Evidence of a Chemical
Reaction
1.Evidences of Chemical Reactions
Some chemical changes are easy to observe. The products formed by the
rearrangement of atoms look different than the reactants. Other changes are not
easy to see but can be detected in other ways.
Chemical reaction, a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.
A. Color Change Substances often change color during a chemical reaction
For example, when gray iron rusts, the product that forms is brown, as shown in
the photograph below.
B. Formation of a Precipitate
Many chemical reactions form products that exist in a different physical
state from the reactants. A solid product called a may form when chemicals
in two liquids react, as shown in the photograph below. Seashells are
often formed this way when a sea creature releases a liquid that reacts
with seawater.
C. Formation of a Gas Chemical reactions may produce a gas, like that formed when antacid pills are
mixed with stomach acid. The photograph below shows an example in which carbon dioxide gas is produced
by a chemical reaction.
D. Temperature Change Most chemical reactions involve a temperature
change. Sometimes you can recognize the change when you see a flame.
Notice the burning of the metal magnesium in the photograph. Other temperature
changes are not immediately obvious. If you touch concrete
before it hardens, you may notice that it feels warm. This warmth is due to a
chemical reaction.
F. Light is given off : Potential energy in valance electrons is
released in the form of light.
All chemical reactions, whether simple or complex, involve cengengesan in
substances. One or more starting substances, the reactants, are changed into
one or more new substances, the products.
Reactants à Products
In
a chemical reaction the ways in which atoms are joined together are changed.
Bonds are broken and new bonds are formed as reactants are converted into
products. The atoms are not created or destroyed. They are just rearranged.
Chemical
reactions can be described in different ways. For example, we could say:
"Iron reacts with oxygen to produce iron (III) oxide (rust)."
Alternatively, we could identify the reactants and product in this reaction by
writing a word equation.
Iron
+ oxygen à iron (III) oxide
In
a word equation, the reactants are written on the left, and the products are
written on the right. They are connected by an arrow ( Ã ) that is read as
"yields" or "reacts to produce." Word equations communicate
the reaction but can get cumbersome. To be more efficient chemists use chemical
formulas for writing equations. For example the rusting of iron would be
represented as follows:
Fe
+ O2 Ã Fe2O3
Equations
that show just the formulas of the reactants and products are called skeleton
equations. A skeleton equation is a chemical equation that does not indicate
the relative amounts of the reactants and products. The physical state of a
substance in a reaction can be indicated in the equation by using the following
symbols: (s) for solid, (l) for liquid, (g) for gas, and (aq) for a solution in
water. These usually follow the substance in the equation and can also be
written as subscripts. To write a skeleton equation, you must write the correct
formulas of the reactants and products with the reactants on the left side of
the yield sign and the products on the right.
Just
writing the skeleton equation of reaction does not always correctly communicate
what is happening in the reaction. To represent chemical reactions correctly,
equations must be balanced so that they show the amount of reactants and
products in the reaction. In every balanced equation each side of the equation
has the same number of atoms of each element. This is necessary to be
consistent with the law of conservation of mass. Remember, in a chemical
reaction, atoms are not created nor destroyed; they are simply
rearranged. Many chemical equations can be balanced by trial and error,
but a few guidelines will make it easier and quicker.
We
can only know with certainty what the products of a chemical reaction are by carrying
out that reaction in the laboratory. The reactants must be allowed to react and
the products of this chemical reaction must be identified. Carrying out each
reaction in the lab is the ideal, but it is both time consuming and costly. It
is possible, however, to predict the products of some chemical reactions. To
achieve this, you must be able to recognize various types of reactions. Five
general types of reactions are: composition, decomposition, single replacement,
double replacement, and combustion.
2. The Chemical Reactions
A chemical reaction is a process that is usually
characterized by a chemical change in which the starting materials
(reactants) are different from the products. Chemical reactions tend to involve
the motion of electrons, leading to the formation and
breaking of chemical bonds. There are several different
types of chemical reactions and more than one way of classifying them. Here are
some common reaction types.
However, if you are asked to name the main 4, 5 or 6 types of chemical
reactions, here is how they are categorized.
