Chemical Quantities and Aqueous Solution. Titration often in the lab, a solution’s concentration is determined by reacting it with another material and.

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



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)




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)




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)


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


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


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


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


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


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


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


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


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


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


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


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)


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)


Chemical Quantities and Aqueous Solution. Titration often in the lab, a solution’s concentration is determined by reacting it with another material and.

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