Chapter -10 CHEMICAL REACTIONS by fjhuangjun

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									Chapter -10                       CHEMICAL REACTIONS

Chemical reactions occur when substances undergo fundamental
changes in identity. The substances that participate in Chemical
reaction are called reactants. The new compounds formed in the
reaction are referred as Products. The reactants are represented
by their formulae on the left side of the chemical equation. The
formula of products are reported on the right side of the equation.
The arrow implies “yield or produced “.

Carbohydrates are broken down into fundamental units called
Glucose. When glucose is being metabolized, carbon-dioxide,
water are formed as products of the reaction with the release of
energy used to run other physiological processes.

The reactants are glucose that combines with oxygen. The
unbalanced chemical equation for this reaction is represented as
follows:

Glucose + O2 _____________>Water + Carbon-dioxide + energy

C6H12O6      + O2______________> H20             + CO2
          (s)     (g)                      (g)           (g)

  Why should you balance the chemical Equation?

  In chemical reactions the matter is conserved. The atoms of
  individual elements that make up the chemical compounds are
  neither gained nor lost. This creates a need to balance the
  chemical equation.

  By trial and error the balanced chemical equation for the
  metabolism of glucose happens to be :
  C6H12O6 + 6O2 (g)__________> 6CO2 (g) + 6 H2O (g)
  If the substance added in a chemical reaction, speeds up the
  chemical reaction , the added substance serves as catalyst which
  is reported above the arrow.

  While balancing the chemical equation, report the physical
  states of reactants and products formed in the reaction . (g)
  stands for gaseous state, (s ) stands for solid state, (l) for liquid
  state and (aq) for substance dissolved in water to form an
  aqueous solution)

  The # of atoms of every element that comes from the
  participating reactants reported on the left side of the equation
  must be counterbalanced with the # of atoms of elements that
  make up the products formed on the right side of the equation.

  While balancing the chemical equation, you are not allowed
  to change the subcripts of reactants and products formed in
  the chemical reaction.

  The whole numbers reported in front of the formula of reactants
  or products in chemical equation are referred as Co-efficient. A
  chemical equation is not complete until it is balanced.

  Review the balanced chemical equation for the following
  ractions:
  a) formation of Magnesium Oxide, pg: 278-279,
  b) formation of water, pg: 280 -281,
  c) Synthesis of Ammonia, pg: 282
  d) Formation of Iron Oxide, pg: 283.

During a chemical reaction, the matter is conserved and there is no
mass change taking place.
Classifying Reactions:
Thre are several types of chemical reaction:

  1. Combustion reaction: organic compound (compound
     containing chain of carbon atoms) burns with Oxygen to
     form CO2 and water as the products of reaction.

  Eg: CH4 + 2O2_____________> CO2 + 2H20

  2. Combination (synthetic ) reaction:

When one element combines with another element to synthesize a
new compound, the reaction is referred as Synthetic or
Combination reaction.
A + B_____________> AB
N2(g) + 3H2(g) _________________> 2 NH3(g)

When H2 gas and O2 gas are ignited by a spark, they combine to
produce water which is an explosive reaction.

2H2(g) + O2(g)__spark___________> 2H2O (l)

Decomposition reaction: A single compound is broken down into
2 or more simpler substances.
Ab___________________> A + B

Electrolysis of water generates H2 and O2 gases.

Decomposition of heavy metal oxides such as HgO and PbO2
generates the metal element and O2 gas.

2HgO( s)___________________> 2Hg(l) + O2(g)
Single Replacement reactions:

The Higher metal element in activity series reacts with salt of
lower metal to produce new element and forms a new salt.

Eg: A + BC___________> AC + B

Cu(s) + 2AgNO3 (aq)________________> Cu(NO3)2 (aq)
+2Ag(s)

A + BC_________________> BA + C (when A is a nonmetal)

Double replacement reactions: The Cations and anions of
participating reactants exchange or swap their partners to form new
ionic compounds.
AB + CD ____________> AD + CB.

Where A and C are cations and Band D are anions.

Case by case analysis of different types of Chemical reaction:

Combustion: hydrocarbon + O2 ______________> CO2 + H2O

Steps to be followed to balance a combustion reaction;

First balance the carbon atoms first, then balance the H atoms
and balance the O atoms last.

Eg: C2H5OH + O2 ________________> CO2 + H2O (not
balanced)

C2H5OH + O2 ________________> 2CO2 + H2O
2C

C2H5OH + O2 ________________> 2CO2 + 3H2O
6H

C2H5OH + O2 ________________> 2 CO2 +3 H2O

                                 4O + 3O
C2H5OH(l) +3O2 (g)________________> 2CO2 (g)+ 3 H2O (g)
(Balanced Equation)

Decomposition Reactions:

2KClO3 (s)___MnO2__________> 2KCl + 3O2(g)

                Heat
Compounds containing nitrates when heated decompose to give a
nitrite compound with the release of O2 gas.
2NaNO3(s) ________Heated______________> 2NaNO2 + O2


2H2O2______MnO2 or I- ion_____________> 2H2O + O 2 (g)
       Decomposition reaction

Reactions of Metals:

Grp. I metals such as Na are very reactive in their elemental state.
Hence they are stored under kerosene or mineral oil. Grp.I metal
elements react rapidly with O2 present in air to form metal
oxides. Metal + oxygen ________> Metal Oxide.

4Na + O2 (g) ____________> 2Na2O (s)
Review the reaction of Mg + O2 and that of Fe + O2 described on
page 293.

