Transcript Chemical Reaction

Chemical Reaction
• What is a chemical reaction?
• A chemical reaction: is a process in which
one set of substances called reactants is
converted to a new set of substances called
products.
•
reactants  products
How do we know a chemical reaction has occurred ?
Some physical evidence to look for that shows a reaction has
occurred:
 a color change
 formation of a solid (precipitate) within a clear solution
 evidence of a gas
evolution or absorption of heat
Types of reactions
Combination reaction
A + B  AB
Decomposition reactions
AB  A + B
Displacement reactions
AB + C  AC + B
Metathesis reactions (double-replacement reactions)
AB + CD  AD + CB
Combustion reactions  reaction with oxygen
How do we represent the chemical reaction in a way that
is convenient and easy to understand?
chemical equation, which is a symbolic way of representing a
chemical reaction in terms of chemical formulas. In a chemical
equation, formulas for the reactants (starting substances in a
chemical reaction) are written on the left sides of the equation and
formulas for products are written on the right. The two sides of
equation are joined by an arrow ().
E. g. Consider the reaction of colorless nitrogen monoxide and
oxygen gas to form red-brown nitrogen dioxide gas.
1 Substitute chemical formulas for names, to obtain the following
expression.
nitrogen
monoxide
gas
react
with
oxygen
gas
to
form
NO
+
O2

nitrogen
dioxide
gas
NO2
2 Balance numbers of atoms to obtain a chemical equation.
In a balanced equation, for each element present, the total number of
atoms of the element is the same on both sides. A chemical reaction
involves simply a recombination of the atoms; none are destroy and none
are created.
Two N atoms
and 4 O atoms
2NO(g) + O2(g)
2 atoms and
4 O atoms
2NO2(g)
The numbers in front of the chemical formula are coefficients required
to balance a chemical equation and they are called stoichiometric
coefficients.
Note: In balancing a chemical equation, the equation can be balanced
only by adjusting the coefficients of formulas, as necessary.
1 Never introduce extraneous atom or molecules into equation
2 Never change formulas for the purpose of balancing an equation
Balancing a Chemical Equation
Balancing by Inspection: means to adjust stoichiometric
coefficients by trial and error until a balanced condition is found.
Useful strategies for balancing equations
1 If an element occurs in only one compound on each side of the
equation, try balancing this element first.
2 When one of the reactants or products exists as the free element,
balance this element last.
3 In some reactions, certain groups of atoms (e.g. polyatomic
ions) remain unchanged. In such case balance these groups as a
unit.
4 It is permissible to use fractional as well as integral numbers as
coefficients. At times, an equation can be balanced most easily
by using one or more fractional coefficients and then, if desired,
clearing the fractions by multiplying all coefficients by a
common multiplier.
E.g. Cu + AgNO3 Cu(NO3)2 + Ag
 Cu + 2AgNO3 Cu(NO3)2 + 2Ag
E.g. Balance the following chemical equations
(a) H2 + O2  H2O
(b) ZnS+ O2  ZnO +SO2
(c) Pb(NO3)2 + KI  PbI2 + KNO3
State of Matter
A complete chemical reaction also includes state of matter and
the reaction condition.
In a chemical equation, the state of matter or physical form of
reactants and products are presented through symbols in
parentheses.
(g) gas
(l) liquid
(s) solid
(aq) aqueous (water)solution
Therefore, the chemical equation of our first example: Consider
the reaction of colorless nitrogen monoxide and oxygen gas to
form red-brown nitrogen dioxide gas is
2NO(g) + O2(g)
2NO2(g)
E.g Solid iron(III) oxide react with carbon monoxide to liberate
carbon dioxide gas and to form iron metal.
E.g. Write a balanced equation for the combustion of thiosalicylic
acid, C7H6O2S, used in the manufacture of indigo dyes.
Reaction Conditions
An important aspect of modern chemical research involves
working out the condition for a reaction. The condition of the
reaction is written above or below the arrow in an equation. 
means that a high temperature is required.
E.g.

