What is a chemical reaction?
A chemical
reaction is the change of a substance into a new one that
has a different chemical identity.
How can I tell if a chemical reaction is
occurring?
A chemical
reaction is usually accompanied by easily observed physical effects, such as
the emission of heat and light, the formation of a precipitate, the evolution
of gas, or a color change. Absolute
confirmation of a chemical change can only be validated by chemical analysis of
the products!
Take a look at the
following image and describe what you see going on? What are the key indicators of a chemical change? The key is observation!
Click on Sherlock Holmes as a warm to test your powers of
observation!
Oh …..don’t forget to check out the barking dog!
There are many different types of
chemical reactions. Chemists have
classified the many different reactions into general categories. The chemical reactions we will explore are a
representation of the types of reactions found in each group. There is a general description of the main
reaction types and specific examples provided in the selection boxes.
Synthesis Reaction (Combination Reaction)
In a synthesis
reaction, two or more substances combine to form a new compound. This type of
reaction is represented by the following equation.
A + B AB
A and B
represent the reacting elements or compounds while AB represents a compound as
the product.
The following examples are representative of synthesis
reactions.
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Formation of Aluminum Bromide:
When Al is
placed on the surface of liquid Br2 an exothermic reaction occurs.
The Al is oxidized to Al3+ by the Br2, which is reduced to Br - ions. The ionic product, AlBr3, can be observed on the watch
glass after the reaction.
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Formation of Sodium Chloride: Molten sodium burns when it is
put into a container of chlorine gas. In the reaction a sodium ion loses an
electron to form a sodium cation and a chlorine atom simultaneously gains an
electron to form a chloride anion. The product of the reaction is the ionic
compound sodium chloride, which is the white solid observed.
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Formation of Zinc Oxide: Oxidation is a loss of electrons
and reduction is a gain of electrons. The oxidation of metallic Zn by O2 to form ZnO(s) is illustrated at the molecular
level. The transfer of electrons from Zn to O2 is shown. Atoms can be observed
to change as they are oxidized or reduced, respectively to their ionic forms.
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Formation of Sodium Hydroxide and Potassium
Hydroxide: When a small piece of Na is added
to a solution containing an indicator, evidence of the reaction can be
observed by the change in the color of the solution as NaOH is formed, by the
melting of the Na and by the movement of the Na caused by formation of hydrogen
gas. K is more reactive than Na as demonstrated by its reaction with water.
This reaction produces enough heat to ignite the H2 produced.
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Single-Replacement Reaction
In a single-replacement
reaction (displacement reaction) one element replaces a similar element
in the compound. Single-replacement
reactions can be represented by the following equations.
AB + C AC + B|
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Thermite Reaction: In the thermite reaction, Al
reduces Fe2O3 to Fe in an extremely exothermic reaction in which Al is oxidized to
Al2O3. The reaction produces enough heat to melt the iron. Because of the
extreme heat produced in the thermite reaction, it is used industrially to
weld iron.
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Reduction of CuO: When black carbon
and black copper oxide are heated together the Cu2+
ions are reduced to metallic Cu and a gas is evolved. When the gas is collected in Ca(OH)2 a white precipitate of CaCO3 is formed. The reaction which
occurs involves the reduction of Cu2+ ions by carbon which is oxidized
to CO2.
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Formation of Silver Crystals:
When a
copper wire is placed in a solution of AgNO3, the Cu reduces Ag+ to metallic Ag. At the same time,
Cu is oxidized to Cu2+. As the reaction progresses Ag crystals can be seen to
form on the Cu wire and the solution becomes blue as a result of the
formation of Cu2+ ions.
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Formation of Tin Crystals:
Oxidation-reduction
chemistry of Sn and Zn. When acidified Sn(II)Cl2 is added to a beaker containing a
piece of Zn, some of the Sn2+ reacts with H+ in the solution to produce H2 gas. Immediate changes can also
be observed on the surface of the Zn as it quickly becomes coated with Sn
crystals. After the reaction has progressed for a time needles of Sn can be
observed on the surface of the Zn.
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Double-Replacement
Reaction
In a
double-replacement reaction, the ions of two compounds exchange places in an
aqueous solution
to form two new compounds.
A double-replacement reaction can be represented by the following
equation.
AB + CD AC + BD|
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This marble statue has been eroded by acid rain. Marble is a
material having CaCO3 as its primary component. Acids react
with and dissolve the marble. The
acid comes from sulfur dioxide in the atmosphere combining with water to form
sulfurous acid.
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An aqueous solution of Potassium Iodide is added to an aqueous
solution of Lead (II) Nitrate forming lead (II) iodide. The formation of a precipitate occurs when
the cations of one reactant combines with the anions of the other reactant to
form an insoluble or slightly insoluble compound.
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An aqueous solution of Sodium Chloride is added to an aqueous
solution of Silver Nitrate forming silver chloride.
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Decomposition
Reaction
In a decomposition reaction, single compound undergoes a
reaction that produces two or more simpler
substances. A decomposition
reaction can be represented by the following equation.
AB A + B|
Electrolysis of Water: When a
direct current is passed through water it decomposes to form oxygen and
hydrogen. The volume of hydrogen gas produced at the negative electrode is
twice the volume of the oxygen gas formed at the positive electrode. This
indicates that water contains twice as many hydrogen atoms as oxygen atoms,
which is an illustration of the law of constant composition.
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Decomposition of
Nitrogen Triiodide: Nitrogen triiodide is
extremely unstable when it is dry. Touching it with a feather causes it to
decompose explosively. The explosion occurs as chemical energy is released by
the decomposition of nitrogen triiodide to N2 and I2.
Violet iodine vapor can be observed after the explosion.
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Combustion
Reaction
In a
combustion reaction, a substance combines with oxygen, releasing a large amount
of energy in the form
of light and heat.
For organic compounds, such as hydrocarbons, the products of the
combustion reaction are carbon dioxide and water.
CH4 + 2
O2 CO2 + 2
H2O|
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The
combustion of hydrogen yields water vapor as a reaction product. Three balloons of hydrogen and one balloon
mixed with hydrogen and oxygen form an explosive mixture
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Reactions
with Oxygen. Magnesium, steel wool, white phosphorous, and sulfur are burned
in oxygen. The resulting reactions are combination reactions in which two
substances react to form one product. The products formed in these reactions
are MgO, Fe2O3, P4O10 and SO2.
All of these combustion reactions are very exothermic.
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The
combustion of yellow phosphorus occurs in an oxygen atmosphere. The
main product of this reaction is phosphorus pentoxide.
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