Chemical Reactions 6

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Chemical Reactions 6

• Chemical reactions occur when substances undergo
  chemical changes to form new substances.
• Burning (Combustion reactions) is always a
  chemical reaction
• it uses O2 (diatomic oxygen) from the air
  to combine with carbon and hydrogen
• forming carbon dioxide and water

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Is this a chemical reaction?
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Signs of chemical reactions

• Production of a gas
• Change in color
• Change in the substances
• Change in the chemical formulas
• Change in energy levels, for example stored
  chemical energy

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Chemical reactions rearrange atoms

• Reactant a substance that undergoes a chemical
  change
• Product A substance that is the result of a
  chemical change.

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Conservation of mass

• Mass cannot be created or destroyed.
• The total number of atoms of reactants must equal
  the total number of atoms of the product.
• Note the total number of atoms is balanced

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Energy in chemical reactions

• It takes energy to start reactions
• It takes energy to break a chemical bond.
• It takes a spark (energy) to ignite a fuel.
• Energy can be heat, electricity, sound, or light
  or (electromagnetic waves)

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When Bonds are formed

• When bonds form energy is released
• Fuel plus oxygen energy released plus New
  products.
• 2C8H18 25 O2 ? 16CO218H2O energy

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Conservation of energy

• Energy is always conserved in the total system
• Stored chemical energy (fossil fuels stored
  energy) changed to heat or thermal energy during
  the reaction.
• 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.
• The surrounding area temperature increases.
• 2H2O2 ? 2H2O Exothermic

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Endothermic reaction

• A reaction in which energy is transferred to
  reactants usually as heat from the surroundings.
• Electrolysis (apply electrical energy to water )
  
• 2H2O? 2H2O2
• Endothermic

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Photosynthesis

• Photosynthesis needs the suns energy.
• It is endothermic.
• 6CO2 6H2O sunlight ? C6H12O6 6O2

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5-2 Reaction typesSynthesis Reactions

• Synthesis reactions combine substances
• 2Na Cl2 ? 2NaCl
• Simpler compounds or elements become more complex
  compounds or even polymers.
• Photosynthesis is a synthesis reaction giving off
  glucose and oxygen

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Decomposition reactions

• Decomposition reactions break substances apart.
• Electrolysis of water is a simple decomposition
  reaction.
• Decomposition reactions take energy.

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Electrolysis of water
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Combustion reactions

• Combustion reactions use oxygen as a reactant.
• Oxygen will be diatomic oxygen O2.
• Products if there is adequate oxygen will be CO2
  and H2O.
• if there is inadequate oxygen then possibly CO or
  sometimes even dark sooty carbon will be given
  off.

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Steel will burn in pure oxygen
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Typical burning reactionCombustion reactions

• Methane ( a hydrocarbon) with oxygen ? carbon
  dioxide and water
• 2CH4 4O2? 2CO2 4H2O
• Note reactions on page 192 without sufficient
  oxygen

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Methane gas venting from a landfill
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single-displacement reactions

• single-displacement reactions, elements trade
  place. Generally in a single-displacement
  reaction, a more reactive element will take the
  place of a less reactive element. Example 158
• Potassium takes the place of one hydrogen atom.

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Double displacement reactions

• Double displacement reactions, ions appear to be
  exchanged between compounds.
• Substances that accept electrons are said to be
  reduced. Fluorine etc.
• Substances that give up electrons are said to be
  oxidized.

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Radicals

• Radicals the fragment of molecules that have at
  least one electron available for bonding.
• Radicals used in the making of polymers

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Redox reactions

• A reaction that occurs when electrons are
  transferred from one reactant to another
• Substances that accept electrons are reduced
• Substances that give up electrons are said to be
  oxidized.
• Iron is oxidized because it will give up electrons

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Balancing equations

• 5-3 Balancing equations
• Chemical Equations equation that uses chemical
  formulas and symbols to show the reactants and
  products in a chemical reaction.

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You love word problems

• Verbal representation
• Methane plus diatomic oxygen from the air with
  a spark ignites giving of heat (exothermic) to
  form carbon dioxide gas and water vapor.
• Chemical equation
• CH4 2O2? CO2 2 H2O

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CH4 2O2? CO2 H2O

• This does not tell you the quantities needed for
  the reaction. It is also impossible due to the
  conservation of mass.
• We must balance the equation.
• When the number of atoms of each element on the
  right-hand side of the equation matches the
  number of atoms of each element on the left, then
  the chemical equation is said to be balanced.

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Dont

• You can not change the chemical formula.
  Subscripts are left alone.
• Math skills page 202

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Do

• Identify the reactants and products
• Write a word equation of the reaction
• Write the equation using formulas for the
  elements and compounds in the word equation
• Balance the equation one element at a time. Look
  for odd even problems. Make a table of each
  element with the number of atoms on each side
• Check your answers

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Burning reactions

• Remember that if given a fuel and adequate oxygen
  that the
• products will be thermal energy and
• carbon dioxide and water vapor.

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Mole ratios

• 2Mg O2 ? 2MgO
• 2 moles of magnesium can react with one mole of
  diatomic oxygen to create two moles of magnesium
  oxide.
• 2(24.3) grams of magnesium 2(16.0)grams of
  oxygn must equal
• 2(24.316.0) grams per mole of magnesium oxide
• Page 167

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Conservation of mass and energy

• conservation of mass applies to moles as well as
  to balancing equations.

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The law of definite proportions

• The law of definite proportions A compound
  always contains the same elements in the same
  proportions, regardless of how the compound is
  made or how much of the compound is formed.

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5.4 Rates of change

• Factors affecting reaction rates
• Size of particles
• well mixed
• temperature
• Large surface area
• Concentrated solutions
• higher pressure
• smaller molecules

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Catalysts

• Catalyst
• A substance that changes the rate of
  chemical reactions without being consumed
• Inhibitors slow reactions down
• Platinum is used in your cars exhuast as a
  positive catalyst

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Catalytic converter on your car
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Enzymes

• Enzymes are proteins that are catalysts for
  chemical reactions in living things.
• Most enzymes are fragile and temperature
  sensitive
• Table 5-1 lists common enzymes. You would not
  live without them
• One minute one molecule of catalyze can catalyze
  the decomposition of 6 million molecules of
  hydrogen peroxide

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Equilibrium Systems

• Reversible reactions
• Example CO2 dissolves increase pressure more
  dissolves
• Open the bottle decreases CO2 quantity
• Temperature reduced increases the solubility of
  the gas.

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Small changes

• Systems in equilibrium respond to minimize change
• Page 174 reverse of limestone to lime
• Le Chateliers principle If a change is made to
  a system in chemical equilibrium, the equilibrium
  shifts to oppose the change until a new
  equilibrium is reached.

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Entropy

• A measure of the disorder or randomness in a
  closed system.
• The tendency for all matter and energy in the
  universe to evolve toward a state of
• inert uniformity.