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Chemical Quantities and Aqueous Solution
Titrationoften in the lab, a solution’s
concentration is determined by reacting it with another material and using stoichiometry – this process is called titration
in the titration, the unknown solution is added to a known amount of another reactant until the reaction is just completed, at this point, called the endpoint, the reactants are in their stoichiometric ratiothe unknown solution is added slowly from
an instrument called a burette a long glass tube with precise volume markings
that allows small additions of solutionTro, Chemistry: A Molecular Approach 2
Acid-Base Titrationsthe difficulty is determining when there has been
just enough titrant added to complete the reactionthe titrant is the solution in the burette
in acid-base titrations, because both the reactant and product solutions are colorless, a chemical is added that changes color when the solution undergoes large changes in acidity/alkalinitythe chemical is called an indicator
at the endpoint of an acid-base titration, the number of moles of H+ equals the number of moles of OH aka the equivalence point
Tro, Chemistry: A Molecular Approach 3
Tro, Chemistry: A Molecular Approach 4
Tro, Chemistry: A Molecular Approach 5
The base solution is thetitrant in the burette.
As the base is added tothe acid, the H+ reacts withthe OH– to form water. But there is still excess acid present so the colordoes not change.
At the titration’s endpoint,just enough base has been added to neutralize all theacid. At this point the indicator changes color.
ExampleThe titration of 10.00 mL of HCl solution of
unknown concentration requires 12.54 mL of 0.100 M NaOH solution to reach the end point. What is the concentration of the unknown HCl solution?
The titration of a 20.0 mL sample of an H2SO4 solution of an unknown concentration requires 22.87 mL of a 0.158M KOH solution to reach the equivalent point. What is the concentration of the unknown H2SO4 solution?
Gas Evolving ReactionsSome reactions form a gas directly from the
ion exchangeK2S(aq) + H2SO4(aq) K2SO4(aq) + H2S(g)
Other reactions form a gas by the decomposition of one of the ion exchange products into a gas and water
K2SO3(aq) + H2SO4(aq) K2SO4(aq) + H2SO3(aq)
H2SO3 H2O(l) + SO2(g)
Tro, Chemistry: A Molecular Approach 7
Tro, Chemistry: A Molecular Approach 8
Tro, Chemistry: A Molecular Approach 9
Reactant
Type
Reacting
With
Ion Exchange
Product
Decom-pose?
Gas
Formed
Example
metalnS,
metal HS
acid H2S no H2S K2S(aq) + 2HCl(aq)
2KCl(aq) + H2S(g)
metalnCO3,
metal HCO3
acid H2CO3 yes CO2 K2CO3(aq) + 2HCl(aq)
2KCl(aq) + CO2(g) + H2O(l)
metalnSO3
metal HSO3
acid H2SO3 yes SO2 K2SO3(aq) + 2HCl(aq)
2KCl(aq) + SO2(g) + H2O(l)
(NH4)nanion base NH4OH yes NH3 KOH(aq) + NH4Cl(aq)
KCl(aq) + NH3(g) + H2O(l)
When an aqueous solution of sodium carbonate is added to an aqueous solution of nitric acid, a gas evolves
Write a molecular equation for the gas-evolution that occurs when you mix aqueous hydrobromic acid and aqueous potassium sulfite
Other Patterns in Reactionsthe precipitation, acid-base, and gas evolving
reactions all involved exchanging the ions in the solution
other kinds of reactions involve transferring electrons from one atom to another – these are called oxidation-reduction reactionsalso known as redox reactionsmany involve the reaction of a substance with
O2(g)
4 Fe(s) + 3 O2(g) 2 Fe2O3(s)
Tro, Chemistry: A Molecular Approach 11
Tro, Chemistry: A Molecular Approach 12
Tro, Chemistry: A Molecular Approach 13
2 Na 2 Na+ + 2 e
Cl2 + 2 e 2 Cl
Reactions of Metals with Nonmetals
consider the following reactions:4 Na(s) + O2(g) → 2 Na2O(s)
2 Na(s) + Cl2(g) → 2 NaCl(s)the reaction involves a metal reacting with a
nonmetalin addition, both reactions involve the conversion
of free elements into ions 4 Na(s) + O2(g) → 2 Na+
2O– (s)
2 Na(s) + Cl2(g) → 2 Na+Cl–(s)
Tro, Chemistry: A Molecular Approach 14
Oxidation and Reductionin order to convert a free element into an ion,
