Chemical Changes

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Chemical Changes

• Physical changes (Ch. 5) involve only changes in
  the physical form of the substance, not in the
  atomic or molecular make-up
• Chemical changes involve conversion into new
  substances with new chemical properties
• Chemical changes can often be observed
• Color change, precipitate (solid forms), bubbles,
  etc.
• In a chemical reaction, reactants go to products
• Atoms of reactants are recombined in products
• 2Ag S ? Ag2S
• (silver reacts with sulfur to form silver
  sulfide)

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Chemical Equations

• Used to represent chemical reactions
• Like a recipe
• - Tells what you need to start with, and how
  much
• - Also tells what you will make, and how much
• Example 2H2 O2 ? 2H2O
• Number of each type of atom must be equal on the
  two sides of the equation
• 4 Hs 2 Os 4 Hs 2 Os
• Use coefficients to balance chemical equations
• Sometimes symbols are used to show physical
  state
• (s) solid, (l) liquid, (g) gas and (aq)
  aqueous (in water)
• Example C(s) O2(g) ? CO2(g)

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Balancing a Chemical Equation

• Write correct formulas for reactants
• Count atoms on both sides (is it balanced?)
• Balance one element at a time (usually C first
  and O or H last, but can be any order)
• Count atoms again to check that its balanced
• Example Propane (C3H8) burns with oxygen to
  form carbon dioxide and water. Write the
  balanced chemical equation.
• C3H8 O2 ? CO2 H2O
• 3 Cs 8 Hs 2 Os ? 1 C 2 Hs
  3 Os
• C3H8 O2 ? 3CO2 H2O
• C3H8 O2 ? 3CO2 4H
• C3H8 5O2 ? 3CO2 4H2O
• 3 Cs 8 Hs 10 Os 3 Cs 8 Hs
  10 Os

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Types of Reactions

• Reactions can be organized into 4 basic types
  combination, decomposition, replacement and
  combustion
• - Combination reactions 2 (or more) reactants
  combine to form a single product
• - Decomposition reactions One reactant splits
  into 2 (or more) products
• - Replacement reactions Elements are exchanged
  between 2 reactants to form 2 products
• - Combustion reactions fuel oxygen ?
  products heat
• Reactions can be more than one type

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Oxidation-Reduction (Redox) Reactions

• Some reactions are also categorized as redox
  reactions
• In these reactions the reactants exchange
  electrons
• - Reduction gain of electrons (GER)
• - Oxidation loss of electrons (LEO)
• Oxidation and reductions reactions are always
  coupled
• (electrons gained electrons lost)
• Example Mg(s) 2HCl(aq) ? MgCl2(aq)
  H2(g)
• - Mg loses 2 electrons to become Mg2 (Mg is
  oxidized)
• - Each Cl gains an electron to become Cl- (Cl is
  reduced)
• - H is not oxidized or reduced (no change in
  of electrons)
• Also, in general, gain of O or loss of H
  oxidation and gain of H or loss of O reduction
  (in biological systems)

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Energy in Chemical Reactions

• In order for a reaction to take place, the
  reactants must contact each other with enough
  energy
• As reactants collide, bonds are broken and new
  bonds are formed
• Example 2H2 O2 ? H2O (the H-H and O-O
  bonds break and two new O-H bonds are formed)
• Between the reactants and the products there is a
  transition state in which bonds are breaking
  and/or forming (highest E point in reaction)
• Energy required to reach transition state is
  called activation energy (EA)
• Transition state is always highest E (higher than
  reactants and products) because it takes E to
  break bonds and E is released when bonds are
  formed

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Exothermic and Endothermic Reactions

• The difference in energy between the reactants
  and the products is called the heat of reaction
• Heat of reaction can be heat released or heat
  consumed, depending on the reaction
• Reactions that release heat are exothermic
• CH4 2O2 ? CO2 2H2O heat (213 kcal)
• Reactions that consume heat are endothermic
• H2 I2 heat (12 kcal) ? 2HI
• Energy diagrams are used to show energy changes
  during a chemical reaction (E vs. reaction
  progress)

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Reaction Rates

• Reaction rate how fast a reaction goes from
  reactants to products
• Rate is based on activation energy and not on
  heat of reaction (lower EA faster reaction)
• Reaction rates are affected by such factors as
• - Reactant concentration (more reactants
  more collisions faster reaction)
• - Temperature (at higher T reactants collide
  more often at higher E faster reaction)
• - Catalyst (addition of a catalyst lowers the
  activation energy faster reaction)
• - a catalyst makes the transition state more
  stable, so it takes less EA to reach it

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Chemical Equilibrium

• Some chemical reactions are reversible (products
  can also go to reactants)
• Example N2(g) O2(g) ? 2NO(g)
• - Forward reaction N2 O2 ? 2NO
• - Reverse reaction 2NO ? N2 O2
• When rate of forward reaction rate of reverse
  reaction, chemical equilibrium has been reached
• When at equilibrium
• - If more products exist in reaction mixture,
  then reaction favors products
• - If more reactants exist in reaction mixture,
  then reaction favors reactants

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LeChâteliers Principle

• The equilibrium can be shifted towards more
  products or more reactants by placing a stress
  on the system
• Add reactants or remove products and equilibrium
  is shifted towards products
• Add products or remove reactants and equilibrium
  is shifted towards reactants
• Heat is also considered a reactant (endothermic
  reactions) or a product (exothermic reactions)
• Example C(s) H2O(g) heat ? CO(g)
  H2(g)
• - Add heat equilibrium shifts towards products
• - Remove H2(g) equilibrium shifts towards
  products
• - Remove H2O(g) equilibrium shifts towards
  reactants