When a substance undergoes a chemical change, it takes part in a chemical reaction.

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Chapter 8 Chemical Equations Reactions
Recognizing Chemical Reactions

• When a substance undergoes a chemical change, it
  takes part in a chemical reaction.

• After it reacts, it no longer has the same
  chemical identity.

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Recognizing Chemical Reactions

• While it may seem amazing that a substance can
  undergo a change and become part of a different
  substance, chemical reactions occur around you
  all the time.

• Many important clues indicate when chemical
  reactions occur.

• None of them alone proves that such a change
  occurs because some physical changes involve one
  or more of these signs.

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Writing Chemical Equations

• In order to completely understand a chemical
  reaction, you must be able to describe any
  changes that take place.

• Part of that description involves recognizing
  what substances react and what substances form.

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Writing Chemical Equations

• A substance that undergoes a reaction is called a
  reactant.

• When reactants undergo a chemical change, each
  new substance formed is called a product.

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Writing Chemical Equations

• For example, a familiar chemical reaction
  involves the reaction between iron and oxygen
  (the reactants) that produces rust, which is
  iron(III) oxide (the product).

• The simplest reactions involve a single reactant
  or a single product, but some reactions involve
  many reactants and many products.

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Word Equations

• The simplest way to represent a reaction is by
  using words to describe all the reactants and
  products, with an arrow placed between them to
  represent change.

• Reactants are placed to the left of the arrow,
  and products are placed to the right.

• Plus signs are used to separate reactants and
  also to separate products.

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Word Equations

• Vinegar and baking soda are common names.

• The compound in vinegar that is involved in the
  reaction is acetic acid, and baking soda is
  sodium hydrogen carbonate.

• These scientific names can also be used in a word
  equation.

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Chemical Equations

• Word equations describe reactants and products,
  but they are long and awkward and do not
  adequately identify the substances involved.

• Word equations can be converted into chemical
  equations by substituting chemical formulas for
  the names of compounds and elements.

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Chemical Equations

• The equation for the reaction of vinegar and
  baking soda can be written using the chemical
  formulas of the reactants and products.

• By examining a chemical equation, you can
  determine exactly what elements make up the
  substances that react and form.

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Chemical Equations

• It may also be important to know the physical
  state of each reactant and product.

• How can we indicate the bubbles we see during
  this reaction are CO2?

• Symbols in the parentheses are put after formulas
  to indicate the state of the substance.

• Solids, liquids, gases, and water (aqueous)
  solutions are indicated by the symbols (s), (l),
  (g), and (aq).

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Chemical Equations

• The following equation shows these symbols added
  to the equation for the reaction of vinegar and
  baking soda.

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Chemical Equations

• Now the equation tells us that mixing an aqueous
  solution of acetic acid (vinegar) with solid
  sodium hydrogen carbonate (baking soda) results
  in the formation of an aqueous solution of sodium
  acetate, liquid water, and carbon dioxide gas.

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Energy and Chemical Equations

• Noticeable amounts of energy are often released
  or absorbed during a chemical reaction.

• Some reactions absorb energy. If energy is
  absorbed, the reaction is known as an endothermic
  reaction.

• For a reaction that absorbs energy, the word
  energy is sometimes written along with the
  reactants in the chemical equation.

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Energy and Chemical Equations

• For example, the equation for the reaction in
  which water breaks down into hydrogen and oxygen
  shows that energy must be added to the reaction.

• Reactions that release heat energy are called
  exothermic reactions.

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Energy and Chemical Equations

• When writing a chemical equation for a reaction
  that produces energy, the word energy is
  sometimes written along with the products.

• Some of this energy is in the form of light.

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Energy and Chemical Equations

• You may have also noticed that the word energy is
  not always written in the equation.

• It is used only if it is important to know
  whether energy is released or absorbed.

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Balancing Chemical Equations

• The mass of the products is always the same as
  the mass of the reactants that react to form them.

• The law of conservation of mass summarizes these
  findings.

• Matter is neither created nor destroyed during a
  chemical reaction.

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Balancing Chemical Equations

• Remember that atoms dont change in a chemical
  reaction they just rearrange.

• The number and kinds of atoms present in the
  reactants of a chemical reaction are the same as
  those present in the products.

• When stated this way, it becomes the law of
  conservation of atoms.

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Balancing Chemical Equations

• For a chemical equation to accurately represent a
  reaction, the same number of each kind of atom
  must be on the left side of the arrow as are on
  the right side.

• If an equation follows the law of conservation of
  atoms, it is said to be balanced.

