Chemical Reactions

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Chemical Reactions

• Chemistry Notes Chapter 10

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Reactions and Equations

• The process by which the atoms of one or more
  substances are rearranged to form different
  substances is called a chemical reaction.
• A chemical reaction is another name for a
  chemical change.
• How can you tell if a chemical reaction has
  taken place?

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Reactions and Equations Cont.

• When a chemical reaction occurs, many signs can
  let you know that one has taken place.
• A change in temperature, in the absence of an
  addition of heat or cold can signify a chemical
  change.
• A permanent change in color like when something
  is burnt can be another sign.
• When smoke appears during an explosion is a clear
  sign of a chemical reaction.

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Representing Chemical Reactions

• Chemical equations are used to represent chemical
  reactions.
• The substances that are present prior to the
  chemical reaction are called reactants.
• The substances that are formed by the reaction
  are called products.

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Representing Chemical Reactions Cont.

• When writing out a chemical equation, the
  products and reactants are separated by an arrow
  that tells the direction of the reaction.
• Reactant 1 Reactant 2 ? Product 1 Product 2
• When the above equation is read the arrow is read
  as yields.

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Representing Chemical Reactions Cont.

• In equations, symbols are used to show the
  physical state of the reactants and products.
• Reactants and products can exist as solids, (s),
  liquids, (l) and gases, (g). They also can be
  dissolved in water, aqueous, (aq).

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Representing Chemical Reactions Cont.

• Word equations can indicate the reactants and
  products of chemical reactions.
• Iron (s) chlorine (g) ? iron (III) chloride
  (g)
• This word equation is read as iron and chlorine
  react to produce iron (III) chloride

                  Iron reacts vigorously with
chlorine gas.
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Representing Chemical Reactions Cont

• Skeleton equations use chemical formulas rather
  than words to identify the reactants and the
  products.
• The skeleton equation for
  Iron (s) chlorine (g) ? iron (III) chloride
  (g) would be, Fe (s) Cl2 (g) ? FeCl3 (g).
• This equation would be read as iron plus chlorine
  yields iron (III) chloride.

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Practice Problems

• Write the skeleton equations for the following
  word equations.
• hydrogen (g) bromine (g) ? hydrogen bromide.
• Carbon monoxide (g) oxygen (g) ? carbon dioxide
  (g)
• Potassium chlorate (s) ? potassium chloride (s)
  oxygen (g)

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Solutions to Practice Problems

• H2 (g) Br2 (g) ? HBr (g)
• CO (g) O2 (g) ? CO2 (g)
• KClO3 (s) ? KCl (s) O2 (g)

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Representing Chemical Reactions Cont

• A skeleton equation does not contain all
  important information about the reaction.
• In order to accurately show what happens in a
  chemical reaction, you must include more
  information.
• Meaning that you must obey scientific laws such
  as the law of conservation of mass.
• So we must balance the reactants with the
  products in order to satisfy this law.

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Representing Chemical Reactions Cont

• A chemical equation is a statement that uses
  chemical formulas to show the identities and
  relative amounts of the substances involved a
  chemical reaction.
• It is a chemical equation that we will use to
  represent chemical reactions.

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Balancing Chemical Equations

• To balance an equation you must find the correct
  coefficients for the chemical formulas in the
  skeleton equation.
• A coefficient in a chemical equation is the
  number written in front of a reactant or product.
  
• Coefficients are usually whole numbers, and are
  usually not written if the value is 1.
• The coefficients in a balance equation describe
  the lowest whole-number ratio of the amounts of
  all the reactants and products.

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Steps For Balancing Equations

• Write the skeleton equation for the reaction.
• Use the correct symbols for the substances.
• Use an arrow to separate the products from the
  reactants.
• Show the physical states of the reactants and the
  products. (solid, liquid, gas, aqueous).

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Steps For Balancing Equations

• Count the atoms of the elements in the reactants.
• a. If a reaction involves identical polyatomic
  ions in the reactants and products, count the
  ions as if they are elements.
• 3. Count the atoms of the elements in the
  products.

