Transcript Chemical Reactions

CHEMICAL REACTIONS
Adapted from www.chemistrygeek.com
Reactants: Zn + I2
Product: Zn I2
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Evidence for a Chemical Reaction
• Color change
• Gas production
– Bubbles, smoke, odor
• Heat transfer
– Spontaneous change in temperature, hot or cold
• Formation of Precipitate
– Solid that settles from mixing two AQUEOUS
solutions
• Change in pH
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Evidence for a Chemical Reaction
Chemical Reactions
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– occur when bonds between the outermost
parts of atoms are formed or broken
– involve changes in matter, the making of
new materials with new properties, and
energy changes.
– Symbols represent elements, formulas
describe compounds, chemical equations
describe a chemical reaction
Chemical Equations
Represent the kind of reactants and
products in a reaction and their
relative amounts.
4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
The numbers in the front are called
stoichiometric coefficients
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Parts of a Chemical Equation
• shows the conversion of reactants into products
• Reactants are the starting materials (left)
• Products are the ending materials (right)
• + sign separates molecules on the same side
• The arrow is read as “yields”
• Example
C(s) + O2(g)
 CO2(g)
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• The charcoal used in a grill is carbon
• The carbon reacts with oxygen to yield carbon dioxide.
• The chemical equation for this reaction, C + O2  CO2
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Symbols Used in Equations
• Solid (s)
• Liquid (l)
• Gas (g)
• Aqueous solution (aq)
H2SO4
• Catalyst
• Escaping gas ()
• Change of temperature ()
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Learning Check
• Write the unbalanced equation (including symbols
for states) for each of the following statements
– Solid magnesium reacts with liquid water to
form solid magnesium hydroxide and hydrogen
gas
– Solid ammonium dichromate decomposes to
solid chromium (III) oxide, nitrogen gas, and
water vapor
– Gaseous ammonia reacts with gaseous oxygen
to form gaseous nitrogen monoxide and water
vapor
Chemical Equations
• Because of the principle
of the conservation of
matter, an equation
must be balanced.
• It must have the same
number of atoms of the
same kind on both sides.
Lavoisier, 1788
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Balancing Equations
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– When balancing a reaction you may add
coefficients in front of the compounds,
but you may not change the subscripts.
– Changing the subscripts changes the
compound.
– Subscripts are determined by the valence
electrons (charges for ionic or sharing for
covalent)
Subscripts vs. Coefficients
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• 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.
Balancing Equations
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4 Al(s) + 3 O2(g) ---> 2 Al2O3(s)
This equation means
4 Al atoms + 3 O2 molecules  2 molecules of Al2O3
Steps to Balancing Equations
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There are three basic steps to balancing a chemical equation.
1. 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.
2. Adjust coefficients in front of formulas so that the left
side has the same number of atoms as the right side for
EACH element.
3. 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
Some 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.
• IF everything balances except for O, and
there is no way to balance O with a whole
number, double all the coefficients and try
again. (Because O is diatomic as an
element)
• (Shortcut) Polyatomic ions that appear on
both sides of the equation should be
balanced as independent units
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Balancing Equations
2 H2(g) + ___ O2(g) ---> ___
2 H2O(l)
___
What Happened to the Other
Oxygen Atom?????
This equation is not balanced!
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Balancing
Equations
2 Al(s) + ___
3 Br2(l) ---> ___ Al2Br6(s)
___
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Balancing Equations
H 2O

H2
NH3 +
O2

NO +
H 2O
SiO2 +
HF

SiF4 +
H2O
NH4NO2

N2
+
H 2O
NO

N 2O +
NO2
K
+
+
KOH
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Balancing
Equations
C3H8 + O2

CO2 +
H2O
B4H10 + O2

B2O3 +
H2O
Sodium phosphate + iron (III) oxide  sodium oxide
+ iron (III) phosphate