chapter 4 electrolytes and aqueous...

Chapter 4Electrolytes and

Aqueous ReactionsDr. Sapna Gupta

Aqueous Solutions

• Solution - a homogeneous mixture of solute + solvent• Solute: the component that is dissolved

• Solvent: the component that does the dissolving (the larger quantity)

• Aqueous solutions are those in which water is the solvent.

• Dissociation - ionic compounds separate into constituent ions when dissolved in solution

• Ionization - formation of ions by molecular compounds when dissolved

Dr. Sapna Gupta/Electrolytes 2

Electrolyte: substance that dissolved in water produces a solution that conducts electricity. Will contain ions.

Strong Electrolyte: substances that dissolve completely in water; 100% dissociation

• All water soluble ionic compounds, strong acids and strong bases

Weak Electrolytes: substances that dissolve partially or dissociate partially in water. This solution does not contain many ions.

• Exist mostly as the molecular form in solution

• Weak acids and weak basesNonelectrolyte: substance that dissolved in water produces a solution that does not conduct electricity and does not contain ions.

Electrolytes


Electrolytes

Method to Distinguish Types of Electrolytes

nonelectrolyte weak electrolyte strong electrolyte


Strong Acids

These acids dissociate completely


Electrolytes – Practical Application

• Our body is about 70% water and we have a number of ionic salts in our body. Some common ions are Na+, Ca2+, K+, Cl-, CO3

2-.

• Electrolytes maintain voltages in our cells; they help with nerve impulses in our nervous system and help with muscle contractions.

• We get electrolytes through our diet and the kidneys are responsible for maintaining an electrolytic balance in the body. If the ions are not in the correct concentration then the above mentioned functions cannot occur.

• One way we lose electrolytes is during sweating. This is two fold – one: we lose water in the body so the concentration of ions change and two: we lose ions also during sweating.

• These ions have to replenished or we can lose muscle control.

• Electrolytes (sports drinks) are commonly used to replace these ions. One has to be careful though – they also have a lot of sugar in them!

• The first electrolyte beverage was invented in University of Florida……


Types of ReactionsTwo classifications: one how atoms are rearrangement and the other is chemicalreaction

1) Atomic Rearrangement

• Synthesis (combination): two substances combine to form one.

2Na(s) + Cl2(g) 2NaCl(s)

• Double Displacement: A reaction in which two elements displaces two elements.

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

• Single displacement: A reaction where one element displaces one other element.

Zn(s) + CuSO4 (aq) ZnSO4(aq) + Cu(s)

• Decomposition: A reaction in which a single compound reacts to give two or more substances.

2HgO(s) 2Hg(l) + O2(g)

2) Chemical Classification: Types of Chemical Reactions

Precipitation Reactions: where a solid is formed when two solutions are mixed.

Neutralization Reactions: when an acid and base react to from salt and water.

Oxidation–Reduction Reactions: addition or removal of oxygen and/or transfer of electrons.


Precipitation Reactions

• Precipitation (formation of a solid from two aqueous solutions) occurs when product is insoluble in water.

• Reaction type: Double displacement

• What is solubility? Solubility is defined as the maximum amount of a solid that can dissolve in a given amount of solvent at a specified temperature

• Prediction of precipitate is based on solubility rules


Solubility Guidelines


Solved Problems

1) Identify the Precipitate

Pb(NO3)2(aq) + 2NaI(aq) 2NaNO3 + PbI2

PbI2 – according to solubility rules

2) Classify the following as soluble or insoluble in water

• Ba(NO3)2 soluble

• AgI insoluble

• Mg(OH)2 insoluble


Writing Equations in Aqueous SolutionsA chemical equation in which the reactants and products are written as if they were molecular substances, even though they may actually exist in solution as ions.

Symbols indicating the states are include: (s), (l), (g), (aq).

For example:

Molecular Equation:

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

Although AgNO3, NaCl, and NaNO3 exist as ions in aqueous solutions, they are written as compounds in the molecular equation.

Ionic Equation:

Ag+(aq) + NO3−(aq) + Na+(aq) + Cl−(aq) AgCl(s) + Na+(aq) + NO3

−(aq)

All compounds that dissociate are shown as ions.

Net Ionic Equation:

In this the spectator ions (ions on both sides of the equation) are eliminated.

Ag+(aq) + NO3−(aq) + Na+(aq) + Cl−(aq) AgCl(s) + Na+(aq) + NO3

−(aq)

Net ionic equation represents the ions reacting. Those will be (g), (l) and (s) products formed.

Ag+(aq) + Cl−(aq) AgCl(s)


Solved Problems

Decide whether the following reaction occurs. If it does, write the molecular, ionic, and net ionic equations.

