Atomic Structure Mrs. Daniels Chemistry.2 September 2002 Revised August 2006.
Chemical Reactions Chemistry.2 Mrs. Daniels rev. December 2006.
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Transcript of 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 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