the atoms must gain or lose electronsof course, if one atom loses electrons, another must
accept themreactions where electrons are transferred
from one atom to another are redox reactionsatoms that lose electrons are being oxidized,
atoms that gain electrons are being reduced
Tro, Chemistry: A Molecular Approach 15
2 Na(s) + Cl2(g) → 2 Na+Cl–(s)Na → Na+ + 1 e– oxidationCl2 + 2 e– → 2 Cl– reduction
Leo
Ger
Electron Bookkeepingfor reactions that are not metal + nonmetal,
or do not involve O2, we need a method for determining how the electrons are transferred
chemists assign a number to each element in a reaction called an oxidation state that allows them to determine the electron flow in the reactioneven though they look like them, oxidation states
are not ion charges! oxidation states are imaginary charges assigned
based on a set of rules ion charges are real, measurable charges
Tro, Chemistry: A Molecular Approach 16
Rules for Assigning Oxidation States rules are in order of priority1. free elements have an oxidation state = 0
Na = 0 and Cl2 = 0 in 2 Na(s) + Cl2(g)2. monatomic ions have an oxidation state equal
to their charge Na = +1 and Cl = -1 in NaCl
3. (a) the sum of the oxidation states of all the atoms in a compound is 0
Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = 0
Tro, Chemistry: A Molecular Approach 17
Rules for Assigning Oxidation States
3. (b) the sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion
N = +5 and O = -2 in NO3–, (+5) + 3(-2) = -1
4. (a) Group I metals have an oxidation state of +1 in all their compounds
Na = +1 in NaCl
4. (b) Group II metals have an oxidation state of +2 in all their compounds
Mg = +2 in MgCl2
Tro, Chemistry: A Molecular Approach 18
Rules for Assigning Oxidation States
5. in their compounds, nonmetals have oxidation states according to the table below
nonmetals higher on the table take priority
Tro, Chemistry: A Molecular Approach 19
Nonmetal Oxidation State Example
F -1 CF4
H +1 CH4
O -2 CO2
Group 7A -1 CCl4
Group 6A -2 CS2
Group 5A -3 NH3
Practice – Assign an Oxidation State to Each Element in the following
Br2
K+
LiF
CO2
SO42-
Na2O2
Tro, Chemistry: A Molecular Approach 20
Oxidation and ReductionAnother Definitionoxidation occurs when an atom’s oxidation
state increases during a reactionreduction occurs when an atom’s oxidation
state decreases during a reaction
Tro, Chemistry: A Molecular Approach 21
CH4 + 2 O2 → CO2 + 2 H2O-4 +1 0 +4 –2 +1 -2
oxidation
reduction
Oxidation–Reductionoxidation and reduction must occur simultaneously
if an atom loses electrons another atom must take them
the reactant that reduces an element in another reactant is called the reducing agentthe reducing agent contains the element that is oxidized
the reactant that oxidizes an element in another reactant is called the oxidizing agentthe oxidizing agent contains the element that is reduced
Tro, Chemistry: A Molecular Approach 22
2 Na(s) + Cl2(g) → 2 Na+Cl–(s)Na is oxidized, Cl is reduced
Na is the reducing agent, Cl2 is the oxidizing agent
Identify the Oxidizing and Reducing Agents in Each of the Following3 H2S + 2 NO3
– + 2 H+ S + 2 NO + 4 H2O
MnO2 + 4 HBr MnBr2 + Br2 + 2 H2O
Tro, Chemistry: A Molecular Approach 23
Combustion ReactionsReactions in which O2(g)
is a reactant are called combustion reactions
Combustion reactions release lots of energy
Combustion reactions are a subclass of oxidation-reduction reactions
Tro, Chemistry: A Molecular Approach 24
2 C8H18(g) + 25 O2(g) 16 CO2(g) + 18 H2O(g)
Combustion Productsto predict the products of a combustion
reaction, combine each element in the other reactant with oxygen
Tro, Chemistry: A Molecular Approach 25
Reactant Combustion Product
contains C CO2(g)
contains H H2O(g)
contains S SO2(g)
contains N NO(g) or NO2(g)
contains metal M2On(s)
Practice – Complete the Reactionscombustion of C3H7OH(l)
combustion of CH3NH2(g)
Tro, Chemistry: A Molecular Approach 26
Practice – Complete the Reactions
C3H7OH(l) + 5 O2(g) 3 CO2(g) + 4 H2O(g)
CH3NH2(g) + 3 O2(g) CO2(g) + 2 H2O(g) + NO2(g)
Tro, Chemistry: A Molecular Approach 27