Transcript A chemical reaction is

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BONDS…Chemical Bonds…
…and Reactions
Mixin’ It Up!
A mixture is matter made up
of two or more substances
that combine physically only.
In Heterogenous Mixtures
substances are not distributed
evenly throughout the
mixture.
In Homogenous Mixtures
substances are distributed
evenly throughout the
mixture.
Solvent is the
substance that
does the
dissolving.
Solute is the
substance that
gets dissolved.
Solutions are homogenous
mixtures in which one
substance dissolves
completely into the other.
Separation Anxiety!
How do you separate mixtures?
Sifting = separation by particle size.
Separation Anxiety!
How do you separate mixtures?
Filtration = separating a solid from a liquid in a heterogenous
mixture.
Separation Anxiety!
How do you separate mixtures?
Evaporation = separating a solid from a liquid in a solution.
Identifying Chemical Reactions
What is a chemical reaction?
A chemical reaction is the
change of a substance into a new
one that has a different chemical
identity.
How can I tell if a chemical
reaction is occurring?
• emission of light
• a change in temperature
• formation of a precipitate
(a solid substance that settles
to the bottom)
• evolution of a gas
• a color change
http://www.youtube.com/watch?v
=gs0j1EZJ1Uc
Glowsticks
Antoine Lavoisier: Law of Conservation of Mass
“Matter is not gained or lost in chemical reactions.”
Discovered the role
oxygen plays in the
combustion of matter.
Annoying girlfriend
Lavoisier
O2
CO2
Chemical formulas tell you what
elements are in a compound…
H
Hydrogen
2
1
Dihydrogen
Hydrogen Oxide
Oxide
O
Oxygen
…and the number of
atoms of each element.
Sometimes element names are
mixed into one name.
CH4 = methane
OH = hydroxide
Sometimes compounds have a
special non-elemental name.
Sometimes compounds have a
chemical name and a familiar
name.
H20 = Dihydrogen oxide = water
When an element has a visible
number following it to indicate
more than one atom, the
number is written in lower case
and placed lower than the text
line. This number is called a
subscript.
CH4
subscript
Noble Gases
All of the elements
in column 8 have
eight valence
electrons.
This makes these
elements very
stable (not likely to
gain, lose, or share
electrons.
They are called the
Noble Gases.
Atoms are unstable if they have valence electrons that are unpaired.
Remember, valence electrons are those that are in the outermost shell.
To reach stability, atoms will bond with each other
so all of the valence electrons are paired.
Not paired
Not paired
Types of Chemical Bonds
Remember, a Chemical Bond is the force that occurs when two or
more atoms hold together.
When one atom gives up an e- to another atom, ions are formed.
The positive and negative charges caused by moving e-’s from one
atom to another causes an attraction.
The attraction bonds the atoms together in an IONIC BOND.
While it is easier to visualize chemical bonds using Bohr diagrams, it
is not always easier to draw them.
We can also demonstrate chemical bonds using Lewis structures.
+
Na
Describe what you just saw.
Cl
Sometimes atoms share electrons instead of gaining and losing them.
This type of bond is called a COVALENT BOND.
H O
H
Covalent sharing is not always equal. In water, the e-’s spend most
of their time around the oxygen.
A covalent bond in which the e-’s spend more time with one atom
than the others is considered to be POLAR.
Water is an example of a polar molecule. This unequal sharing
creates a slight negative charge to one side of the molecule.
The polarity of water makes it a very good solvent (can dissolve many
other substances) because it can bond with lots of other atoms.
Why does salt dissolve in water so easily?
Multiple Bonds
Triple bond
N
N
When e-’s shift so that more than one pair is shared between two
atoms, multiple bonds are formed.
Remember, bonds are typically indicated with a line.
Sometimes those lines are connected to form shapes.
Chemical Bonding Animated Tutorials
http://www.kentchemistry.com/links/bonding/bondingflashes/bond_types.swf
Let’s Practice Making Bonds
http://www.learner.org/interactives/periodic/groups_interactive.html
http://www.drkstreet.com/r
esources/metallic-bondinganimation.swf
Assignments:
• Study Guide: Chemical Bonds
I’m diggin’ for
questions!
Got any?
CHEMICAL REACTIONS
Writing Formulas and Naming Compounds
The charge on an element is called its oxidation number.
+
Na
Cl
A binary compound is made of two elements. (Bi=2)
NaCl
Writing Formulas and Naming Compounds
Metals always lose electrons. Nonmetals always gain electrons.
METALOIDS
NONMETALS
METALS
Gain e-’s become more negative
Lose e-’s become more positive
Oxidation numbers are the number of e-’s an element can
gain (and become -) or lose (and become +).
+1
+2
Oxidation numbers
can be added to the
periodic table….
METALS
NONMETALS
METALOIDS
+3
+4
-3
-2
-1
Writing Formulas and Naming Compounds
Special elements with more than one possible oxidation number.
NAME
OXIDATION NUMBER
Copper (I)
Copper (II)
Iron (II)
Iron (III)
Chromium (II)
Chromium (III)
Lead (II)
Lead (IV)
1+
2+
2+
3+
2+
3+
2+
4+
Writing Formulas and Naming Compounds
Ions alone have oxidation numbers…
+
Na
Cl
But when they combine, their compounds are neutral.
