CH. 4 Aqueous Solutions
Solutions in which water is the dissolving medium are called __aqueous solutions_
A _solution__ is a homogeneous mixture of two or more substances. The solution present in greater quantity is usually called the _solvent____________. The other substance in solution is known as the _solute________.
* The __solute___ is dissolved in the __solvent____
A substance whose aqueous solutions contain ions is called an _electroylte_. A substance that does not form ions in solution is called a _nonelectrolyte___.
How Compounds Dissolve in Water
NaCl(s) →
As an ionic compound dissolves , the ions become surrounded by H2O molecules. The ions are said to be solvated. Solvation helps stabilize the ions in solution and prevents the ions from combining.
Na2SO4(s)
C6H12O6(s)
Strong Electrolytes are those solutes that exist in solution __completely__ or __nearly completely___ as ions.
* Essentially all soluble __ionic compounds_ and a few molecular compounds (acids) are strong electrolytes.
Weak electrolytes are those solutes that exists in solution mostly in the form of _molecules__ with a small fraction in the form of ions.
*Be careful to not confuse the __extent__ to which an electrolyte dissolves with whether it is __strong_ or __weak___.
When a weak electrolyte such as acetic acid ionizes in solution, we write the reaction:
The double arrow means that the reaction is significant in both directions. This balance produces a state of _chemical equilibrium _that varies from one weak electrolyte to another.
Chemists use a double arrow to represent ionization of___weak electroyltes__ and a single arrow to represent the ionization of __strong electroyltes__.
The single arrow indicates that the H+ and Cl- ions have no tendency to recombine in water to form HCl molecules.
Practice Problem 4.1
The diagram to the left represents an aqueous solution of one of the following compounds: MgCl2, KCl or K2SO4. Which solution does it best represent?
Precipitation Reactions
Reactions that results in the formation of an insoluble product (not dissolved) are known as _precipation reactions_.
A _precipitate_is an insoluble solid formed by a reaction in solution. Example:
*Precipitation reactions occur when certain pairs of oppositely charged ions attract each other so strongly that they form an insoluble ionic solid.
The _solubility___ of a substance is the amount of that substance that can be dissolved in a given quantity of solvent.
~ Any substance with a solubility less than 0.01mol/L will be referred to as insoluble.
SOLUBILITY RULES |
Always soluble: alkali metal ions (Li+, Na+, K+, Rb+, Cs+), NH4+, NO3–, ClO3–, ClO4–, C2H3O2– Generally soluble: (mnemonics) Cl–, Br–, I– Soluble except Ag+, Pb2+, Hg22+ (AP/H) F– Soluble except Ca2+, Sr2+, Ba2+, Pb2+ , Mg2+ (CBS-PM) SO42– Soluble except Ca2+, Sr2+, Ba2+, Pb2+ (CBS/PBS) Generally insoluble: O2–, OH– Insoluble except alkali metal ions and NH4+ Ca2+, Sr2+, Ba2+ (CBS) somewhat soluble
CO32–, PO43–, S2–, SO32–, C2O42–, CrO42– Insoluble except alkali metals and NH4+ |
To predict whether a precipitate forms when we mix aqueous solutions of two strong electrolytes, we must (1) note the ions present in the reactant (2) consider the possible combinations of the cations and anions (3) determine if any of these combinations in insoluble.
Example: Mg(NO3)2 and NaOH are mixed
The reaction above is known as _exchange reactions___or __metathesis _ reactions. Precipitation reactions conform in this pattern; which positive ions and negative ions appear to exchange partners.
Practice: Write the balanced chemical equation for the reaction between BaCl2 and K2SO4
Ionic Equations
Consider the equation:
Pb(NO3)2(aq) + 2 KI(aq) PbI2(s) + 2KNO3(aq)
An equation written in this fashion, showing the complete chemical formulas of the reactants and products, is called the __molecular equation__ because it shows the chemical formulas of the reactants & products without indicating their ionic character.
Now if we write the chemical equation to indicate explicitly the ions that are in solution:
An equation written in this form, with all the soluble strong electrolytes shown as ion, is known as a
_compete ionic equation ___.
