Chemical Reactions

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Chapter 7

• Chemical Reactions

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Sec 7.1

• The Nature of Chemical Reactions.

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Chemical reaction

• The process by which the atoms of one or more
  substances are rearranged to form different
  substances.

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Combustion of methane
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Reactants

• Starting substances.

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Where are the reactants?
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Products

• Substances formed during a chemical reaction.

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Where are the products?
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Chemical equations

• Represent chemical reactions.

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Write the total number of atoms present

• 4CO2 ( )
• 6H2O ( )
• 2NaHSO4 ( )

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Evidence that a chemical reaction has occurred

• Color change.
• Formation of gas.
• A solid precipitates.
• Temperature change.
• Odor.

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Energy change
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Formation of gas
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Rust
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Tarnish
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Rot on fruits
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Odor
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Chemical energy

• The energy stored within atoms and molecules that
  can be released when a substance reacts.

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Exothermic reaction

• A reaction that transfers energy from the
  reactants to the surroundings usually as heat.

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Exothermic
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Endothermic reaction

• A reaction in which energy is transferred to the
  reactants usually as heat from the surroundings.

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Endothermic
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• Photosynthesis is an endothermic reaction.
• Digestion of food is an exothermic reaction.

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Photosynthesis is endothermic
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Cellular respiration is exothermic
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Discussion

• Is burning a candle a chemical reaction?
• melting ice?
• Rusting iron?
• Rubbing a marker on paper?

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• Classify as endothermic or exothermic
• Paper burning.
• A firecracker exploding.
• Water evaporating.

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Sec 7.2

• Balancing Chemical Reactions.

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• A balanced chemical equation follows the law of
  conservation of mass.

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• Unbalanced chemical equation
• CH4 O2 ? CO2 H2O
• Balanced
• CH4 2O2 ? CO2 2H2O
• Balance by adding coefficients.

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• NEVER CHANGE THE SUBSCRIPTS!
• Why not?

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Sec 7.3

• Reaction Types

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Synthesis reaction

• A reaction of at least two substances that form a
  new, more complex compound.
• A B ? AB

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Synthesis reactions

• 2Fe 3Cl2 ? 2FeCl3
• C O2 ? CO2
• CaO H2O ? Ca(OH)2

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Decomposition reaction

• A reaction in which one compound breaks into at
  least two products.
• AB? A B

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Decomposition reactions

• NH4NO3 ? N2O 2H2O
• 2NaN3 ? 2Na 3N2

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Electrolysis

• The decomposition of a compound by an electric
  current.

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Electrolysis of water

• 2H2O ? 2H2 O2

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Combustion reaction

• A reaction in which a compound and oxygen burn.
• Example combustion of methane.

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Combustion of methane
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Single-displacement reaction

• A reaction in which atoms of one element take the
  place of atoms of another element in a compound.

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• A BX ? AX B

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• 3CuCl2 2Al ? 2AlCl3 3Cu
• K H2O ? KOH H2

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Double-displacement reaction

• A reaction in which a gas, a solid precipitate,
  or a molecular compound is formed from the
  apparent exchange of ions between two compounds.

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• AX BY ? AY BX
• Pb(NO3)2 K2CrO4 PbCrO4 2KNO3

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Redox reaction

• A reaction that occurs when electrons are
  transferred from one reactant to another.
• Also known as oxidation/reduction reactions.

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Radicals

• The fragments of molecules that have at least one
  electron available for bonding.

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Sec 7.4

• Rates of Change.

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Factors Affecting Reaction Rates

• High temperatures.
• Large surface area.
• Concentration.
• High pressure.
• Nature of the reactants.
• Catalysts.

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Catalyst

• A substance that changes the rate of chemical
  reactions without being consumed.
• Enzymes are biological catalysts.

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• Catalysts speed up chemical reactions by lowering
  activation energy.

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Catalysts lower activation energy
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Catalase

• An enzyme that breaks down hydrogen peroxide
• 2H2O2? 2H2O O2