Chapter 4 1

Chapter 4

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Reactions between ions and molecules in aqueous solutions

4.1 Solution Terminology

• Solution – homogeneous mixture in which the two or more components mix freely

• Solvent – the component present in the

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p plargest amount

• Solute – is any substance dissolved in the solvent

• Concentration – the ratio of solute to solvent in a solution

Solution formation

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Chapter 4 2

Solution Concentration• Percentage

concentration: number of grams of solute per 100 g of

l i

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solution

• The dilute solutionon the left has less solute per unit volume than the (more) concentrated solution on the right.

Solubility

• Solubility – the number of grams that dissolve in 100g of solvent at a given temperature.

• A solution is said to be saturated when no more solute can be dissolved at the current temperature.

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can be dissolved at the current temperature.• A solution containing less solute is called unsaturated

because it is able to dissolve more solute.• Supersaturated solutions contain more solute than

required for saturation at a given temperature.

Supersaturated Solutions

• How are they prepared?– Lower the temperature of a

saturated solution

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• They are not stable.• If a single crystal of solute

is added to a supersaturated solution, the extra solute crystallizes.

Chapter 4 3

Precipitates

• Many ionic compounds dissolve in water.• Those that do not form a solid.• Precipitate a solid substance that separates

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• Precipitate – a solid substance that separates from solution.

• Precipitates can also form during reactions, specifically precipitation reactions.

4.2 Ionic compounds conduct electricity when dissolved in water

• Ionic compounds that are soluble in water will conduct l i i

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electricity.• Why?

– Separation of charge– The charged ions

dissociate.

• These solutions are called electrolytes.

Types of Electrolytes

• Strong electrolyte – dissociates completely, totally soluble in water

N l l d di i b i ll

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• Nonelectrolyte – does not dissociate, but is totally soluble in water.

Chapter 4 4

Dissociation equations

• Represent what happens when an ionic compound dissolves in water.

• Remember ionic compounds dissociate into

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Remember, ionic compounds dissociate into ions! Don’t make up new ions when doing so.

)(SO )(Na 2 )(SONa -2442 aqaqs +→ +

-2442 SO Na 2 SONa +→ +

Practice Problem

• Write the dissociation reactions for the following compounds when dissolved in water:

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– (NH4)2SO4

– MgCl2

– Al(NO3)3

Equations for Ionic Reactions

• Ionic compounds often react when their aqueous solutions combine.

• Let’s look at Pb(NO3)2 reacting with KI

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Let s look at Pb(NO3)2 reacting with KI

Chapter 4 5

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Representing Ionic Reactions• Complete (molecular equation):

• Ionic:3223 2KNO)(PbI2KI)Pb(NO +→+ s

---2 2NO2K)(PbI2I2K2NOPb ++→+++ +++

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• Net Ionic:

• The most compact notation is the net ionic equation.

• It eliminates the spectator ions from the equation.

3232 2NO2K)(PbI 2I2K2NOPb ++→+++ +++ s

)(PbI 2IPb 22 s→+ −+

Ionic and Net Ionic Equations

• Criteria for balanced ionic and net ionic equations:– Material balance – the same number of each

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Material balance the same number of each type of atom on each side of the arrow

– Electrical balance – the net electrical charge on the left side of the arrow must equal the net electrical charge on the right side of the arrow

Chapter 4 6

Practice Problem

• Write the ionic and net ionic equations for the following reaction:

CdCl2 + Na2S CdS (s) + 2 NaCl

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CdCl2 + Na2S CdS (s) + 2 NaCl

4.3 Acids and bases are classes of compounds with special properties

• Acids– Have a tart (sour) taste– Turn litmus red– Vinegar and lemon juice

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g j– Corrode metals

• Bases– Have a bitter taste– Turn litmus blue– Milk of magnesia– Can feel slippery

Neutralization reactions

• Arrhenius gave first comprehensive theory of acids and bases.

• He proposed that acids form hydrogen ions (H+) and bases released hydroxide ions (OH-) in solution.

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bases released hydroxide ions (OH ) in solution.– Acids H+

– Bases OH-

• The characteristic reaction between acids and bases is neutralization.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)• In general, the reaction of an acid and a base produces

water and a salt.

