chemistry 101 : chap. 4 aqueous reactions and solution stoichiometry (1) general properties of...

Chemistry 101 : Chap. 4Chemistry 101 : Chap. 4

Aqueous Reactions and Solution Stoichiometry

(1) General Properties of Aqueous Solutions

(2) Precipitation Reactions

(3) Acid-Base Reactions

(4) Concentration of Solutions

(5) Solution Stoichiometry and Chemical Analysis

Aqueous SolutionsAqueous Solutions

Solution : A homogeneous mixture of two or more substance

Solvent : The substance present in the largest amount

Solute(s) : The other substance in the solution

Aqueous Solutions : Solutions in which water is the dissolving medium (or solvent)

Ionic Compounds in WaterIonic Compounds in Water

When ionic compounds dissolve in water, they dissociate completely into ions. The ions are surrounded by water molecules

--

-

--+

++

+

-

-

+

++ H

2O

NaCl (s)

NaCl (aq) or Na+ (aq) + Cl- (aq)

[ Solvation of Na+ and Cl- ]

Molecular Compounds in WaterMolecular Compounds in Water

When molecular compounds dissolve in water, they do not dissociate into ions in general. The molecules remain intact and are surrounded by water molecules

+ H2O

CH3OH (l)

CH3OH (aq)

Molecular Compounds in WaterMolecular Compounds in Water

Some molecular compounds, when dissolved in water, dissociate (ionoized) into ions.

Strong Acid, such as hydrochloric acid, dissociate completely

HCl (aq) H+ (aq) + Cl- (aq)

Weak Acid, such as acetic acid, dissociate only partially

CH3COOH (aq) H+ (aq) + CH3COO- (aq)

NOTE: All three species are present in solutions

Electrolyte vs. NonelectrolyteElectrolyte vs. Nonelectrolyte

Electrolyte : A substance whose aqueous solutions contains ions

Ionic compounds

Strong acids

Weak acids

strong electrolyte

strong electrolyte

weak electrolyte

Nonelectrolyte : A substance that does not form ions in solution

NOTE: Don’t confuse the extent to which an electrolyte dissolves with whether it is strong or weak

Ex. CH3COOH is extremely soluble, but it is a weak electrolyte.

Ba(OH)2 is not very soluble, but it is a strong electrolyte.

the dissolved Ba(OH)2 completely dissociate

Electrolyte SolutionsElectrolyte Solutions

Electrolyte solutions conduct electricity !!

Electrical conductivity is the flow of charged particles through

substance, such as the flow of electrons through a wire or

the flow of ions through a solution.

Testing electrical conductivity of electrolyte solutions

Electrolyte SolutionsElectrolyte Solutions

Precipitation ReactionsPrecipitation Reactions

Recall : Three types of chemical reactions studied in Ch. 3

Combination Reaction : A + B C

Decomposition Reaction : A B + C

Combustion Reaction : CxHy + O2 CO2 + H2O

Exchange Reaction (Metathesis Reaction)

AB + CD AD + CB

Reactants “exchange partners” to form the products


Precipitation Reactions : Reactions that result in the formation of insoluble products

Pb(NO3)2 (aq) + 2KI (aq) PbI2 (s) + 2KNO3 (aq)yellow ppt

AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)white ppt

Mg(NO3)2 (aq) + 2NaOH (aq) Mg(OH)2 (s) + 2NaNO3 (aq)

white ppt


What is happening in molecular level (nanoscale view)?

AgNO3 (aq) + NaI (aq) AgI (s) + NaNO3 (aq)brown ppt

+

NOTE: Water molecules are omitted in these pictures

Solubility GuidelinesSolubility Guidelines

Question: How can you tell if a precipitation will form during an

exchange reaction?


Example: Which of the following compounds is likely soluble in water?

