Chemistry I Notes Chapter 9

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Chemistry I Notes Chapter 9

• Chemical Reactions and Equations

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9-1 The Nature of Chemical Reactions

• A chemical reaction is a process in which one or
  more substances is converted into new substances
  with different chemical and physical properties.
• The substance or substances entering the reaction
  are called reactants.
• The substance or substances made in the reaction
  are products.
• Atoms and compounds undergo reactions to become
  more stable by obtaining a more stable electron
  configuration

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9-2 Chemical Equations

• Chemical equations give information in two major
  areas.
• First, they tell us what substances are reacting
  (those being used up) and what substances are
  products (those being made).
• Second, the coefficients of a balanced equation
  tell us in what ratio the substances react or are
  produced.
• Word equations just give the names of reactants
  and products
• Formula equations contain the chemical symbols
  and formulas
• The reactants are on the left side of a chemical
  equation and the products are on the right side.
• However, you might ask, "On the left and right
  side of what?
• Answer - the arrow. 2 H2 O2 ? 2 H2O
• Be aware that there can be one, two, three, or
  more substances on either side of the arrow, as
  in this more complex equation
• Ca(H2PO4)2 CaSO4 HF ? Ca10F2(PO4)6 H2SO4
• Typically the arrow is replaced with "produces"
  or "yields" when the equation is said out loud.

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Balancing Equations

• Coefficients are the numbers in front of the
  formulas.
• 2 H2 O2 ? 2 H2O
• Note the presence of a two in front of the
  hydrogen and also the water. These are called the
  coefficients. These numbers give two very
  important pieces of information about the
  equation. You must understand both in order to
  read and to use chemical equations successfully.
• First the coefficients give the number of
  molecules (or atoms) involved in the reaction. In
  the example reaction, two molecules of hydrogen
  react with one molecule of oxygen and produce two
  molecules of water.
• Second the coefficients give the number of moles
  of each substance involved in the reaction. In
  the example reaction, two moles of hydrogen react
  with one mole of oxygen and produce two moles of
  water.

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Balancing Equations cont

• For mass to be conserved the number of atoms of
  each element on the reactant side must equal the
  number of atoms of each element on the product
  side
• What goes in must come out!
• Chemical equations do not come already balanced.
  This must be done before the equation can be used
  in a chemically meaningful way.

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Balancing Equations cont

• H2 O2 ? H2O
• It is an unbalanced equation (sometimes also
  called a skeleton equation). This means that
  there are UNEQUAL numbers at least one atom on
  each side of the arrow.
• In the example equation, there are two atoms of
  hydrogen on each side, BUT there are two atoms of
  oxygen on the left side and only one on the right
  side.
• Remember this A balanced equation MUST have
  EQUAL numbers of EACH type of atom on BOTH sides
  of the arrow.
• An equation is balanced by changing coefficients
  in a somewhat trial-and-error fashion. It is
  important to note that only the coefficients can
  be changed, NEVER a subscript.
• The coefficient times the subscript gives the
  total number of atoms.

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Balancing Equations cont

• Two things you CANNOT do when balancing an
  equation.
• 1) You cannot change a subscript.
• You cannot change the oxygen's subscript in water
  from one to two, as in
• H2 O2 ? H2O2
• True, this balances the equation, but you have
  changed the substances in it. H2O2 is a
  completely different substance from H2O.
• 2) You cannot place a coefficient in the middle
  of a formula.
• The coefficient goes at the beginning of a
  formula, not in the middle, as in
• H2 O2 ? H22O
• Water only comes as H2O and you can only use
  whole formula units of it.
• There is another thing you should avoid. Make
  sure that your final set of coefficients are all
  whole numbers with no common factors other than
  one. For example, this equation is balanced
• 4 H2 2 O2 ? 4 H2O
• However, all the coefficients have the common
  factor of two. Divide through to eliminate common
  factors like this..

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Writing Complete Chemical Equations

• Include the state symbols as subscripts in
  parentheses
• (s) solid, (l) liquid, (g) gas, (aq) aqueous
  (water) solution
• In most common reactions the reactants are gases,
  liquids, or aqueous solutions.

