PowerPoint Presentation by V42pElM

VIEWS: 3 PAGES: 45

									Effects of chemical reactions:

• Chemical reactions rearrange atoms in the
  reactants to form new products.

• The identities and properties of the
  products are completely different from
  that of the reactants.

• Production of gases and color changes are
  signs of chemical reactions.
  Energy and Reactions
Energy must be ADDED to BREAK bonds.

Energy is RELEASED when bonds are
 FORMED.

Chemical energy is CONSERVED in chemical
 reactions.
       Exo- vs. Endo-
EXOTHERMIC REACTIONS:
 release energy (More energy is released as the
 products form bonds than is absorbed to break
 the bonds in the reactants.)


ENDOTHERMIC REACTIONS:
 absorb energy
Chemical equations are used to represent or
describe chemical reactions.
For example when hydrogen H2 burns, it
reacts with oxygen, O2, in the air to form
water. We write the chemical equation for
this reaction as follows:
         2H2 + O2 —> 2H2O
   Chemical Equations
An equation shows…


 Formulas of reactants
 Formulas of products
 Molar ratios of all compounds in the reaction.
Chemical Equations
The (+) sign as “reacts with”


The arrow () means “yields” or “reacts to
produce”.

            2H2 + O2  2H2O


             Reactants          Products
To show physical states of each
substance:
• (s)  solid
• (l)  liquid
• ( g) gas
• (aq) aqueous
  • aqueous means dissolved in water
To show physical states of each
substance:
• Consider the reaction of iron with oxygen
  to form iron (III) oxide, or rust.


• Fe(s) + O2(g)  Fe2O3(s)   (unbalanced)
  Coefficients & Subscripts
COEFFICIENTS: numbers in front of
compound that represents the number of
molecules/moles of that compound
SUBSCRIPTS: small numbers that help
define the compound.

 Coefficient

               2H2SO4        Subscript
H2O   One molecule of water




2H2O Two molecules of water




H2O2 One molecule of Hydrogen Peroxide
•During a chem. rxn.; atoms are rearranged
(NOT created or destroyed!)
•Chemical equations must be balanced to
show the relative amounts of all
substances.
•Balanced means: each side of the equations
has the same # of atoms of each element.
CH4 + O2 —> H2O + CO2         Unbalanced
CH4 + 2O2 —> 2H2O + CO2       Balanced
RULES to follow in balancing:
1. Correct formulas for all reactants and
   products
2. Reactants  Products
3. Count the number of atoms of each
   element in reactants & products.
4. Balance one at a time using coefficients.
5. Check for balance
6. Are the coefficients in the lowest
   possible ratio?
Always balance C, H and O FIRST if they are
in an equation. C, then H, then O, then
everything else. I call it the Santa Claus Rule
   Balancing Equations

NOTE: When balancing equations, you
may change coefficients as much as
you need to, but you may never
change subscripts because you can’t
change what substances are involved.
Balancing equations involves a great deal of
“trial and error” at first,
     but there are some tricks…
Examples
  CuCl2(aq) + Al(s)  Cu(s) +AlCl3(aq)

3CuCl2(aq) + 2Al(s)  3Cu(s) +2AlCl3(aq)

               (3:2:3:2)
Examples
Propane, C3H8, burns in oxygen, O2, to form
carbon dioxide and water.

            C3H8 + O2 CO2 + H2O

          Balance C – then H – then O

          C3H8 + 5O2 3CO2 + 4H2O

                   (1:5:3:4)
Examples
Pentane, C5H12, burns in oxygen, O2, to form
carbon dioxide and water.

            C5H12 + O2 CO2 + H2O

          Balance C – then H – then O

          C5H12 + 8O2 5CO2 + 6H2O

                    (1:8:5:6)
Examples
Silver nitrate reacts with copper to produce
silver and copper (II) nitrate.

         AgNO3 + Cu Ag + Cu(NO3)2

        2AgNO3 + Cu 2Ag + Cu(NO3)2

                    (2:1:2:1)
Examples
Phosphorus reacts with oxygen gas to produce
diphosphorus pentoxide.

                P + O2  P2O5

              4P + 5O2  2P2O5

                   (4:5:2)
Examples
           C7H14 + O2 CO2 + H2O

      Balance C – then H – then O

      C7H14 + 10½O2 7CO2 + 7H2O

     2C7H14 + 21O2 14CO2 + 14H2O


                (2:21:14:14)
      Types of Chemical Reactions

•   Synthesis / Combination
•   Decomposition
•   Single Replacement
•   Double Replacement
•   Combustion


           PASS OUT HANDOUT!!!
Synthesis / Combination Reactions
Definition: Reaction where 2 or more
substances react to form 1 product.
                 A + B  AB
Examples:
2K(s) + Cl2(g)  2KCl(s)
SO2(g) + H2O(l)  H2SO3(aq)
Decomposition Reactions
  Definition: 1 reactant decomposes to form 2
  or more products.
                 AB  A + B
  Examples:
  2H2O(l)  2H2(g) + O2(g)
  CaCO3  CaO + CO2
Single-Replacement Reactions
 Definition: Reaction where atoms of one element
 replace atoms of a second element in a compound.
                           XA + B  BA + X
 Note: A reactive metal will replace any metal listed below it in the activity
 series. Generally, nonmetal replacement is limited to the halogens. The
 activity of the halogens decreases as you go down Group 7A of the periodic
 table. See handout.