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Balancing Chemical Equations

• The easiest way to count atoms is to
  practicefirst with a simple reaction and then
  with some that are more complex.

• For example, consider the equation that
  represents breaking down carbonic acid into water
  and carbon dioxide.

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Balancing Chemical Equations

• Because a subscript after the symbol for an
  element represents how many atoms of that element
  are found in a compound, you can see that there
  are two hydrogen, one carbon, and three oxygen.

• All of the atoms in the reactants are the same as
  those found in the products.

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Balancing Chemical Equations

• Examine the equation for the formation of sodium
  carbonate and water from the reaction between
  sodium hydroxide and carbon dioxide.

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Balancing Chemical Equations

• One carbon atom is on each side of the arrow, but
  the sodium, oxygen, and hydrogen atoms are not
  balanced.

• The equation, as written, does not truly
  represent the reaction because it does not show
  conservation of atoms.

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Balancing an Equation

• To indicate more than one unit taking part or
  being formed in a reaction, a number called a
  coefficient is placed in front of it to indicate
  how many units are involved.

• Look at the previous equation with a coefficient
  of 2 in front of the sodium hydroxide formula.

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Balancing an Equation

• Is the equation balanced now?

• Two sodium atoms are on each side. How many
  oxygen atoms are on each side?

• You should be able to find four on each side.
  How about hydrogen atoms?

• Now two are on each side.

• Because one carbon atom is still on each side,
  the entire equation is balanced it now
  represents what happens when sodium hydroxide and
  carbon dioxide react.

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Balancing an Equation

• The balanced equation tells us that when sodium
  hydroxide and carbon dioxide react, two units of
  sodium hydroxide react with each molecule of
  carbon dioxide to form one unit of sodium
  carbonate and one molecule of water.

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Major Classes of Reactions

• If you can classify a reaction into one of five
  major categories by recognizing patterns that
  occur, you already know a lot about the reaction.

• In one type of reaction, two substanceseither
  elements or compoundscombine to form a compound.

• Whenever two or more substances combine to form a
  single product, the reaction is called a
  synthesis reaction.

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A Synthesis Reaction

• When iron rusts, iron metal and oxygen gas
  combine to form one new substance, iron(III)
  oxide.

• The balanced equation for this synthesis reaction
  shows that there is more than one reactant but
  only one product.

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A Synthesis Reaction
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Major Classes of Reactions

• In a decomposition reaction, a compound breaks
  down into two or more simpler substances.

• The compound may break down into individual
  elements, such as when mercury(II) oxide
  decomposes into mercury and oxygen.

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Major Classes of Reactions

• The products may be an element and a compound,
  such as when hydrogen peroxide decomposes into
  water and oxygen.

• The compound may break down into simpler
  compounds.

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A Decomposition Reaction

• When ammonium nitrate is heated to a high
  temperature, it explosively breaks down into
  dinitrogen monoxide and water.

• The decomposition reaction taking place is
  represented by a balanced equation that shows one
  reactant and more than one product.

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A Decomposition Reaction
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Major Classes of Reactions

• In a single-displacement reaction, one element
  takes the place of another in a compound.

• The element can replace the first part of a
  compound, or it can replace the last part of a
  compound.

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Single Displacement

• If an iron nail is placed into an aqueous
  solution of copper(II) sulfate, the iron
  displaces the copper ions in solution, and copper
  metal forms on the nail.

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Single Displacement
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Major Classes of Reactions

• In double-displacement reactions, the positive
  portions of two ionic compounds are interchanged.

• For a double-displacement reaction to take place,
  at least one of the products must be a
  precipitate or water.

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Double Displacement

• When clear aqueous solutions of lead(II) nitrate
  and potassium iodine are mixed, a
  double-displacement reaction takes place and a
  yellow solid appears in the mixture.

• This solid is lead(II) iodine, and it
  precipitates out because it is insoluble in
  water, unlike the two reactants and the other
  product.

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Double Displacement
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Major Classes of Reactions

• A combustion reaction is one in which a substance
  rapidly combines with oxygen to form one or more
  oxides.

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Combustion

• When welding is done with an acetylene torch,
  acetylene combines with oxygen to form carbon
  dioxide and water.

• This combustion reaction is exothermic, and
  enough energy is released to melt metal.