Fe(NO3)3

AgCl

CaCO3

(NH4)2SO4

CuSO4

Cu(OH)2


Empirical Rule : Salts containing following ions are always soluble

Group 1A ion (alkali metal):

Li+, Na+, K+, …

Ammonium ion :

NH4+

Nitrate ion :

NO3-

Acetate : CH3COO-

Insoluble Ionic compounds :

Ag+, Hg2+, Pb2+ + Halogen anions (Cl-, Br-, I-)

Predicting Precipitation ReactionsPredicting Precipitation Reactions

Example : Complete the following equation and determine whether

or not a precipitation reaction (exchange reaction) will

occur. If a reaction will occur, balance the equation

Pb(NO3)2 (aq) + KI (aq)

(1) Determine the products of exchange reaction:

(2) Determine if there is any insoluble product

(3) Balance the chemical equation

Predicting Precipitation ReactionsPredicting Precipitation Reactions

Example : Complete the following equation and determine whether

or not a precipitation reaction (exchange reaction) will

occur. If a reaction will occur, balance the equation

Mg(NO3)2 (aq) + KCl (aq)

(1) Determine the products of exchange reaction:

(2) Determine if there is any insoluble product

(3) Balance the chemical equation (?)

Net Ionic EquationNet Ionic Equation

Pb(NO3)2 (aq) + 2 KI (aq) PbI2 (s) +2 KNO3 (aq)

Molecular equation :

(complete) Ionic equation:

Pb2+(aq) + 2NO3-(aq) + 2K+(aq) + 2I- (aq)

PbI2 (s) + 2K+(aq) + 2NO3-(aq)

spectator ions Net ionic equation :

Pb2+(aq) + 2I- (aq) PbI2 (s)

NOTE: Mg(NO3)2 (aq) + 2 KCl (aq) All ions are spectators ! No net reaction !!


Summary:

Spectator Ions : Ions present in solution before and after

the reaction.

Net Ionic Equation : Spectators are omitted and only the ions

involved in forming the precipitation are shown

NOTE: Net ionic equation must be mass balanced and charge balanced.

Writing net ionic equation :

(1) Write a balanced molecular equation for the reaction

(2) Write the complete ionic equation [precipitation or not]

(3) Identify and remove the spectator ions


Example : Write the net ionic equation for the precipitation reaction

that occurs when solutions of nickel nitrate and

sodium hydroxide are mixed.

(1) Write the balanced molecular equation

(2) Write the complete ionic equation

(3) Write the net ionic equation


Example : Write the net ionic equation for the precipitation reaction

that may occur when solutions of calcium nitrate and

sodium acetate are mixed.

(1) Write the balanced molecular equation

(2) Check the solubility of products

(3) Net ionic equation:

Acids and BasesAcids and Bases

Properties of Acids

Acids ionize to produce protons: HCl (aq) H+(aq) + Cl-(aq)

Turn blue litmus red

Taste sour Aqueous solution has a pH less than 7

Properties of Bases

Bases produce OH- in water: NaOH (aq) Na+(aq) + OH-(aq)

Turn red litmus blue

Bitter taste Aqueous solution has a pH more than 7

React with metal to produce H2 gas

Acids and Bases Acids and Bases

Strong acids and bases

Strong acids and bases ionize completely in aqueous solution.

Strong acids and bases are strong electrolytes.

HCl (aq) H+(aq) + Cl-(aq) 100% ionized

NaOH (aq) Na+(aq) + OH-(aq) 100% ionized

HCl, HBr, HI, HNO3, H2SO4, HClO4, HClO3 strong acids

group 1A hydroxide [LiOH, NaOH, KOH]

group 2A hydroxide [Ca(OH)2, Sr(OH)2, Ba(OH)2]strong bases

Acids and BasesAcids and Bases

Weak acids and bases

Weak acids and bases ionize partially in aqueous solution.

Weak acids and bases are weak electrolytes.