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Steps For Balancing Equations

• Change the coefficients to make the number of
  atoms of each element equal on both sides of the
  equation.
• a. Never change a subscript in a chemical
  formula to balance an equation because doing so
  changes the identity of the substance.

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Steps For Balancing Equations

• Write the coefficients in their lowest possible
  ratio.
• The coefficients should be the smallest possible
  whole numbers.
• Check your work.
• Make sure that chemical formulas are written
  correctly.
• Check that the number of atoms of each element is
  equal on both sides of the reaction.

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Law of Conservation of Mass

• All chemical reactions must obey the law of
  conservation of mass.
• Meaning that the mass of the products of a
  chemical reaction is equal to that of the mass of
  the reactants.

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Balance these equations.

• Molten sodium chloride
• produces in the presence
• of electricity sodium and
• chlorine gas.
• 2NaCl (l) ? 2Na (s) Cl2 (g)
• Liquid pentane (C5H12)
• burns, producing water
• vapor and carbon dioxide.
• C5H12(l) 8O2(g)?6H2O(g)
• 5CO2(g)
• Magnesium burns in air to
• form magnesium oxide.
• Mg (s) 02(g) ? 2MgO(s)

• SnS2(s) O2(g) ?SnO2 SO2
• SnS2 (s) _O2 (g)?SnO2 (s) 2SO2
• C2H6(g) O2(g) ? CO2(g) H2O
• 2C2H6(g)7O2 (g) ?4CO2(g)6H2O(g)
• Al (s) HCl (aq) ?AlCl3 (aq) H2(g)
• 2Al(s) 6HCl(aq) ? 2Alcl3(aq)
• 3H2(g)
• CoCO3 (s) ? CoO (s) CO2 (g)
• CoCO3 (s) ?CoO (S) CO2
• When aluminum nitrate and sodium
• hydroxide solutions are mixed, solid
• aluminum hydroxide forms. The other
• product is sodium nitrate.
• Al(NO3)3(aq)3NaOH(aq)?Al(OH)3
• (s) 3NaNO3 (aq)
• When solid copper (II) oxide and
• hydrogen react, metallic copper and
• water form.
• CuO(s) H2 (g) ? Cu(s) _H2O (l)

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Classifying Chemical Reactions

• Synthesis Reactions are reactions in which two or
  more substances react to produce a single
  product.
• So substances A and B combine to produce compound
  AB.
• A B ? AB
• Substances A and B can be single elements or
  compounds.
• Element A plus element B ? Compound AB
• Compound A plus Compound B ? Compound AB
• Compound A plus element B ? Compound AB

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Classifying Chemical Reactions

• In a combustion reaction, oxygen combines with a
  substance and releases energy in the form of heat
  and light.
• In order for a combustion reaction to occur,
  oxygen must be one of the reactants.

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Decomposition Reactions

• A decomposition reaction is one in which a single
  compound breaks down into two or more elements
  or new compounds.
• Decomposition reactions often look like
• AB ? A b
• This type of reaction often requires an energy
  source such as heat, light or electricity, to
  occur.
• For example NH4NO3 (s) ? N2O (g) 2H2O (g)

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Types Of Replacement Reactions

• Singlereplacement reactions are reactions in
  which the atoms of one element replace the atoms
  of another element in a compound
• A BX ? AX B

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Replacement reactions and metals

• A metal will not always replace another metal in
  a compound dissolved in water.
• This is because metals differ in their
  reactivities.
• A metals reactivity is its ability to react with
  another substance.
• The most active metals, which are those that do
  replace the metal in a compound are at the top of
  the list, the least active metals are at the
  bottom of the bottom of the list on page 288.

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Double Replacement Reactions

• A double replacement reaction is a reaction that
  involves an exchange of ions between two
  compounds.
• For example AX BY ? AY BX
• In this generic equation A and B represent
  positively charged ions (cations), and X and Y
  represent negatively charged ions (anions).
• A key characteristic of a double replacement
  reactions is the type of product that is formed
  when the reaction takes place.
• All double replacement reactions produce either a
  precipitate, a gas, or water.

