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BALANCING EQUATIONS. LEARNING OUTCOMES. 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

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slide2

LEARNING OUTCOMES

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

slide3

VOCABULARY

  • Subscript
  • Coefficient
  • Law of Conservation of Mass
  • Molecule
  • Atom
  • Skeleton Equation
  • Balanced Equation
  • Word Equation
slide4

Balancing Equations

  • 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.

Products

Reactants

(c) McGraw Hill Ryerson 2007

slide5

Balancing Equations

Products

Reactants

(c) McGraw Hill Ryerson 2007

slide6

Balancing Equations

  • Chemical reactions can be written in different ways.
  • 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.

(c) McGraw Hill Ryerson 2007

slide7

Law of Conservation of Mass

  • 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, there MUST 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).

(c) McGraw Hill Ryerson 2007

slide8

Writing & Balancing Equations

  • 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
slide9

Writing & Balancing Equations

  • 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 reaction arrow.
      • 4K + O2 2K2O

K

K

O

O

K

O

K

K

O

K

K

K

slide10

Counting Atoms to Balance Equations

  • 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.

(c) McGraw Hill Ryerson 2007

slide11

Counting Atoms to Balance Equations

Skeleton equation: CH4 + O2  H2O + CO2

Carbon = 1 Carbon = 1

Hydrogen = 4 Hydrogen = 2

Oxygen = 2 Oxygen = 3

Balanced equation: CH4 + 2O2  2H2O + CO2Carbon = 1 Carbon = 1

Hydrogen = 4 Hydrogen = 4

Oxygen = 4 Oxygen = 4

The same number of atoms must be on each side.

(c) McGraw Hill Ryerson 2007

balancing equations
Balancing Equations

hydrogen + oxygen  water

O2

H2 +

H2O

balancing equations1
Balancing Equations

X

hydrogen + oxygen  hydrogen peroxide

H2 + O2 H2O2

YOU CANNOT CHANGE THE SUBSCRIPTS

balancing equations2

2

Balancing Equations

hydrogen + oxygen  water

H2 + O2 H2O

balancing equations3
Balancing Equations

hydrogen + oxygen  water

H2 + O2 H2O

2

balancing equations4
Balancing Equations

hydrogen + oxygen  water

2 H2 + O2 H2O

2

(l)

(g)

(g)

slide17

Birkeland’s Big Bag ‘O Balancing Tricks

  • 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, treat polyatomic ions (such as SO42–) as a group & balance them as such.
      • If ‘OH’ and H2O are in the equation, write water as HOH.
      • Balance one compound at a time & rewrite the # of atoms in the chart as things change.
      • Only add coefficients; NEVER change subscripts!!!
      • If H and O appear in more than one place, attempt to balance them 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!!!

(c) McGraw Hill Ryerson 2007

slide18

WATCH THIS LESSON

ON BALANCING EQUATIONS

Turn sound “on”

http://www.authorstream.com/Presentation/kellymdeters-86103-section-2-7-balancing-chemical-equations-education-ppt-powerpoint/

slide19
Balance the following:
    • Fe + Br2 FeBr3
    • Sn(NO2)4 + K3PO4  KNO2 + Sn3 (PO4)4
    • C2H6 + O2  CO2 + H2O
slide20
____Ba + ____H2O  ____Ba(OH)2 + ____H2

____CO2 + ____H2O  ____H2CO3

____Fe2O3 + ____C  ____Fe + ____CO

____Fe + ____H2O  ____H2 + ____Fe2O3

slide21

Turn Word Equations into

Balanced Equations

  • 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 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

(c) McGraw Hill Ryerson 2007

slide22

Turn Word Equations into

Balanced Equations

  • Several common covalent molecules containing hydrogen have common names that you should know and MEMORIZE!!
    • methane = CH4
    • glucose = C6H12O6
    • ethane = C2H6
    • ammonia = NH3

See page 208

(c) McGraw Hill Ryerson 2007

slide23

Turn Word Equations into

Balanced Equations

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

See page 208

(c) McGraw Hill Ryerson 2007