2NaNO3(s) 2NaNO2(s) + O2(g)
CO2(g) + 2H2(g) — CH3OH(g)
ZnO, Cr2O3
Review Questions
• What is a balanced equation?
• Why does a chemical equation has to be
balanced?
Chemical equations are written in terms of molecules or formula
units. But to plan for real reaction, you can’t count molecules; they
are too tiny. Since the moles can be converted to mass and the
number of moles involved in a reaction is proportional to the
coefficients in the balanced chemical equation, we can convert the
equation to the amounts we can weigh, or measure by some other
means.
Number of molecules
Stoichiometry
Amounts you can
measure e.g. mass
Chemical Equations and Stoichiometry
The coefficients in the chemical equation
2H2(g) + O2(g)  2H2O(l)
mean that two molecules of H2 react with one molecules of O2 and
produce 2 molecules of H2O. A similar statement involving multiples
of these numbers of molecules is also correct.
2 x molecules H2 + x molecules O2  2x molecules H2O
Suppose that we let x = Avogadro’s number. Then x molecules
represents one mole. Thus the chemical equation also mean that
2 moleH2 + 1 mol O2  2 mol H2O
Therefore, on the atomic or molecular scale, the coefficients of a
balanced equation represent atoms and molecules. On the
macroscopic scale, the coefficients of a balanced equation represent
moles of atoms and molecules.
Summary
Because moles can be converted to mass, we can also give a
mass interpretation of a chemical equation. We may interpret a
chemical equation either in terms of numbers of molecules (or
ions or formula units) or in terms of numbers of moles,
depending on our needs. We summarized these three
interpretations as follows:
2H2(g)
+ O2(g)
 2H2O(l)
2 molecule H2 + 1 molecule O2  2 molecules H2O
2 mole H2
+ 1 moleO2
 2 moleH2O
2 x2.02g H2 + 32.0g O2
 2 x18.0g H2O
E.g. How many grams of CO2 is produce by burning 2.50 mole of
CH4 in an excess of O2?
Chemical Reactions in Solution
A solution is a homogeneous mixture of two or more substances.
Type of solution: see handout.
The common type of solution is a solid dissolved in a liquid.
The solid that is dissolved is called solute, and the liquid in
which it is dissolved is called solvent.
The most common method of expressing the concentration of the
solute is Molarity which is defined as
Molarity(M) = amount of solute in mole __
volume of solution in liter
E.g 15mL of acetic acid, CH3COOH (d=1.048g/mL), is dissolved
in enough water to produce 500.0 mL of solution. What is the
molarity of acetic acid in the solution?
E.g. Commercial sulfuric acid is sold as a solution that is 96.7%
H2SO4(l) and 3.3% H2O by mass. What is the concentration in
molarity of this solution? Density of the solution is 1.84g/mL.
Solution Dilution
~ all solute taken from the initial, more concentrated solution
appears in the final diluted solution.
~ When a solution is diluted, the amount of solute remains
constant between the initial(i) solution taken and the final(f)
solution produced. That is
MiVi =ni = nf = MfVf
MiVi = MfVf
E.g Compute the volume of concentrated (18.1M) H2SO4(aq)
required to produce 500mL of 0.30M H2SO4(aq).
E.g. A 15mL sample of 0.450 M K2CrO4 is diluted to 100.00 mL.
What is the concentration of the new solution?
E.g. How many mL of water must be added to 30.0 mL of 0.520 M
NaCl to produce a 0.195 M solution?
Stoichiometry of Reactions in Solution
Molarity is used in stoichiometric calculation for reaction that
take place in solution.
E.g. A 25.00mL of 0.250M of K2CrO4(aq) is added to an excess
of AgNO3(aq). What mass of Ag2CrO4(s) will precipitate from the
solution?
Limiting Reactant
When all the reactants are completely and simultaneously
consumed in a chemical reaction, the reactants are said to be in
stoichiometric proportions-in the mole ratios dictated by the
coefficients in the balanced equation. In reality, many reactions
are carried out using an excess amount of one reactant-more than
is actually needed according to stoichiometric. In such case, one
of the reactants may be completely consumed at the end of
reaction, whereas some amounts of other reactants will remain
unreacted.
The reactant that is completely consumed when a reaction goes to
completion and thereby limits the amount of product formed is
called the limiting reactant (or limiting reagent). Once one of
the reactants is used up, the reaction stops. This means that the
moles of product are always determined by the starting moles of
limiting reactant.
E.g. Reaction of sodium sulfate and carbon yield sodium sulfide and
carbon monoxide. What mass of CO(g) forms in the reaction of 30.0g
sodium sulfate and 20.0g carbon?
First write a balanced equation
Then determine the limiting reactant
•There are more than one ways to answer this. Here we are going
to use a somewhat tidies but maybe less confusing way to you. We
take each reactant in turn and ask how much product would be
obtained if each were totally consumed. The reactant that gives the
smaller amount of product is the limiting reactant.
Other Practical Matters in Reaction Stoichiometry
Theoretical Yield, Actual Yield, and Percent Yield
For many chemical reactions the amount of the desired product
obtained is less than the theoretical amount. The percent yield is
defined as
the quantity of product
that is actually produced
percent yield =
actual yield of product
x 100%
theoretical yield of product
the calculated quantity of product that
one expects from given quantities of reactants
E.g. What is the percentage yield if the reaction of 25.0g P4 with 91.5g
of Cl2 produces 104g PCl3?
Consecutive Reactions, Simultaneous Reactions, and
Overall Reactions
Most of time it takes more than one steps to get desired product
~ Reactions that are carried out one after another in sequence to
yield a final product are called consecutive reactions.
~ In simultaneous reaction, two or more substances react
independently of one another in separate reactions occurring at the
same time.
~Any substance that is produced in one step and consumed in
another step of a multistep process is called an intermediate.
E.g. Purification of TiO2
2TiO2(impure) + 3C(s) +4Cl2(g) 2TiCl4(g) + CO2(g) + 2CO(g)
2x[ TiCl4(g) + O2(g) TiO2(s) +2Cl2(g)]_______
overall reaction: 3C(s) + 2O2(g) CO2(g) + 2CO(g)
E.g. How many gram of NaBr is produced if 55.85g of Fe is
consumed?
3Fe + 3Br2 3FeBr2
3FeBr2 + Br2 Fe3Br8
Fe3Br8 + 4Na2CO3  8NaBr + 4CO2 + Fe3O4
Overall reaction 3Fe + 4Br2 + 4Na2CO3  8NaBr + 4CO2 + Fe3O4
E.g Simultaneous reaction
Magnalium alloys contain 70% Al and 30.0% Mg by mass. How
many grams of H2(g) are produce in the reaction of a 0.710g
sample of this alloy with excess HCl(aq)?