Chemical Formulas and Equations

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Chemical Formulas and Equations
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Physical or Chemical Changes Matter can undergo
two kinds of changes

• Physical changes in a substance only affect
  physical properties such as size, shape or
  whether a solid, liquid or a gas

• Chemical changes
• Produce new substances that have properties
  different form those of the original substances

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Chemical Changes

• The rust on a bikes handlebars, for example, has
  properties different from those of the metal
  around it.
• A process that produces chemical change is a
  Chemical Reaction.
• Folding a newspaper in half- C or P
• Lighting a fire- C or P

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Chemical Equations

• To describe a chemical reaction you must know
  which substances react and which are produced
• The substances that react are called REACTANTS
• The substances that are produced are called
  PRODUCTS
• Baking Soda Vinegar Chemical or Physical

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Chemical Rxn cont.

• The bubbles that form produces a gas and
  something else. What goes on in a chemical rxn is
  more than what you can see with your eyes.
• Chemists are interested in which reactants are
  used and the products that are produced.
• To do this, a shorthand way of indicating what
  happens is called a chemical equation.
• C12H22O11 H2O
  CO2

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Chemical Equations

• The Law of Conservation of Matter states that
  nothing is created or destroyed. Therefore, the
  mass of the chemical reactants must be the same
  mass as the products
• Ex Nacl -------- NaCl2 is are the
  elements ?
• When you write the chemical equation you must
  observe the law of conservation of matter,
  sometimes this easy
• Count the of atoms of each type in the
  reactants and the products

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Balancing Chemical Equations

• Ag H2S -------- Ag2 S H2
• 2 Ag H2S ------- Ag2 H2
• Any Questions? What do you think?
• Complete the following exercise

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Energy in Chemicals

• Often, energy is released or absorbed during a
  chemical rxn. The energy for the welding torch is
  released when H and O combine to form water 2H2
  O2 ---- 2H2O energy.
• This energy comes from chemical bonds that break
  to form when atoms gain, lose or share electrons
  in the reactants and new bonds form in the
  product

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Energy in chemical rxn

• In rxns that release energy, the products are
  more stable and their bonds have less energy than
  those of the reactants. The extra energy is
  released in other forms such as light, sound and
  heat
• When energy is absorbed, the reactants are more
  stable and their bonds have less energy than
  those of the products
• 2H2 O2 energy--- 2H2 O2.

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Energy of Rxns

• Endothermic rxns occur when there is an
  absorption of heat energy
• Ex Heating a home, cold pack (AlNO3 H20)
• Exothermic rxns occur when there is a release of
  heat energy
• Ex Burning materials, skid marks from an
  accident

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Activation Energy.

• Before a rxn can start, molecules of the reactant
  must bump into each other or collide
• This means that there must be a strong collision
  that will cause the molecules to speed up
• The amount of energy needed to speed up a
  chemical rxn is called the Activation Energy
• Ex Burning of gasoline.. Thats why signs are
  posted at gas stations with warnings!!

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Reaction Rate

• The rate of Rxn tells how fast a rxn occurs. To
  find the rate of a rxn, you can measure either
  how quickly one of the reactants is disappearing
  or how quickly one of the products is appearing
• Factors that affect Reaction rate are
• Temperature changes, Raising Temp, Concentration
  of molecules and particle size

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Energy in Chemicals

• Factors that slow down chemical rxns
• Substances that slow down rxn are called
  inhibitors.
• Inhibitors do not completely stop a rxn, they
  simply slow down its progression of the product.
• Substances that speed up a rxn are called
  catalyst. It is used to produce the same amount
  of product faster

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Enzymes

• Enzymes are substances that can speed a rxn or
  slow down a rxn.
• Enzymes make it possible for your body to
  function. All enzymes are proteins, but not all
  proteins are enzymes.
• An enzyme exists to carry out each type or rxn in
  your body.
• Other uses they contain proteases found in meat
  tenderizers, and contact lens

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Homework

• Complete section assessment 1, pg46 (1-6).
• Complete Physical/chemical Changes activity
  handout in 6.1 pH packet
• All packets are due 9/23/2010