Chemical Quantities and Aqueous Solution

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Chapter 4

• Chemical Quantities and Aqueous Solution

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Titration

• often in the lab, a solutions 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 ratio
• the unknown solution is added slowly from an
  instrument called a burette
• a long glass tube with precise volume markings
  that allows small additions of solution

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Acid-Base Titrations

• the difficulty is determining when there has been
  just enough titrant added to complete the
  reaction
• the 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/alkalinity
• the 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

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Titration
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Titration
The base solution is the titrant in the burette.
As the base is added to the acid, the H reacts
with the OH to form water. But there is still
excess acid present so the color does not change.
At the titrations endpoint, just enough base has
been added to neutralize all the acid. At this
point the indicator changes color.
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Example

• The 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?

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Gas Evolving Reactions

• Some reactions form a gas directly from the ion
  exchange
• K2S(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)

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NaHCO3(aq) HCl(aq) ? NaCl(aq) CO2(g) H2O(l)
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Compounds that UndergoGas Evolving Reactions
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)
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• 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

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Other Patterns in Reactions

• the 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 reactions
• also known as redox reactions
• many involve the reaction of a substance with
  O2(g)
• 4 Fe(s) 3 O2(g) ? 2 Fe2O3(s)

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Combustion as Redox2 H2(g) O2(g) ? 2 H2O(g)
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Redox without Combustion2 Na(s) Cl2(g) ? 2
NaCl(s)
2 Na ? 2 Na 2 e?
Cl2 2 e? ? 2 Cl?
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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
  nonmetal
• in addition, both reactions involve the
  conversion of free elements into ions
• 4 Na(s) O2(g) ? 2 Na2O (s)
• 2 Na(s) Cl2(g) ? 2 NaCl(s)

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Oxidation and Reduction

• in order to convert a free element into an ion,
  the atoms must gain or lose electrons
• of course, if one atom loses electrons, another
  must accept them
• reactions where electrons are transferred from
  one atom to another are redox reactions
• atoms that lose electrons are being oxidized,
  atoms that gain electrons are being reduced

2 Na(s) Cl2(g) ? 2 NaCl(s) Na ? Na 1 e
oxidation Cl2 2 e ? 2 Cl reduction
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Electron Bookkeeping

• for 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
  reaction
• even 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

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Rules for Assigning Oxidation States

• rules are in order of priority
• free elements have an oxidation state 0
• Na 0 and Cl2 0 in 2 Na(s) Cl2(g)
• monatomic ions have an oxidation state equal to
  their charge
• Na 1 and Cl -1 in NaCl
• (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

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Rules for Assigning Oxidation States

• (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
• (a) Group I metals have an oxidation state of 1
  in all their compounds
• Na 1 in NaCl
• (b) Group II metals have an oxidation state of
  2 in all their compounds
• Mg 2 in MgCl2

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Rules for Assigning Oxidation States

• in their compounds, nonmetals have oxidation
  states according to the table below
• nonmetals higher on the table take priority

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
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Practice Assign an Oxidation State to Each
Element in the following

• Br2
• K
• LiF
• CO2
• SO42-
• Na2O2

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Oxidation and ReductionAnother Definition

• oxidation occurs when an atoms oxidation state
  increases during a reaction
• reduction occurs when an atoms oxidation state
  decreases during a reaction

CH4 2 O2 ? CO2 2 H2O -4 1
0 4 2 1 -2
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OxidationReduction

• oxidation 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 agent
• the reducing agent contains the element that is
  oxidized
• the reactant that oxidizes an element in another
  reactant is called the oxidizing agent
• the oxidizing agent contains the element that is
  reduced

2 Na(s) Cl2(g) ? 2 NaCl(s) Na is oxidized, Cl
is reduced Na is the reducing agent, Cl2 is the
oxidizing agent
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Identify the Oxidizing and Reducing Agents in
Each of the Following

• 3 H2S 2 NO3 2 H 3 S 2 NO 4 H2O
• MnO2 4 HBr MnBr2 Br2 2 H2O

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Combustion Reactions

• Reactions 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

2 C8H18(g) 25 O2(g) ? 16 CO2(g) 18 H2O(g)
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Combustion Products

• to predict the products of a combustion reaction,
  combine each element in the other reactant with
  oxygen

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)
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Practice Complete the Reactions

• combustion of C3H7OH(l)
• combustion of CH3NH2(g)

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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)