Chemical Reactions

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Chemical Reactions

• Reactions involve chemical changes in matter
  resulting in new substances being formed.
• Reactions involve rearrangement and exchange of
  atoms to produce new substances.

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Evidence of Chemical Reactions

• Look for evidence of a new substance
• Visual clues (permanent) that indicate a reaction
  has occurred
• color change
• precipitate formation
• solid that forms when liquid solutions are mixed
• gas bubbles
• large energy changes
• emission of light
• Production of flame
• Other indications
• new odor
• whooshing sound from a tube
• permanent new state
• temperature change

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Describing chemical reactions by formulas

• We want to put enough information into the
  formula so that we can understand exactly what
  happens.
• Sodium reacts with water to give off hydrogen and
  produces sodium hydroxide solution.
• Na(s) H2O(l) ? H2(g) NaOH(aq)
• We show that solid sodium reacts with liquid
  water to give off gaseous H2 and aqueous sodium
  hydroxide.

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Chemical Equation

• Shorthand way to describe a reaction
• Provides information about the reaction
• Formulas of the reactants and products
• Gives the states of the reactants and products
• Relative numbers of reactant and product
  molecules that are required
• Can be used to determine amounts of reactants
  used and amounts of products produced in the
  reaction

• Matter cannot be created or destroyed
• Therefore the total mass cannot change
• and the total mass of the reactants will be the
  same as the total mass of the products
• In a chemical reaction, all the atoms present at
  the beginning must still be present at the end of
  the reaction
• if all the atoms are still there, then the mass
  cannot change

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Atoms are conserved in a chemical reaction!
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Law of Conservation of Mass
Na(s) H2O(l) H2(g) NaOH(aq)
The number of each type of element in the
reactants is the same as the number of each type
of element in the products. To achieve this we
need to balance the chemical equation. In
balancing a chemical equation we can change the
number of reactants or products, but never their
chemical composition (identities or formulas)
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Balancing a Chemical Equation
Allowed Balance the equation by adding
coefficients to the left of the chemical
formulas Na(s) H2O(l)
H2(g) 2 NaOH(aq) Not allowed Change a
chemical formula to make an element balance
(balancing H) Na(s) H3O(l)
H2(g) NaOH(aq) Not allowed Add more
chemical species for convenience (balancing all
elements) Na(s) H2O(l) H
H2(g) NaOH(aq)
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Balanced Chemical Equation
2 Na(s) 2 H2O(l) H2(g) 2 NaOH(aq)
Atoms in reactants 2 4 2
Atoms in products 2 4 2
Element Na H O
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Balancing Chemical Equations Formal Approach
To balance a chemical equation, do not change any
molecules or atoms, but change the number of
molecules KClO4 KCl O2 1.
Place a 1 in front of the molecule with the
greatest number of atoms. If two molecules have
the same number of atoms, then choose the
molecule with the greatest number of different
elements (this number may change) 1 KClO4
KCl O2
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Balancing Equations
2. Insert coefficients that balance elements
that are in compounds with more than one
different element 1 KClO4 1
KCl O2 3. Place coefficients in front of all
else to balance remaining elements 1 KClO4
1 KCl 2 O2
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4. Clear fractions, if any, and multiply/divide
by a common denominator. If, for instance, we
get the following balanced equation... 4 KClO4
4 KCl 8 O2 ...then divide by
4 to get... 1 KClO4 1 KCl
2 O2 5. Check to be sure the final equation is
balanced Reactants
Products 1 K, 1 Cl, 4 O 1 K, 1
Cl, 4 O Balanced Chemical Equation KClO4
KCl 2 O2
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Simple, Systematic Approach
Balance the following equation MgCO3 HCl
MgCl2 CO2 H2O
1. In how many compounds does each element
appear? Choose the element with the lowest
number.
Element Number of compounds in equation
Mg 2 C 2 O 3 H 2 Cl 2
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2. After choosing the element that appears in
the fewest number of compounds, balance the
number of atoms of that element (i.e., balance
the number of Mg, C, H, Cl atoms)
1 MgCO3 HCl 1 MgCl2
CO2 H2O 1 MgCO3 HCl
1 MgCl2 1 CO2 H2O 1 MgCO3
2 HCl 1 MgCl2 1 CO2
H2O
Mg
C
H
Note that you can coincidentally balance more
than one element at a time (such as Cl)
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MgCO3 2 HCl MgCl2
CO2 H2O 3. Always check to confirm that the
equation is balanced Reactants
Products 1 Mg, 1 C, 3 O, 2 H, 2 Cl
1 Mg, 2 Cl, 1 C, 3 O, 2 H
Balanced Chemical Equation MgCO3 2 HCl
MgCl2 CO2 H2O
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Balance by Inspection
Cu S Cu2S
1. Determine the required number of compounds by
visually inspecting the chemical equation. This
is often the fastest method for simple equations.
2 Cu S Cu2S
2. Always check to confirm that the equation is
balanced Reactants
Products 2 Cu, 1 S 2
Cu, 1 S
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Now Your Turn
Balance the following chemical equations CO O2
CO2 P4 Cl2 PCl3 P O2
P2O3 C3H8 O2 CO2 H2O Notice that on
at least one reaction, you may be able to balance
the reaction just by inspecting it