1.
Oxidation-Reduction or Redox Reaction
In a redox reaction, the oxidation
numbers of atoms are changed. Redox
reactions may involve the transfer of electrons between chemical
species.
The reaction that occurs when In which I2 is reduced to I-
and S2O32- (thiosulfate anion) is oxidized to
S4O62- provides an example of a
redox reaction:
2 S2O32−(aq) + I2(aq) → S4O62−(aq)
+ 2 I−(aq)
2.
Direct Combination or Synthesis Reaction
In a synthesis reaction, two or more chemical
species combine to form a more complex product.
A + B → AB
The combination of iron and sulfur to form iron
(II) sulfide is an example of a synthesis reaction:
8 Fe + S8 → 8 FeS
3.
Chemical Decomposition or Analysis Reaction
In a
decomposition reaction, a compound is broken into smaller chemical
species.
AB → A + B
The electrolysis of water into oxygen and hydrogen gas
is an example of a decomposition reaction:
2 H2O → 2 H2 + O2
4.
Single
Displacement or Substitution
Reaction
A substitution or single
displacement reaction is characterized by one element being
displaced from a compound by another element.
A + BC → AC + B
An example of a
substitution reaction occurs when zinc combines with hydrochloric
acid. The zinc replaces the hydrogen:
Zn + 2 HCl → ZnCl2 + H2
5.
Metathesis or Double Displacement Reaction
In a double displacement or metathesis reaction
two compounds exchange bonds or ions in order to form different
compounds.
AB + CD → AD + CB
An example of a
double displacement reaction occurs between
sodium chloride and silver nitrate to form sodium nitrate and silver
chloride.
NaCl(aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)
6.
Acid-Base Reaction
An acid-base reaction is type of double
displacement reaction that occurs between an acid and a base. The H+
ion in the acid reacts with the OH- ion in the base to form water
and an ionic salt:
HA + BOH → H2O + BA
The reaction between hydrobromic acid (HBr) and sodium
hydroxide is an example of an acid-base reaction:
HBr + NaOH → NaBr + H2O
7.
Combustion
A combustion reaction is a type of redox reaction
in which a combustible
material combines with an oxidizer to form oxidized products and
generate heat (exothermic reaction). Usually, in a combustion
reaction oxygen combines with another compound to form
carbon dioxide and water. An example of a
combustion reaction is the burning of naphthalene:
C10H8 + 12 O2 → 10 CO2 + 4 H2O
8. Isomerization
In an isomerization reaction, the structural
arrangement of a compound is changed but its net atomic composition remains the
same.
9. Hydrolysis
Reaction
A hydrolysis
reaction involves water. The general form for a hydrolysis reaction
is:
X-(aq) + H2O(l) ↔ HX(aq) + OH-(aq)
3.
The chemical Reaction in Dailly Life
a.
Photosynthesis
Is a Reaction To Make Food
Potential energy is the energy an
object has due to its position relative to other objects. It is the amount of
energy stored in something because of its relative position. To understand how
the position of atoms is connected to energy, consider the reaction between the
elements hydrogen and oxygen to form water. The process of forming water
releases a large amount of heat.
Plants
apply a chemical reaction called
photosynthesis to convert carbon dioxide and water into food (glucose) and oxygen. It's one of
the most common everyday chemical reactions and also one of
the most important since this is how plants produce food for themselves
and animals and convert carbon dioxide into oxygen.
6
CO2 + 6 H2O + light → C6H12O6 +
6 O2
b. Rust Is a Common Chemical Reaction
Over
time, iron develops a red, flaky coating called rust. This is an example
of an oxidation reaction. Other everyday examples include formation of
verdigris on copper and tarnishing of silver. Here is the chemical
equation for the rusting of iron:
Fe
+ O2 + H2O → Fe2O3. XH2O
Source :
https://www.thoughtco.com/examples-of-chemical-reactions-in-everyday-life-604049
http://www.oocities.org/tdwarnick/rxnnotes.htm
http://www.classzone.com/vpg_ebooks/ml_sci_gr8/accessibility/ml_sci_gr8/page_274.pdf
