Unit 7 Chemical Reactions

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Unit 7 Chemical Reactions

• Chapter 7
• pp. 175 - 200

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What is a chemical reaction?

• It is the process by which atoms of one or more
  substances are rearranged to form NEW different
  substances.
• Consider the following demonstration
• What happens when an antacid tablet is added to
  water?
• Lets take a look

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How can you tell if a chemical reaction has
occurred?

• Temperature change Adding a strong acid to
  water causes a dramatic increase in temp.
• Color change A rusty nail changes from silver
  to orange/brown
• Odor The smell of rotten eggs when you burn
  sulfur in the lab
• Gas bubbles When baking bread, the bread rises
  because of gas production
• Precipitate or formation of a solid A solid
  appears after adding two liquids together

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Chemical Equations

• In every chemical equation there are three parts.
• Reactants
• Yield symbol or ?
• Products
• Reactant 1 Reactant 2 ? Product 1 Product 2
• Sometimes there are symbols that go above the
  Yield symbol. Often times these symbols
  represent catalysts or energy added or released
  from a reaction.

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Symbols in chemical equations

• Physical States
• (s) - solid
• (l) - liquid
• (g) - gas
• (aq) aqueous or in solution

• Symbols above the ?
• Heat heat energy
• ? energy (often heat)
• ? - electricity
• Sy Elemental symbol acting as a catalyst

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An important reminder

• The following elements exist as diatomic
  molecules in their natural state.
• H2
• F2
• Cl2
• Br2
• I2
• O2
• N2

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Representing Chemical Reactions

• Word Equations
• Writing equations in the English language to
  explain the process of a chemical reaction.
• Example
• iron(s) chlorine(g) ? iron(III)chloride(s)
• Solid Iron and gaseous chlorine react to produce
  solid iron(III)chloride.

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Writing Word Equations

• Lets look at another example.
• Sodium(s) Water(l) ? Sodium Hydroxide(aq)
  Hydrogen(g)
• Now you try to write it correctly in English.
• Answer
• Solid sodium and liquid water reacted to form
  aqueous sodium hydroxide and hydrogen gas.

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Writing Skeleton Equations

• Word equations are useful for describing chemical
  reactions, however, they are cumbersome and lack
  important information.
• Lets look at an example of the earlier iron
  reaction
• iron(s) chlorine(g) ? iron(III)chloride(s)
• Skeleton equations use symbols in place of the
  names of atoms and compounds
• Fe(s) Cl2(g) ? FeCl3(s)

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Writing Skeleton Equations

• Lets look at an example
• Carbon(s) Sulfur(s) ? Carbon disulfide(l)
• Now you try to write the skeleton equation
• Answer
• C(s) S(s) ? CS2 (l)

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Practice

• Write the following as Word Equations
• hydrogen(g) bromine(g) ? hydrogen bromide(g)
• carbon monoxide(g) oxygen(g) ? carbon
  dioxide(g)
• Write the following as Skeleton Equations
• Solid barium and oxygen gas react to produce
  solid barium oxide.
• Solid iron and aqueous hydrogen sulfate (sulfuric
  acid) react to produce aqueous iron(III)sulfate
  and gaseous hydrogen.

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Answers

• Hydrogen gas reacted with gaseous bromine
  producing gaseous hydrogen bromide.
• Gaseous carbon monoxide reacted with oxygen gas
  to produce carbon dioxide gas.
• Ba(s) O2(g) ? BaO(s)
• Fe(s) H2SO4(aq) ? Fe2(SO4)3(aq) H2(g)

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Chemical Reactions

• Both of these types of equations are useful, but
  still something is missing
• When a reaction occurs it is important to
  remember the Law of Conservation of Mass.
• Mass (atoms) cannot be created nor destroyed.
  Therefore, the previous types of equations wont
  do for chemical analysis.

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Balanced Chemical Equations

• Lets look at the iron equation one more time.
• Fe(s) Cl2(g) ? FeCl3(s)
• Notice anything strange?
• Where did the extra chlorine atom come from in
  the iron(III)chloride?
• To accurately represent this equation it is
  important to show that the number of atoms in the
  reactants is equal to the atoms of product.

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Balancing Chemical Equations

• Whenever the number of atoms of reactant(s) are
  equal to the number of atoms of product(s) we say
  that the equation is a Balanced Chemical
  Equation.
• How do we get the reactants and products to
  balance out?
• Coefficients (different than subscripts!) Whole
  numbers placed in front of an atom or compound to
  indicate more than one of each.

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Balancing Chemical Equations

• Lets look at the previously discussed iron
  equation when it is balanced
• 2Fe(s) 3Cl2(g) ? 2FeCl3(s)
• Notice that there are 2 atoms of iron and 6 atoms
  of chlorine. (or 3 molecules of chlorine gas)
• The same can be said for the atoms of iron and
  chlorine in the iron(III)chloride compound.

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Steps for balancing equations.

• Step 1 Write the skeleton equation for the
  reaction.
• Step 2 Count the atoms of the elements in the
  reactants.
• Step 3 Count the atoms of the elements in the
  products.
• Step 4 Change the coefficients to make the
  number of atoms of each element equal on both
  sides of the equation.
• Step 5 Write the coefficients in their lowest
  possible ratio.
• Step 6 Check your work.

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Steps for balancing equations example.