KBr + MgSO4

Determine the product formulas by double displacement method

• K+ and SO42− make K2SO4

• Mg2+ and Br − make MgBr2

Determine whether the products are soluble:

K2SO4 is soluble and MgBr2 is soluble

KBr + MgSO4 no reaction



NaOH + MgCl2

Determine the product formulas by double displacement method• Na+ and Cl− make NaCl• Mg2+ and OH− make Mg(OH)2

Determine whether the products are soluble• NaCl is soluble and Mg(OH)2 is insoluble

Molecular Equation

Balance the reaction and include state symbols

2NaOH(aq) + MgCl2(aq) 2NaCl(aq) + Mg(OH)2(s)

Ionic Equation

2Na+(aq) + 2OH−(aq) + Mg2+(aq) + 2Cl−(aq) 2Na+(aq) + 2Cl−(aq) + Mg(OH)2(s)

Net Ionic Equation

2OH−(aq) + Mg2+(aq) Mg(OH)2(s)

Solved Problems


One more….


K3PO4 + CaCl2

Determine the product formulas:

• K+ and Cl− make KCl; Ca2+ and PO43− make Ca3(PO4)2

Determine whether the products are soluble:

• KCl is soluble and Ca3(PO4)2 is insoluble

Molecular Equation(Balance the reaction and include state symbols)2K3PO4(aq) + 3CaCl2(aq) 6KCl(aq) + Ca3(PO4)2(s)

Ionic Equation6K+(aq) + 2PO4

3−(aq) + 3Ca2+(aq) + 6Cl−(aq) 6K+(aq) + 6Cl−(aq) + Ca3(PO4)2(s)

Net Ionic Equation2PO4

3−(aq) + 3Ca2+(aq) Ca3(PO4)2(s)


Another one….

Aqueous solutions of silver nitrate and sodium sulfate are mixed. Write the net ionic reaction.

2AgNO3(aq)+Na2SO4(aq)2NaNO3(?)+Ag2SO4(?)

Determine solubility of salts. All nitrates are soluble but silver sulfate is insoluble

Molecular Equation

2AgNO3(aq)+Na2SO4(aq)2NaNO3(aq)+Ag2SO4(s)

Ionic equation

2Ag+(aq) + 2NO3(aq) + 2Na+(aq) + SO4

2(aq) 2Na+(aq) + 2NO3(aq) + Ag2SO4(s)

Cancel spectators

2Ag+(aq) + 2NO3(aq) + 2Na+(aq) + SO4

2(aq) 2Na+(aq) + 2NO3(aq) + Ag2SO4(s)

Net ionic equation

2Ag+(aq) + SO42(aq) Ag2SO4(s)


Neutralization Reactions (acid-base)

Acids Bases

Arrhenius AcidA substance that produces hydrogen ions, H+, when dissolved in water.

Arrhenius BaseA substance that produces hydroxide ions, OH−, when dissolved in water.

Brønsted–Lowry Acid

The species (molecule or ion) that donates a proton, H+, to another species in a proton−transfer reaction.

Brønsted–Lowry BaseThe species (molecule or ion) that accepts a proton, H+, from another species in a proton−transfer reaction.

Sour Bitter

Corrosive Caustic, slippery

pH value 1-7 pH value 7-14

Strong acids (inorganic acids) – ionize completely in water, e.g.: HNO3, H2SO4, HClO4, HCl, HBr, HI

Strong bases (inorganic bases) –ionize completely in water; most are hydroxides, e.g.: NaOH, KOH, Ca(OH)2

Weak acids – ionize partially in water, e.g. HFOrganic acid: HC2H3O2 (CH3COOH)

Weak bases– ionize partially in water, e.g.: NH4OH, Na2CO3, NaHCO3

organic bases: CH3NH2Dr. Sapna Gupta/Electrolytes 16

Common Acids and Bases


More on Acids-Bases

Indicators: these are chemicals used to determine if an acid or base is strong or weak by changing colors.

Polyprotic Acid: An acid that results in two or more acidic hydrogens per molecule.

For example: H2SO4, sulfuric acid

Neutralization Reaction:

• Almost all acid base reactions are double displacement reactions.

• Most will produce a salt and water as product.

• Carbonates and sulfites give CO2 and SO2 gases in product.


Acid-Base Neutralization Reactions

Neutralization: Reaction between an acid and a base

Acid + Base Salt + Water

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

Ionic equation:H+(aq)+ Cl(aq) + Na+(aq) + OH(aq) Na+(aq) + Cl(aq) + H2O(l)

Net ionic equation:H+(aq) + OH(aq) H2O(l)


Neutralization Reactions

Write the molecular, ionic, and net ionic equations for the neutralization of sulfurous acid, H2SO3, by potassium hydroxide, KOH

The reaction is a double displacement reaction.Molecular Equation(Balance the reaction and include state symbols)

H2SO3(aq) + 2KOH(aq) 2H2O(l) + K2SO3(aq)

Ionic EquationH2SO3(aq) + 2K+(aq) + 2OH−(aq) 2H2O(l) + 2K+(aq) + SO3

2−(aq)

Net Ionic EquationH2SO3(aq) + 2OH−(aq) 2H2O(l) + SO3

2−(aq)


Neutralization Reactions Producing Gases

Sulfides, carbonates, sulfites react with acid to form a gas.