Group I and Group II Metal oxides react with water to form the
corresponding Metal Hydroxides which are basic in nature.
Metal Oxide + Water _______________> Metal Hydroxide
MgO + H2O___________> Mg(OH)2

Oxidation-Reduction reaction:
The metal looses or gives up its valance electrons to form Metal
cation in an oxidation reaction. Thus the metal is oxidized to
metal cation as its oxidation # increases form zero to what ever is
the charge carried by the metal cation depending upon the # of
electrons lost during oxidation reaction.
M(s) gives up its valence electrons      > M+ncation + ne electron
                  Oxidation reaction
(Zero oxidn. #) oxidation # increases (oxidation # = charge of
                                          cation)
 M stands for metal, n stands for charge of cation and n stands for
# of electrons lost during oxidation reaction.
The substance that looses electrons serves as Reducing agent.

Oxidation reaction is accompanied by Reduction reaction. The
substance that gains electrons is getting reduced . The substance
that gains elctrons servesc as Oxidizing agent.

X + ne___________> X n- (During reduction electrons are gained)

Review Example 10.8

Activity Series
Some metals are more reactive than others. Certain metal elements
have tendency to loose their valence electrons more readily than
others eg: while The other metal cations have tendency to gain
electrons more readily than others. Eg: Cu +2 ion gains electrons
to form cu while Na prefers to loose electron to form Na+ cation
. The reactivity of various metals are compiled in a table called
activity series. The different metals are arranged in order of
decreasing reactivity as you descend the series from top. As you
walk down the series from top, the tendency of metal to loose
electron decreases. in other words the tendency of metals to
undergo oxidation reaction decreases. The activity series is used to
predict whether a chemical reaction between metal and acid or
with other compound occurs spontaneously or not. A higher metal
in the activity series will react with the salt of lower metal
spontaneously. The lower metal of the salt is displaced by the
higher metal in the reaction. Such reactions are single
replacement reactions which also happen to be oxidation –
reduction reaction.

Zn + CuSO4 _________________> ZnSO4 + Cu (spontaneous
reaction)

Zn is oxidized to Zn +2cation, Cu +2 cation is reduced to Cu.
Higher metal is oxidized to form metal cation while the lower
metal is reduced to form a metal.

Fe + CaSO4_________________> no reaction.

Group I metals are sometimes referred as active metals because
they react with cold water to form the corresponding metal
hydroxides with the release of H2 gas. They also react with steam
and acids to produce H2 gas. Refer to reaction between Na +
water, Ca + water on pg: 295.
Review examples 10.10 and 10.11.
Metals placed above Hydrogen in the activity series also react with
acids to form metal salt and produce H2 gas.

Reactions of Nonmetals:
Nonmetals react with Oxygen present in air to form nonmetal
oxides.
S8(s) + 8O2(g)_______________> 8 SO2 (g)
Carbon forms CO 2, P forms P2O5
Nonmetal oxides react with water to form acids.
CO2 + H2O_____________> H2CO3
SO2 + H2O_____________> H2SO3
Review example 10.12
Single replacement reactions of Nonmetals:
Most reactive halogen element reacts with compounds containing
less reactive halide anions . As you walk down the halogen group,
the reactivity of halogen element decreases. The order of
reactivity is F> Cl> Br> I . F is most non metallic and most
electronegative in character.
Cl2 (g) + NaF aq) ________________> no reaction

Cl2(g) + 2HI (g) ______________> I2(s) + 2HCl(g)

Metathesis or double replacement reactions:

When the aqueous solutions containing two different ionic
compounds are mixed, the participating cations swap their anions
and exchange their partners to form new ionic compound products
and one of the product formed happens to be insoluble in water and
precipitates out of solution. Such reactions are called as
Precipitation reactions.

Eg: NaCl(aq) + AgNO3 (aq)__________> NaNO3(aq) + AgCl(s)

Review other precipitation reactions listed on bottom of page 300.

Alternatively, the aqueous solution of ionic compound reacts
with acid to form a stable covalent compound such as water
and other common gases.

H2SO4 + Na2S_____________> H2S(g) + Na2SO4. Review other
reactions listed on page 300.
The driving force for metathesis reaction is the formation of stable
covalent compound.
The chemical equation in which reactants and products are
represented in the form of ionic species is referred as Ionic
equation.

Pb(NO3)2 (aq) + 2KI (aq)______________> PbI2(s) + 2KNO3(aq)
Molecular equation

Pb+2 (aq)+ 2NO3-(aq) + 2K 1+ + 2I-(aq)_______> PbI2(s) + 2K +1 (aq) +
2NO3- (aq) This equation is called Ionic Equation.

The ions that appear as same species on both sides of equation ,
and do not participate in chemical reaction. These ions are called
Spectator ions. Identify the spectator ion. Cancel out the spectator
ions from both sides of equation. Ionic equation without
spectator ions becomes net ionic equation.

NO3- and K+1 serve as spectator ions in the above mentioned
chemical Equation for the reaction between Lead Nitrate and
Potassium Iodide. So the Net Ionic equation for this precipitation
reaction is written as follows:

Pb+2 (aq) + 2I-(aq)_______________> PbI2(s)

Neutralization reaction is a double replacement reaction:
Acid reacts with base to form salt and water.

Acid + Base _____________> Salt + Water.

HCl(aq) + NaOH (aq) _____________> H2O(l) + NaCl (aq)

*******Announcements*******

Exam -2 will be administered on March 27th. Review
Text,notes and Home work problems on Chapter 7,8, 9, 10 and
11 to score well on the exams. Solutions for home work Questions
will be posted on bulletin board across from my office on the 4th
floor
HW Assignment due Thursday
10.5, 10.7, 10.11, 10.14,10.21,10.27,10.33, 10.35, 10.39, 10.45,
10.47 and 10.55.

								
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