aqueous reactions and stoichiometry lecture

8/10/2019 Aqueous Reactions and Stoichiometry Lecture

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Chapter 4 Aqueous Reactions and

Solution StoichiometryJohn D. Bookstaver

St. Charles Community CollegeSt. Peters, MO

2006, Prentice Hall, Inc.

Chemistry, The Central Science , 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.;

and Bruce E. Bursten


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Solutions:

Homogeneousmixtures of two ormore pure

substances. The solvent is

present in greatestabundance.

All other substancesare solutes .


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Dissociation

When an ionicsubstance dissolves

in water, the solventpulls the individualions from the crystaland solvates them.

This process is calleddissociation .


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Electrolytes

Substances thatdissociate into ions

when dissolved inwater. A nonelectrolyte may

dissolve in water, butit does not dissociateinto ions when it doesso.


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Electrolytes andNonelectrolytes

Soluble ionic

compounds tendto be electrolytes.


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Electrolytes andNonelectrolytes

Molecular

compounds tend tobe nonelectrolytes,except for acids andbases.


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Electrolytes

A strong electrolyte dissociates completely

when dissolved inwater. A weak electrolyte

only dissociatespartially whendissolved in water.


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Strong Electrolytes Are

Strong acids


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Strong acids Strong bases


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Strong acids Strong bases Soluble ionic salts


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Precipitation Reactions

When one mixes ionsthat form compounds

that are insoluble (ascould be predicted bythe solubilityguidelines), a

precipitate is formed.


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Metathesis (Exchange) Reactions

Metathesis comes from a Greek word thatmeans to transpose

AgNO 3 ( aq ) + KCl (aq ) AgCl (s ) + KNO 3 ( aq )


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It appears the ions in the reactantcompounds exchange, or transpose, ions

Ag NO 3 ( aq ) + K Cl (aq ) AgCl (s ) + KNO 3 ( aq )


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It appears the ions in the reactantcompounds exchange, or transpose, ions

Ag NO 3 ( aq ) + K Cl (aq ) AgCl (s ) + KNO 3 ( aq )


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Solution Chemistry

It is helpful to pay attention to exactly what species are present in a reaction

mixture (i.e., solid, liquid, gas, aqueoussolution).

If we are to understand reactivity, we

must be aware of just what is changingduring the course of a reaction.


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Molecular Equation

The molecular equation lists the reactantsand products in their molecular form.

AgNO 3 (aq ) + KCl (aq ) AgCl (s ) + KNO 3 (aq )


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Ionic Equation

In the ionic equation all strong electrolytes (strongacids, strong bases, and soluble ionic salts) aredissociated into their ions.

This more accurately reflects the species that arefound in the reaction mixture.

Ag+

(aq ) + NO 3-

(aq ) + K+

(aq ) + Cl-

(aq )

AgCl (s ) + K+ (aq ) + NO 3- (aq )


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Net Ionic Equation

To form the net ionic equation, cross out anythingthat does not change from the left side of theequation to the right.

Ag +(aq) + NO 3-(aq ) + K +(aq) + Cl -(aq )

AgCl (s ) + K +(aq) + NO 3-(aq )


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Net Ionic Equation


The only things left in the equation are those thingsthat change (i.e., react) during the course of thereaction.

Ag +(aq) + Cl -(aq ) AgCl (s )


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Net Ionic Equation


The only things left in the equation are those thingsthat change (i.e., react) during the course of thereaction.

Those things that didnt change (and were deleted

from the net ionic equation) are called spectator ions .

Ag +(aq) + NO 3-(aq ) + K +(aq) + Cl

-(aq ) AgCl (s ) + K +(aq) + NO 3

-(aq )


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Writing Net Ionic Equations

1. Write a balanced molecular equation.2. Dissociate all strong electrolytes.3. Cross out anything that remains

unchanged from the left side to theright side of the equation.

4. Write the net ionic equation with thespecies that remain.


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Writing Net Ionic Equations


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Acids:

Substances thatincrease the

concentration of H+

when dissolved inwater (Arrhenius).

Proton donors(Brnsted Lowry).


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Acids

There are only sevenstrong acids:

Hydrochloric (HCl) Hydrobromic (HBr) Hydroiodic (HI) Nitric (HNO 3)

Sulfuric (H 2SO 4) Chloric (HClO 3) Perchloric (HClO 4)


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Bases:

Substances thatincrease the

concentration ofOH when dissolvedin water (Arrhenius).

