Physical and Chemical Changes

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Chapter 20 and 21

• Physical and Chemical Changes
• Balancing Equations

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Physical Properties

• Characteristic of a material that can be observed
  without changing the identity of its substances
• Density is unchanged
• Ratio of substances is unchanged

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• Appearance color, shape, state of matter
• Measurable size, mass, volume, temperature,
  odor, density
• Behavior magnetism, melting or boiling point

4
Evidence of a Physical Change

• Any change in the above characteristics that does
  not change the actual substance
• Examples evaporation, crushing

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Chemical Properties

• Characteristic of a substance that tells whether
  it can go through a certain chemical change
• Flammable, combustible, photosensitive

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Evidence of a Chemical Change

• Substance changes its identity
• Change in odor
• bubbling./foaming without heat source
• Giving off heat, light or sound
• Formation of a precipitate
• Solid particles form in a solution and settle out
• Homogeneous to heterogeneous
• Change in temperature or color
• EX burning, rusting, digestion, respiration

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Molecular Difference

• Physical change change in molecular movement or
  arrangement
• Chemical change change in molecular bonding
  (atomic arrangement)
• What elements make up a material and how they are
  combined determines what physical and chemical
  properties it will have and what changes it can
  undergo

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Weathering

• Physical
• Dunes shifting
• Waves cutting into cliffs
• Erosion of river banks
• Chemical
• Acid rain
• Metals tarnish faster

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Definitions

• Chemical Change A change in a substance that
  involves the breaking and reforming of chemical
  bonds to make a new substance or substances.
  Change can not be undone.
• Physical Change Change in the physical
  properties of a substance. Change can be undone.

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Chemical Reactions

• Process that led to the chemical change
• Break chemical bonds in one or more substance and
  reform to make new ones
• Verbal description possible
• Now called a chemical equation
• State of matter for each substance is usually
  indicated

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• EX Na (s) Cl2 (g) ? NaCl (s)
• Says that some solid sodium has combined with
  some chlorine gas to form sodium chloride (table
  salt)
• Na and Cl are the REACTANTS
• NaCl is the PRODUCT

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Conservation of Mass

• States that there are the same number of atoms of
  each element AFTER the reaction as BEFORE it!
• Matter is neither created nor destroyed
• Neither process results in a change in number of
  total atoms!
• Means that we need to balance equations
• Use coefficients, not subscripts!
• Gives ratio of compounds in reaction
• EX 2Na Cl2 ? 2NaCl

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Rules for Balancing

• (1) Identify all elements involved in the
  equation.
• (2) Compare total number of atoms of each element
  on both sides of the equation (reactants vs.
  products)
• (3) Conservation of Mass is a must
• Choose coefficients that will make all atoms
  equal
• Finding least common multiple
• NEVER change the subscripts!
• Changes the identity of the compound
• Coefficients only change quantity

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Hints for Balancing

• If the same element is in more than one formula
  on the same side of the equation, add
  coefficients to it LAST.
• If there is an even number of an element on one
  side of the equation and an odd number on the
  other, try addiing the coefficient 2
• Practice!

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Practice Balancing Equations

• 2K Br2 ? 2KBr
• P4 5O2 ? 2P2O5
• 3Mg N2 ? Mg3N2
• 4Al 3O2 ? 2Al2O3
• PCl54H2O ? 5HCl H3PO4
• 4H3PO3 ? 3H3PO4 PH3

• K Br2 ? KBr
• P4 O2 ? P2O5
• Mg N2 ? Mg3N2
• Al O2 ? Al2O3
• PCl5H2O ? HCl H3PO4
• H3PO3 ? H3PO4 PH3

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Conservation of Mass in terms of grams of
material

• Same number of grams on each side of the equation
• Use algebra to solve for missing quantity
• Ex if I have 12.5g of reactants that completely
  react, what mass of products can I expect to form?

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Example

• 890g of fat reacts totally with 120g of NaOH to
  form 92g glycerin and some soap. How many grams
  of soap can I expect to form?
• Write out the chemical equation
• Write the quantity of each compound above its
  formula
• Use a variable for the unknown
• Use these numbers to form a mathematical equation
  and solve

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What do Equations tell you?

