Transcript Unit 14 - CHEMICAL REACTIONS Reactants: Zn + I2 Product: Zn I2

Unit 14 - CHEMICAL REACTIONS
Reactants: Zn + I2
Product: Zn I2
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Introduction
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• Chemical reactions 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: 2 H2 + O2 → 2 H2O
• Chemical reactions occur
when bonds between the
outermost parts of atoms
are broken and new bonds
are formed.
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A chemical reaction is a process in
which a substance (or substances)
called reactants are changed into
one or more new substances which
are called products.
4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
Parts of a Reaction Equation
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Chemical equations show the conversion of
reactants (the molecules shown on the left of
the arrow) into products (the molecules shown
on the right of the arrow).
• A + sign separates molecules on the same
side
• The arrow is read as “yields”
• Example
C + O2  CO2
• This reads “carbon plus oxygen react to yield
carbon dioxide”
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The charcoal used in a grill is basically carbon. The
carbon reacts with oxygen to yield carbon dioxide.
The chemical equation for this reaction,
C + O2  CO2
contains the same information as the English
sentence but has quantitative meaning as well.
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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A chemical reaction is written as a
chemical equation using chemical
formulas with symbols which represent
the chemical elements and numbers
written as sub-indexs, which indicate
the number of atoms of each element.
4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)
The numbers in the front are called
stoichiometric coefficients.
The letters (s), (g) are the physical
states of compounds.
Symbols Used in Equations: Chemical formulas
The chemical formulas are
represented by letters and numbers:
Na2ClO4
• The letters represent the symbols of the
chemical elements present (as in the
Periodic Table).
Na: sodium, Cl: Chloride, O:oxygen
• The numbers indicate the number of
atoms of each element involved.
2 atoms Na, 1 atoms Cl, 4 atoms O
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Symbols Used in Equations: Chemical elements
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Symbols Used in Equations:
Subindexs and Coefficients
Subindexs indicate the number of atoms of
each element involved. The absence of
subindex means that there is 1 atom.
MgBr2 1 atom Mg, 2 atoms Br
Coefficients represent the number of units
of that substance.
2 MgBr2 2 x (1 atom Mg, 2 atoms Br) =
2 atoms Mg, 4 atoms Br
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Symbols Used in Equations: Chemical formulas
If there is a parenthesis applies to
everything inside it.
Fe2 ( S O4 ) 3
Fe: iron S:sulphur, O:oxygen
2 atoms Fe
3 x 1 = 3 atoms S
3 x 4 = 12 atoms O
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PRACTISE: Count the number of atoms in a
chemical formula
• H2
• KCl
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Na2O
Be(OH)2
P2O5
Li2SO4
Ni(BrO3)2
Ca3(PO4)2
2 Co2O3
3 C5H10
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PRACTISE: Count the number of atoms in a
chemical formula
• H2
• KCl
• H2 : 2 atoms H
• KCl : 1 atom K, 1 atom Cl
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Na2O
Be(OH)2
P2O5
Li2SO4
Ni(BrO3)2
Ca3(PO4)2
2 Co2O3
3 C5H10
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Na2O : 2 atoms Na, 1 atom O
Be(OH)2 : 1 atom Be, 2 atoms O, 2 atoms H
P2O5: 2 atoms P, 5 atoms O
Li2SO4 : 2 atoms Li, 1 atom S, 4 atoms O
Ni(BrO3)2 : 1 atom Ni, 2 atoms Br, 6 atoms O
Ca3(PO4)2 : 3 atoms Ca, 2 atoms P, 8 atoms O
2 Co2O3 : 4 atoms Co, 6 atoms O
3 C5H10 : 15 atoms C, 30 atoms H
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Symbols Used in Equations
• Solid (s)
• Liquid (l)
• Gas (s)
• Aqueous solution (aq)
• Escaping gas ()
• Precipitating solid ()
Balancing Equations
• When balancing a chemical reaction you
may add coefficients in front of the
compounds to balance the reaction, but you
not change the
sub-indexs.
may
• Changing the sub-indexs changes the
compound.
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Subindexs vs. Coefficients
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The sub-indexs
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.
Chemical Equations
4 Al(s) + 3 O2(g)
---> 2 Al2O3(s)
This equation means
4 Al atoms + 3 O2
molecules ---produces--->
2 molecules of Al2O3
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Steps to Balancing Equations
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There are four basic steps to balancing a chemical
equation.
1. Write the correct formula for the reactants and
the products. DO NOT TRY TO BALANCE IT YET!
You must write the correct formulas first. And
most importantly, once you write them correctly
DO NOT CHANGE THE FORMULAS!
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. (reduced)
Some Suggestions to Help You
• 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)
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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!
Two hydrogen atoms from a hydrogen
molecule (H2) combines with one of the
oxygen atoms from an oxygen molecule
(O2) to form H2O. Then, the remaining
oxygen atom combines with two more
hydrogen atoms (from another H2 molecule)
to make a second H2O molecule.
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Balancing
Equations
2 Al (s) + ___
3 Br2(l) ---> ___ Al2Br6(s)
___
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PRACTISE:
Balancing
Equations
____C3H8(g) + _____ O2(g) →
____CO2(g) + _____ H2O(g)
____B4H10(g) + _____ O2(g) →
___
B2O3(g) + _____ H2O(g)
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PRACTISE:
Balancing
Equations
C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)
2 B4H10(g) + 11 O2(g) → 4 B2O3(g) + 10 H2O(g)
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Balancing Equations
C4H8 + O2 → CO2 + H2O
Mg + HI → Mgl2 + H2
Ca(OH)2 + HCl→ CaCl2 + H2O
Na3PO4 + Fe2O3 → Na2O + FePO4
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