Chemical Reactions
CHEMICAL
REACTIONS
EVIDENCES
FOR
CHEMICAL
REACTIONS
TYPES OF
CHEMICAL
REACTIONS
Evidences for a Chemical Reaction
COLOR
CHANGE
HEAT ABSORBED
OR RELEASED
FORMATION OF A
PRECIPITATE
CHEMICAL
REACTION
GAS
RELEASE
IRREVERSIBLE
NEW
PRODUCT
FORMATION
Evidence - A chemical change is accompanied
by new product formation.
Plants absorb water and Carbon-dioxide and make their
own food in the presence of sunlight. This is known as
“photosynthesis.”
CO2 + H2O
C6H1206 + O2
Evidence - A chemical change is irreversible.
Evidence - A chemical change can be accompanied
by gas release.
We usually use, baking powder/baking soda in baking
cakes and muffins. Baking powder/baking soda releases
carbon-dioxide (gas) on heating and this makes our cakes
and muffins rise.
Evidence - A chemical change can be
accompanied by color change.
Leaves change color in Autumn from green to reddishorange and this is due to a chemical reaction.
Evidence - A chemical change can be accompanied by
heat release or absorption (exothermic vs
endothermic).
Burning of candles results in the release of heat to the
surrounding (“exo”-outside and “thermic” – heat) and cold
packs operate by means of a chemical reaction which
absorbs heat from the surrounding (“endo” –inside and
“thermic” –heat).
Evidence - A chemical change can be accompanied by
precipitate formation [precipitate = an insoluble solid
compound formed when two soluble compounds are mixed
with each other.]
When vinegar is added to warm milk, it results in the
removal of milk solids and this is known as “curdling of
milk.” Curdling is a type of precipitation reaction.
TYPES OF
CHEMICAL
REACTIONS
SYNTHESIS
REDOX
DECOMPOSITION
REPLACEMENT
RXNS
ACID-BASE
COMBUSTION
SINGLE
DOUBLE
Combustion reactions
Image Courtesy of
http://www.cdc.gov/eLCOSH/docs/d0400/d000485/imag
e3.jpg
Features of Combustion Reactions
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●
●
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Fuel and oxygen required by all combustion reactions.
If hydrocarbons (compounds containing carbon and
hydrogen) are the fuel, then carbon-dioxide and water are
formed as products.
All combustion reactions are “exothermic” reactions as they
release heat to the surroundings.
Unless specifically stated, it is always assume combustion
to be complete and predict products of complete
combustion.
Combustion Reactions contd
•
Combustion reactions always use oxygen of the air as a
reactant.
•
Combustion of hydrocarbons (compounds containing
hydrogen and carbon) produces CO2 and H2O.
•
H2O state whether gas or liquid depends on the temperature
conditions of the reaction.
•
Products of incomplete combustion include CO and
sometimes C also called soot.
Synthesis Reactions
•
In synthesis, two or more substances react to form a single
product.
•
Many elements react with one another to form compounds.
•
When a metal reacts with a non-metal, an ionic compound is
formed.
•
2 Mg + O2
•
CaO + H2O
2 MgO
Ca(OH)2
Decomposition
•
A single substance breaks apart to form two or more
substances.
•
AB
•
Example: Sodium azide decomposes to form sodium and
nitrogen. This reaction is used to inflate safety bags in
automobiles. The system is designed in such a way that an
impact results in ignition of sodium azide.
•
NaN3
A+B
Na + N2
Single Replacement Reactions
• A reaction in which one element replaces another
element in a compound is called single replacement
reaction.
• In this type of reactions, a metal replaces a metal and
a non-metal replaces a non-metal.
• General equation for the reaction is:
• A + BC
AC + B
• Activity Series- arrangement of elements in
decreasing order of chemical reactivity.
• Halogens – Each halogen will react to replace any of
the halogens below it in the periodic table.
• Cl2 + NaBr
NaCl + Br2
• Metals – can be listed in a series in which each metal
will replace all metals below it in the series and not he
metals above.
• Fe(s) + CuSO4(aq)
• Fe(s) + MgSO4(aq)
FeSO4 + Cu
N.R (No Reaction)
Precipitation Reactions
• Also called metathesis or exchange reactions.
