6.3 Nature of Reactions

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6.3 Nature of Reactions

• Objectives
• Demonstrate factors that influence the directions
  of a reaction.
• Classify factors that influence the rate of a
  reaction.

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Reversible Reactions

• Many reactions can change direction.
• These reactions are called reversible reactions.
• They are reversible based on energy flow.
• However, not all reactions are reversible
• Formation of caves, drying of paint and the
  burning of fuel.

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Automobile Battery

• Pb(s) PbO2(s) 2 H2SO4(aq) 2 PbSO4 (s)
  2 H2O(l) energy
• When a car isnt running the battery releases the
  energy as it moves to the right.
• If you need to jump start the battery the
  reaction moves to the left while the car engine
  runs

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Equilibrium

• Equilibrium term for a system where no net
  change occurs in the amount of reactants or
  products
• Dynamic Equilibrium term describing a system in
  which opposite reactions are taking place at the
  same rate.

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Reactions at Equilibrium

• Reactants are never used up, b/c they are
  constantly being formed by the products
• Reactants and products are being formed at the
  same rate.
• Example-Lime
• CaCO3(s) CaO(s) CO2(g)
• Reactions at equilibrium do not necessarily have
  the same amount of reactants and products

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Changing Direction

• Le Chateliers principle-disturbing an
  equilibrium will make a system readjust to reduce
  the disturbance and regain equilibrium.
• Chemical engineers apply this to obtain a larger
  product yield
• They will remove some product so the reaction
  will produce additional product in order to reach
  equilibrium.

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CaCO3(s) CaO(s) CO2(g)

• If carbon dioxide is removed as it is produced
  the reaction will produce more to achieve
  equilibrium
• When one produce is a gas and the other products
  and reactants are not, it is easy to see how the
  gas can be removed
• A product that does not dissolve in water can be
  removed if all other products and reactants
  dissolves in water.
• Soluble-it dissolves in a liquid
• Insoluble-does not dissolve in a liquid
• An insoluble product will form a precipitate and
  sink to the bottom

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Adding Reactants or Energy

• Adding more reactants has the same effect as
  removing products
• Adding or removing energy (usually as heat) can
  also influence the direction of a reaction
• Because energy is part of any reaction it can
  thought of as either a reactant or a product

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3 C 2 Al2O3(s) energy ?? 4 Al(l) 3 CO2(g)

• If more energy is added then the reaction goes to
  the right forming more aluminum and carbon dioxide

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N2(g) 3 H2 (g) ?? 2 NH3(g) energy

• Adding energy to the right will cause the
  reaction to move to the left
• This will produce more nitrogen and hydrogen

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Reaction Rate

• The rate at which a reaction occurs
• Activation Energy the amount of energy the
  particles in a reaction must have when they
  collide for the reaction to occur
• Reaction can be sped up or slowed down.

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Temperature

• Temperature can speed up or slow down reactions
• A rise in temperature caused the baking of a cake
  to speed up
• Decreasing temperature can slow down a reaction
  such as keeping film cool so that film has a
  better self life

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Concentration

• Concentration the amount of a substance present
  in a unit volume
• Concentration also effects the reaction rate
• A rise in concentration will increase the rate of
  a reaction
• A decrease in concentration will slow down the
  reaction

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Limiting Reactant

• Limiting Reactant the reactant of which there
  is not enough when it is used up, the reaction
  stops and no new product is formed
• Limiting reactants control the amount of product
  that a reaction produces, once it is used up the
  reaction ceases

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Catalysts

• Catalysts- a substance that speeds up the rate of
  a reaction without being permanently changed or
  used up itself
• Catalysts do not change the position of
  equilibrium and therefore does not affect how
  much product is formed
• Enzymes biological catalysts
• Enzymes are involved in almost every process in a
  cell
• Figure 6.22 page 222 Jello and Pineapple

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Slowing Down Reactions

• Inhibitor a substance that slows down a
  reaction
• Inhibitors do not completely stop a reaction
• An inhibitor is placed in bottles of hydrogen
  peroxide to prevent the decomposition

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Review

• Define
• Equilibrium
• Dynamic Equilibrium
• Activation Energy
• What does a catalyst do to a reaction?
• What does a inhibitor do to a reaction?

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Homework

• Page 223 1-3
• Page 225 7
• Extra Credit
• Read page 216-217 and answer 1-2
• Read page 221 and answer 1-2