Chapter 4 7

Acids (what’s really happening)• Acid – a substance that reacts with water to produce

hydronium ion, H3O+.• In general, acids are molecular compounds that react with

water to produce ions

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• This is called ionization:Acid molecule + H2O H3O+ + anion

Acids continued

• It is common to encounter the hydrogen ion

)(Cl )(H H )HCl( -32 aqaqOOg +→+ +

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It is common to encounter the hydrogen ion (H+) instead of the hydronium ion (H3O+)

)(Cl )(H )HCl( -OH2 aqaqg +⎯⎯→⎯ +

Acids and Protons

• Monoprotic acids are capable of furnishing only one hydrogen ion per molecule– HCl, HNO3, HF, HCO3

-, etc…

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• Acids that can furnish more than one hydrogen ion per molecule are called polyprotic acids – H2SO4, H3PO4, H2PO4, H2CO3, etc…

Chapter 4 8

Bases• Base – a substance that produces hydroxide ion

, OH-, in water.• Typically, they will be ionic compounds

containing either OH- or O2-.

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• Exceptions: molecular basesB + H2O HB+ + OH-

• For exampleNH3 + H2O NH4

+ + OH-

Strong Acids and Strong Bases• Acids and bases that are strong electrolytes conduct

electricity very well.• Complete ionization

STRONG ACIDS STRONG BASESHCl Hydrochloric acid LiOH Lithium hydroxide

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HCl Hydrochloric acid LiOH Lithium hydroxideHBr Hydrobromic acid NaOH Sodium hydroxideHI Hydroiodic acid KOH Potassium hydroxideHNO3 Nitric acid RbOH Rubidium hydroxideHClO4 Perchloric acid CsOH Cesium hydroxideH2SO4 Sulfuric acid Ca(OH)2 Calcium hydroxide

Ba(OH)2 Barium hydroxideSr(OH)2 Strontium hydroxide

Weak Acids and Bases• Acids and bases that are weak electrolytes.

• They do not ionize completely in water, and

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y p y ,therefore only conduct electricity a little.

• What does it mean “to ionize”

Chapter 4 9

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Dynamic Equilibrium

• An dynamic

−+ +⇔+ OHCOHOHOHHC 323232

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• An dynamic equilibrium exists between the weak acid and ion.

• Note the double arrow indicating the equilibrium

Same with weak bases

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−+ +⇔+ OHNHOHNH 423

Chapter 4 10

How to recognize…

• An acid: look for a polyatomic anion with extra hydrogens– H3PO4

We are skipping section 4.4

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– HC2H3O2– H2CO3

• A base: look for hydroxide ion (OH-) or an organic compound containing nitrogen– NH3– CH3NH2

How to write dissociation RXNS for Acids/Bases

Acid/Base Strong or Weak? Dissociation RXN

NH3

NaOH

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NaOH

Ca(OH) 2

HC2H3O2

H3PO4

H2SO4

4.5 Ionic reactions can often be predicted

• In general, a net ionic equation will exist (a reaction occurs) if one of the following occurs:– A precipitate forms– An acid reacts with a base– A weak electrolyte is from strong electrolytes– A gas is formed

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Chapter 4 11

Metathesis Reactions

• Also called double replacement reactions (think of switching dance partners)

• Must form a product = usually ppt or water

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• Solubility rules allow the prediction of when a precipitation reaction will occur.

• Water will form from the reaction of strong acids/bases.

The Solubility Rules

1. All compounds of alkali metals are soluble.2. All salts containing NH4

+, NO3-, ClO4

-, ClO3-

and C2H3O2- are soluble.

3 All hl id b id d i did (Cl B I )

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3. All chlorides, bromides and iodides (Cl-, Br-, I-) are soluble except when combined with Ag+, Hg2

2+ and Pb2+.4. All sulfates (SO4

2-) are soluble except those of Ca2+, Sr2+, Ba2+, Hg2

2+ and Pb2+.5. Ca(OH)2, Sr(OH)2 and Ba(OH)2 are soluble.

You need to memorize these.