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9-3 Types of Chemical Reactions

• Single Replacement - one element replaces another
  element in a compound. There are two different
  possibilities
• One cation replaces another. Written using
  generic symbols, it is
• AX Y ? YX A
• Cu AgNO3 ? Ag Cu(NO3)2Fe Cu(NO3)2 ?
  Fe(NO3)2 CuCa H2O ? Ca(OH)2 H2Zn HCl ?
  ZnCl2 H2
• One anion replaces another. Written using generic
  symbols, it is
• A XY ? XA Y
• Cl2 NaBr ? NaCl Br2Br2 KI ? KBr I2
• In single replacement, one reactant is always an
  element. It does not matter if the element is
  written first or second on the reactant side. The
  other reactant will be a compound

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9-3 Types of Chemical Reactions cont

• Double Replacement - During double replacement,
  the cations and anions of two different compounds
  switch places. In double replacement, both
  reactants are compounds, each with a cation part
  and an anion part. Diatomic elements do not
  count they are included in the single
  replacement category.
• Written using generic symbols, it is AB XY ?
  AY XB
• A and X are the cations (postively-charged ions)
  in this example, with B and Y being the anions
  (negatively-charged ions).
• KOH H2SO4 ? K2SO4 H2O FeS HCl ? FeCl2
  H2S NaCl H2SO4 ? Na2SO4 HCl AgNO3 NaCl ?
  AgCl NaNO3
• These three are also examples of double
  replacement, but there is something special about
  them
• CaCO3 HCl ? CaCl2 CO2 H2O K2SO3 HNO3 ?
  KNO3 SO2 H2O NH4Cl NaOH ? NaCl NH3 H2O

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9-3 Types of Chemical Reactions cont

• Decomposition (analysis)-During decomposition,
  one compound splits apart into two (or more)
  pieces. These pieces can be elements or simpler
  compounds
• Written using generic symbols, it is usually
  shown as AB ? A B
• HgO ? Hg O2 MgCl2 ? Mg Cl2 CaCO3 ? CaO
  CO2 FeS ? Fe S
• Na2CO3 ? Na2O CO2 KClO3 ? KCl
  O2 Ba(ClO3)2 ? BaCl2 O2 H2O ? H2 O2
• Notice how, in every case so far, there is only
  one substance on the left-hand (reactant) side.
  This is always the case in a decomposition
  reaction.

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9-3 Types of Chemical Reactions cont

• Direct Combination (synthesis)-Syntheses are, at
  this introductory level, almost always the
  reverse of a decomposition reaction. That means
  that two pieces join together to produce one,
  more complex compound. These pieces can be
  elements or simpler compounds. Complex simply
  means that the product compound has more atoms
  than the reactant molecules. Usually!!
• Written using generic symbols, it is usually
  shown as A B? AB
• Mg O2 ?MgO H2 O2 ? H2O K Cl2 ? KCl
  Fe O2 ? Fe2O3
• CaO CO2 ? CaCO3 Na2O CO2 ? Na2CO3
  KCl O2 ? KClO3
• Notice how, in every case so far, there is only
  one substance on the right-hand (product) side.
  This is not always the case in a synthesis
  reaction. Sometimes there will be two products.
  Here's an example
• CO2 H2O ? C6H12O6 O2

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9-3 Types of Chemical Reactions cont

• Combustion, at its most general, can mean the
  reaction of oxygen gas (O2) with anything.
• However, we will understand combustion to mean
  the reaction of oxygen with any compound
  containing carbon and hydrogen. A common synonym
  for combustion is burn.
• Written using generic symbols, it is usually
  shown as
• CxHy O2 ? CO2 H2O
• Balance these examples
• CH4 O2 ? CO2 H2O
• C2H6 O2 ? CO2 H2OC6H12O6 O2 ? CO2
  H2OC2H5OH O2 ? CO2 H2O

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Video Clip
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Predicting Products

