aqueous reactions general properties of aqueous solutions -electrolytes -concentration -dilutions...

AqueousReactions

• General Properties of Aqueous Solutions- Electrolytes - Concentration - Dilutions

• Reactions- Precipitation Reactions

- Acid-Base Reactions

- Oxidation-Reduction Reactions

© 2009, Prentice-Hall, Inc.

Topic 4Topic 4Aqueous Reactions and Solution Aqueous Reactions and Solution

StoichiometryStoichiometry

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General Properties of Aqueous Solutions

• Solutions are defined as homogeneous mixtures of two or more pure substances.

• The solvent is present in greatest abundance.

• All other substances are solutes.

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• One of the most important substances on Earth.

• Can dissolve many different substances.

• A polar molecule because of its unequal charge distribution.

Aqueous solutionsWater is the solvent

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Dissociation

• When an ionic substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them.

• This process is called dissociation.

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Dissolution of a solid in a liquid


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Dissociation

• An electrolyte is a substance that dissociates into ions when dissolved in water.

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Electrolytes

• An electrolyte is a substances that dissociates into ions when dissolved in water.

• A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so.

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Electrolytes

• A strong electrolyte dissociates completely when dissolved in water.

• A weak electrolyte only dissociates partially when dissolved in water.

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Strong Electrolytes Are…

• Strong acids• Strong bases

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Strong Electrolytes Are…

• Strong acids• Strong bases• Soluble ionic salts

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Concentration of Solutions• Two solutions can contain the same

compounds but be quite different because the proportions of those compounds are different.

• Molarity is one way to measure the concentration of a solution.

moles of solute

volume of solution in litersMolarity (M) =

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Concentration of Electrolytes

• 1.0 M solution of NaCl = 1.0 M in Na+ ions

and 1.0 M in Cl- ions

• 1.0 M solution of Na2SO4 = 2.0 M in Na+ ions

and 1.0 M in SO4 ions


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Mixing a Solution

• To create a solution of a known molarity, one weighs out a known mass (and, therefore, number of moles) of the solute.

• The solute is added to a volumetric flask, and solvent is added to the line on the neck of the flask.

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Dilution• One can also dilute a more concentrated

solution by– Using a pipet to deliver a volume of the solution to

a new volumetric flask, and– Adding solvent to the line on the neck of the new

flask.

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DilutionThe molarity of the new solution can be determined from the equation

Mc Vc = Md Vd,

where Mc and Md are the molarity of the concentrated and dilute solutions, respectively, and Vc and Vd are the volumes of the two solutions.

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A 0.50 M solution of sodium chloride in an open beaker sits on a lab bench. Which of the following would decrease the concentration of the salt solution?

a) Add water to the solution.b) Pour some of the solution down the sink drain.c) Add more sodium chloride to the solution.d) Let the solution sit out in the open air for a

couple of days.e) At least two of the above would decrease the

concentration of the salt solution.


CONCEPT CHECK!CONCEPT CHECK!

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What is the minimum volume of a 2.00 M NaOH solution needed to make 150.0 mL of a 0.800 M NaOH solution?


EXERCISE!EXERCISE!

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Solution Stoichiometry and Chemical Analysis

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Precipitation Reactions

When one mixes ions that form compounds that are insoluble (as could be predicted by the solubility guidelines), a precipitate is formed.

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Solubility Rules

• Soluble – solid dissolves in solution; (aq) is used in reaction equation.

• Insoluble – solid does not dissolve in solution; (s) is used in reaction equation.


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Simple Rules for Solubility1. Most nitrate (NO3

) salts are soluble.

2. Most alkali metal (group 1A) salts and NH4+ are soluble.

3. Most Cl, Br, and Isalts are soluble (except Ag+, Pb2+, Hg2

2+).

4. Most sulfate salts are soluble (except BaSO4, PbSO4, Hg2SO4, CaSO4).

5. Most OH are only slightly soluble (NaOH, KOH are soluble, Ba(OH)2, Ca(OH)2 are marginally soluble).

6. Most S2, CO32, CrO4

2, PO43 salts are only slightly soluble,

except for those containing the cations in Rule 2.


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Which of the following ions form compounds with Pb2+ that are generally soluble in water?

a) S2–

b) Cl–

c) NO3–

d) SO42–

e) Na+



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Metathesis (Exchange) Reactions• Metathesis comes from a Greek word that

means “to transpose.”

AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq)

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Metathesis (Exchange) Reactions• Metathesis comes from a Greek word that

means “to transpose.”• It appears the ions in the reactant

compounds exchange, or transpose, ions.


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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, aqueous solution).

• If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction.

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Formula Equation• Gives the overall reaction stoichiometry but not

necessarily the actual forms of the reactants and products in solution.

• Reactants and products generally shown as compounds.• Use solubility rules to determine which compounds are

aqueous and which compounds are solids.


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Complete Ionic Equation• All strong electrolytes (strong acids, strong bases,

and soluble ionic salts) are dissociated into their ions.

• This more accurately reflects the species that are found in the reaction mixture.

Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq) AgCl (s) + K+ (aq) + NO3

- (aq)

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Net Ionic Equation• Includes only those solution components undergoing a

change.• To form the net ionic equation, cross out anything

that does not change from the left side of the equation to the right.

Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl (s) + K+(aq) + NO3

-(aq)

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Net Ionic Equation• To form the net ionic equation, cross out anything


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

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

Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl (s) + K+(aq) + NO3

-(aq)

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Net Ionic Equation• To form the net ionic equation, cross out anything


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

• Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions.

Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)

AgCl (s) + K+(aq) + NO3-(aq)

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Writing Net Ionic Equations

1. Write a balanced formula equation.

2. Dissociate all strong electrolytes.

3. Cross out anything that remains unchanged from the left side to the right side of the equation.

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

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Write the correct formula equation, complete ionic equation, and net ionic equation for the reaction between cobalt(II) chloride and sodium hydroxide.

Formula Equation:CoCl2(aq) + 2NaOH(aq) Co(OH)2(s) + 2NaCl(aq)

Complete Ionic Equation:

Co2+(aq) + 2Cl(aq) + 2Na+(aq) + 2OH(aq)

Co(OH)2(s) + 2Na+(aq) + 2Cl(aq)

Net Ionic Equation:Co2+(aq) + 2OH(aq) Co(OH)2(s)



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Solving Stoichiometry Problems for Reactions in Solution

1. Identify the species present in the combined solution, and determine what reaction occurs.

2. Write the balanced net ionic equation for the reaction.

3. Calculate the moles of reactants.4. Determine which reactant is limiting.5. Calculate the moles of product(s), as required.6. Convert to grams or other units, as required.


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10.0 mL of a 0.30 M sodium phosphate solution reacts with 20.0 mL of a 0.20 M lead(II) nitrate solution (assume no volume change).

– What precipitate will form?– What mass of precipitate will form?– What is the concentration of nitrate ions left

in solution after the reaction is complete? – What is the concentration of phosphate ions

left in solution after the reaction is complete?



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Acid-Base Reactions

• Arrhenius defined acids as substances that increase the concentration of H+ when dissolved in water.

• Brønsted and Lowry defined them as proton donors.

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Acids

There are only seven strong acids:• Hydrochloric (HCl)• Hydrobromic (HBr)• Hydroiodic (HI)

• Nitric (HNO3)

• Sulfuric (H2SO4)

• Chloric (HClO3)

• Perchloric (HClO4)

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Bases• Arrhenius defined bases

as substances that increase the concentration of OH− when dissolved in water.

• Brønsted and Lowry defined them as proton acceptors.

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Bases

The strong bases are the soluble metal salts of hydroxide ion:• Alkali metals• Calcium• Strontium• Barium

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Acid-Base Reactions

In an acid-base reaction, the acid donates a proton (H+) to the base.

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Neutralization ReactionsGenerally, when solutions of an acid and a base are combined, the products are a salt and water.

CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + H2O (l)

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Neutralization ReactionsWhen a strong acid reacts with a strong base, the net ionic equation is…

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

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Neutralization ReactionsWhen a strong acid reacts with a strong base, the net ionic equation is…

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

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

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

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Titration

Titration is an analytical technique in which one can calculate the concentration of a solute in a solution.

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Acid–Base Titrations

• Titration – delivery of a measured volume of a solution of known concentration (the titrant) into a solution containing the substance being analyzed (the analyte).

• Equivalence point – enough titrant added to react exactly with the analyte.

