CHEMICAL REACTIONS

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CHEMICAL REACTIONS
Reactants Zn I2
Product Zn I2
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Chemical Equations

• Depict the kind of reactants and products and
  their relative amounts in a reaction.
• 4 Al (s) 3 O2 (g) ---gt 2 Al2O3 (s)
• The numbers in the front are called
• Stoichiometric units
• The letters (s), (g), and (l) are the physical
  states of compounds.

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Introduction

• Chemical reactions occur when bonds between the
  outermost parts of atoms are formed or broken
• Chemical reactions involve changes in matter, the
  making of new materials with new properties, and
  energy changes.
• Symbols represent elements, formulas describe
  compounds, chemical equations describe a chemical
  reaction

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• The charcoal used in a grill is basically carbon.
  The carbon reacts with oxygen to yield carbon
  dioxide. The chemical equation for this reaction,
  C O2 ? CO2, contains the same information as
  the English sentence but has quantitative meaning
  as well.

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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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Symbols Used in Equations

• Solid ___
• Liquid (l)
• Gas ___
• Aqueous solution (aq)
• Catalyst H2SO4
• Escaping gas (?)
• Change of temperature (?)

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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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Chemical Equations

• 4 Al(s) 3 O2(g) ---gt 2 Al2O3(s)
• This equation means
• 4 Al atoms 3 O2 molecules
• Produces ---gt 2 molecules of Al2O3
• AND/OR
• 4 moles of Al 3 moles of O2 ---produces ?
  2 moles of Al2O3

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Steps to Balancing Equations

• There are four basic steps to balancing a
  chemical equation.
• Write the correct formula for the reactants and
  the products. DO NOT TRY TO BALANCE IT YET! You
  must write the correct formulas first. And most
  importantly, once you write them correctly DO NOT
  CHANGE THE FORMULAS!
• Find the number of atoms for each element on the
  left side. Compare those against the number of
  the atoms of the same element on the right side.
• Determine where to place coefficients in front of
  formulas so that the left side has the same
  number of atoms as the right side for EACH
  element in order to balance the equation.
• 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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Some Suggestions to Help You

• Some of the Helpful Hints for balancing
  equations
• Take one element at a time, working left to right
  except for H and O. Save O for next to last, and
  H until last.
• IF everything balances except for O, and there is
  no way to balance O with a whole number, double
  all the coefficients and try again. (Because O is
  diatomic as an element)
• (Shortcut) Polyatomic ions that appear on both
  sides of the equation should be balanced as
  independent units

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More Rules

• ? means yields and shows direction of action
• ? above arrow shows heat has been added
• ?? shows reaction is reversible
• MINOH try Metals first, then Ions, then
  Nonmetals, and Oxygen, then Hydrogen last

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Rules

• 1. LEARN THESE DIATOMIC MOLECULES
• O2 H2 N2 F2 Cl2 Br2 I2
• 2. The same number of atoms of each element
  appearing in reactants must appear in the
  products.
• 3. The subscripts of a formula cannot be changed
  to balance the equation. Each compound must
  retain its stableness via oxidation states.
• 4. Choose compound with greatest number of atoms
  first.
• Balance by placing necessary coefficients in
  front of entire molecules.
• NO SUBSCRIPT CHANGES!

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Rules cont.

• 6. Be certain each side of equation has same
  number of atoms of each type.
• 7. Make sure coefficients are in lowest whole
  number ratios.
• 8. (aq.) stands for aqueous which is Latin for
  water solution
• (g) gas (l) liquid (s) solid
  
• (c) crystalline solid

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• Balancing chemical equations using the ones and
  twos technique.
•  Example
• Balance the equation
• K O2 ? K2O
• Solution
• Step 1 On the left side of the equation there
  are 2 oxygen atoms and on the right side of the
  equation there is one oxygen atoms. Multiply K2O
  by the coefficient of 2 to balance the oxygen
  atoms.
• K O2 ? 2K2O
• Step 2 Balance the K by placing the coefficient
  of 4 in front of K
• 4K O2 ? 2K2O
• Step 3 Check that all the atoms balance and make
  sure that all coefficients are in the
  lowest-possible ratio.

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• Balancing chemical equations using the twos and
  threes technique.
•  Example
• Balance the equation
• Fe O2 ? Fe2O3
• Step 1 On the left side of the equation there
  are 2 oxygen atoms and on the right side of the
  equation there are 3 oxygen atoms. To balance the
  oxygen atoms, multiply Fe2O3 by the coefficient
  of 2 and the O2 by the coefficient of 3
• Fe 3O2 ? 2Fe2O3
• Step 2 Balance the Fe by placing the coefficient
  of 4 in front of Fe
• 4Fe 3O2 ? 2Fe2O3
• Step 3 Check that all the atoms balance and make
  sure that all coefficients are in the
  lowest-possible ratio.

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• Balancing chemical equations using the even
  technique.
• If you have an even number of a certain element
  on one side of the equation and an odd number of
  the same element on the other side of the
  equation, multiply both sides of the equation
  through by the coefficient of 2. This will give
  an even number on both sides and make the
  equation easier to balance.
•  Example
• Balance the equation
• CH3OH O2 ? H2O CO2
• Step 1 Using the CHO technique, we start with
  carbon, one on each side, so carbon is balanced.
  There are four H on the left and two on the
  right, so we place the coefficient of 2 in front
  of the H2O on the right
• CH3OH O2 ? 2H2O CO2
• Step 2 When we try to balance the oxygen, we
  find three on the left and four on the right. We
  multiply both sides of the equation through by
  two.
• 2CH3OH 2O2 ? 4H2O 2CO2
• Step 3 The C and H are still balanced, and now
  there are six O on the left and eight on the
  right. Change the coefficient in front of the O2
  to 3 to give eight O on the left.
• 2CH3OH 3O2 ? 4H2O 2CO2
• Step 4 Check that all the atoms balance and make
  sure that all coefficients are in the
  lowest-possible ratio.

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•  Balancing chemical equations using the CHO
  technique.
• If you are balancing equations that have carbon,
  hydrogen and oxygen atoms then balance the carbon
  atoms first, the hydrogen atoms second and the
  oxygen atoms last.
• Example
• Balance the chemical equation
• C5H12 O2 ? CO2 H2O
• Step 1 Start with C. To balance the C put the
  coefficient of 5 for CO2
• C5H12 O2 ? 5CO2 H2O
• Step 2 Then, balance the H by placing the
  coefficient of 6 for H2O
• C5H12 O2 ? 5CO2 6H2O
• Step 3 Lastly, balance the O by placing the
  coefficient of 8 for O2
• C5H12 8O2 ? 5CO2 6H2O
• Step 4 Check that all the atoms balance and make
  sure that all coefficients are in the
  lowest-possible ratio.

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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! Two hydrogen atoms from
a hydrogen molecule (H2) combines with one of the
oxygen atoms from an oxygen molecule (O2) to form
H2O. Then, the remaining oxygen atom combines
with two more hydrogen atoms (from another H2
molecule) to make a second H2O molecule.
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Balancing Equations
2
3

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

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Balancing Equations

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

____B4H10(g) _____ O2(g)
----gt ___ B2O3(g) _____ H2O(g)
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Balancing Equations

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

Na3PO4 Fe2O3 ----gt
Na2O FePO4