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

Chemical Reactions

Chemistry .2Mrs. 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 ice2. A Bumper rusting3. Melting glass4. Distilling coffee5. 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 - H2O before and H2O after

2. A bumper rusting - iron before and iron (III) oxide after

3. Melting glass - solid glass to liquid glass4. Distilling coffee - coffee solution to solid

coffee without water5. 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 (H2)

• 2 molecules of methane (CH4)

• 6 molecules of molecular oxygen (O2)

Balancing Equations - Activity

1. React one H2 with one O2 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

• _____ H2 + _____ O2 --> _____ H2O

• __2__ H2 + _____ O2 --> __2__ H2O

Balancing Equations - Activity

2. Now react methane (CH4) with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O)

• 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

• ___ CH4 + ___ O2 --> ___ CO2 + ___ H2O

• ___ CH4 + _2_ O2 --> ___ CO2 + _2_ H2O

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) + H2O (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. • BaCl2 (aq) + K2CO3 (aq) BaCO3 (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• CH4 (g) + 2 O2 (g) CO2 (g) + 2H2O (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