Chemical Reactions and Equations Chapter 9

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Chemical Reactionsand EquationsChapter 9
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Write the chemical reaction

• Write the formula equation for the following
• Ammonia reacts with hydrochloric acid to form
  ammonium chloride.

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Learning Objectives

• Be able to balance chemical equations by applying
  the law of conservation of mass.
• Be able to recognize direct combination
  (synthesis), decomposition, single displacement,
  double displacement, combustion and
  neutralization reactions.

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Law of Conservation of Matter

• Conservation of Matter in all chemical and
  physical changes, matter is neither created or
  destroyed
• The total mass in a chemical reaction remains
  constant
• Antoine Lavoisier
• Made accurate and precise measurements during
  chemical reactions

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Chemical Reactions

• A chemical change is really a chemical reaction
• Has 2 parts
• Reactants the substances you start with
• Products the substances you end up with
• The reactants turn into the products
• 4 Fe 3 O2 ? 2 Fe2O3
• Reactants ? products

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In a chemical reaction

• Atoms are rearranged into new molecules
• Can be described several ways
• 1. In a sentence
• Copper reacts with chlorine to form copper (II)
  chloride.
• 2. In a word equation
• Copper chlorine copper (II) chloride
• 3. In chemical formulas
• Cu Cl2 ? CuCl2

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Symbols in equations-

• the arrow ? separates the reactants from the
  products
• (s) after the formula solid
• (g) after the formula gas
• (l) after the formula liquid
• (aq) after the formula - aqueous solution,
  dissolved in water.

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Symbols used in equations

• Double arrow indicates a reversible
  reaction
• shows that
  heat is supplied to the reaction
• is used to indicate a catalyst
  is supplied, in this case, platinum.

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What is a catalyst?

• A substance that speeds up a reaction, without
  being changed or used up by the reaction.
• Enzymes are biological or protein catalysts.

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Balanced Equation

• Atoms cant be created or destroyed
• All the atoms at the beginning must appear in the
  end
• A balanced equation has the same number of atoms
  of each element on both sides of the equation

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O

C
C
O
O
O

• C O2 CO2
• This equation is already balanced
• What do we do when an equation is not balanced?

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O

C
C
O
O

• C O2 CO
• We need one more oxygen in the products.
• Cant change the formula, because it describes
  what the substance is (carbon monoxide in this
  example)

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C
O

O

C
O
C
O

• Must be used to make another CO
• But where did the other C come from?

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C
C
O

O

O
C
O
C

• Must have started with two C
• 2 C O2 2 CO

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Rules for balancing

• Assemble, write the correct formulae for all the
  reactants and products
• Count the number of atoms of each type appearing
  on both sides
• Balance the elements one at a time by adding
  coefficients (the numbers in front) - save H and
  O until last!
• Check to make sure it is balanced.

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Never

• Never change a subscript to balance an equation.
• If you change the formula you are describing a
  different reaction.
• H2O is a different compound than H2O2
• Never put a coefficient in the middle of a
  formula
• 2 NaCl is okay, Na2Cl is not.

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Balancing Equations Examples

• H2 (g) O2 (g) ? H2O (l)
• Zn HCl ? H2 ZnCl2
• Pb (NO3)2 K2S ? PbS KNO3

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Try a couple

• Write the balanced chemical equation for
  dinitrogen pentoxide reacts with water to produce
  nitric acid.

N2O5 H2O ? HNO3
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Try a couple

• Write the balanced chemical equation for
  magnesium reacts with titanium (IV) chloride to
  produce magnesium chloride and titanium.

Mg TiCl4 ? MgCl2 Ti
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Balance the following

• iron(II) chloride sodium phosphate ? sodium
  chloride iron (II) phosphate
• FeCl2 Na3PO4 ? NaCl Fe3(PO4)2

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Balance the following- Combustion Reaction (CO2
and H2O are the products)

• Write a balanced chemical equation for the
  reaction in which benzene (C6H6) reacts with
  oxygen gas to produce carbon dioxide and water.
• C6H6 O2 ? CO2 H2O

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Five General Types of Chemical Reactions

• Direct Combination (Synthesis)
• Decomposition
• Single-Replacement
• Double-Replacement
• Combustion
• By knowing the type of reaction that is
  occurring, you can predict the products that will
  be formed.

