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Unit 10:
Energy in Chemical
Reactions
Cypress Creek High School
Chemistry 1L
Chapter 10
States of Matter
• Some chemical equations indicate the state of matter of the
substances

–
–
–
–


Solid (s) - baking soda
Liquid (l) - water
Gas (g) - carbon dioxide
Aqueous (aq) - vinegar and sodium acetate
• Aqueous means the substance is dissolved in water. For example,
NaCl (aq) means table salt dissolved in water.
Chemical Equations
– The simplest way to represent this reaction is by using
words
– Chemical equations replace chemical names with
chemical formulas
Reactants (LEFT)
substances that
undergo a reaction
Yield sign (MIDDLE)
represents the
change/reaction
Products (RIGHT)
new substances formed
in a reaction
Chemical Equations
Practice
• What is the chemical equation for the following
reactions?
• What are the reactants and products?
1) Water and sulfur trioxide combine to form sulfuric
acid
H O + SO → H SO
2
reactants
3
2
4
product
2) Potassium chlorite decomposes to form potassium
chloride and oxygen
KClO2 → KCl + O2
reactant
products
Review of Chemical Equations
• Label the following chemical equation:
reactants
products
SnO2(s) + 2H2(g)  Sn(s) + 2H2O(g)
subscript
state of
matter
coefficient
yield sign
Law of Conservation of Mass
• The LCM says matter is neither created
nor destroyed
– The mass of the reactants is always the same as the
mass of the products.
– The number of atoms of each element is always the
same in the reactants and the products.
Law of Conservation of Mass
Examples
Hydrogen gas and oxygen gas combine to create water.
2H2 + O2  2H2O
• The large numbers, known as coefficients, indicate how many
moles of each compound there are. This helps to satisfy the LCM!
• They apply to every element in the compound (2H2O means 4 H’s
& 2 O’s)
• They are the coefficients
should be
in the lowest
whole number
ratio
Particles are rearranged, not
created or destroyed!
Law of Conservation of Mass
Examples
This should be a review - just count moles of atoms!
How many carbon atoms are conserved? Hydrogen atoms? Oxygen
atoms?
LCM Examples
• Calcium chloride and sodium sulfate combine to form calcium
sulfate and sodium chloride. CaCl2 + Na2SO4  CaSO4 + 2NaCl
Before and After:
• Does the mass change?
• Do the number of atoms change?
# atoms in the
reactants
# atoms in the
products
Ca
1
1
Cl
2
2
Na
2
2
S
1
1
O
4
4
Total
10
10
Balancing Chemical Equations
Because the Law of Conservation of Mass (LCM) states that matter
is not created or destroyed, the number of atoms of each element
must be identical in the reactants and products. This means the
chemical equation is balanced.
Example 1:
# atoms in
# atoms in
the reactants the products
H
2
2
C
1
1
O
3
3
Total
6
6
• This equation is balanced
because it satisfies the LCM
• Writing coefficients is not
necessary because each
compound has a coefficient
of 1
Balancing Chemical Equations
Example 2:
# atoms in
# atoms in
the reactants the products
Na
1
2
O
3
4
H
1
2
C
1
1
Total
6
9
• This equation is not balanced
because it does not satisfy the
LCM
• Writing coefficients is
necessary
So we have to balance the equation…
Balancing Chemical Equations
Example 2 continued:
• The process of balancing chemical equations requires trial and error.
• You may not alter the subscripts because it changes the substance!
• You may only place coefficients in front of the compounds. These act as
multipliers to balance the number of atoms.
# atoms in
the reactants
# atoms in
the products
Na
2
2
O
4
4
H
2
2
C
1
1
Total
9
9
• By placing a 2 in front of NaOH,
the equation now satisfies the
LCM and is balanced
***Tip: balance hydrogen and especially oxygen last they often fall into place!
Balancing Chemical Equations
Practice
• Write a balanced chemical equation for the following reactions.
Place coefficients in the blanks. Chemical equations never actually
write the coefficient “1”, but for this activity write a number in
every blank.
A) ___ K(s) + ___ H2O(l)  ___ H2(g) + ___ KOH(aq)
B) ___ CaCl2(aq) + ___ Na2CO3(aq)  ___ CaCO3(s) + ___ NaCl(aq)
C) ___ N2(g) + ___ H2(g)  ___ NH3(g)