Chemical Reactions

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Chemical Reactions
Chapter 14
Preview
Section 1 Forming New Substances Section 2
Chemical Formulas and Equations Section 3 Types
of Chemical Reactions Section 4 Energy and
Rates of Chemical Reactions
Concept Mapping
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Chapter 14
Section 1 Forming New Substances
Bellringer
What do baking bread, launching the space
shuttle, and digesting food have in
common? Write your answer in your science
journal.
3
Chapter 14
Section 1 Forming New Substances
Objectives

• Describe how chemical reactions produce new
  substances that have different chemical and
  physical properties.
• Identify four signs that indicate that a
  chemical reaction might be taking place.
• Explain what happens to chemical bonds during a
  chemical reaction.

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Chapter 14
Section 1 Forming New Substances
Chemical Reactions

• A chemical reaction is a process in which one or
  more substances change to make one or more new
  substances.
• The chemical and physical properties of the new
  substances differ from those of the original
  substances.

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Chapter 14
Section 1 Forming New Substances
Chemical Reactions, continued

• Signs of Chemical Reactions include color
  changes, gas formation, the formation of a
  precipitate (a solid substance formed in a
  solution), and energy given off as light, thermal
  energy, or electrical energy.
• A Change of Properties The most important sign
  of a chemical reaction is the formation of new
  substances that have different properties.

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Chapter 14
Section 1 Forming New Substances
Bonds Holding Molecules Together

• A chemical bond is an attraction that holds
  atoms together in a molecule.
• Breaking and Making Bonds If molecules bump
  into each other with enough energy, the chemical
  bonds in the molecules break. The atoms then
  rearrange, and new bonds form to make new
  substances.

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Chapter 14
Section 1 Forming New Substances
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Chapter 14
Section 1 Forming New Substances
Bonds Holding Molecules Together, continued

• New Bonds, New Substances Whenever new
  substances form, their properties differ from the
  properties of the starting substances.
• For example, sodium is a violently reactive
  metal and chlorine is a greenish poisonous gas.
  The two elements combine to form a white solid
  called sodium chloride, or table salt.

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Chapter 14
Section 2 Chemical Formulas and Equations
Bellringer
Write the following chemical symbols in your
science journal. Then, try to write the names of
the elements matching the symbols. H O C Na N Cl
K Mg Ca Al Au F Cu Fe
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Chapter 14
Section 2 Chemical Formulas and Equations
Objectives

• Interpret and write simple chemical formulas.
• Write and balance simple chemical equations.
• Explain how a balanced equation shows the law of
  conservation of mass.

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Chapter 14
Section 2 Chemical Formulas and Equations
Chemical Formulas

• A chemical formula is a combination of chemical
  symbols and numbers to represent a substance. A
  chemical formula shows how many atoms of each
  kind are present in a molecule.

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Chapter 14
Section 2 Chemical Formulas and Equations
Chemical Formula
Click below to watch the Visual Concept.
Visual Concept
13
Chapter 14
Section 2 Chemical Formulas and Equations
Chemical Formulas, continued

• Writing Formulas for Covalent Compounds You can
  use the name of a covalent compound to write its
  chemical formula. The names of covalent compounds
  use prefixes. Each prefix represents a number, as
  shown below.

mono- 1 hexa- 6
di- 2 hepta- 7
tri- 3 octa- 8
tetra- 4 nona- 9
penta- 5 deca- 10
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Chapter 14
Section 2 Chemical Formulas and Equations
Chemical Formulas, continued

• Writing Formulas for Ionic Compounds To write
  the formula for an ionic compound, make sure the
  compounds charge is 0.
• The formula must have subscripts that cause the
  charges of the ions to cancel out.

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Chapter 14
Section 2 Chemical Formulas and Equations
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Chapter 14
Section 2 Chemical Formulas and Equations
Chemical Equations

• Describing Reactions by Using Equations A
  chemical equation uses chemical symbols and
  formulas as a shortcut to describe a chemical
  reaction.
• From Reactants to Products The starting
  materials in a reaction are reactants. The
  substances formed from a reaction are products.

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Chapter 14
Section 2 Chemical Formulas and Equations
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Chapter 14
Section 2 Chemical Formulas and Equations
Chemical Equations, continued

• The Importance of Accuracy The symbol or
  formula for each substance in a chemical equation
  must be written correctly or it will not
  correctly describe the reaction. Some formulas
  and symbols can be confused.

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Chapter 14
Section 2 Chemical Formulas and Equations
Chemical Equations, continued

• The Reason Equations Must Be Balanced Atoms are
  never lost or gained in a chemical reaction, they
  are just rearranged. Every atom in the reactants
  becomes part of the products.
• When writing a chemical equation, make sure the
  number of atoms of each element in the reactants
  equals the number of atoms of those same elements
  in the products. This is called balancing the
  equation.

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Chapter 14
Section 2 Chemical Formulas and Equations
Chemical Equations, continued

• In the 1700s, French chemist Antoine Lavoisier
  found that the total mass of the reactants was
  always the same as the total mass of the
  products.
• Lavoisiers work led to the law of conservation
  of mass, which states that mass is neither
  created nor destroyed in ordinary chemical and
  physical changes.

