CHEMICAL REACTIONS

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CHEMICAL REACTIONS
Reactants Zn I2
Product Zn I2
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Introduction

• Chemical reactions occur when bonds between atoms
  are formed or broken
• Chemical reactions involve changes in matter, the
  making of new materials with new properties, and
  energy changes.
• Chemical reactions are written in symbolic form
• How can you tell a chemical reaction is
  happening?

• Color, heat/light, bubbles

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Energy is in the bonds!

• Exothermic rxns bonds broken and energy is
  released

• Endothermic rxns bonds made and energy is stored

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Parts of a Reaction Equation

• reactants (molecules on left of arrow)
• products (molecules on right of arrow).
• sign separates molecules on same side
• ? is read as yields
• Ex carbon plus oxygen react to yield carbon
  dioxide
• C O2 ? CO2

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Other symbols

• Coefficients - The numbers in front
• Show relative amounts of reactants products
• Shows the recipe
• 4 Al (s) 3 O2 (g) ---gt 2 Al2O3 (s)
• Letters (s), (g), and (l) are the physical states
  of compounds.

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Chemical Equations

• Because of the principle of the conservation of
  matter,
• an equation must be balanced.
• It must have the same number of atoms of the
  same kind on both sides.

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Balancing Equations

• When balancing a chemical reaction you may add
  coefficients in front of the compounds to balance
  the reaction, but you may not change the
  subscripts.
• Changing the subscripts changes the compound.
  Subscripts are determined by the valence
  electrons (charges for ionic or sharing for
  covalent)

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Subscripts vs. Coefficients

• The subscripts tell you how many atoms of a
  particular element are in a compound. The
  coefficient tells you about the quantity, or
  number, of molecules of the compound.

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Steps to Balancing Equations

• DO NOT CHANGE THE FORMULAS!
• Always start on the left.
• Compare the number of atoms for each element on
  either side of the reaction
• Add or change COEFFICIENTS to balance.
• Every time you make a change, go back to the
  beginning.
• If polyatomic ions are present on both sides,
  treat them as one unit.
• Check your answer to see if
• The numbers of atoms on both sides of the
  equation are now balanced.
• The coefficients are in the lowest possible whole
  number ratios. (reduced)

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Balancing Equations
2
2

• ___ H2(g) ___ O2(g) ---gt ___ H2O(l)

What Happened to the Other Oxygen Atom????? This
equation is not balanced! What coefficients need
to be added to balance this equation?
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2
3

• ___ Al(s) ___ Br2(l) ---gt ___ Al2Br6(s)

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Balancing Equations
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• ____C3H8(g) _____ O2(g)
  ----gt _____CO2(g) _____ H2O(g)

3
4
2
11
5.5
____B4H10(g) _____ O2(g)
----gt ___ B2O3(g) _____ H2O(g)
4
10
2
5
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Balancing Equations

• Sodium phosphate iron (III) oxide ? sodium
  oxide iron (III) phosphate

Na3PO4 Fe2O3 ----gt
Na2O FePO4
2
3
2
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Now Try These
Which Is Balanced?
Mg O2 ? MgO 2Mg O2 ? 2MgO
Mg ½O2 ? MgO Mg2 O2 ? 2MgO
4Mg 2 O2 ? 4MgO
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a) P4 O2 ? P4O10 b) Li H2O ? H2
LiOH c) Bi(NO3)3 K2S ? Bi2S3
KNO3 d) C2H6 O2 ? CO2 H2O
2
2
2
2
6
3
2
3
3.5
C2H6 O2 ? CO2 H2O
2
7
4
6
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Balance these skeleton equations

• a) Mg 2HCl ? MgCl2 H2
• b) 3Ca N2 ? Ca3N2
• c) NH4NO3 ? N2O 2H2O
• d) 2BiCl3 3H2S ? Bi2S3 6HCl
• e) 2C4H10 13O2 ? 8CO2 10H2O
• f) 6O2 C6H12O6 ? 6CO2 6H2O
• g) 3NO2 H2O ? 2HNO3 NO
• h) Cr2(SO4)3 6NaOH ? 2Cr(OH)3 3Na2SO4
• i) Al4C3 12H2O ? 3CH4 4Al(OH)3

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Here are some more to balance

• a) 2KNO3 ? 2KNO2 O2
• b) 2Pb(NO3)2 ? 2PbO 4NO2 O2
• c) P4 6I2 ? 4PI3
• d) 3MgO 2H3PO4 ? Mg3(PO4)2 3H2O
• e) Br2 2KI ? I2 2KBr
• f) Ca(OH)2 2HNO3 ? Ca(NO3)2 2H2O
• g) Bi2O3 3H2 ? 2Bi 3H2O
• h) 3Fe 2O2 ? Fe3O4
• i) 2CaO 5C ? 2CaC2 CO2

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• a) 2Li 2H2O H2 2LiOH
• b) P4 5O2 P4O10
• c) 2C2H6 7O2 4CO2 6H2O
• d) CS2 3O2 CO2 2SO2
• e) 2AsCl3 3H2S As2S3 6HCl
• f) 3AgNO3 FeCl3 3AgCl Fe(NO3)3
• g) 2KClO3 2KCl 3O2
• h) 2SO2 O2 2SO3