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I. Direct Combination Reactions (also called
synthesis reactions).

• General form A B ? AB
• (two reactants make a single product)
• A, B elements or compounds
• AB compound consisting of A and B
• This is the only type of chemical reaction in
  which there is a single product formed. This
  single product is always more complex than the
  reactants.

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Examples of Synthesis Reactions

• calcium oxygen yields calcium oxide 2Ca O2
  ? 2CaO
• carbon dioxide water yields carbonic acid
• CO2 H2O ? H2CO3
• Notice All equations show two (or more)
  reactants, but only one product.
• http//www.ric.edu/ptiskus/reactions/Index.htm

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II. Decomposition Reactions

• General form AB ? A B
• (one reactant makes two or more products)
• AB compound
• A, B elements or simpler compounds
• This is the only type of chemical reaction in
  which there is a single reactant. This single
  reactant is always more complex than the
  products.

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Decomposition Reactions Examples

• water yields hydrogen and oxygen
• 2H2O ? 2H2 O2
• marble (calcium carbonate) yields calcium oxide
  and carbon dioxide
• CaCO3 ? CaO CO2
• Notice all equations show a single reactant
  decomposing into two (or more) products.
• http//www.ric.edu/ptiskus/reactions/Index.htm

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III. Single-Replacement Reactions

• General Form A BX ? AX B
• One element and one compound recombine (switch
  partners)
• AX, BX ionic compounds
• A, B elements
• X ion that switches partners
• This is the only type of chemical reaction where
  a compound reacts with an element to make another
  compound and element.

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Single-Replacement Examples

• magnesium metal and copper (II) sulfate
• Mg CuSO4 ? MgSO4 Cu
• iron metal and copper (II) sulfate)
• Fe CuSO4 ? FeSO4 Cu
• Notice In both reactions, an element combines
  with a compound to create a new compound and a
  different element
• www.ric.edu/ptiskus/reactions/Index.htm

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IV. Double-Replacement Reactions

• General form AX BY ? AY BX
• (Positive ions in two compounds are exchanged)
• A,B positive ions
• X,Y negative ions
• This is the only type of chemical reaction with
  two compounds as reactants and two compounds as
  products.

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Double Replacement Examples

• calcium carbonate and hydrochloric acid yield
  calcium chloride and carbonic acid
• CaCO3 2HCl ? CaCl2 H2CO3
• Notice in this reaction, two ionic compounds
  exchange ions to form two new ionic compounds
• www.ric.edu/ptiskus/reactions/Index.htm

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Rules of Double-Displacement Reactions

• Reactants must be dissolved in water (releasing
  the ions).
• Will occur if one of the products
• is a molecule (covalent),
• a precipitate (solid comes out of solution), or
• an insoluble gas.

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V. Combustion Reactions

• General Form
• CxHy O2 ? H2O CO2
• (hydrocarbon and oxygen react to form carbon
  dioxide and water)
• This is the only type of chemical reaction where
  something reacts with oxygen and forms carbon
  dioxide and water

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Combustion Examples

• Methane reacts with oxygen
• CH4 (methane) 2O2 ? 2H2O CO2
• Gasohol reacts with oxygen
• C2H5OH (gasohol) 3O2 ? 3H2O 2CO2
• Notice in both cases, water and carbon dioxide
  are the products.
• www.ric.edu/ptiskus/reactions/Index.htm

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Convert these to equations

• Solid iron (III) sulfide reacts with gaseous
  hydrogen chloride to form iron (III) chloride and
  hydrogen sulfide gas.
• Nitric acid dissolved in water reacts with solid
  sodium carbonate to form liquid water and carbon
  dioxide gas and sodium nitrate dissolved in
  water.