Chapter 4 Reactions in Aqueous Solution Lecture Presentation © 2012 Pearson Education, Inc. Ashley Warren Kings High School AP Chemistry Lecture Presentation

Chapter 4

Reactions in Aqueous Solution

Lecture Presentation

© 2012 Pearson Education, Inc.

Ashley Warren Kings High School

AP Chemistry

Lecture Presentation


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Solutions• An aqueous solution is

a solution in which water is the dissolving medium.

• 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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Dissociation

• 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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Chemical Equilbrium

• HCl (aq) H+ (aq) + Cl- (aq)

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

• What is the biggest difference between the two reactions above?

• Which represents a strong electrolyte and which represents a weak?

• Why?


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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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Solvation• Solvation helps stabilize

the ions in the solution and prevents cations and anions from recombining. – Why does water help do

this?

• Since ions are free to move, they become dispersed uniformly.


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Electrolytes and Nonelectrolytes

Soluble ionic compounds tend to be electrolytes. Why?

Molecular compounds tend to be nonelectrolytes, except for acids and bases.

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Strong Electrolytes Are…

• Strong acids• Strong bases

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Precipitation Reactions

Reactions that result in the formation of an insoluble product are called precipitation reactions

Precipitates are insoluble solids formed by a reaction in solution.

We know if ppts. form bc we can predict them by the solubility guidelines.

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

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Solubility

• The solubility of a substance at a given temperature is the amount of the substance that can be dissolved in a given quantity of solvent at the given temperature.

• Any substance with a solubility less than 0.01 mol/L will be referred to as insoluble.

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Metathesis (Exchange) Reactions

• Metathesis comes from a Greek word that means “to transpose.”

• It appears as though the ions in the reactant compounds exchange, or transpose, ions.

• Reactions in which cations and anions appear to exchange partners conform to the general equation: – AX + BY AY + BX

AgNO3(aq) + KCl(aq)

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Metathesis Reactions• Steps to complete and balance these

reactions: – Use the chemical formulas to determine which

ions are present. – Write the chemical formulas of the products by

combining the cation from one reactant with the anion of the other, using ionic charges to determine the correct subscripts.

– Check the solubilities of the products. – Balance.


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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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Molecular Equation

The molecular equation lists the reactants and products in their molecular form:

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

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Complete Ionic Equation

• In the 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

• 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)

What is the name of the ions that we cross out on both sides of the equation? And why do we cross them out?

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Net Ionic Equation

• To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right.

• 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)

This is your net ionic equation. The equation that shows only the ions and molecules directly

involved in the reaction.

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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 the right side of the equation.

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

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Acids

• The Swedish physicist and chemist S. A. Arrhenius defined acids as substances that increase the concentration of H+ when dissolved in water.

• Both the Danish chemist J. N. Brønsted and the British chemist T. M. Lowry defined them as proton donors.

• Lewis defined them as electron acceptors.

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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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Acids

• Molecules of different acids ionize to form different numbers of H+ ions. – Monoprotic Acids (yield one H+ ion per acid)– Diprotic Acids (yields 2 H+ ions per acid)

• Polyprotic Acids ionize in steps

H2SO4 (aq) H+ (aq) + HSO4- (aq)

HSO4- (aq) H+ (aq) + SO4

2- (aq)


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Acids• Ionizable Hydrogens


How many hydrogen ions can be generated by each citric acid molecule dissolved in water?

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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.

• Lewis defined them as electron donors.

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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.

Label Acid, Base, Conjugate Acid, and Conjugate Base

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Reactivity of Strong and Weak Acids/Bases

• When reactivity depends only on H+ (aq) concentrations, strong acids are more reactive than weak acids.

• The reactivity of an acid, however, can depend on the anion concentration. – HF is a weak acid, but it is very reactive and

attacks many substances.


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Comparing Acid Strengths

• The following represent aqueous solutions of acids HX, HY, and Hz, with water molecules omitted. Rank the acids from strongest to weakest.


