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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 conservation of atoms, using molecular models 3. Write and balance (using the lowest whole number coefficients) chemical equations from formulae, word equations, or descriptions of experiments
Subscript Coefficient Law of Conservation of Mass Molecule Atom Skeleton Equation Balanced Equation Word Equation
(c) McGraw Hill Ryerson 2007
• Chemical reactions result in chemical changes.– 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
(c) McGraw Hill Ryerson 2007
• Chemical reactions can be written in different ways.
– A word equation:• Nitrogen monoxide + oxygen nitrogen dioxide
– A symbolic equation:• 22NO(gg) + O2(gg) 22NO2(gg)
STATE OF MATTERSTATE OF MATTER
- Letters indicate the state of each compound.
(aq) = aqueous/dissolved in water
(s) = solid
( ) = liquid
(g) = gas
COEFFICIENTSCOEFFICIENTS
- Indicates how many of each molecule there is.
-Ie: there are 2 molecules of NO.
(c) McGraw Hill Ryerson 2007
• When a chemical reaction occurs, new compounds are created, BUT…– No new matter is created or destroyedNo new matter is created or destroyed; atoms are
just rearrangedrearranged as the atoms change partners change partners to form new compounds.
– If there are 3 atoms of oxygen in the reactants, there MUST be 33 atoms of oxygen in the products.
– Number of each atom in reactants = number of each Number of each atom in reactants = number of each atom in products.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).
• The simplest form of chemical equation is a word equation.– Potassium metal + oxygen gas potassium oxide
• A skeleton equation shows the formulas of the elements/compounds.– A skeleton equation shows which atoms are involved, but
not how many molecules are involved.• K + O2 K2O
• A balanced chemical equation balanced chemical equation shows all atoms and the coefficientscoefficients tells us how many molecules (and atoms) there are.– Balancing ensures ensures that the number of each number of each
atom atom is the same on both sides same on both sides of the reaction arrow.
44K + O2 22K2O
K
K
K
K
K
K
K
K
O O O
O
(c) McGraw Hill Ryerson 2007
• Using the law of conservation of mass, we can count atoms to balance 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
(c) McGraw Hill Ryerson 2007
Skeleton equation: CH4 + O2 H2O + CO2
Carbon = 11 Carbon = 11
Hydrogen = 44 Hydrogen = 22
Oxygen = 22 Oxygen = 33
Balanced equation: CH4 + 22O2 22H2O + CO2 Carbon = 11 Carbon = 11
Hydrogen = 44 Hydrogen = 44
Oxygen = 44 Oxygen = 44
The same number of atoms must be on
each side.
See Page 207
Balancing Equations
ReactantsReactants ProductsProducts
HH 22 22
hydrogen + oxygen water
OO22 HH22OOHH2 2 ++
OO 22 11
Balancing Equations
H2 + O2 H2O2
ReactantsReactants ProductsProducts
HH 22 22
OO 22 22
hydrogen + oxygen hydrogen peroxide
XYOU CANNOT CHANGE THE SUBSCRIPTS
Balancing Equations
H2 + O2 H2O
ReactantsReactants ProductsProducts
HH 22 22
OO 22 11
hydrogen + oxygen water
22
Balancing Equations
H2 + O2 H2O
ReactantsReactants ProductsProducts
HH 22 44
OO 22 22
hydrogen + oxygen water
22
Balancing Equations
22 H2 + O2 H2O
ReactantsReactants ProductsProducts
HH 44 44
OO 22 22
hydrogen + oxygen water
22(g) (g) (l)
(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 USE A TABLE (write atoms underneath reactants and products)• If they look the same on both sides of the equation, treat treat
polyatomic ions polyatomic ions (such as SO42–) as a group as a group & balance them as
such.• If ‘OH’ and H2O are in the equation, write water as HOH. write water as HOH. • Balance one compound at a time Balance one compound at a time & rewrite the # of atoms rewrite the # of atoms in
the chart as things changeas things change.• Only add coefficientsOnly add coefficients; NEVER change subscripts!!!• If H and O H and O appear in more than one place, attempt to balance
them LASTLAST.– Balance everything that isn’t ‘H’ or ‘O’ 1st.– Balance the ‘H’s 2nd to last.– Balance the ‘O’s last.
– Always double-check Always double-check after you think you are finished.– CHECK YOUR ANSWERS!!!
See pages 209 - 211
http://www.authorstream.com/Presentation/kellymdeters-86103-section-2-7-balancing-chemical-equations-education-ppt-powerpoint/
____Ba + ____H2O ____Ba(OH)2 + ____H2
____CO2 + ____H2O ____H2CO3
____Fe2O3 + ____C ____Fe + ____CO
____Fe + ____H2O ____H2 + ____Fe2O3
(c) McGraw Hill Ryerson 2007
• If you don’t transform your word into a skeleton equation properly, you won’t be able to balance the equation correctly.– Change chemical names into chemical
formulas. 4 types4 types:• Simple ionic compounds• Multivalent ionic compounds• Ionic compounds with polyatomic ions• Covalent compound
• Be careful of diatomic elements -- remember the special sevenspecial seven!! HH22, N, N22, O, O22, F, F22, Cl, Cl22, Br, Br22, I, I22
See page 208
(c) McGraw Hill Ryerson 2007 See page 208
Several common covalent molecules containing hydrogen have common names that you should know and MEMORIZE!! methane = CHmethane = CH44
glucose = Cglucose = C66HH1212OO66
ethane = Cethane = C22HH66
ammonia = NHammonia = NH33
(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