• Ratio of substances involved in each reaction
• Ratio of relative masses
• Conservation of mass and atoms
• State of matter for each substance

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What do equations NOT tell you?

• If the reaction will even occur
• How fast the reaction occurs
• Conditions necessary for reaction to occur
• Activation energy
• Smallest amount of energy needed to start a
  chemical reaction
• Varies from reaction to reaction
• Several potential sources
• Light, electricity, direct heat

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Other Factors

• Limiting Reactants
• Used up first in a chemical reaction
• Usually the more expensive substance
• Excess Reactants
• Substance not completely used up during the
  reaction
• Usually the cheaper substance

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Yield

• Percent Yield usually the amount of product
  formed is less than what you expected
• Predicted yield can be found from the balanced
  equation
• Actual yield can be found by measuring the
  product actually recovered at the end of the
  reaction
• Percent yield can then be found using
• Percent Yield (actual/predicted) X 100

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Classifying Reactions

• There are five major categories of reactions
• Each has a specific way that atoms rearrange
  themselves during the reaction
• MUST memorize their general pattern

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Addition Reaction

• A B ? AB
• Also called synthesis reaction
• Two or more substances combine to form one new
  substance
• Ex S8(s) 8Cu(s) ? 8CuS(s)

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Polymers- Addition RXN

• Polymers are a large molecule composed of many
  smaller, repeating molecules
• Monomer monomer ? polymer monomer
• Ex glucose glucose ? cellulose glucose
• Polystyrene (foam cups), polyester, and polyvinyl
  acetate (chewing gum)

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Decomposition Reaction

• AB ? A B
• A substance breaks down into two or more simpler
  substances
• Reverse of addition reaction
• Most require heat, light or electricity
• Ex H2CO3 ? CO2 H2O

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Single-Displacement Reaction

• A BC ? AC B
• A more active element replaces a lesser active
  element
• Does not have to have the same oxidation number
• Clue presence of a different metal compound on
  both sides of the equation
• Ex Cu(cr) AgNO3(aq) ? Cu(NO3)2(aq) Ag(cr)

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• To remove tarnish, react with coating of Zinc
  (more active)
• ZnS forms and can be wiped off
• Leaves original metal behind
• Used on the Statue of Liberty

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Double-Displacement Reaction

• AB CD ? AD CB
• Two elements from two different compounds swap
  places with each other to form two new compounds
• Usually aqueous compounds
• Look for a precipitate, gas or water
• IMPORTANT the positive oxidation number element
  always swaps with the positive oxidation number
  element!
• Ex Pb(NO2)2(aq) NaI(aq) ? PbI(cr) NaNO3(aq)

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Combustion Reaction

• Carbon Compound O2 ? CO2 H2O
• Releases a large amount of energy (light or heat)
• Primary use of hydrocarbons is as a fuel resource
• Non-renewable
• Ex C3H8(g) 5O2(g) ? 3CO2(g) 4H2O(l)

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Energy in Reactions

• During every chemical reaction, energy is both
  absorbed and released
• Chemical bonds need to be broken
• Uses energy
• New chemical bonds need to be formed
• Releases energy
• Net change in energy is decided by which of the
  these two is greater

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Exothermic Reactions

• Includes MOST reactions
• Gives off more energy than it uses
• Net release of energy
• Usually given off as heat or light
• If occurs slowly, may not be able to tell by
  touch that heat is given off
• Ex burning wood, explosions, rusting, furnaces,
  car engines, cooking, light sticks, hot packs
• Usually occur spontaneously

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Endothermic Reactions

• Net absorption of energy
• Uses more energy to occur than it gives off
  afterward
• Indicates compounds are very stable
• Do not occur spontaneously
• May occur too slow to feel a decrease in
  temperature
• Ex separating a metal from its ore, cold packs

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Two Ways to control chemical Reactions

• Catalyst used to speed up a reaction so its more
  useful or occurs at a lower temperature
• Is not, itself, permanently changed during
  reaction
• Complete reaction with same quantity
• Ex enzymes

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• Inhibitor helps to prevent or slow a reaction
• Actually binds with one of the reactants
• Quantity decreases as reaction progresses
• Ex food preservatives