MH+VH+ = MOH-VOH-

• Endpoint – the indicator changes color so you can tell the equivalence point has been reached.


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For the titration of sulfuric acid (H2SO4) with sodium hydroxide (NaOH), how many moles of sodium hydroxide would be required to react with 1.00 L of 0.500 M sulfuric acid to reach the endpoint?



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Gas-Forming Reactions

• Some metathesis reactions do not give the product expected.

• In this reaction, the expected product (H2CO3) decomposes to give a gaseous product (CO2).

CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)

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When a carbonate or bicarbonate reacts with an acid, the products are a salt, carbon dioxide, and water.

CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)

NaHCO3 (aq) + HBr (aq) NaBr (aq) + CO2 (g) + H2O (l)

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Similarly, when a sulfite or sulfide reacts with an acid, the products are a salt, sulfur dioxide, and water.

SrSO3 (s) + 2 HI (aq) SrI2 (aq) + SO2 (g) + H2O (l)

2HCl (aq) + Na2S (aq) H2S (g) + 2NaCl (aq)

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Oxidation-Reduction Reactions

• An oxidation occurs when an atom or ion loses electrons.

• A reduction occurs when an atom or ion gains electrons.

• One cannot occur without the other.

OIL RIGor LEO says GER

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Oxidation Numbers

To determine if an oxidation-reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity.

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Rules for Assigning Oxidation States

1. Oxidation state of an atom in an element = 02. Oxidation state of monatomic ion = charge of the ion3. Oxygen = 2 (except in peroxides where it = 1, or with F

where it = +2)4. Hydrogen = -1 when bonded to metal, +1 when bonded

to a nonmetal5. Sum of oxidation states = 0 in compounds 6. Sum of oxidation states = charge of the ion in ions


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Find the oxidation states for each of the elements in each of the following compounds:

• K2Cr2O7

• CO32-

• MnO2

• PCl5

• SF4


EXERCISE!EXERCISE!

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Displacement Reactions

• In displacement reactions, ions oxidize an element.

• The ions, then, are reduced.

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In this reaction,

silver ions oxidize

copper metal.

Cu (s) + 2 Ag+ (aq) Cu2+ (aq) + 2 Ag (s)

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The reverse reaction,

however, does not

occur.

Cu2+ (aq) + 2 Ag (s) Cu (s) + 2 Ag+ (aq) x

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Which of the following are oxidation-reduction reactions? Identify the oxidizing agent and the reducing agent.

a)Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

b)Cr2O72-(aq) + 2OH-(aq) 2CrO4

2-(aq) + H2O(l)

c)2CuCl(aq) CuCl2(aq) + Cu(s)

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Balancing Oxidation–Reduction Reactions by Oxidation States

1. Write the unbalanced equation.2. Determine the oxidation states of all atoms in

the reactants and products.3. Show electrons gained and lost using “tie lines.”4. Use coefficients to equalize the electrons gained

and lost.5. Balance the rest of the equation by inspection.6. Add appropriate states.


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• Balance the reaction between solid zinc and

aqueous hydrochloric acid to produce aqueous zinc(II) chloride and hydrogen gas.


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1. What is the unbalanced equation?

• Zn(s) + HCl(aq) Zn2+(aq) + Cl–(aq) + H2(g)


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2. What are the oxidation states for each atom?

• Zn(s) + HCl(aq) Zn2+(aq) + Cl–(aq) + H2(g) 0 +1 –1 +2 –1 0


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3. How are electrons gained and lost?

1 e– gained (each atom)

• Zn(s) + HCl(aq) Zn2+(aq) + Cl–(aq) + H2(g) 0 +1 –1 +2 –1 0 2 e– lost

• The oxidation state of chlorine remains unchanged.


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4. What coefficients are needed to equalize the electrons gained and lost?

1 e– gained (each atom) × 2

• Zn(s) + HCl(aq) Zn2+(aq) + Cl–(aq) + H2(g) 0 +1 –1 +2 –1 0 2 e– lost

• Zn(s) + 2HCl(aq) Zn2+(aq) + Cl–(aq) + H2(g)


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5. What coefficients are needed to balance the remaining elements?

• Zn(s) + 2HCl(aq) Zn2+(aq) + 2Cl–(aq) + H2(g)


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