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Identifying Strong/Weak Electrolytes

• Classify the following as strong, weak, or nonelectrolyte:

– CaCl2, HNO3, C2H5OH (ethanol), HCOOH (formic acid), KOH


Rank the following compounds that are dissolved in water in order of increasing electrical conductivity: Ca(NO3)2, C6H12O6, NaCH3COO, CH3COOH

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Neutralization Reactions

Generally, when solutions of an acid and a metal hydroxide base are combined, the products are a salt and water:

CH3COOH(aq) + NaOH(aq) NaCH3COO(aq) + H2O(l)

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When 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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H+(aq) + Cl−(aq) + Na+(aq) + OH−(aq)

Na+(aq) + Cl−(aq) + H2O(l)

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H+(aq) + Cl−(aq) + Na+(aq) + OH−(aq)

Na+(aq) + Cl−(aq) + H2O(l)

H+(aq) + OH−(aq) H2O(l)

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What happens if the base is not water-soluble?

Mg(OH)2 (s) + 2HCl (aq) MgCl2 (aq) + 2H2O (l)


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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 reacts with an acid, the products are a salt, sulfur dioxide, and water:

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

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• This reaction gives the predicted product, but you had better carry it out in the hood, or you will be very unpopular!

• But just as in the previous examples, a gas is formed as a product of this reaction:

Na2S(aq) + H2SO4(aq) Na2SO4(aq) + H2S(g)

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Oxidation-Reduction Reactions

• 2Ca (s) + O2 (g) 2CaO (s)

• 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.

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Oxidation Numbers

To determine if an oxidation–reduction reaction has occurred, we assign an oxidation number (or you may hear oxidation state) to each element in a neutral compound or charged entity.

Of course there are rules!

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Oxidation NumbersThe Rules

• Elements in their elemental form have an oxidation number of 0.– This includes diatomics

• The oxidation number of a monatomic ion is the same as its charge.– Alkali metals always have +1– Alkaline earth metals always have +2– Aluminum is always +3

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• Nonmetals tend to have negative oxidation 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 has an oxidation number of −1.

– Hydrogen is −1 when bonded to a metal, but it is usually +1 because it’s typically bonded to a nonmetal.

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• Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions.– Fluorine always has an oxidation number

of −1.– The other halogens have an oxidation

number of −1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions.

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• The sum of the oxidation numbers in a neutral compound is 0.

• The sum of the oxidation numbers in a polyatomic ion is the charge on the ion.

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Displacement Reactions

• In displacement reactions, ions oxidize an element.• The ions, then, are reduced.

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• Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g)


0 +1 -1 +2 -1 0

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In this reaction,

silver ions oxidize

copper metal:

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

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The reverse reaction,

however, does not

occur:

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

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Activity Series

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Analyzing Chemical Reactions for Patterns

• Imperative to recognize/identify patterns in reactions so that you gain a chemical intuition. Helpful questions can help you determine the reaction: – What are the reactants? – Are they electrolytes or nonelectrolytes?– Are they acids and bases?– Will metathesis produce a ppt? Water? Gas?– Do reactants engage in redox?


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Molarity

• 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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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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Using Molarities inStoichiometric Calculations

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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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Titration


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Titration

• 45.7 mL of 0.500 M H2SO4 is required to neutralize 20.0 mL of NaOH solution. What is the concentration of the NaOH solution?


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Titration

• Ag+ (aq) + Cl- (aq) AgCl (s)• How many grams of chloride ion are in a sample of

the water if 20.2 mL of 0.100 M Ag+ is needed to react with all the chloride in the sample?

• If the sample has a mass of 10.0 g, what percent Cl- does it contain?


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Chapter 4 Homework Problems

• 1-4, 7-9, 11-13, 15, 17, 19, 21-23, 25, 27, 29, 30, 32, 33, 35, 36, 38-40, 42, 43, 46-49, 51-53, 55, 57, 58, 60, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 85, 87, 89, 94b, 97


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Chapter 4 Reactions in Aqueous Solution Lecture Presentation © 2012 Pearson Education, Inc. Ashley Warren Kings High School AP Chemistry Lecture Presentation