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Combustion
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Basic Assessment Questions
Question 1
Write a word equation and a skeleton equation for
each of the following descriptions of chemical
reactions.
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Basic Assessment Questions
Question 1a
Solid lithium reacts with chlorine gas to produce
solid lithium chloride.
Answer 1a
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Basic Assessment Questions
Question 1b
Nitrogen gas reacts with oxygen gas to produce
nitrogen dioxide gas.
Answer 1b
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Basic Assessment Questions
Question 2
Write a balanced chemical equation for the
following reactions, making sure coefficients are
in their lowest possible ratio.
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Basic Assessment Questions
Question 2a
Solid potassium reacts with liquid water to
produce hydrogen gas and a water solution of
potassium hydroxide.
Answer 2a
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Basic Assessment Questions
Question 2b
Calcium chloride and sodium carbonate in water
solution produce solid calcium carbonate and a
water solution of sodium chloride.
Answer 2b
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Basic Assessment Questions
Question 3
Identify each of the following skeleton equations
as representing a synthesis reaction, a
combustion reaction, both synthesis and
combustion, or decomposition reaction. Balance
each equation if necessary.
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Basic Assessment Questions
Question 3a
Answer 3a
synthesis
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Basic Assessment Questions
Question 3b
Answer 3b
decomposition already balanced
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Reactions in Aqueous Solutions

• When aqueous solutions that contain ions are
  mixed, the ions may react in a double-replacement
  reaction.

• The product is typically a solid precipitate,
  water, or a gas.

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Reactions in Aqueous Solutions

• An example of a double-replacement reaction that
  produces a precipitate occurs when aqueous
  solutions of sodium chloride and silver nitrate
  are mixed to form a precipitate of solid silver
  chloride.

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Reactions in Aqueous Solutions

• To show all of the particles in solution as they
  really exist, a complete ionic equation can be
  written.

• The sodium and nitrate ions are on both sides of
  the equation.

• Such ions that do not participate in a reaction
  are called spectator ions.

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Reactions in Aqueous Solutions

• An ionic equation that does not show spectator
  ions but only the particles that participate in a
  reaction is called a net ionic equation.

• In the case of the reaction above, the net ionic
  equation from which the sodium and nitrate ions
  have been removed is as follows.

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Writing Ionic Equations

• Write the balanced chemical equation for the
  reaction between aqueous solutions of strontium
  nitrate and potassium sulfate, which forms the
  precipitate strontium sulfate.

• Then write the complete ionic and net ionic
  equations.

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Writing Ionic Equations

• Write the correct skeleton equation.

• Use coefficients to produce the balanced chemical
  equation.

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Writing Ionic Equations

• Write the complete ionic equation.

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Writing Ionic Equations

• Cross out the spectator ions, which are those
  that are on both sides of the equation.

• That leaves the net ionic equation.

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Reactions that form water or a gas

• Some double-replacement reactions in aqueous
  solution produce water or a gas (or both) rather
  than a precipitate.

• In such cases, the water or gas is shown as a
  product in the net ionic equation, as are the
  ions that produced it.

• The remaining ions are eliminated as spectator
  ions.

• The following example problem illustrates this
  concept.

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Reactions that form water or a gas

• When hydrochloric acid and potassium hydroxide
  solutions are mixed, water results, together with
  an aqueous solution of potassium chloride.

• Write the balanced chemical equation, a complete
  ionic equation, and a net ionic equation for this
  reaction.

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Reactions that form water or a gas

• The balanced chemical equation is the same as the
  skeleton equation.

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Reactions that form water or a gas

• Write the complete ionic equation, which includes
  all of the ions.

• Remove the spectator ions to produce the net
  ionic equation.

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Additional Assessment Questions
Question 1
Write a balanced chemical, complete ionic, and
net ionic equations for the following reaction
Aqueous solutions of lead(II) nitrate and
ammonium chloride are mixed, forming a
precipitate of lead(II) chloride.
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Additional Assessment Questions
Answer
Chemical
Complete Ionic
Net ionic
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Additional Assessment Questions
Question 2
Write a balanced chemical, complete ionic, and
net ionic equations for the reaction between the
following substances, which produce water
nitric acid (HNO3) and aqueous barium hydroxide
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Additional Assessment Questions
Answer
Chemical
Complete ionic
Net ionic
or, with coefficients reduced to lowest terms,
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Additional Assessment Questions
Question 3
Write a balanced chemical, complete ionic, and
net ionic equations for the reaction between the
following substances, which produce a gas
hydrochloric acid and aqueous sodium cyanide,
with production of hydrogen cyanide gas (HCN)
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Additional Assessment Questions
Answer
Chemical
Complete ionic
Net ionic