Chemical Equations and Reactions

1
Chemical Equations and Reactions

• Section 8.1

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What is a chemical reaction?

• Any change where a new chemical is formed.
• Indications of a chemical reaction
• Change in temperature
• Production of light/color change
• Change in state

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Changes in State

• Not all changes in state indicate a chemical
  reaction. When water melts it is just a physical
  change.
• When two liquids are mixed and a solid is formed
  that solid is called a precipitate.

4
What is a chemical equation?

• Chemical equations represent chemical reactions
• They
• Identify reactants and products
• Contain the correct chemical formulas
• Satisfy the Law of Conservation of Mass.
  Coefficients are used to tell how many of each
  chemical is present.

5
Types of chemical equations

• Word Equations
• methane oxygen --gt carbon dioxide water
• Formula Equations
• CH4(g) 2O2(g) --gt CO2(g) 2H2O(g)

6
State Symbols

• State symbols - subscripted after each element or
  compound.
• (s) - Solid
• (l) - Liquid
• (g) - Gas
• (aq) - Aqueous. This means that a substance is
  dissolved in water.

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Other Symbols

• Yields sign - arrow that separates reactants from
  products.
• Triangle above yields sign - reactants must be
  heated.
• Up arrow- a gas is formed
• Down arrow - a precipitate is formed
• lt--------gt - reversible reaction

8
Law of Conservation of Mass

• Matter cannot be created nor destroyed in a
  chemical reaction so we have to have the same
  number of each element on both sides of the
  equation.
• This is done by balancing the equation with
  coefficients.
• Zn(s) 2HCl(aq) --gt H2(g) ZnCl2(aq)

9
Lets balance some equations

• The rusting of Iron
• Fe O2 ----gt Fe2O3

4 Fe 3 O2 ----gt 2 Fe2O3

• Combustion of octane in an engine
• C8H18 O2 ----gt H2O CO2

2 C8H18 25 O2 ----gt 18 H2O 16 CO2
10
The Diatomic Elements

• There are seven diatomic elements. They are never
  found as individual atoms but are bonded as a
  pair of atoms
• H2, N2, O2, F2, Cl2, Br2, I2
• You must memorize these!

11
Lets write some equations

• Calcium reacts with sulfur to produce solid
  calcium sulfide.
• Ca (s) S (s) ----gt CaS (s)
• Hydrogen reacts with fluorine to produce hydrogen
  fluoride gas.
• H2 (g) F2 (g) ----gt 2 HF (g)

12
Here is another

• Aluminum reacts with a solution of zinc chloride
  to produce zinc and aqueous aluminum chloride
• 2 Al(s) 3 ZnCl2(aq) ----gt 3 Zn(s) 2 AlCl3(aq)