Transcript Document

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)
Next slides are extra
practice
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 (table is
also in textbook)!!!
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
Ca
Na
Mg
Activity Series: HANDOUT
Increasing Activity
Any element will replace
any element below it.
Al
Zn
Fe
Pb
(H)*
Cu
Hg
Ag
*Metals from Li to Na will replace H
from acids and water; from Mg to Pb
they will replace H from acids only
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
HONORS
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+ + IAn aqueous solution
is ions dissolved in
water
Ionic Equations
• Consider the reaction…
• Pb(NO3)2(aq) + 2 NaI(aq)  PbI2(s) +2 NaNO3(aq)
• What is really going on is…
• Pb2+ +2 NO3- + 2 Na+ + 2 I-  PbI2(s) +2 Na+ + 2 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.
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