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Chemical Equations and
Chemical Reactions
http://www.personal.kent.edu/~cearley/ChemWrld/balance/balance.htm
Writing A Chemical Equation
• The general format for chemical
equations is:
•a + b  c + d
• The substances on the left side of the
equation (a and b in this case) are
called the reactant(s).
• The + sign means “reacts with”.
• The  means “produces or yields”.
• The substance(s) on the right side of
the equation (c and d in this case) are
called the product(s).
Lets work with the statement
Hydrogen gas reacts with oxygen
gas to form liquid water.
• Word Equation:
• hydrogen + oxygen  water
• Skeleton Equation:
• H2(g)+ O2(g)  H2O(l)
• Balanced Equation:
• ___H2(g)+ ___O2(g)  ___H2O(l)
• 2H2(g)+ O2(g)  2H2O(l)
Why must we balance equations?
• The Law of Conservation of Mass states
that the total mass of reactants is always
equal to the total mass of the products.
(Lavoisier’s law)
Antoine-Laurent Lavoisier
experimenting to determine
the composition of water,
French engraving, 19th
century.
http://www.britannica.com/eb/art-15464/AntoineLaurent-Lavoisier-experimenting-to-determinethe-composition-of-water
Why must we balance equations?
• Dalton’s atomic theory
states that atoms can not
be created or destroyed.
Compounds are formed
when atoms of different
elements combine in fixed
proportions.
John Dalton, F.R.S., engraved by Worthington after an 1814
painting by William Allen, published 25 June 1823 in Manchester
and London. Note the charts with Dalton's atomic symbols lying
on the table. Fisher Collection, Chemical Heritage Foundation.
To balance equations numbers called
coefficients are put before the
formulas. This changes the number of
molecules. You cannot add or change
subscripts. You do not write
coefficients of 1 (one).
Sometimes you need to know the state
of the chemicals that are involved in a
chemical reaction.
State
solid
liquid
gas
aqueous solution
Abbreviation
State
Abbreviation
solid
(s)
liquid
gas
aqueous solution
State
Abbreviation
solid
(s)
liquid
(l)
gas
aqueous solution
State
Abbreviation
solid
(s)
liquid
(l)
gas
(g)
aqueous solution
State
Abbreviation
solid
(s)
liquid
(l)
gas
(g)
aqueous solution
(aq)
Final Balanced Chemical Equation
2H2(g) + O2(g)  2H2O(l)
Rules For Writing Balanced
Chemical Equations
This requires a lot of practice and
patience. These rules will not
apply in all equations.
1. Write the skeleton equation.
2. Look for a polyatomic ion that
appears once on each side of the
equation. Balance these first.
3. Pick single elements that appears
on each side. Choose coefficients
to balance this element. The terms
you balanced in the previous step
have to remain balanced. Repeat
for any other elements.
4. Check for any term you have not yet
looked at. Adjust the coefficient of
the term so that the numbers of the
elements appearing in that term are
balanced across the whole equation.
If you have any fraction coefficients
clear them by multiplying. Also,
make sure you have the smallest
possible coefficients by reducing.