Ions that appear in identical forms among both the reactants & products of a complete ionic equation are called _specator ions__. * They are present but play no direct role in the reaction. If we omit the spectator ions:
We are left with the ___net ionic equation____. A net ionic equation includes only the ions and molecules directly involved in the reaction. Charge is conserved in reactions, so the sum of the charges of the ions must be same on both sides.
* If every ion in a complete ionic equation is a spectator ion, then no reaction occurs!
Writing Net Ionic Equations
1) Write the balanced molecular equation
2) Rewrite the equation to show the ions that form in solutions when each soluble strong electrolyte dissociates into its component ions. *Only strong electrolytes dissolved in aqueous solutions are written in ionic form.
3) Identify and cancel spectator ions.
Practice Problem
Write the net ionic equation for the precipitation reaction that occurs when solutions of calcium chloride and sodium carbonate are mixed.
Acid-Base Reactions
_Acids_ are substances that ionize in aqueous solutions to form _hydrogen ions__, thereby increasing the concentration of ___________ .
* Because a hydrogen atom consists of a proton and an electron, H+ is simply a __proton__.
Acids are often called proton donors.
__Monoprotic acids_ yield one H+ per molecules of acid. Examples:
Diprotic acids yield two H+ per molecules of acid. The ionization of diprotic acids occurs in two steps:
_Bases_ are substances that accept (react with) H+ ions. Bases produce __hydroxide ions__ when they dissolve in water.
Compounds that do not contain OH- ions can also be bases. For example:
Because only a small fraction of the NH3 forms NH4+ and OH- ions, ammonia is a weak electrolyte.
Acids and bases that are strong electrolytes are called _strong_____ acid and __strong_ bases. Those that are weak electrolytes are called __weak_acids and __weakbases.
The reactivity of acids depends on the anions as well as the H+. If the reactivity depends on the concentration of H+, then strong acids are _more_ reactive than weak acids.
Strong Acids | Strong Bases |
HCl hydrochloric acid HBr hydrobromic acid HI hydroiodic acid HClO3 chloric acid HClO4 perchloric acid H2SO4 sulfuric acid (1st H) HNO3 nitric acid | Group 1A metal hydroxides (LiOH, NaOH, KOH, RbOH, CsOH) Heavy group 2A metal hydroxides [ Ca(OH)2, Sr(OH)2, Ba(OH)2 ] *MEMORIZE- all others are weak! * |
Summary of Electrolytic Behavior | |||
| Strong Electrolyte | Weak Electrolyte | Nonelectrolyte |
Ionic | | | |
Molecular | | | |
Classify the following as strong, weak or nonelectrolyte: CaCl2, HNO3, C2H5OH, HCOOH (formic acid), KOH
When solutions of an acid and a base are mixed, a _neutralization reaction occurs. The products of the reaction have none of the characteristics properties of either the acidic or basic solutions.
Example:
The term _salt_ has come to mean any ionic compound whose cation comes from a base and whose anion comes from an acid. In general, a neutralization reaction between an acid and a metal hydroxide produces a salt__ and __water___________.
If we write the complete ionic equation above, we get:
Therefore the net ionic equation is,
The essential feature of a neutralization reaction between any strong acids and any strong base: H+(aq) and OH-(aq) ions combine to form H2O.
Oxidation-Reduction Reactions
Reactions in with electrons are transferred between reactant are called _oxidation-reduction __ or __redox_____________ reactions.
What we call ___corrosion__ is the conversion of a metal into a metal compound. When a metal corrodes, it __loses____ electrons and forms _cation_. When an atom, ion or molecule has become more positively charged , we say that it has been __oxidized___.
*Loss of electrons by a substance is called __oxidation____.
When an atom, ion, molecule has become more negatively charged (gained electrons), we say that it is __reduced_______________.
*Gain of electrons by a substance is called __reduction________________.
When one reactant __loses___ electrons, another reactant must _gain___ them; the __oxidation___ of one substance is always accompanied by the _reduction___ of another as electrons are transferred between them.
Oxidation Numbers
The __oxidation number_of an atom in a substance is the actual charge of the atom if it is a monatomic ion; otherwise, it is the hypothetical charge assigned to the atom using a set of rules.
1) For an atom in its elemental form the oxidation number is always zero.
example:
2) For any monatomic ion the oxidation number equals the charge on the ion.
example:
Note: we will write the sign before the number to distinguish them from the actual electronic charge.
