Chemical Reactions Chemistry.2 Mrs. Daniels rev. December 2006

Journal #14 : Chemical vs. Physical Change Review Identify each of the following as chemical or physical changes. Determine whether the result is a “new product” or not 1.Melting ice 2.A Bumper rusting 3.Melting glass 4.Distilling coffee 5.Boiling water off a hydrated salt

Chemical Changes are called REACTIONS A NEW product is always formed Let’s look at which of the following have new products: 1.Melting ice - H 2 O before and H 2 O after 2.A bumper rusting - iron before and iron (III) oxide after 3.Melting glass - solid glass to liquid glass 4.Distilling coffee - coffee solution to solid coffee without water 5.Boiling water off a hydrated salt - hydrate salt to anhydrous salt

So, only one of the five examples was a chemical reaction Every reaction can be represented by an equation The equation involves writing out what is reacting and the resulting product(s)

The items reacting go on the left These are called the reactants The products go on the right of the equation The arrow in between means “produces or results in” Reactant + Reactant --> Product

Law of Conservation of Mass/Matter Does a chemical reaction actually “create” new matter? NO, a chemical reaction is simply a rearrangement of atoms The number of each type of atom on the left must equal the number of each type of atom on the right In other words, they must BALANCE!

Balancing Equations - Activity Each paper clip represents a single atom On your half sheet of paper, designate one of your colors to be oxygen, one to be hydrogen, and one to be carbon. Make: 2 molecules of molecular hydrogen (H 2 ) 2 molecules of methane (CH 4 ) 6 molecules of molecular oxygen (O 2 )

Balancing Equations - Activity 1.React one H 2 with one O 2 by splitting the molecules and joining one oxygen with two hydrogen atoms What do you have? What do you have left over? How many hydrogen molecules would you need to react with the oxygen that is left? 1 molecule (with two atoms of hydrogen)

Balancing Equations - Activity Write out an equation for this first reaction _____ H 2 + _____ O 2 --> _____ H 2 O __2__ H 2 + _____ O 2 --> __2__ H 2 O

Balancing Equations - Activity 2. Now react methane (CH 4 ) with oxygen (O 2 ) to produce carbon dioxide (CO 2 ) and water (H 2 O) Start by writing out the equation without coefficients…then, determine how many of each you’ll need Start with one molecule of methane and one molecule of oxygen and continue until all of the reactants have been used to make products

Balancing Equations - Activity Summarize what happened in this reaction by completing your equation ___ CH 4 + ___ O 2 --> ___ CO 2 + ___ H 2 O ___ CH 4 + _2_ O 2 --> ___ CO 2 + _2_ H 2 O

Types of Chemical Reactions We already know how to write a chemical equation There are several different ways that elements can combine during a chemical reaction We’ll look at 5 different types of reactions

Types of Chemical Reactions Synthesis Reactions Decomposition Reactions Single-Displacement Reactions Double-Displacement Reactions Combustion Reactions

Synthesis Reactions Also called a combination reaction 2 or more substances combine to form 1 single new substance Reactant A + reactant B  product Ex. CaO (s) + H 2 O (l)  Ca(OH) 2 (aq)

Decomposition Reactions The opposite of combining 2 things is breaking 1 apart into its component parts In a decomposition reaction, a single compound is broken down into two or more products Very rapid decomposition reactions can cause explosions Most decomposition reactions require energy in the form of heat, light, or electricity

Single-Displacement Reactions One element replaces a second element in a compound What determines who will push who out? There is a list of metals called the activity series. The higher up on the list, the more reactive (meaning the more power it has to take the place of a lower ranking metal)

Double-Displacement Reactions Involve an exchange of cations between two reacting compounds Like a double date, but the two cations decide to trade dates Ex. BaCl 2 (aq) + K 2 CO 3 (aq)  BaCO 3 (s) + 2KCl (aq)

Combustion Reactions An element or compound reacts with oxygen to produce heat and light Often referred to as “burning” Lighting a Bunsen burner is an example of a combustion reaction CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2H 2 O (g)

Chemical Reactions & Energy Chemical reactions involve energy exchange Energy must either go into or be released from a chemical reaction This energy could be in the form of heat

Chemical Reactions & Energy When heat energy is absorbed into a chemical reaction, the reaction is called endothermic Endo- means into Thermic- means heat So, endothermic means that heat is put into the reaction This type of reaction would feel cool to the touch for us

Chemical Reactions & Energy When a chemical reaction releases or gives off heat, it is called an exothermic reaction or process Exo- means coming off or being released Thermic- means heat Exothermic- means releasing or giving off heat These reactions feel quite hot to the touch

Catalysts Some reactions happen so slowly that they are useless Catalysts are chemicals that appear on both the reactant and the product side of the equation They simply “observe” the reaction and help to speed it up They are NOT used up or combined in the reaction

Inhibitors In order to prevent a non-favorable reaction from occurring, an inhibitor can be used to stop the reaction Think of it as a hostage taker who keeps the reactant(s) from joining with something else It keeps the reactants from reacting It is able to be removed later and is not “used” up

Other factors Some other factors may slow down or speed up the reaction In many cases, heat will speed up a reaction Mixing could also be responsible for speeding up a reaction