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BALANCING EQUATIONS. LEARNING OUTCOMES. You Should Be Able To… 1. Define and explain the law of conservation of mass 2. Write and balance chemical equations from formula, word equations, or descriptions of experiments. DEFINE: VOCABULARY. Subscript Coefficient

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BALANCING EQUATIONS

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Balancing equations

BALANCING EQUATIONS


Balancing equations

LEARNING OUTCOMES

You Should Be Able To…

1. Define and explain the law of conservation of mass

2. Write and balance chemical equations from formula, word equations, or descriptions of experiments


Balancing equations

DEFINE:VOCABULARY

  • Subscript

  • Coefficient

  • Law of Conservation of Mass

  • Molecule

  • Atom

  • Skeleton Equation

  • Balanced Equation

  • Word Equation


Balancing equations

4.3: 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

See pages 202 - 203


Balancing equations

4.3: Balancing Equations

Products

Reactants

(c) McGraw Hill Ryerson 2007

See pages 202 - 203


Balancing equations

4.3: 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


Balancing equations

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


Balancing equations

Writing & Balancing Equations

  • The simplest form of chemical equation is a word equation, a DESCRIPTION

    • 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


Balancing equations

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


Balancing equations

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

See Page 207


Balancing equations

Counting Atoms to Balance Equations

Skeleton equation:CH4 + O2  H2O + CO2

Carbon = 1Carbon = 1

Hydrogen = 4Hydrogen = 2

Oxygen = 2Oxygen = 3

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

Hydrogen = 4Hydrogen = 4

Oxygen = 4Oxygen = 4

The same number of atoms must be on each side.

(c) McGraw Hill Ryerson 2007

See Page 207


Balancing equations

Balancing Equations

hydrogen + oxygen  water

O2

H2 +

H2O


Balancing equations1

2

Balancing Equations

hydrogen + oxygen  water

H2 + O2 H2O


Balancing equations2

Balancing Equations

hydrogen + oxygen  water

H2 + O2 H2O

2


Balancing equations3

Balancing Equations

hydrogen + oxygen  water

2 H2 + O2 H2O

2

(l)

(g)

(g)


Balancing equations

McG’sBig 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)

      • 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!!!

See pages 209 - 211

(c) McGraw Hill Ryerson 2007


Balancing equations

  • Balance the following:

    • Fe + Br2 FeBr3

    • Sn(NO2)4 + K3PO4  KNO2 + Sn3 (PO4)4

    • C2H6 + O2  CO2 + H2O


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