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Precipitate

• When a solid is produced during a chemical
  reaction involving liquids as the reactants, the
  solid that is formed is called a precipitate.

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Guidelines For Double Replacement Reactions

• Step
• Write the components of the reactants in a
  skeleton equation.
• Identify the cations and anions in each compound.
• Pair up each cation with the anion from the other
  compound.
• Write the formulas for the products using the
  pairs from step 3
• Write the complete equation for the reaction
• Balance the equation

• Example
• AL(NO)3 H2SO4
• AL(NO)3 has Al3 and NO3- , H2SO4
• has H and SO42-
• Al 3 pairs with SO42- H pairs with
• NO3-
• Al2(SO4)3 and HNO3
• AL(NO)3 H2SO4 ?Al2(SO4)3
• HNO3
• 2AL(NO)3 3 H2SO4 ?Al2(SO4)3
• 6HNO3

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Predicting Products of Chemical ReactionsClass
of reaction Reactants Probable
products

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Aqueous Solutions

• A solution is a homogeneous mixture.
• A solution contains one or more substances called
  solutes dissolved in water.
• Water would be the solvent, (dissolving agent)
  and it would be the most plentiful substance in
  the solutions.
• An aqueous solution is a solution in which the
  solvent is water.
• Water is always the solvent in aqueous solutions,
  but there can be many solutes.

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Aqueous Solutions Continued

• An ionic equation that shows all of the particles
  in a solution as they realistically exist is
  called a complete ionic equation.
• Ions that do not participate in a reaction are
  called spectator ions and are usually not shown
  in a reaction.
• Ionic equations that include only the particles
  that participate in the reaction are called net
  ionic equations.
• Net ionic equations are written from a complete
  ionic equation by crossing out all spectator
  ions. (see example on page 293

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Test Review

• A reaction in which a compound breaks down into
  two or more elements or new compounds.
• A number written in front of a chemical
  formula.
• A solid produced during a chemical reaction in a
  solution.
• A solution in which water is the solvent.
• A statement that uses chemical formulas to show
  the identities and relative amounts of the
  substances involved in a chemical reaction.
• An equation that shows all of the particles in
  solution as they actually exist.
  
• Substances dissolved in a solution.

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• An equation that includes only the particles that
  participate in a reaction.
• An ion that is present but does not participate
  in a reaction.
• A reaction in which oxygen combines with a
  substance and releases heat and light energy.

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• A reaction in which the atoms of one element
  replace the atoms of another element in a
  compound.
• A reaction involving the exchange of positive
  ions between two compounds dissolved in water.
• The process by which the atoms of one or more
  substances are rearranged to form different
  substances.
• A starting substance in a chemical reaction.
• A substance formed during a chemical reaction.
• A reaction in which two or more substances react
  to produce a single product.
• The most plentiful substance in a solution.

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• The process by which the atoms of one or more
  substances are rearranged to form different
  substances is called a chemical reaction.

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• When a chemical reaction occurs, many signs can
  let you know that one has taken place.
• A change in temperature, in the absence of an
  addition of heat or cold can signify a chemical
  change.
• A permanent change in color like when something
  is burnt can be another sign.
• When smoke appears during an explosion is a clear
  sign of a chemical reaction

38

• Chemical equations are used to represent chemical
  reactions.
• The substances that are present prior to the
  chemical reaction are called reactants.
• The substances that are formed by the reaction
  are called products.

39

• A skeleton equation does not contain all
  important information about the reaction.
• In order to accurately show what happens in a
  chemical reaction, you must include more
  information.
• Meaning that you must obey scientific laws such
  as the law of conservation of mass

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• All chemical reactions must obey the law of
  conservation of mass.
• Meaning that the mass of the products of a
  chemical reaction is equal to that of the mass of
  the reactants.

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• Study charts on pages 290 and 291
• Be able to balance equations and identify the
  type of reaction