Proton acceptors(Brnsted Lowry).


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Bases

The strong basesare the soluble salts

of hydroxide ion: Alkali metals Calcium Strontium

Barium


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Acid-Base Reactions

In an acid-basereaction, the acid

donates a proton(H+) to the base.


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Neutralization Reactions

Generally, when solutions of an acid and a base arecombined, the products are a salt and water.

HCl (aq ) + NaOH (aq ) NaCl (aq ) + H2O (l )


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When a strong acid reacts with a strong base, the netionic equation is


H+ (aq ) + Cl - (aq ) + Na + (aq ) + OH -(aq ) Na + (aq ) + Cl - (aq ) + H 2O ( l )


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When a strong acid reacts with a strong base, the netionic equation is


H+ (aq ) + Cl - (aq ) + Na + (aq ) + OH -(aq ) Na + (aq ) + Cl - (aq ) + H 2O ( l )

H+ (aq ) + Cl - (aq) + Na + (aq ) + OH - (aq ) Na + (aq ) + Cl - (aq ) + H 2O ( l )


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Observe thereaction between

Milk of Magnesia,Mg(OH) 2, and HCl.


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Gas-Forming Reactions

These metathesis reactions do not give theproduct expected.

The expected product decomposes to give agaseous product (CO 2 or SO 2).

CaCO 3 (s ) + HCl (aq ) CaCl 2 (aq ) + CO 2 (g ) + H 2O (l ) NaHCO 3 (aq ) + HBr (aq ) NaBr (aq ) + CO 2 (g ) + H 2O (l )

SrSO 3 (s ) + 2 HI (aq ) SrI 2 (aq ) + SO 2 (g ) + H 2O (l )


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Gas-Forming Reactions

This reaction gives the predicted product, butyou had better carry it out in the hood, or you

will be very unpopular! Just as in the previous examples, a gas isformed as a product of this reaction:

Na 2S (aq ) + H 2SO 4 (aq ) Na 2SO 4 (aq ) + H 2S (g )


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Oxidation-Reduction Reactions

An oxidation occurswhen an atom or ion

loses electrons. A reduction occurswhen an atom or iongains electrons.


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Oxidation-Reduction Reactions

One cannot occur

without the other.


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Oxidation Numbers

To determine if an oxidation-reductionreaction has occurred, we assign an

oxidation number to each element in aneutral compound or charged entity.


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Oxidation Numbers

Elements in their elemental form havean oxidation number of 0.

The oxidation number of a monatomicion is the same as its charge.


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Oxidation Numbers

Nonmetals tend to have negativeoxidation numbers, although some are

positive in certain compounds or ions.Oxygen has an oxidation number of 2,except in the peroxide ion in which it hasan oxidation number of 1.Hydrogen is 1 when bonded to a metal,+1 when bonded to a nonmetal.


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Oxidation Numbers

Nonmetals tend to have negativeoxidation numbers, although some are

positive in certain compounds or ions.Fluorine always has an oxidation numberof 1.

The other halogens have an oxidationnumber of 1 when they are negative; theycan have positive oxidation numbers,however, most notably in oxyanions.


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Oxidation Numbers

The sum of the oxidation numbers in aneutral compound is 0.

The sum of the oxidation numbers in apolyatomic ion is the charge on the ion.


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Oxidation Numbers


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Displacement Reactions

In displacementreactions, ions

oxidize an element. The ions, then, arereduced.


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In this reaction,silver ions oxidizecopper metal.

Cu (s ) + 2 Ag + (aq ) Cu 2+ (aq ) + 2 Ag (s )


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The reverse reaction,however, does notoccur.

Cu 2+ (aq ) + 2 Ag (s ) Cu (s ) + 2 Ag + (aq )x


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Activity Series


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Molarity

Two solutions can contain the samecompounds but be quite different because theproportions of those compounds are different.

Molarity is one way to measure theconcentration of a solution.

moles of solutevolume of solution in litersMolarity ( M ) =


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Mixing a Solution


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Dilution


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Using Molarities inStoichiometric Calculations


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Titration

The analyticaltechnique inwhich one cancalculate theconcentration ofa solute in asolution.


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Titration

aqueous reactions and stoichiometry lecture

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