3) Nonmetals usually have negative oxidation numbers.
a) The oxidation number of _oxygen__ is usually -2 in both ionic and molecular compounds. The major exception is in compounds called peroxides, which contain O2-2 ion, giving each oxygen an oxidation number of -1.
b) The oxidation number of _hydrogen is +1 when bonded with nonmetals, and -1 when bonded with metals.
c) The oxidation number of fluorine is -1 in all compounds. The other _halogens____________ have an oxidation number of -1 in most binary compounds. When combined with oxygen, as in oxyanions, however, they have positive oxidation states.
4) The sum of the oxidation numbers of all the atoms in a neutral compound is __zero__________. The sum of the oxidation numbers in a polyatomic ion equals the charge of the ion.
example:
Practice: determine the oxidation state of sulfur in each of the following
a) H2S b) S8 c) SCl2 d) Na2SO4 e) SO4-2
Oxidation of Metals by Acids & Salts
A reaction of a metal with either an acid or a metal salt conforms to the following general pattern:
These reactions are called ___displacement reactions__ because the ion in solution is displaced or replaced through oxidation of an element.
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
The oxidation number of Mg changes from 0 to +2. The increase in the oxidation number indicates that the atom has lost electrons and therefore been __oxidized_______.
The H+ ion of the acid decreases in oxidation number from +1 to 0, indicating that this ion has gained electrons and has therefore been ____reduced_______________.
The oxidation number of Cl- ion remains -1, and it is a __spectator ion_ in the reaction.
The __activity series __ of metals helps to predict whether a certain metal will be oxidized.
* Any metal on the list can be oxidized by the ions of elements below it.
__Concentration__ designates the amount of solute dissolved in a given quantity of solvent or solution.
The greater the amount of solute dissolved in a certain amount of solvent, the more concentrated the resulting solution.
__Molarity__ (symbol M) expresses the concentration of a solution as the number of moles of solution in a liter of solution:
Practice: Calculate the molarity of a solution made by dissolving 23.4g of sodium sulfate (Na2SO4 molar mass= 142g/mol) in enough water to form 125 mL of solution.
Expressing the Concentration of an Electrolyte
When an ionic compound dissolves, the relative concentration of the ions introduced into the solution depends on the chemical formula of the compound.
Example: A 1.0M solution of NaCl is _________ in Na+ and _________ in Cl- ions.
A 1.0M solution of Na2SO4 is __________ in Na+ and __________ in SO4-2 ions.
Interconverting Molarity, Moles, and Volume
The definition of molarity contains three quantities- molarity, moles solute, and L of solution. If we know any two of these, we can calculate the third.
Calculation of the number of moles of HNO3 in 2.0L of 0.200M HNO3 solution:
Moles HNO3 =
Calculate the volume of 0.30M HNO3 solution require to supply 2.0 mol of HNO3:
Liter soln =
Solutions of lower concentrations can then be obtained by adding water, a process called __dilution ______.
When a solvent is added to dilute a solution, the number of moles of solute remains unchanged.
Moles solute before dilution = moles solute after dilution
or
Moles solute in conc soln = moles solute in dil soln
*Any volume unit can be used so long as that same unit is used on both sides of the equation
Practice: We want to prepare 250mL of 0.100M CuSO4 solution by diluting a stock solution containing 1.00M CuSO4.
Mconc = Mdil =
Vconc = Vdil =
Practice: How many grams of Ca(OH)2are needed to neutralize 25.0 mL of 0.100 M HNO3?
The determine the concentration of a particular solute in a solution, chemists often carry out a __titration__, which involves combining a sample of the solution with a reagent solution of known concentration.
The point at which stoichiometrically equivalent quantities are brought together is known as the __equivalence point____ (equal number of moles in relation to balanced equation).
In order to titrate an unknown with a standard solution, there must be some way to determine when the equivalence point of the titration has been reached. In acid-base titrations, dyes known as acid-base __indicators_ are use for this purpose.
* The color change singles the _end point_ of the titration, which usually coincides very nearly with the equivalence point.
Practice: The quantity of Cl- is a water supply is determined by titrating the sample with Ag+.
Ag+(aq) + Cl-(aq) → AgCl(s)
a) How many grams of chloride ion are in a sample of the water if 20.2 mL of 0.100M Ag+ is needed to react with all the chloride in the sample?