• Single Replacement - ZnS O2 ?
• (1) Decide if the reactant element (O2 in this
  case) is normally positive or negative. (It is a
  negative 2. Just use O, not O2.)
• (2) Identify the opposite charged (from step one)
  portion of the compound and its charge. (Zn has a
  charge of 2.)
• (3) Write a formula using information from step
  one two.(ZnO since Zn 2 and O -2.)
• (4) Write the left over element as the second
  product. Write it as diatomic if it is. So the
  final answer looks like this ZnS O2 ? ZnO
  S
• Double Replacement - Ca(OH)2 HCl ?
• (1) Identify the cations and anions in each
  compound
• Ca(OH)2 has Ca2 and OH and HCl has H and Cl
  All you have to do is identify each, you need not
  worry about amounts yet.
• (2) Pair up each cation with the anion from the
  OTHER compound
• Ca2 pairs with Cland H pairs with OH (Write
  two new (CORRECT!!) formulas using the pairs from
  step two.
• (3) CaCl2 since Ca is positive 2 and Cl is minus
  one and H2O since H is plus one and OH is
  negative one So the final answer looks like this
  Ca(OH)2 HCl ? CaCl2 H2O

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Predicting Products cont

• Analysis (decomposition) - NaClO3 ?
• Figuring out what the products are in
  decomposition is harder (maybe you'll think it's
  easier!!) because you will have to recognize
  several categories of decomposition reactions.
  Here are your first three
• 1) All binary compounds will break down into
  their elements.2) All carbonates break down to
  the oxide and carbon dioxide.3. Chlorates will
  break down to the binary salt and oxygen.
• (1)Identify the type of compound decomposing
  NaClO3 is a chlorate Notice that you have to be
  able to "read" a formula and identifiy the parts
  (cation and anion) that make it up.
• (2) Apply the rule for that type chlorates
  decompose to the binary salt and oxygen gas
• (3) Write two new (CORRECT!!) formulas using the
  rule from step two. NaCl since Na is positive 1
  and Cl is minus one O2 since oxygen is a
  diatomic gas So the final answer looks like this
  NaClO3 ? NaCl O2

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Predicting Products cont

• Synthesis (direct combination) LiCl O2 ?
• Since synthesis reactions are the reverse of
  decomposition, you might ask if the decomposition
  categories apply, just in reverse. The answer is
  yes!
• 1) Direct union of two elements will produce a
  binary compound.2) Metallic oxides and carbon
  dioxide react to produce carbonates.3. Binary
  salts and oxygen react to produce a chlorate.

(1) Ask yourself what type of decomposition
produces these products LiCl O2 are the
products of a chlorate decomposing. Notice that
you have to be able to "read" a formula and
identifiy the parts (cation and anion) that make
it up. (2) Write the reactant formula using the
compounds from step one. Chlorate is always
ClO3 Li is plus one So the final answer looks
like this LiCl O2 ? LiClO3
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Predicting Products cont

• Combustion-C7H6O O2 ?
• (1) Identify the reaction as combustion A
  carbon-hydrogen compound reacting with oxygen
• (2) Know that the combustion products are always
  CO2 and H2O So the final answer looks like this
• C7H6O O2 ? CO2 H2O
• Now Balance!

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Acid Base Reactions

• Neutralization Acid Base ? Salt Water or
  HCl NaOH ? NaCl H2O
• Here are acid base decomposition reactions
• Ca(OH)2 ? CaO H2O HNO3 ? N2O5 H2O NaOH ?
  Na2O H2O H3PO4 ? P2O5 H2O
• The first two substances are bases and the last
  two are acids. In each case, the acid or base
  breaks down into the oxide of the metal (in the
  case of bases) or the oxide of the nonmetal (in
  the case of acids) plus water.
• Here are acid base synthesis reactions
• CaO H2O ? Ca(OH)2 N2O5 H2O ? HNO3 Na2O
  H2O ? NaOH P2O5 H2O ? H3PO4
• The first two substances are metallic oxides and
  the last two are nonmetallic oxides. In each
  case, the oxide plus water will produce a base
  (in the case of the metallic oxide) or an acid
  (in the case of the nonmetallic oxide).

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HOFBrINCl FriendsMemorize us!

• The Seven Diatomic Substances (plus two friends)
• Hydrogen - H2
• Oxygen - O2
• Fluorine - F2
• Bromine - Br2
• Iodine - I2
• Nitrogen - N2
• Chlorine - Cl2
• In addition to the above, phosphorous is P4 and
  sulfur is S8.

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Video Quiz