Transcript H 2 S:O 2 :SO 2 :H 2 O = 2:3:2:2

Stochiometry I
SECTION 12.1: THE ARITHMETIC OF
EQUATIONS
Objectives
 Upon completing this presentation, the student will
be able to:



Explain how balanced equations apply to both chemistry and
everyday situations.
Interpret balanced chemical equations in terms of moles,
representative particles, mass, and gas volume at STP.
Identify the quantities that are always conserved in chemical
reactions.
Introduction
 When we bake cookies, we need to know the
amounts of materials we need.








How much flour?
How much sugar?
How many eggs?
How much milk?
How much butter?
How much salt?
How much baking powder?
How many chocolate chips?
Introduction
 Likewise, a chemist needs to know the number of
reactant molecules are needed to produce a given
number of product molecules.
 For example:



We use one molecule of methane gas, CH4(g), with two
molecules of oxygen gas, O2(g) to produce one molecule of
carbon dioxide gas, CO2(g), and two molecules of liquid water,
H2O(l).
This is written as:
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)
This is a balanced equation.
Using Balanced Chemical Equations
 Chemists use balanced chemical equations as a basis
to calculate how much reactant is needed or
product is formed in a reaction.
 The calculation of quantities in chemical reactions is
a subject of chemistry called stoichiometry.



It is a form of chemical bookkeeping.
The reactants are the assets and the products are the debits.
Everything must balance out at the end.
Interpreting Chemical Equations
 A balanced chemical equation can be interpreted in
terms of many different quantities.





numbers of atoms
numbers of molecules
moles
mass
volume
 We will learn to interpret chemical equations in each
of these terms.
Interpreting Chemical Equations
 Interpreting in terms of number of atoms
 At the atomic level, the balanced equation represents the
number and type of each atom in a reaction.
 The number of each kind of atom is the same on each side of
the balanced equation.
 For example:
N2(g) + 3 H2(g) → 2 NH3(g)
 On the reactant side, there are two atoms of N combined with
the six atoms of H.
 On the product side, there are two atoms of N combined with
the six atoms of H.
Interpreting Chemical Equations
 Interpreting in terms of number of molecules
 At the molecular level, the balanced equation represents the
number and type of each molecule in a reaction.
 We can determine the ratios of the molecules needed to
produce the desired product by looking at the coefficients.
 For example:
N2(g) + 3 H2(g) → 2 NH3(g)
 With one nitrogen molecule,N2, and three hydrogen molecules,
H2, we get two ammonia molecules, NH3.
 The ratios of molecules for this reaction will always be 1:3:2
for N2:H2:NH3.
Interpreting Chemical Equations
 Interpreting in terms of number of moles
 We can expand the molecular interpretation to include moles.
 We can determine the ratios of the moles needed to produce
the desired product by looking at the coefficients.
 For example:
N2(g) + 3 H2(g) → 2 NH3(g)
 With one mole of N2 and three moles of H2 give us two moles
of NH3.
 The ratios of moles for this reaction will always be 1:3:2 for
N2:H2:NH3.
Interpreting Chemical Equations
 Interpreting in terms of mass
 A balanced chemical equation obeys the law of the
conservation of mass.
Mass is neither created nor destroyed in a chemical reaction.
 The mass of the products must equal the mass of the products.




For example:
N2(g) + 3 H2(g) → 2 NH3(g)
The mass of ammonia produced must equal the mass of
hydrogen and nitrogen consumed.
1 mol of N2 (28 g) + 3 mol of H2 (6 g) is equal to 2 mol of NH3
(34 g).
Interpreting Chemical Equations
 Interpreting in terms of volume
 At STP (0°C and 1 atmosphere), 1 mol of gas occupies 22.4 L.
 Therefore, the volume of the reactants and products are
related to the number of mols of each.
 For example:
N2(g) + 3 H2(g) → 2 NH3(g)
 1 mol of N2 (22.4 L) and 3 mol of H2 (67.2 L) are consumed
and produce 2 mol of NH3 (44.8 L).
 Notice that the volume is not conserved like the mass.
Mass Conservation in Chemical Reactions
 Mass and atoms are conserved in every chemical
reaction.

