Transcript Chemical Reactions

CHEMICAL REACTIONS
Chapter 6
Reactants: Zn + I2
Product: Zn I2
Introduction
–Chemical rxns occur when bonds
between valence electrons of atoms
are formed or broken
–Chemical rxns involve changes in
matter, the making of new materials
with new properties&energy changes
–Symbols represent elements,
–Formulas represent compounds,
–Chemical equations represent
chemical rxns
Signs of Chemical Reactions
1. Color change (ex: KMnO4)
2. Formation of precipitate
3. Odor change (ex: spoiled milk)
4. Gas release (baking soda &
vinegar)
5. Energy change- (vermiculite)
–heat absorbed
–heat or light released
.
•Equations: Show what
is happening during a
reaction
Rules All Equations Follow
• Show reactants (“before”) on the left,
products (“after”) on the right
• Use an arrow() to separate reactants
from products
• Use + signs between reactants
• Use + signs between products
Ex:aluminum+oxygenaluminum oxide
There are 2 ways to write an
equation for a rxn.
• Word Equations- use names of
compounds & elements.
• Chemical Equations-use
formulas of compounds &
symbols of elements.
Word Equations
(Label example from prior slide)
aluminum+oxygenaluminum oxide
Problem: word eq’ns don’t tell us enough.
Chemical Equations
Their Job: Show the relative
amounts of reactants and
products in a reaction.
4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
The numbers in the front are called
Coefficients & show how many units
(atoms, molecules, etc.)
Additional Rules Chemical
Equations Follow
• Physical state of chemicals is shown after
the formula or symbol
(s) = solid
(l) = liquid
(g) = gas
(aq) = in aqueous solution
• Energy is shown when it is an important
part of a rxn.
• With the reactants in endothermic rxns
• With the products in exothermic rxns
• Balance-equations must be balanced
• Charcoal, which is basically carbon, reacts with
oxygen to yield carbon dioxide.
• Word eq’n: carbon + oxygen  carbon dioxide
+ energy
• Chemical equation: C(s) + O2(g)  CO2(g)
+ energy
has a quantitative meaning as well.
Balancing Chemical Equations
Because of the principle of
the conservation of
matter, an equation
must be balanced.
Def: It must have the
same number of atoms of the
Lavoisier, 1788
same kind on both sides.
Balancing Chemical Equations,
cont.
4 Al(s) + 3O2(g)--->2 Al2O3(s)
This equation means
4 Al atoms + 3 O2 molecules
---produces--->
2 units of Al2O3
Balancing Equations, cont.
–When balancing a chemical
equation you may ONLY add
coefficients in front of the
compounds & elements to
balance the reaction.
–You may not change the
subscripts.
(Changing the subscripts
changes the compound.)
Subscripts vs. Coefficients
• Subscripts tell
you how many
atoms of an
element are in a
compound. Ex:
the “2” in O2.
• Coefficients tell
you the quantity, or number,
of units of a
chemical. Ex:
the “3” in front
of CO2.←
4 Steps to Balancing Equations
Step #1: Write the correct
formula for the reactants and
the products.
• (DO NOT TRY TO BALANCE
IT YET! Just write the correct
formulas first.
• And most importantly, once
you write them correctly, DO
NOT CHANGE THE
FORMULAS!)
HINT: WHEN COMPLETING STEP #1
When writing formulas,
remember
•The 7 diatomic elements
–H2, N2, O2, F2, Cl2, Br2, I2
4 Steps to Balancing Equations
Step #2. Count the
number of atoms for
each element on the left
side.
Count the number of the
atoms of each element
on the right side.
4 Steps to Balancing Equations
Step #3: Determine
where to place
coefficients in front of
symbols & formulas so
that the left side has the
same number of atoms
as the right side for
EACH element.
Steps to Balancing Equations
Step # 4: Check your answer
to see if:
–The numbers of atoms on
both sides of the
equation are now
balanced.
–The coefficients are in the
lowest possible whole
number ratios. (reduced)
Some Suggestions to Help You
Helpful Hints for balancing
equations:
• Take one element at a time,
working left to right except for H
and O. Save H for next to last,
and O until last.
• (Shortcut) Polyatomic ions that
appear on both sides of the
equation should be balanced as
independent units.
Balancing Equations
2 H (g) + ___ O (g) ---> __
2 H O(l)
__
2
2
2
LEFT SIDE
H=2
O=2
What Happened to
the Other Oxygen
Atom?????
This equation is not
balanced!
RIGHT SIDE
H=2
O=1
ANSWER TO PRIOR SLIDE
• Re-Count the atoms
_2_H2(g) + ___O2(g) --->_2_H2O(l)
• LEFT SIDE
RIGHT SIDE
• H=4
H=4
• O=2
O=2
Balancing
Equations
2 Al(s) + ___
3 Br2(l) ---> ___ Al2Br6(s)
___
Practice
Balancing
Equations
____C3H8(g) + _____ O2(g) ---->
_____CO2(g) + _____ H2O(g)
____B4H10(g) + _____ O2(g) ---->
___ B2O3(g) + _____ H2O(g)
Balancing Equations
Sodium phosphate + iron (III) oxide 
sodium oxide + iron (III) phosphate
Na3PO4 +
Na2O +
Fe2O3 ---->
FePO4
5 Types of Reactions
1. Synthesis-2 or more reactants
combine to form 1 product.
Ex: 2H2 + O2  2H2O
2. Decomposition- 1 reactant
breaks down into 2 or more
products. Ex: 2H2O  2H2 + O2
5 Types of Reactions
3. Single Replacement- 1
element replaces another in a
compound
Ex: 2K + H2O  2KOH + H2
(K replaced H in water)
5 Types of Reactions
4. Double Replacementexchange of cations between 2 ionic compounds
Ex:
2NaCN +H2SO42HCN + Na2SO4
5 Types of Reactions
5. Combustion-the rapid
combination of a substance
w/oxygen to form heat & light.
• Ex 1: CH4 + 2O2  CO2 + 2H2O
+ energy
• Ex 2: Mg + O2 2MgO +energy
(note: Ex2 is also a synthesis rxn)
What type of reaction?
1. Na3PO4 + Fe2O3 Na2O +FePO4
2. C3H8(g)+O2(g)CO2(g) + H2O(g)
3. 2Al(s) + 3Br2(l) ---> Al2Br6(s)
What type of reaction?
1. Na3PO4 + Fe2O3 Na2O +FePO4
Double replacement
2. C3H8(g)+O2(g)CO2(g) + H2O(g)
combustion
3. 2Al(s) + 3Br2(l) ---> Al2Br6(s)
synthesis
Practice Problems
• Complete Q#1- from p
____ of text.
• Q& A please!