NaCl
Assignment:
• Study Guide: Chemical Bonds
• L3, L4 and L5
Writing Formulas and Naming Compounds
The oxidation numbers of the combining positive and
negative ions must balance with each other.
+
Na
Cl
So that when they combine, their compounds cancel each
other out.
NaCl
Writing Formulas and Naming Compounds
What do you do if you have two ions with
oxidation numbers that don’t balance with each other?
2+
Ca
+
F
?
Add more of the element that requires additional charge.
2+
Ca
+
2F
 CaF2
The number you added indicating more than one atom of a
particular element in an equation is called a coefficient.
Balancing Equations
Sometimes equations require a little more thought.
3+
Al
+
2O
?
Find the lowest common multiple of 3 and 2 to figure out how
many of each ion you need.
3+
2Al
2x3=6
+
23O
3x2=6
Al2O3
Writing Formulas and Naming Compounds
Practice: Write the formula for combining sodium and oxygen.
Hint: Use the periodic table to get oxidation numbers, see if they
match, then look for any imbalance.
Check your work…
+
2Na
+
2O
Na2O
Reduction-Oxidation Chemical Reactions
Reduction is the opposite
of oxidation.
Reduction involves the
gaining of electrons and
the element is negative.
A reduced material takes
electrons and heat is
absorbed.
The element that gains e-’s
is reduced.
Which element
is reduced?
Fe2O3
Reduction-Oxidation Chemical Reactions
Oxidation refers to a chemical
reaction that commonly involves
oxygen.
More generally, oxidation is a
reaction in which an atom loses
electrons while combining with
other elements.
The atom that gives away
electrons is said to be oxidized.
Examples: rusting of iron,
burning of paper. Heat is usually
given off.
Which element
is oxidized?
Fe2O3
OIL RIG
What this stands for is:
Oxidation Is Loss of electrons
Reduction Is Gain of electrons
Reduction-Oxidation Chemical Reactions
Redox is the shortened term used to identify a reductionoxidation reaction.
+
Na
Cl
What element was oxidized and what element was reduced?
Sodium was oxidized (loses an electron).
Chlorine was reduced (gained an electron).
Chemical Equations
Scientists write out the reactions of combining chemicals as
chemical equations using symbols from the periodic table.

+
+
Na
+
Cl
 NaCl
Chemical Equations
Remember…
The elements that are reacting with each other are called the
reactants and are shown on the left side of the action arrow.
+
Na
+
Cl
Reactants
 NaCl
Products
The compounds formed by the reacton are called products and
are shown on the right side of the action arrow.
Balancing Equations
Matter cannot be created or destroyed. But it can be rearranged!
Red Bull x Milk  YUCK!
Magnesium mixed with oxygen burns with such a brilliant light
that it is used in emergency flares.
Balancing Equations
When magnesium burns it reacts with the
oxygen in the air to produce a white powder
(magnesium oxide).
How can you write a chemical equation for the
reaction? Here are four steps to follow:
Step 1: Write a starter chemical equation for the reaction using
formulas and symbols. Remember that oxygen gas is diatomic.
Mg + O2  MgO
Balancing Equations
Step 2: Count the atoms in the reactants and in the products.
Mg + O2  MgO
1
2
Magnesium is balanced…but Oxygen is not.
1
1
Balancing Equations
Step 3: Choose a coefficient that balance the element that is not
balanced.
Mg + O2  2 MgO
1
2
1
2
Now oxygen is balanced, but magnesium is not.
2
1
Balancing Equations
Step 4: Keep the first coefficient. Re-evaluate and now choose a
coefficient that balance the second element.
2 Mg + O2  2MgO
21
2
2
2
Congratulations! Now everything is balanced!
Balancing Equations
Polish your skill…Let’s play ChemBalancer.
http://funbasedlearning.com/chemistry/chemBalancer/default.htm
Polish your skill…Balance the following equation.
Li + H2O  LiOH + H2
Answer:
2Li + 2H2O  2LiOH + H2
Energy and Chemical Reactions
Reactant
Reactants
ENTER the
reaction.
Reactant
Energy is
absorbed or
released by
the reaction.
Products are
PRODUCED by
the reaction.
EXOthermic Reaction = RELEASES energy
Reactant
Reactant
Reactants
ENTER the
reaction.
EXOTHERMIC
Energy is
RELEASED by
the reaction.
Products are
PRODUCED by
the reaction.
ENDOthermic Reaction = ABSORBS energy
Reactant
Reactant
Reactants
ENTER the
reaction.
ENDOTHERMIC
Energy is
ABSORBED by
the reaction.
Products are
PRODUCED by
the reaction.
Exothermic Reaction = Releases Energy
ACTIVATION ENERGY = energy required
ENERGY
to cause the reaction to occur.
Energy
RELEASED
TIME
Endothermic Reaction = Absorbs Energy
ACTIVATION ENERGY = energy required
to cause the reaction to occur.
Energy
ENERGY
ABSORBED
TIME
Chemical Reactions and Catalysts
Catalysts are special molecules that aid in making chemical
reactions happen but are not used up themselves
Catalysts reduce the activation energy!
ACTIVATION ENERGY = energy required
ENERGY
to cause the reaction to occur.
TIME
Assignments:
•
•
•
•
Study Guide: Chemical Bonds
L3, L4 and L5
Study Guide: Chemical Reactions
L6
Any Questions?
Comprehensive TEST on Coach
Book Chapter 1 (L1-L6).