5. Check the equation!
Example 1
• Word Equation:
• sodium + water  sodium hydroxide + hydrogen gas
•
•
•
•
•
Skeletal Equation:
Na + H2O  NaOH + H2(g)
Balanced Equation:
___Na + ___H2O  ___NaOH +___H2(g)
2Na + 2H2O  2NaOH + H2(g)
Example 2
• Balanced Chemical Equation
• 2H2O__ 2Na__ +  2NaOH__ + H2___
• 2H2O(l) + 2 Na(s)  2NaOH(aq) + H2(g)
• Verification
Reactants
2 Na atoms
Products
2 Na atoms
4 H atoms
4 H atoms
2 O atoms
2 O atoms
Example 2
• Word Equation:
• copper + silver nitrate  copper(II) nitrate + silver
• Skeletal Equation:
• ___ + ______  ______ + ___
• Cu + AgNO3  Cu(NO3)2 + Ag
Example 2…
• Balanced Equation:
• __Cu +__ AgNO3  __Cu(NO3)2 + __Ag
• Cu + 2AgNO3  Cu(NO3)2 + 2Ag
• Balanced Chemical Equation:
• Cu__ + 2AgNO3__  Cu(NO3)2__ + 2Ag__
• Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Verification
Reactants
Products
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Verification
Reactants
1 Cu atom
Products
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Verification
Reactants
Products
1 Cu atom
1 Cu atom
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Verification
Reactants
Products
1 Cu atom
1 Cu atom
2 Ag atoms
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Verification
Reactants
Products
1 Cu atom
1 Cu atom
2 Ag atoms
2 Ag atoms
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Verification
Reactants
Products
1 Cu atom
1 Cu atom
2 Ag atoms
2 Ag atoms
2 NO3
- ions
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Verification
Reactants
Products
1 Cu atom
1 Cu atom
2 Ag atoms
2 Ag atoms
- ions
NO3 ions
2 NO3
2
Example 3
• Word Equation:
• calcium nitrate + sodium hydroxide  calcium
hydroxide + sodium nitrate
• Skeletal Equation:
• Ca(NO3)2 + NaOH  Ca(OH)2 + NaNO3
• Balanced Equation:
• __Ca(NO3)2 + __NaOH  __Ca(OH)2 + _NaNO3
• Ca(NO3)2 + 2NaOH  Ca(OH)2 + 2NaNO3
Balanced Chemical Equation
• Ca(NO3)2__ + 2NaOH__  Ca(OH)2__ + NaNO3__
• Ca(NO3)2(aq) + 2NaOH(aq)  Ca(OH)2(s) + NaNO3(aq)
Ca(NO3)2(aq) + 2NaOH(aq)  Ca(OH)2(s) + 2NaNO3(aq)
Reactants
Products
Ca(NO3)2(aq) + 2NaOH(aq)  Ca(OH)2(s) + 2NaNO3(aq)
Reactants
1 Ca atom
Products
Ca(NO3)2(aq) + 2NaOH(aq)  Ca(OH)2(s) + 2NaNO3(aq)
Reactants
Products
1 Ca atom
1 Ca atom
Ca(NO3)2(aq) + 2NaOH(aq)  Ca(OH)2(s) + 2NaNO3(aq)
Reactants
Products
1 Ca atom
1 Ca atom
2 NO3
- ions
Ca(NO3)2(aq) + 2NaOH(aq)  Ca(OH)2(s) + 2NaNO3(aq)
Reactants
Products
1 Ca atom
1 Ca atom
2 NO3
- ions
2
NO3 ions
Ca(NO3)2(aq) + 2NaOH(aq)  Ca(OH)2(s) + 2NaNO3(aq)
Reactants
Products
1 Ca atom
1 Ca atom
2 NO3
- ions
2 Na atoms
2
NO3 ions
Ca(NO3)2(aq) + 2NaOH(aq)  Ca(OH)2(s) + 2NaNO3(aq)
Reactants
Products
1 Ca atom
1 Ca atom
2 NO3
- ions
2 Na atoms
2
NO3 ions
2 Na atoms
Ca(NO3)2(aq) + 2NaOH(aq)  Ca(OH)2(s) + 2NaNO3(aq)
Reactants
Products
1 Ca atom
1 Ca atom
2 NO3
- ions
2 Na atoms
2
OH
ions
2
NO3 ions
2 Na atoms
Ca(NO3)2(aq) + 2NaOH(aq)  Ca(OH)2(s) + 2NaNO3(aq)
Reactants
Products
1 Ca atom
1 Ca atom
2 NO3
- ions
2 Na atoms
2
OH
ions
2
NO3 ions
2 Na atoms
2
OH
ions
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