 Examples:
 2AgNO3 + Mg  Mg(NO3)2+2Ag
 Mg+LiNO3  no reaction
Li
K
       Activity Series: HANDOUT
Ca         Increasing Activity
         Any element will replace
Na
          any element below it.
Mg
Al
Zn     *Metals from Li to Na will replace H
Fe     from acids and water; from Mg to Pb
       they will replace H from acids only
Pb
(H)*
Cu
Hg
Ag
For Example…

    Ca + MgO  CaO + Mg

      The Ca will replace the Mg
 because Ca is more active than Mg.
 That is to say…Ca is above Mg on the
 activity list.
Double-Replacement Reactions
 Definition: 2 positive ions “switch places” between
 two compounds to form 2 new ionic compounds.
                      XA + BY  BA + XY
 Note: These reactions generally take place between two ionic compounds in
 aqueous solution, and are often characterized by one of the products
 coming out of solution in some way.

 Examples:
 2NaCN(aq)+H2SO4(aq)  2HCN(g)+Na2SO4(aq)
 Na2S(aq)+Cd(NO3)2(aq)  CdS(s)+2NaNO3(aq)
Combustion Reactions
 Definition: a hydrocarbon (containing C
 and H) or other substance burns in the
 presence of oxygen gas (O2) to produce
 CO2 and H2O
 Examples:
 CH4+2O2  CO2+2H2O + heat + light
 2Mg(s)+O2(g)  2MgO(s)
Combustion of Hydrocarbons

      If the reactant is a hydrocarbon, the
 products are always carbon dioxide and
 water.
      CH4 + 2O2  CO2 + 2H2O
DEMOS

Cu (s) + AgNO3 (aq)  ????
          KI
H2O2 (l)  ????




*** see next slide
Copper Silver Nitrate

Determine the products and type of reaction
 for:
Cu (s) + AgNO3 (aq) 



We will let this sit for a minute to let the
 reaction take place and do…
Elephant’s Toothpaste

Determine the products and type of reaction
 for:
        KI
H2O2 (l) 



 See next slide
Which type of gas was produced? Hydrogen gas
 (H2) or Oxygen gas (O2)?



Let’s test…
  Glowing wooden splint will test for the type of gas.
     H2 gas is HIGHLY flammable
     O2 gas is not flammable, but will support the burning of
      other substances
Therefore…
            KI
2 H2O2 (l)  2 H2O (g) + O2 (g)



What type of reaction is this?
DECOMPOSITION
Go back to copper…

Let’s see what happened…

Cu (s) + AgNO3 (aq)  produced what?
Cu (s) + 2 AgNO3 (aq)  2 Ag (s) + Cu(NO3)2 (aq)



What type of reaction is this?
SINGLE REPLACEMENT
ANOTHER TYPE of RXN:
please write this UNDER
“combustion” in notetakers

HELPFUL IN DOUBLE REPLACEMENT
 REACTIONS
Ionic Equations
• When a soluble substance is dissolved
  in water, the substance often breaks
  into ions. This solution is said to be an
  aqueous solution.

• Pb(NO3)2(aq)  Pb2+ + 2NO3-
• NaI(aq)  Na+ + I-    An aqueous solution
                        is ions dissolved in
                        water
    Ionic Equations
• Consider the reaction…
  • Pb(NO3)2(aq) + NaI(aq)  PbI2(s) + NaNO3(aq)


• What is really going on is…
  • Pb2+ + NO3- + Na+ + I-  PbI2(s) + Na+ + NO3-


• Note that the Na+ ion and the NO3- ion are
  not reacting. They are said to be spectator
  ions.
Net Ionic Equations
• It is often useful to write an equation
  showing only the species that are
  actually reacting. This is called a net
  ionic equation. It does not show the
  spectator ions.
 Pb2+ + NO3- + Na+ + 2I-  PbI2(s) + Na+ + NO3-
    becomes….
          Pb2+ + 2I-  PbI2(s)
WORKSHEET (types of rxns)

DO THE SIDE THAT SAYS: types of rxns

ONLY determine the type of reaction for #1-15
 (do not have to balance)

#16,17: do the whole question:
  Products
  Balance
  Type of reaction

								
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