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When%20a%20substance%20undergoes%20a%20chemical%20change,%20it%20takes%20part%20in%20a%20chemical%20reaction.

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These scientific names can also be used in a word equation. ... you can determine exactly what elements make up the substances that react and form. ... – PowerPoint PPT presentation

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Title: When%20a%20substance%20undergoes%20a%20chemical%20change,%20it%20takes%20part%20in%20a%20chemical%20reaction.


1
Chemical Reactions and Equations Basic Concepts
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.

2
Chemical Reactions and Equations Basic Concepts
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.

3
Chemical Reactions and Equations Basic Concepts
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.

4
Chemical Reactions and Equations Basic Concepts
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.

5
Chemical Reactions and Equations Basic Concepts
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.

6
Chemical Reactions and Equations Basic Concepts
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.

7
Chemical Reactions and Equations Basic Concepts
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.

8
Chemical Reactions and Equations Basic Concepts
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.

9
Chemical Reactions and Equations Basic Concepts
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.

10
Chemical Reactions and Equations Basic Concepts
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).

11
Chemical Reactions and Equations Basic Concepts
Chemical Equations
  • The following equation shows these symbols added
    to the equation for the reaction of vinegar and
    baking soda.

12
Chemical Reactions and Equations Basic Concepts
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.

13
Chemical Reactions and Equations Basic Concepts
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.

14
Chemical Reactions and Equations Basic Concepts
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.

15
Chemical Reactions and Equations Basic Concepts
Energy and Chemical Equations
  • Reactions that release heat energy are called
    exothermic reactions.

16
Chemical Reactions and Equations Basic Concepts
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.

17
Chemical Reactions and Equations Basic Concepts
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.

18
Chemical Reactions and Equations Basic Concepts
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.

19
Chemical Reactions and Equations Basic Concepts
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.

20
Chemical Reactions and Equations Basic Concepts
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.

21
Chemical Reactions and Equations Basic Concepts
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.

22
Chemical Reactions and Equations Basic Concepts
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.

23
Chemical Reactions and Equations Basic Concepts
Balancing Chemical Equations
  • Examine the equation for the formation of sodium
    carbonate and water from the reaction between
    sodium hydroxide and carbon dioxide.

24
Chemical Reactions and Equations Basic Concepts
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.

25
Chemical Reactions and Equations Basic Concepts
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.

26
Chemical Reactions and Equations Basic Concepts
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.

27
Chemical Reactions and Equations Basic Concepts
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.

28
Chemical Reactions and Equations Basic Concepts
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.

29
Chemical Reactions and Equations Basic Concepts
Major Classes of Reactions
  • 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.

30
Chemical Reactions and Equations Basic Concepts
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.

31
Chemical Reactions and Equations Basic Concepts
A Synthesis Reaction
32
Chemical Reactions and Equations Basic Concepts
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.

33
Chemical Reactions and Equations Basic Concepts
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.

34
Chemical Reactions and Equations Basic Concepts
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.

35
Chemical Reactions and Equations Basic Concepts
A Decomposition Reaction
36
Chemical Reactions and Equations Basic Concepts
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.

37
Chemical Reactions and Equations Basic Concepts
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.

38
Chemical Reactions and Equations Basic Concepts
Single Displacement
39
Chemical Reactions and Equations Basic Concepts
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.

40
Chemical Reactions and Equations Basic Concepts
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.

41
Chemical Reactions and Equations Basic Concepts
Double Displacement
42
Chemical Reactions and Equations Basic Concepts
Major Classes of Reactions
  • A combustion reaction is one in which a substance
    rapidly combines with oxygen to form one or more
    oxides.

43
Chemical Reactions and Equations Basic Concepts
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.

44
Chemical Reactions and Equations Basic Concepts
Combustion
45
Chemical Reactions and Equations Additional
Concepts
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.

46
Chemical Reactions and Equations Additional
Concepts
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.

47
Chemical Reactions and Equations Additional
Concepts
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.

48
Chemical Reactions and Equations Additional
Concepts
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.

49
Chemical Reactions and Equations Additional
Concepts
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.

50
Chemical Reactions and Equations Additional
Concepts
Writing Ionic Equations
  • Write the correct skeleton equation.
  • Use coefficients to produce the balanced chemical
    equation.

51
Chemical Reactions and Equations Additional
Concepts
Writing Ionic Equations
  • Write the complete ionic equation.

52
Chemical Reactions and Equations Additional
Concepts
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.

53
Chemical Reactions and Equations Additional
Concepts
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.

54
Chemical Reactions and Equations Additional
Concepts
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.

55
Chemical Reactions and Equations Additional
Concepts
Reactions that form water or a gas
  • The balanced chemical equation is the same as the
    skeleton equation.

56
Chemical Reactions and Equations Additional
Concepts
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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