Soluble or Insoluble?• KCl

• MgCO3

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• MnO2

• CaI2

• BaSO4

Chapter 4 12

Reactions of ionic solutions

• What happens when we mix two different ionic solutions?– Either they react to form a precipitate or they

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don’t.

– Let’s see what happens when we mix some solutions and write the balanced equations (complete, ionic and net ionic) for any reactions that occur.

Predicting Acid/Base Reactions

• Strong acid with strong base– Always forms water and a salt (soluble ionic

compound)

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HNO3 + KOH H2O + KNO3

• What would be the ionic and net ionic equations? (best to use H+ instead of H3O+)

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Chapter 4 13

More neutralization reactions

• When one reactant is weak:

2322232 OHNaCOHNaOHOHHC +→+

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– Does not always form a salt and water

• Weak acid with weak base

2322232

ClNHHClNH 43 →+

23243232 OHCNHNHOHHC →+

Practice Problems• Write the complete, ionic and net ionic equations

for the reaction of – HCl with KOH– H2CO3 with LiOH

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2 3– NH3 with HNO3– CH3NH2 with HC2H3O2

We will be skipping the section on “Predicting reactions in which a gas is formed”. You are not responsible for this section.

Concentration

• Concentration – relationship of solute to solvent in a solution.

• The molar concentration or molarity (M) is

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The molar concentration or molarity (M) is defined as

• The molarity of a solution gives an equivalence relation between the moles of solute and volume of solution.

solution of literssolute of moles (M)molarity =

Chapter 4 14

Concentration continued

• Notice that molarity has two units: mol/L• It can be used as a conversion factor

between moles and volume

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between moles and volume.

Diluting a solution

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• Solutions of high concentration can be diluted to make solutions of lower concentration

edconcentratedconcentratdilutedilute M V MV ×=×

Use only for dilution calculation.

What is the molarity of a solution made by dissolving 1.461 g of NaCl in 250.0 mL of water? (MM of NaCl = 58.443 g/mol)

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Chapter 4 15

How many grams of AgNO3 are needed to prepare 250. mL of 0.0125 M AgNO3? (MM of = 169.87 g/mol)

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How could 100. mL of 0.125 M H2SO4 solution be made from 0.500 M H2SO4 solution?

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4.7 Molarity is used for problems in solution stoichiometry

• Very similar to our previous calculations.• Instead of starting with grams we will usually start

with volume (L or mL)

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• Follow the same steps as we did in Chapter 4.

How many milliliters of 0.124M NaOH contain enough NaOH to react with 15.4 mL of 0.108M H2SO4?

2NaOH + H2SO4 2 H2O + Na2SO4

Chapter 4 16

How many milliliters of 0.124M NaOH contain enough NaOH to react with 15.4 mL of 0.108M H2SO4?

2 NaOH + H2SO4 2 H2O + Na2SO4

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How many moles of BaSO4 will form if 20.0 mL of 0.600 M BaCl2 is mixed with 30.0 mL of 0.500 M MgSO4?

BaCl2 + MgSO4 BaSO4 + MgCl2

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4.8 Chemical analysis and titrations

• Titration is a technique used to make quantitative measurements of the amounts of solutions

• The end-point is often determined visually

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Chapter 4 17

Titration Tools• A buret allows us to measure the volume of the

known solution.– Always record to two decimal places

• An indicator is used to tell us when the reaction is complete– In lab, the indicator was phenolphthalein.– It goes from colorless to light pink when you have

reached the end point.• The indicator changes colors at the end point.• The end point is the point at which the acid is

exactly neutralized.

Titration Calculations1. Write a balanced equation.2. Write down what you know.3. Write down what you are looking for.4 Start with what you know (not where the question marks

Do not use the dilution equation!

4. Start with what you know (not where the question marks are!)….also, don’t start with a number that has 2 sets of units.

5. Use stoichiometry to determine moles of question mark6. Determine the concentration of the question mark.

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10.00 mL of an unknown H2CO3 solution is titrated with 0.0500 M NaOH and it requires 15.76 mL to reach the end point. What is the concentration of the H2CO3?

H2CO3 + 2 NaOH Na2CO3 + 2 H2O

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Chapter 4 18

Stoichiometry Flow Chart

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