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Chapter 14
Section 2 Chemical Formulas and Equations
Chemical Equations, continued

• How to Balance an Equation To balance an
  equation, you must use coefficients. A
  coefficient is a number that is placed in front
  of a chemical symbol or formula.
• For an equation to be balanced, all atoms must
  be counted. So, you multiply the subscript of
  each element in a formula by the formulas
  coefficient.
• The next slide shows how to use coefficients to
  balance an equation.

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Chapter 14
Section 2 Chemical Formulas and Equations
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Chapter 14
Section 3 Types of Chemical Reactions
Bellringer
Are the products of a reaction always more
complex than the reactants? Could products be
simpler than the reactants? Explain your
answers. Write your responses in your science
journal.
24
Chapter 14
Section 3 Types of Chemical Reactions
Objectives

• Describe four types of chemical reactions.
• Classify a chemical equation as one of four
  types of chemical reactions.

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Chapter 14
Section 3 Types of Chemical Reactions
Synthesis Reactions

• A synthesis reaction is a reaction in which two
  or more substances combine to form one new
  compound.
• For example, a synthesis reaction takes place
  when sodium reacts with chlorine to produce
  sodium chloride, which you know as table salt.

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Chapter 14
Section 3 Types of Chemical Reactions
Decomposition Reactions

• A decomposition reaction is a reaction in which
  a single compound breaks down to form two or more
  simpler substances.
• Decomposition is the reverse of synthesis.

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Chapter 14
Section 3 Types of Chemical Reactions
Single-Displacement Reactions

• A single-displacement reaction is a reaction in
  which an element replaces another element that is
  part of a compound. The products of
  single-displacement reactions are a new compound
  and a different element.
• Reactivity of Elements In a single-displacement
  reaction, a more reactive element can displace a
  less reactive element in a compound.

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Chapter 14
Section 3 Types of Chemical Reactions
Double-Displacement Reactions

• A double-displacement reaction is a reaction in
  which ions from two compounds exchange places.
  One of the products of this type of reaction is
  often a gas or a precipitate.
• The next slide shows models of each of the four
  types of chemical reactions.

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Chapter 14
Section 3 Types of Chemical Reactions
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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Bellringer
Now that you know a little about chemical
reactions, think about the many chemical
reactions that take place around you every day.
Describe your favorite chemical reaction. How do
you think energy is involved in the
reaction? Write your answer in your science
journal.
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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Objectives

• Compare exothermic and endothermic reactions.
• Explain activation energy.
• Interpret an energy diagram.
• Describe five factors that affect the rate of a
  reaction.

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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Reactions and Energy

• By comparing the chemical energy of the
  reactants with the chemical energy of the
  products, you can decide if energy is released or
  absorbed in the reaction.
• Exothermic Reactions are reactions in which
  energy is released. Energy can be released as
  light, electrical energy, or thermal energy.

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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Reactions and Energy, continued

• Endothermic Reactions are reactions in which
  energy is taken in.
• The Law of Conservation of Energy states that
  energy cannot be created or destroyed. The energy
  released in exothermic reactions was first stored
  in the chemical bonds of the reactants. And the
  energy taken in during endothermic reactions is
  stored in the products.

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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Rates of Reactions

• The speed at which new particles form is called
  the rate of a reaction.
• Activation Energy is the smallest amount of
  energy that molecules need to react. A chemical
  reaction needs a boost of energy greater than or
  equal to the activation energy before the
  reaction can start.

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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Rates of Reactions, continued

• Sources of Activation Energy Friction is one
  source of activation energy. In a match, friction
  provides the energy needed to break the bonds in
  the reactants and allow new bonds to form.
• An electric spark in a cars engine is another
  source of activation energy. This spark begins
  the burning of gasoline.
• Light can also be a source of activation energy
  for a reaction.

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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Activation Energy and Chemical Reactions
Click below to watch the Visual Concept.
Visual Concept
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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Factors Affecting Rates of Reactions

• Temperature A higher temperature causes a
  faster rate of reaction. At high temperatures,
  particles of reactants move quickly and collide
  hard and often.
• At low temperatures, particles move slowly and
  collide less often.

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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Factors Affecting Rates of Reactions, continued

• Concentration In general, a high concentration
  of reactants causes a fast rate of reaction.
  Concentration is a measure of the amount of one
  substance when it is dissolved in another
  substance.
• When concentration is high, there are many
  reactant particles in a given volume. So, there
  is little distance between particles and the
  particles collide more often and react faster.

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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Factors Affecting Rates of Reactions, continued

• Surface Area is the amount of exposed surface of
  a substance. Increasing the surface area of solid
  reactants increases the rate of a reaction.
• Inhibitors are substances that slow down or stop
  a chemical reaction. The rate of a reaction
  decreases in the presence of an inhibitor.

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Chapter 14
Section 4 Energy and Rates of Chemical Reactions
Factors Affecting Rates of Reactions, continued

• Catalysts are substances that speed up a
  reaction without being permanently changed.
  Because it is not changed, a catalyst is not a
  reactant.
• A catalyst lowers the activation energy of a
  reaction, which allows the reaction to happen
  more quickly.
• Catalysts called enzymes speed up most reactions
  in your body.

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Chapter 14
Chemical Reactions
Concept Mapping
Use the terms below to complete the Concept
Mapping on the next slide.
products reactants chemical equations coefficients chemical reactions chemical formulas

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Chapter 14
Chemical Reactions
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Chapter 14
Chemical Reactions