Chemical Equations

1
Chemical Equations Reactions

• Chapter 8

2
Chemical Reactions

• A chemical reaction is a process by which one or
  more substances is changed into one more
  different substances
• Reactants original substances
• Products resulting substances

3
Indications of a chemical reaction

• Evolution of heat light
• Production of a gas
• Formation of a precipitate
• Precipitate a solid that is produced as a result
  of a chemical reaction

4
Chemical equation

• Describes a chemical reaction
• Represents with symbols formulas the identities
  relative amounts of the reactants products

5
Characteristics of a chemical equation

• Must represent known facts
• Must contain the correct formulas
• Law of conservation of mass must be satisfied
• The same number of atoms of each element must
  appear on each side
• To equalize number of atoms, coefficients are
  added
• Placing a coefficient in front of a formula
  specifies the number of moles of a substance

6
Equations

• Word equation an equation in which the reactants
  products in a chemical reaction are represented
  by words
• Only qualitative meaning

• Formula equation represents the reactants
  products by their symbols or formulas
• Must be balanced!

7
ProblemWrite word formula equations for the
chemical reaction that occurs when solid sodium
oxide is added to water at room temperature and
forms sodium hydroxide.
8
Significance of a chemical equation

• Coefficients indicate relative, not absolute,
  amounts of reactants products as moles
• The relative masses of the reactants products
  of a chemical reaction can be determined from the
  reactions coefficients.
• The reverse reaction for a chemical equation has
  the same relative amounts of substances as the
  forward reaction.

9
Contd

• An equation gives no indication of whether a
  reaction will actually occur.
• An equation can be written for a reaction that
  may not even occur.
• An equation gives no info about the speed of the
  reaction.

10
Balancing equations

• Write a word equation.
• Write a formula equation.
• Balance the formula equation
• Balance atoms that appear only once on each side.
• Balance polyatomic ions that appear on both sides
  as single units.
• Balance the different types of atoms one at a
  time.
• Balance H O atoms after all other atoms are
  balanced.
• Count atoms to make sure the equation is balanced.

11
Common mistakes

• Writing incorrect formulas
• Trying to balance an equation by changing the
  subscripts

12
ProblemThe reaction of zinc with aqueous
hydrochloric acid produces a solution of zinc
chloride hydrogen gas. Write a balanced
chemical equation for the reaction.
13
Balance the equations

• _Al _Fe2O3 ? _Al2O3 _Fe
• _Zn(NO3)2 ? _ZnO _NO2 _O2
• _KClO3 ? _KCl _O2
• _H3PO4 ? _H2O _P4O10
• _C4H10 _O2 ? _CO2 _H2O
• _Cu _AgNO3 ? _Cu(NO3)2 _Ag
• _N2 _H2 ? _NH3

14
Types of chemical reactions
15
(1) Synthesis

• Two or more substances combine to form a new
  compound
• A X ? AX
• Na Cl? NaCl

16
(2) Decomposition

• A single compound breaks down into two or more
  simpler substances
• AX ? A X
• NaCl? Na Cl

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(3) Single-replacement

• One element replaces a similar charged element in
  a compound
• A BX ? AX B
• Na KCl ? K NaCl

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(4) Double-replacement

• The ions of two compounds exchange places in an
  aqueous solution to form two new compounds
• AX BY ? AY BX
• NaCl KBr ? NaBr KCl

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(5) Combustion

• A substance combines with oxygen, releasing a
  large amount of energy in the form of light
  heat.
• A compound reacts with oxygen to form carbon
  dioxide water
• C2H8 O2 ? CO2 H2O

20
Identify the type of reaction

• C3H6O 4O2 ? 3CO2 3H2O
• 2KClO3 ? 2KCl 3O2
• 2CaO MnI4 ? MnO2 2CaI2
• O2 2H2 ? 2H2O
• 5. LiNO3 Ag ? AgNO3 Li