Download CHEMICAL REACTIONS Chapter 4

CHEMICAL REACTIONS
Balancing all things
Reactants: Zn + I2
Product: ZnI2
Introduction
 Chemical reactions occur when bonds between
the outermost parts of atoms (valence electrons)
are broken then reformed.
 Chemical reactions involve changes in matter, the
making of new materials with new properties, and
energy changes.
 Symbols represent elements (C, O2), Formulas
describe compounds (H2O, NaCl), Chemical
equations describe a chemical reaction.
2H2 + O2
2H2O
Chemical Equations
Their Job: a chemical shorthand for
the reactants and products and
their relative amounts in a reaction.
4 Al(s) + 3 O2(g) → 2 Al2O3(s)
The numbers in the front are called
“coefficients”
The letters (s) and (g) are the
physical states of the compounds
during the reaction.
Parts of a Reaction Equation
Chemical equations show the conversion of
• Reactants - the atoms and molecules on the left of the
arrow (ORDER DOES NOT MATTER) , into
• Products - the atoms and molecules on the right of the
arrow. (ORDER DOES NOT MATTER)
• A + sign separates molecules on the same side
• The arrow is read as “yields” or “reacts to form”
• Example:
Reactants
Products
2Na(s) + Cl2(g)  2NaCl(s)
• This reads “two atoms of solid sodium plus one
molecule of chlorine gas react to form two molecules of
solid sodium chloride”
• Note that the coefficients are understood to be 1 when
no coefficients are written.
Symbols Used in Equations
• Solid = (s)
• Pure Liquid = (l)
• Gas = (g)
• Aqueous solution = (aq)
–Dissolved in water
Pt
• Catalyst =
• Escaping gas = ( )
• Precipitate (solid) formed = (↓)
• Heat is required
=
Δ
Chemical Equations
Because of the principle of the
Conservation of Matter,
“Cannot create or destroy matter”
Equations must be balanced.
Lavoisier, 1788
It must have the same
number of atoms on both
reactant and product sides.
John Dalton, 1790
Chemical Equation =
Chemical Shorthand
•
2H2(g) + O2(g)
→ 2H2O(l)
• The subscript 2 on hydrogen tells
how many atoms of hydrogen are in
one molecule of hydrogen gas
• The 2 in front of hydrogen is called a
coefficient and shows there are two
molecules of hydrogen required to
maintain conservation of mass in
this reaction.
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Balancing Equations
 When balancing a chemical reaction you
may change the value of coefficients in
front of the compounds
not
 but you may
change the
subscripts on the compounds.
• Changing the subscripts changes the
compound. Subscripts are determined by the
ratio of charges on the anion and cation in
the compound.
Subscripts vs. Coefficients
• The subscripts
tell you how
many atoms of
a particular
element are in a
compound. The
coefficient tells
you about the
quantity, or
number, of
molecules of
the compound.
The Seven Diatomic Elements
N2, O2, F2, Cl2, Br2, I2, H2
• Seven of the elements on the periodic table
are diatomic (two atoms) naturally.
• The theme of the seven diatomics is “7”
– There are 7,
– They begin with element atomic number 7,
Nitrogen
– They form a “7” from Nitrogen to Iodine
– The 7th is Hydrogen
• Remember, you will always have a periodic
table so just remember the pattern of 7.
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Chemical Equations
2Al(s) + Fe2O3(s) →Al2O3(s) + 2Fe(s)
This equation means:
2 Al atoms + 1 Fe2O3 molecule produces
1 molecule of Al2O3 and 2 molecules of Fe
This reaction is called the thermite reaction and
is reported to create a temperature of 3000 C!
Steps to Balancing Equations
There are four basic steps to balancing an equation:
1. Write the correct formula for the reactants and the
products. DO NOT TRY TO BALANCE IT YET!
2. Find the number of atoms for each element on the left
side. Compare those against the number of the atoms
of the same element on the right side.
3. Determine where to place coefficients in front of
formulas so that the left side has the same number of
atoms as the right side for EACH element in order to
balance the equation.
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. (No fractions)
Some Suggestions to Help You
• Take one element at a time, working left side to
right side.
• Start with the most complicated and progress
to the simplest.
• Leave free elements in the reaction for last.
• (Shortcut) Polyatomic ions that appear on both
sides of the equation should be balanced as
independent units
Balancing Equations
2 Al(s) + ___
3 Br2(l) → ___ Al2Br6(s)
___
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Balancing Equations
6 O2(g)
___C4H8(g) + ___
→
4
4 2O(g)
___CO
2(g) +___H
9 2 → ___CO
___C6H12 + ___O
6
6 2O
2 + ___H
Balancing Equations
__Na
2 3PO4 + __Fe2O3 → __Na
2
3 2O + __FePO
4
___C3H6
9
+ ___O
2
2
→ ___CO
3
3 2O
2 + ___H
Can’t leave a fraction so multiply by 2x
___C
2 3H6 + ___O
9 2 → ___CO
6
6 2O
2 + ___H
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Balancing Equations
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5 2O(g)
2
2 2O3(g) + ___H
__B4H10(g) + ___O
2(g) → ___B
Can’t leave a fraction so multiply by 2x
10 2O(g)
11 2(g) → ___B
4 2O3(g) + ___H
2 4H10(g) + ___O
__B
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END