HF (aq) H+(aq) + F-(aq) ~10% ionized

HF, CH3COOH, HNO2, … weak acids

NH3, … weak bases

NOTE: NH3 do not have OH. However, they produce OH- by

accepting a proton from H2O.

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

Acid-Base ReactionsAcid-Base Reactions

acid-base neutralization reactions:

An exchange reaction in which a molecular compound (H2O)

is formed.

molecular equation: HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)

complete ionic equation :

H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) H2O(l) + Na+(aq) + Cl-(aq)

net ionic equation :

H+(aq) + OH-(aq) H2O(l)


NOTE: (1) The products of simple acid-base reactions are water and ionic compounds called “salts”.

HNO3(aq) + NaOH (aq) H2O(l) + NaNO3(aq)

acid base water salt

(2) The formation of water is what drives the acid-base neutralization reaction

H+(aq) + OH-(aq) H2O(l)

possible, but much less favorable than the other way


Example : Complete and balance the following equation for an

acid-base neutralization reaction: H3PO4(aq) + KOH(aq)

What is the net ionic equation?


Example : Complete and balance the following equation for an

acid-base neutralization reaction: Mg(OH)2(s) + HCl(aq)



acid-base reactions with gas formation :

Acids reacting with certain weak bases produce a salt (ionic

compound) and gas (molecular compound). The most common

reaction of this type involves carbonates (CO32-).

2 HCl(aq) + Na2CO3(aq) H2CO3 (aq) + 2NaCl(aq)

unstable

H2CO3(aq) H2O (l) + CO2(g)

The overall reaction will be

2 HCl(aq) + Na2CO3(aq) H2O(l) + 2NaCl(aq) + CO2(g)


Example : What is the net ionic equation for the reaction between

Na2CO3(aq) and HCl(aq)?


Example: Complete and balance the following equation.

HCl(aq) + CaCO3(s)


Concentrations of SolutionsConcentrations of Solutions

Concentration: The amount of solute dissolved in a given amount

of solvent or solution

Molarity: The most common way of expressing concentration

in chemistry

literin solution of volume

solute moles(M)Molarity

A 1.00 molar solution (= 1.00M) contains 1.00 mol of solute in every liter of solution.


1 mol 1 liter 1M

1 mol ? liter 1M


Example : Calculate the molarity of a solution made by dissolving

23.4 g of Na2SO4 (MW= 142) in enough water to form

125 mL of solution.

Concentrations of ElectrolyteConcentrations of Electrolyte

1 mole of Na2SO4 (s)

1M solutionof Na2SO4

1L

2Na+(aq) + SO42-(aq)

Each formula unit of Na2SO4 that dissolve produces two Na+ ions

and one SO42- ion. The concentration of Na+ (2M) will be twice

the SO42- concentration (1M).

ConcentrationsConcentrations

Example: How many grams of Na2SO4 (MW=142) are required

to make 0.350L of 0.500M Na+(aq)?

DilutionsDilutions

add solvent

number of solutemolecules before dilution

number of solute molecules after dilution=

number of moles of solute before dilution

number of moles of solute after dilution=

Mconc , Vconc Mdilute , Vdilute

Mconc Vconc = Mdilute Vdilute

DilutionsDilutions

Example : What is the molarity of the resulting solution when

15 mL of 0.65M solution is diluted to 315mL?

Solution StoichiometrySolution Stoichiometry

Example : What volume of a 0.100 M HNO3 solution is needed

to completely neutralize 0.1 g of Ca(OH)2 ?

Stratagy: (1) What is the chemical reaction involving HNO3(aq)

and Ca(OH)2 (s)?

(2) How many moles of HNO3 is needed to neutralize

0.1 g of Ca(OH)2?

(3) What is the volume of 0.100M HNO3 solution

neutralizing that many moles of Ca(OH)2?

Solution StoichiometrySolution Stoichiometry

aA + bB cC + dD

chemistry 101 : chap. 4 aqueous reactions and solution stoichiometry (1) general properties of...

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