• Two soluble ionic compounds react to form an
insoluble product called a precipitate.
• Why does a precipitate form?
• The electrostatic force of attraction outweighs the
tendency of the ions to remain solvated so the ions
cluster together and come out of the solution as a
solid.
• Solvation is a process where ions freely move about in
solution. This is possible because most ionic
compounds are soluble in nature and dissociate in
solution to form the corresponding ions.
• How to predict whether precipitation occurs or not?
• Note the ions present.
• Consider the possible cation-anion combinations.
• Decide whether any combination is insoluble by
referring to the solubility chart.
• Write both ionic and molecular equations.
• Aqueous solutions of silver nitrate and barium chloride
mixed together.
• The ions present are Ag+ (silver), Ba2+ (barium), Cl-1
(chloride) and nitrate (NO3-1).
• The possible combinations include AgCl (silver
chloride), Ba(NO3)2.
• All nitrates are soluble but silver chloride is insoluble.
• Molecular Equation:
• AgNO3 + BaCl2
AgCl + Ba(NO3)2
Acid-Base Reactions
• Arrhenius definition – Acid is a substance which
releases H+1 ions in solution.
• A base is a substance which releases OH-1 ions in
solution.
• When an acid and base react, the H+1 and OH-1 ions
combine to form neutral H2O molecules.
• The reaction between an acid and a base is called a
neutralization reaction.
• Acids and bases, in general, are referred to as
“electrolytes.”
• Strong acids and bases completely dissociate into
ions whereas weak acids/bases partially dissociate
into ions.
• A solution of acid, base can conduct electricity.
• Common acids/bases in household
• Tartaric acid – grapes
• Citric acid – citrus fruits
• Vinegar or acetic acid
• Sting of bees and insects – formic acid
• Milk of magnesia – Magnesium hydroxide
• Mylanta – Magnesium hydroxide and Aluminum
hydroxide
• Viewing acid-base reactions as proton-transfer
reactions.
• An acid is a proton donor and a base is a proton
acceptor. (Lowry-Bronsted theory)
• Example – gas forming reactions.
• Reaction between an ionic carbonate and an acid
• Such reactions result in the formation of a gas
(carbon-dioxide) and water.
Acid – Base Titrations
• Titration – quantitative procedure.
• One solution of known concentration is used to
determine the concentration of another solution
through a monitored reaction.
• Solution whose concentration is known is called a
standardized solution.
• A standardized solution is one whose concentration is
known.
• Usually the base is chosen as the standardized
solution.
• The acid concentration has to be determined.
• A few drops of indicator added to the acid solution.
• An indicator is a chemical substance which changes
color in the presence of an acid or a base.
• Examples of indicator – phenolphthalein, bromothymol
blue.
• The standardized solution is slowly added from the
buret to the acid solution of unknown concentration.
• The solution changes color when all the acid is
consumed by the base and a few drops of excess is
present.
• Equivalence point – in a titration is when all moles of
H+1 present in the solution have reacted with all moles
of OH-1 added from the buret.
• End point – in a titration is when a tiny excess of base
is added to change the color of the indicator.
• Usually end point and equivalence point are close to
each other.
Redox Reactions
Oxidation – a process which involves loss of electrons.
Reduction – a process which involves gain of electrons.
Oxidizing agent – a substance which helps oxidation take
place – removes electrons from a different substance
and as a result gets reduced.
Reducing agent – a substance which helps reduction take
place – donates electrons to a different substance and
as a result is oxidized.
• Use oxidation number to monitor the movement of electrons.
• Oxidation number /oxidation state tells us the number of
electrons gained or lost by an atom.
• Rules for assigning oxidation number• All elements in group 1 have a +1 oxidation # in all
compounds.
• All elements in group 2 have a +2 oxidation # in all
compounds.
• Hydrogen has a +1 oxidation # in combination with all nonmetals and a -1 oxidation # in combination with metals and
boron.
• Fluorine has a -1 combination in all compounds.
• All other halogens have a -1 in combination with metals, nonmetals (except O) and other halogens lower in the group.)
• Oxygen has an oxidation # of -2 in all compounds except in
combination with fluorine.