Molecules, formula units, moles, and volumes are not
necessarily conserved.
N2(g) + 3 H2(g) → 2 NH3(g)
N2
+
3 H2
→
2 NH3
Conserved?
2 N atoms
+
6 H atoms
→
2 N atoms + 6 H atoms
Yes
28 g N2
+
6 g H2
→
34 g NH3
Yes
1 mol N2
+
3 mol H2
→
2 mol NH3
No
22.4 L N2
+
67.2 L H2
→
44.8 L NH3
No
Conceptual Problem 12.1
Hydrogen sulfide, which smells like rotten eggs, is found in volcanic gases.
The balanced equation for the burning of hydrogen sulfide is:
2 H2S(g) + 3 O2(g) → 2 SO2(g) + 2 H2O(g)
Interpret this equation in terms of
a.numbers of representative particles and moles.
b.masses of reactants and products.
The coefficients tell us the relative numbers of representative particles
and moles. Therefore, the ratio of H2S:O2:SO2:H2O = 2:3:2:2
Conceptual Problem 12.1
Hydrogen sulfide, which smells like rotten eggs, is found in volcanic gases.
The balanced equation for the burning of hydrogen sulfide is:
2 H2S(g) + 3 O2(g) → 2 SO2(g) + 2 H2O(g)
Interpret this equation in terms of
a.numbers of representative particles and moles. H2S:O2:SO2:H2O = 2:3:2:2
b.masses of reactants and products.
The coefficients tell us the relative numbers of representative particles
and moles. Therefore, the ratio of H2S:O2:SO2:H2O = 2:3:2:2
Conceptual Problem 12.1
Hydrogen sulfide, which smells like rotten eggs, is found in volcanic gases.
The balanced equation for the burning of hydrogen sulfide is:
2 H2S(g) + 3 O2(g) → 2 SO2(g) + 2 H2O(g)
Interpret this equation in terms of
a.numbers of representative particles and moles. H2S:O2:SO2:H2O = 2:3:2:2
b.masses of reactants and products.
The balanced equation obeys the law of the conservation of mass.
Therefore, the mass of reactants is equal to the mass of products.
mass = (number of mols)(molar mass)
⇒ mH2S = (2)(34) g = 68 g
⇒ mO2 = (3)(32) g = 96 g
⇒ mreactants = mH2S + mO2 = 72 g + 96 g = 164 g
Conceptual Problem 12.1
Hydrogen sulfide, which smells like rotten eggs, is found in volcanic gases.
The balanced equation for the burning of hydrogen sulfide is:
2 H2S(g) + 3 O2(g) → 2 SO2(g) + 2 H2O(g)
Interpret this equation in terms of
a.numbers of representative particles and moles. H2S:O2:SO2:H2O = 2:3:2:2
b.masses of reactants (164 g) and products.
The balanced equation obeys the law of the conservation of mass.
Therefore, the mass of reactants is equal to the mass of products.
mass = (number of mols)(molar mass)
⇒ mH2S = (2)(34) g = 68 g
⇒ mO2 = (3)(32) g = 96 g
⇒ mreactants = mH2S + mO2 = 72 g + 96 g = 164 g
Conceptual Problem 12.1
Hydrogen sulfide, which smells like rotten eggs, is found in volcanic gases.
The balanced equation for the burning of hydrogen sulfide is:
2 H2S(g) + 3 O2(g) → 2 SO2(g) + 2 H2O(g)
Interpret this equation in terms of
a.numbers of representative particles and moles. H2S:O2:SO2:H2O = 2:3:2:2
b.masses of reactants (164 g) and products.
The balanced equation obeys the law of the conservation of mass.
Therefore, the mass of reactants is equal to the mass of products.
mass = (number of mols)(molar mass)
⇒ mSO2 = (2)(64) g = 128 g
⇒ mH2O = (2)(18) g = 36 g
⇒ mproducts = mSO2 + mH2O = 128 g + 36 g = 164 g
Conceptual Problem 12.1
Hydrogen sulfide, which smells like rotten eggs, is found in volcanic gases.
The balanced equation for the burning of hydrogen sulfide is:
2 H2S(g) + 3 O2(g) → 2 SO2(g) + 2 H2O(g)
Interpret this equation in terms of
a.numbers of representative particles and moles. H2S:O2:SO2:H2O = 2:3:2:2
b.masses of reactants (164 g) and products (164 g).
The balanced equation obeys the law of the conservation of mass.
Therefore, the mass of reactants is equal to the mass of products.
mass = (number of mols)(molar mass)
⇒ mSO2 = (2)(64) g = 128 g
⇒ mH2O = (2)(18) g = 36 g
⇒ mproducts = mSO2 + mH2O = 128 g + 36 g = 164 g
Summary
 A balanced chemical equation can be interpreted in
terms of many different quantities.





numbers of atoms
numbers of molecules
moles
mass
volume
 The quantities number of atoms and mass are
conserved.
 The quantities number of molecules, moles, and
volume may not be conserved.