• Hydrogen gas reacted with chlorine gas to produce
  hydrogen chloride.
• Step 1 H2(g) Cl2(g) ? HCl(g)
• Step 2 H2 Cl2 (2
  atoms of H) (2 atoms of Cl)
• Step 3 HCl ( 1 atom H 1 atom Cl)
• Step 4 H2(g) Cl2(g) ? 2HCl(g)
  (2 atoms H) (2
  atoms Cl) (2 atoms H
  2 atoms Cl)

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Steps for balancing equations example.

• Step 5 The ratio 1 hydrogen molecule to 1
  chlorine molecule to 2 hydrogen chloride
  molecules (112) is the lowest possible ratio
  because the coefficients cannot be reduced and
  still remain whole numbers.
• Step 6 Make sure the chemical formulas are
  written correctly. Then, check that the number of
  atoms of each element is equal on both sides of
  the equation.
• Thats it!

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OK, maybe thats not all

• Sometimes equations are more complex than others,
  especially when polyatomic ions are involved.
  Here are a couple of extra tips.
• Begin balancing with the most complex formula.
• Balance polyatomic ions as a single unit.

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Practice Time

• Write the balanced chemical equations for the
  following
• Aqueous sodium hydroxide and aqueous calcium
  bromide react to produce solid calcium hydroxide
  and aqueous sodium bromide.
• Liquid carbon disulfide reacts with oxygen gas,
  producing carbon dioxide gas and sulfur dioxide
  gas.

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Answers

• FeCl3(aq) 3NaOH(aq) ? Fe(OH)3(s) 3NaCl(aq)
• CS2(l) 3O2(g) ? CO2(g) 2SO2(g)

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Classifying Chemical Reactions
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Why differentiate?

• When you go to the library, you see many
  different types of books that are organized in a
  way to help you find them
• Fiction, Nonfiction, Mysteries, Biographies,
  History, Fantasy, etc.
• Like books, there are also many different kinds
  of chemical reactions and scientists need a way
  to organize them.

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Types of Chemical Reactions

• Synthesis
• Combustion
• Decomposition
• Single-replacement
• Double-replacement

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Synthesis Reactions

• A chemical reaction in which two or more
  substances react to produce a single product.
• A B ? AB
• Example 1 - 2 elements
• 2Fe(s) 3Cl2(g) ? 2FeCl3(s)
• Example 2 - 2 compounds
• CaO(s) H2O(l) ? Ca(OH)2(s)
• Example 3 - 1 element 1 compound
• 2SO2(g) O2(g) ? 2SO3(g)

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Combustion Reactions

• Oxygen combines with a substance and releases
  energy in the form of heat and light.
• Example 1
• 2H2(g) O2(g) ? 2H2O(g)
• Example 2
• C(s) O2(g) ? CO2(g)
• Notice that both of these reactions are also
  Synthesis reactions as well...

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Combustion contd.

• Not all combustion reactions are also synthesis
  types.
• Sometimes oxygen will react with compounds called
  hydrocarbons that contain carbon and hydrogen.
• In these reactions, such as the one below, carbon
  dioxide and water are produced.
• CH4(g) 2O2(g) ? CO2(g) 2H2O(g)
• This is the same type of reaction that powers a
  car. In that instance octane (C8H18), another
  hydrocarbon, is a reactant, but the products are
  the same.

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Decomposition Reactions

• A reaction in which a single compound breaks down
  into two or more elements or new compounds.
• AB ? A B
• Example
• NH4NO3(s) ? N2O(g) 2H2O(g)
• These types of reactions often require an energy
  source, such as heat, light, or electricity, to
  occur.

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Decomposition contd.

• One of the best examples of decomposition is in
  automobile airbags.
• Airbags are inflated when an electrical signal
  starts a reaction with sodium azide (NaN3)
  pellets within the bag.
• These pellets produce nitrogen gas which quickly
  inflates the bag.
• 2NaN3(s) ? 2Na(s) 3N2(g)

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Single Replacement Reactions

• Atoms of one element replace atoms of another
  element in a compound.
• When studying this type of reaction we must look
  closely at the element that we believe will
  replace another element
• Some metals are more reactive than others.
• If a metal is more reactive than the element it
  is replacing, it will take its place.
• IF a metal is less reactive than the element it
  is replacing, ultimately, it will not replace it.

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Single Replacement Contd.

• How do we know if one element will replace
  another?
• Activity Series of Metals A table indicating
  the general level of reactivity of different
  elements

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Single Replacement

• Example 1
• Mg Zn(NO3)2 ? Mg(NO3)2 Zn
• Example 2
• Mg 2AgNO3 ?Mg(NO3)2 2Ag
• Example 3
• Mg LiNO3 ? No Reaction

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Double Replacement

• An exchange of positive ions between two
  compounds in a chemical reaction
• Generally, these types of reactions occur in
  solution
• Often times, in these types of reactions, one of
  the products will come out of solution as either,
  a gas, a pure liquid, or a solid.

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Double Replacement Contd.

• Example 1 One product is soluble, the other is
  a precipitate.
• Na2S(aq) Cd(NO3)2(aq) ? CdS(s) 2 NaNO3(aq)
• Example 2 One product is a gas that bubbles out
  of the mixture
• 2NaCN(aq) H2SO4(aq) ? 2HCN(g) Na2SO4(aq)
• Example 3 One product is a molecular compound,
  such as water, which separates from the compounds
  in solution.
• Ca(OH)2(aq) 2HCl(aq) ? CaCl2(aq) 2H2O(l)