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You Should Be Able To…
1. Define and explain the law of conservation of mass
2. Represent chemical reactions and the conservationof atoms, using molecular models
3. Write and balance (using the lowest whole number
coefficients) chemical equations from formulae, wordequations, or descriptions of experiments
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(c) McGraw Hill Ryerson 2007
• Chemical reactions result in chemicalchanges.
– Chemical changes occur when new substances
are created. – The original substance(s), called reactants,
change into new substance(s) called products.
See pages 202 - 203
Reactants Products
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(c) McGraw Hill Ryerson 2007 See pages 202 - 203
Reactants Products
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(c) McGraw Hill Ryerson 2007
• Chemical reactions can be written in differentways.
– A word equation:
• Nitrogen monoxide + oxygen nitrogen dioxide
– A symbolic equation:
• 2NO( g ) + O2( g ) 2NO2( g ) STATE OF MATTER
- Letters indicate the state of
each compound.
(aq) = aqueous/dissolved in
water
( s ) = solid
( ) = liquid
( g ) = gas
COEFFICIENTS
- Indicates how many of each
molecule there is.
-Ie: there are 2 molecules of NO.
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(c) McGraw Hill Ryerson 2007
• When a chemical reaction occurs, new compounds are
created, BUT… – No new matter is created or destroyed; atoms are
just rearranged as the atoms change partners to form
new compounds.
– If there are 3 atoms of oxygen in the reactants, thereMUST be 3 atoms of oxygen in the products.
– Number of each atom in reactants = number of each
atom in products.
• The law of conservation of mass: – Mass of reactants = mass of products
If you could collect and measure all of the exhaust from
this car, you would find that mass of reactants (gas + O2) =
mass of products (exhaust).
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• The simplest form of chemical equation is a word
equation. – Potassium metal + oxygen gas potassium oxide
• A skeleton equation shows the formulas of theelements/compounds. – A skeleton equation shows which atoms are involved, but
not how many molecules are involved.• K + O2 K2O
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• A balanced chemical equation shows all
atoms and the coefficients tells us how many
molecules (and atoms) there are.
– Balancing ensures that the number of each
atom is the same on both sides of the reactionarrow.
4K + O2 2K2O
K
K
K
K
K
K
K
K
O O O
O
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(c) McGraw Hill Ryerson 2007
• Using the law of conservation of mass, we
can count atoms to balance the number of
atoms in chemical equations.
– Word equation: methane + oxygen water +
carbon dioxide – Skeleton equation: CH4 + O2 H2O + CO2
• To balance the compounds, take note of how many
atoms of each element occur on each side of the
reaction arrow.
See Page 207
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(c) McGraw Hill Ryerson 2007
Skeleton equation: CH4 + O2 H2O + CO2
Carbon = 1 Carbon = 1Hydrogen = 4 Hydrogen = 2
Oxygen = 2 Oxygen = 3
Balanced equation: CH4 + 2O2 2H2O + CO2 Carbon = 1 Carbon = 1
Hydrogen = 4 Hydrogen = 4
Oxygen = 4 Oxygen = 4
The same number
of atoms must be on
each side.
See Page 207
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Balancing Equations
Reactants Products
H 2 2
hydrogen + oxygen water
O2 H2OH2 +
O 2 1
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Balancing Equations
H2
+ O2
H2
O2
Reactants Products
H 2 2
O 2 2
hydrogen + oxygen hydrogen peroxide
YOU C NNOT CH NGE THE SUBSCRIPTS
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Balancing Equations
H2 + O2 H2O
Reactants Products
H 2 2
O 2 1
hydrogen + oxygen water
2
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Balancing Equations
H2 + O2 H2O
Reactants Products
H 2 4
O 2 2
hydrogen + oxygen water
2
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Balancing Equations
2 H2 + O2 H2O
Reactants Products
H 4 4
O 2 2
hydrogen + oxygen water
2(g) (g) (l)
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(c) McGraw Hill Ryerson 2007
• Balance chemical equations by following these steps: –
Trial and error will work but can be very inefficient.• USE A TABLE (write atoms underneath reactants and products)
• If they look the same on both sides of the equation, treatpolyatomic ions (such as SO4
2 –) as a group & balance them assuch.
• If ‘OH’ and H2O are in the equation, write water as HOH.
• Balance one compound at a time & rewrite the # of atoms inthe chart as things change.
• Only add coefficients; NEVER change subscripts!!!
• If H and O appear in more than one place, attempt to balancethem LAST.
– Balance everything that isn’t ‘H’ or ‘O’ 1st.
– Balance the ‘H’s 2nd to last.
– Balance the ‘O’s last.
– Always double-check after you think you are finished.
– CHECK YOUR ANSWERS!!! See pages 209 - 211
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http://www.authorstream.com/Presentation/kellymdeters-86103-section-
2-7-balancing-chemical-equations-education-ppt-powerpoint/
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• Balance the following:
– Fe + Br 2 FeBr 3
– Sn(NO2)4 + K3PO4 KNO2 + Sn3 (PO4)4
– C2H6 + O2 CO2 + H2O
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____Ba + ____H2O ____Ba(OH)2 + ____H2
____CO2 + ____H2O ____H2CO3
____Fe2O3 + ____C ____Fe + ____CO
____Fe + ____H2O ____H2 + ____Fe2O3
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(c) McGraw Hill Ryerson 2007
• If you don’t transform your word into askeleton equation properly, you won’tbe able to balance the equationcorrectly. – Change chemical names into chemical
formulas. 4 types:• Simple ionic compounds
• Multivalent ionic compounds
• Ionic compounds with polyatomic ions
• Covalent compound
• Be careful of diatomic elements --remember the special seven!!
H2, N2, O2, F2, Cl2, Br2, I2
See page 208
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(c) McGraw Hill Ryerson 2007
See page 208
Several common covalent molecules containing hydrogen have
common names that you should know and MEMORIZE!!
methane = CH4
glucose = C6H12O6
ethane = C2H6
ammonia = NH3
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(c) McGraw Hill Ryerson 2007
See page 208
Example #1:
Word Equation: Solutions of lead nitrate react with potassium iodide to
produce solid lead iodide and a solution of potassium nitrate.
Skeleton Equation: Pb(NO3)2(aq) + KI(aq) PbI2(s) + KNO3(aq)
Balanced Equation: Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)
Example #2:
Word Equation: Copper reacts with hydrogen nitrate to produce copper (II)
nitrate plus hydrogen.
Skeleton Equation: Cu + H(NO3) Cu(NO3)2 + H2
Balanced Equation: Cu + 2H(NO3) Cu(NO3)2 + H2
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