Transcript Chemical Equations

Molecular rep of a rxn
CH4 + O2  CO2 + H2O
Notice that there are 4 hydrogen atoms in the reactants
and only 2 in the products.
Notice that there are only 2 oxygen atoms in the reactants
and 3 in the products
Chemical Equation
A representation of a balanced chemical
reaction:
CH4 + 2O2  CO2 + 2H2O
reactants
products
Molecular rep of a rxn
CH4 + 2O2  CO2 + 2H2O
Chemical Equation
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CH4 + 2O2  CO2 + 2H2O
The equation is balanced – there are
the same number of each atom on each
side of the arrow.
1 molecule of methane reacts with 2
molecules of oxygen
To produce:
 1 molecule of carbon dioxide and 2
molecules of water
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To Balance Equations
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Start with the element that is most complex,
usually
Make sure the same numbers of atoms of that
element are on both sides of the equation
You CANNOT change the formula of a molecule
in an equation
You must change the COEFFICIENT of the
molecule to make there be the same numbers
Balance the Reaction 1
Ca + AlCl3  CaCl2 + Al
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Chlorine is the most complex
There are 3 Cl’s in the reactants and 2 Cl’s in the
products
The least common multiple between 3 and 2
is…6
In order to make 6 Cl’s on each side, put the
coefficient 2 in front of the AlCl3 and the
coefficient 3 in front of the CaCl2.
Balance the Reaction 1
Ca + 2AlCl3  3CaCl2 + Al
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Now you have changed the number of
Aluminum’s in the reactants to 2. There is only
1 Al in the products
In order to make there be 2 Al’s in the products,
put the coefficient 2 in front of the Al
You have also changed the number of Calcium’s
in the products to 3. There is only 1 Ca in the
reactants
Put the coefficient 3 in front of the Ca.
Balance the Reaction 1
3Ca + 2AlCl3  3CaCl2 + 2Al
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Check to see that there are equal numbers
of each atom on each side of the
reaction…
The reaction is Balanced!!
Balance the Reaction 2
C2H5OH + O2  CO2 + H2O
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Hydrogen is the most complex
There are 6 H’s in the reactants and only
2 in the products
In order to make 6 H’s in the products,
put the coefficient 3 in front of the H2O
Balance the Reaction 2
C2H5OH + O2  CO2 + 3H2O
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Carbon is the next most complex
There are 2 C’s in the reactants and only 1
in the products
In order to make 2 C’s in the products, put
the coefficient 2 in front of the CO2
Balance the Reaction 2
C2H5OH + O2  2CO2 + 3H2O
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Now you have changed the numbers of Oxygen
atoms in the products – there are 7 O’s
Changing the number of C2H5OH’s will force you
to change the coefficients on the products
One of the O’s is used up in the C2H5OH, so
there are 6 O’s that need to be accounted for by
the O2’s.
Put the coefficient 3 in front of the O2 so there
are 7 total O’s in the reactants.
Balanced Reaction 2!
C2H5OH + 3O2  2CO2 + 3H2O
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Check to make sure that there are the
same numbers of each atom on each side!
Which
of the following correctly balance
the chemical equation given below?
CaO + C  CaC2 + CO2
1.
2.
3.
4.
CaO2 + 3C  CaC2 + CO2
2CaO + 5C  2CaC2 + CO2
CaO + (2.5)C  CaC2 + (0.5)CO2
4CaO + 10C  4CaC2 + 2CO2
Notice
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The number of atoms of each element must be
the same on both sides of a balanced equation.
Subscripts must not be changed to balance an
equation.
A balanced equation tells us the ratio of the
number of molecules that react and are
produced in a chemical reaction.
Trial and error is a valid method to balance a
chemical equation.
Balance the following Reactions
N2 +
H2 
H2O 
NH3
H2 +
O2
Mg +
FeCl3 
MgCl2 +
Fe
Br2 +
FeCl3 
FeBr3 +
Cl2
Check your answers
N2 + 3H2  2NH3
2H2O  2H2 + O2
3Mg + 2FeCl3  3MgCl2 + 2Fe
3Br2 + 2FeCl3  2FeBr3 + 3Cl2
Balance the following reactions
NaCl + MgBr2  MgCl2 + NaBr
Pb(NO3)2 + Na2SO4  NaNO3 + PbSO4
CH4 + O2  CO2 + H2O
C4H10 + O2  CO2 + H2O
Check Your Answers
2NaCl + MgBr2  MgCl2 + 2NaBr
Pb(NO3)2 + Na2SO4  2NaNO3 + PbSO4
CH4 + 2O2  CO2 + 2H2O
C4H10 + 13/2O2  4CO2 + 5H2O or
2C4H10 + 13O2  8CO2 + 10H2O
Balance the Reactions
Al + O2  Al2O3
C3H8 + O2  CO2 + H2O
LiCl + CuSO4  CuCl2 + Li2SO4
NaCl + F2  NaF + Cl2
Check Your Answers
4Al + 3O2  2Al2O3
C3H8 + 5O2  3CO2 + 4H2O
2LiCl + CuSO4  CuCl2 + Li2SO4
2NaCl + F2  2NaF + Cl2