms of a compound together. Food, wood, batteries, and fossil fuels contain chemical potential energy.

 An exothermic reaction is a chemical reaction that releases heat.  Examples: The setting of cement and concrete, adding concentrated acid to water, and adding water to anhydrous copper(II) sulfate.  An endothermic reaction is a chemical reaction that absorbs heat from its environment. It gets colder, and takes the heat to make the chemical bonds.  Examples: dissolving ammonium chloride in water, mixing water with potassium chloride, and mixing water and ammonium nitrate.

 In a synthesis reaction, two or more substances (reactants) combine to form a new product or compound.  An example of a synthesis reaction is hydrogen gas combined with oxygen. This results in a more complex substance, water. The equation for this is 2H 2 + O 2  2H 2 O.  An example of an equation for a synthesis reaction is A+B  AB.

 In a decomposition reaction a chemical compound separates into elements or smaller compounds. One reactant yields 2 or more products.  Basically, synthesis and decomposition reactions are opposites.  For example, water can be broken down into hydrogen gas and oxygen gas. The chemical equation for this decomposition reaction is:

 A combustion reaction is a chemical reaction that involves oxygen and produces energy (heat) so rapidly that a flame results.  This reaction occurs between certain compounds, especially organic compounds, oxygen, and hydrocarbons, which are carbon and hydrogen attached.  The products of this reaction include carbon dioxide and water.

Single ReplacementDouble Replacement  In a single replacement reaction, a single uncombined element replaces another in a compound. Two reactants yield two products.  Ex. when zinc combines with hydrochloric acid, the zinc replaces hydrogen.  In a double replacement reaction parts of two compounds switch places to form two new compounds. Two reactants yield two products.  Ex. when silver nitrate combines with sodium chloride, two new compounds--silver chloride and sodium nitrate are formed because the sodium and silver switched places.

 Synthesis A+B AB  Decomposition AB A+B  Combustion O 2 will always be a reactant (O 2 must be on the left side)  Single Replacement Ax + B AB + x (Switch partners)  Double Replacement Ax + B y AY + BX Courtesy of Mr. W

 Discovery Eduation: Curriculum Center- school.discoveryeducation.com/curriculumcenter/chemistry/glossary.htmlhttp://dev- school.discoveryeducation.com/curriculumcenter/chemistry/glossary.html  Chemical Properties | Introduction: Science Experiments- projects/chemical-properties/ projects/chemical-properties/  Science Knowledge: Science 3 Materials and Their Properties-  Chemical Reactions: Signs of a Reaction- ps/chm/100/dgodambe/thedisk/chemrxn/signs.htmhttp:// ps/chm/100/dgodambe/thedisk/chemrxn/signs.htm  Chemical Equations-  Energy in Chemical Reactions-  Chemical Reactions-  K-12 Energy Education Program: Definitions-  Exothermic Reaction- Wikipedia-  Types of Chemical Reactions-  Combustion Reaction-  Endothermic Reaction Examples-  Wiki Answers: What happens when a chemical reaction occurs?-  Chemical Decomposition-  Types of Reactions-