Reactions in Chemistry
Conservation of Mass
Chemical Equations
• Chemical equations are supposed to accurately
reflect the reaction taking place.
• Remember that in a chemical reaction, matter is
neither created nor destroyed.
• What does this mean when writing chemical
reactions?
▫ It means that the same elements must be present
both before and after reaction, and that the
number of atoms of each element should remain
constant during the reaction.
Chemical Equations
• To ensure that this is the case requires counting
the number of each type of atom on the reactants
side and on the products side. These two counts
should be identical.
• In order to do this, begin by understanding the
role of subscripts in formulas as well as
coefficients in balanced chemical equations.
The role of subscripts
• Remember that subscripts in a formula can tell
us the number of atoms of a particular element
in one unit of the substance:
NaCl  AgNO 3  NaNO3  AgCl
NaCl:
Na –
Cl –
AgNO3
Ag –
N–
O–
The role of subscripts
• What happens when there are parentheses in the
formula, followed by a subscript?
FeSO4  Ba(NO3 ) 2  BaSO 4  Fe(NO3 ) 2
• The number outside the parentheses tells you
how many units of what is in parentheses there
are in one unit of the substance:
(NO3 ) 2 means NO3  NO3
N–2
O-6
The role of subscripts
• This count can be arrived at by taking the
subscript outside parentheses times the
subscript of each element in parentheses
(NO3 ) 2
N: 2(1) = 2
O: 2(3) = 6
Counting reactants
• Looking at just the reactants in this reaction:
FeSO4  Ba(NO3 ) 2  BaSO 4  Fe(NO3 ) 2
FeSO4:
Fe –
S–
O–
Ba(NO3)2
Ba –
N–
O–
Counting atoms
• Sometimes the same element shows up more
than once in a particular formula, such as in the
formula for ethanol in the following combustion
reaction:
C2 H5OH  3O2  2CO2  3H 2O
C2H5OH:
C–
H–
O–
www.personal.psu.edu
upload.wikimedia.org
The impact of coefficients
• This reaction also contains coefficients.
Coefficients are whole numbers that tell how
many units there are of a substance in a reaction.
C2 H5OH  3O2  2CO2  3H 2O
• Understanding that 3O2 means there are 3
molecules of oxygen:
O
O
O
O
O
O
3O 2 means O 2  O 2  O 2
so there are 6 oxygen atoms
The impact of coefficients
• This means that if you know the number of
atoms in one unit of the substance, all you need
to do is multiply that number by the coefficient
of the formula to find the total number
represented:
3O2 means there are 3  2  6 O atoms
2CO2 means there are
2 1  2 C atoms
2  2  4 O atoms
Putting it all together
• Count the number of each type of atom indicated
by each of the following:
4H2O
2P2O5
3Ba(NO3)2
Challenge:
2CH3COOH
Resources
• http://www.personal.psu.edu/amd16/images/Et
hanol.gif
• http://upload.wikimedia.org/wikipedia/commo
ns/0/04/Ethanol-displayed.png