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

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

Na2SO3(aq) + 2HCl(aq) 2NaCl(aq) + H2O(l) + SO2(g)

The photo below shows baking soda (sodium hydrogen carbonate) reacting with acetic acid in vinegar to give bubbles of carbon dioxide.

+


Neutralization Reaction – another one

Molecular Equation

(Balance the reaction and include state symbols)

CuCO3(s) + 2HCl(aq) CuCl2(aq) + H2O(l) + CO2(g)

Ionic Equation

CuCO3(s) + 2H+(aq) + 2Cl−(aq) Cu2+(aq) + 2Cl−(aq) + H2O(l) + CO2(g)

Net Ionic Equation

CuCO3(s) + 2H+(aq) Cu2+(aq) + H2O(l) + CO2(g)


Redox Reactions

Oxidation Reduction

Addition of oxygen Removal of oxygen

Removal of hydrogen Addition of hydrogen

Loss of electrons (LEO) Gain of electrons (GER)

Metals lose electrons hence undergo oxidation

Non metals gain electrons hence undergo reduction

Reducing agents – something that causes reduction of another element and gets oxidized (loses electrons) itself

Oxidizing agent – an element that causes oxidation of another element and gets reduced (gains electrons) itself


Writing Redox Reactions

Example

Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)

gaining 2 electrons, reduction

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

loses 2 electrons, oxidation

Half Reactions: Zinc is losing 2 electrons and oxidized. It is the reducing agent

Zn(s) Zn2+(aq) + 2e

Copper ions are gaining the 2 electrons. It is the oxidizing agent

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


Rules for Assigning Oxidation Numbers

1. Elements: The oxidation number of an element is zero.

2. Monatomic ions: The oxidation number of a monatomic ion equals the charge on the ion.

3. Oxygen: The oxidation number of oxygen is −2 in most compounds. (An exception is O in H2O2 and other peroxides, where the oxidation number is −1.)

4. Hydrogen: The oxidation number of hydrogen is +1 in most of its compounds. (The oxidation number of hydrogen is −1 in binary compounds with a metal such as CaH2.)

5. Halogens: The oxidation number of fluorine is −1. Each of the other halogens (Cl, Br, I) has an oxidation number of −1 in binary compounds, except when the other element is another halogen above it in the periodic table or the other element is oxygen.

6. Compounds and ions: The sum of the oxidation numbers of a compound is zero. The sum of the oxidation numbers of the atoms in a polyatomic ion equals the charge on the ion.


Oxidation Numbers on the Periodic Table

(most common in red)


Activity Series

Loses electrons easily

Does not lose electrons easily


Assigning Oxidation States

Assign oxidation numbers for all elements in each species

1) MgBr2: Mg +2, Br 1 x 2= -2; +2 +(-2) = total charge of 0

2) ClO2 : O 2 x 2 = -4; Cl +3; (-4) + (+3) = -1 (charge left over on ion)

3) Assign oxidation number of Mn in KMnO4

K Mn O

1(+1) + 1(oxidation number of Mn) + 4(−2) = 0

1 + 1(oxidation number of Mn) + (−8) = 0

(−7) + (oxidation number of Mn) = 0

Oxidation number of Mn = +7

4) What is the oxidation number of Cr in dichromate, Cr2O72−?

Cr O

2(oxidation number of Cr) + 7(−2) = −2

2(oxidation number of Cr) + (−14) = −2

2(oxidation number of Cr) = +12

Oxidation number of Cr = +6Dr. Sapna Gupta/Electrolytes 28

Balancing Redox Equations (electronically)

Balance the following reaction.

Zn(s) + Ag+(aq) Zn2+(aq) + Ag(s)

Oxidation Numbers 0 + +2 0

Next, write the unbalanced half−reactions.

Zn(s) Zn2+(aq) (oxidation)

Ag+(aq) Ag(s) (reduction)

Now, balance the charge in each half reaction by adding electrons.

Zn(s) Zn2+(aq) + 2e− (oxidation)

e− + Ag+(aq) Ag(s) (reduction)

Each half−reaction should have the same number of electrons. To do this, multiply each half−reaction by a factor so that when the half−reactions are added, the electrons cancel.

Zn(s) Zn2+(aq) + 2e− (oxidation)

2e− + 2Ag+(aq) 2Ag(s) (reduction)

Lastly, add the two half−reactions together.

Zn(s) + 2Ag+(aq) Zn2+(aq) + 2Ag(s)


One more Balancing Redox Equation

FeI3(aq) + Mg(s) Fe(s) + MgI2(aq)

The oxidation numbers are given below the reaction.

FeI3(aq) + Mg(s) Fe(s) + MgI2(s)

+3 −1 0 0 +2 −1

Now, write the half−reactions. Since Iodide is a spectator ion it is omitted at this point.

Mg(s) Mg2+(aq) (oxidation)

Fe3+(aq) Fe(s) (reduction)

Balancing the half−reactions:

Mg(s) Mg2+(aq) + 2e− (oxidation)

Fe3+(aq) + 3e− Fe(s) (reduction)

Multiply the oxidation half−reaction by 3 and the reduction half−reaction by 2.

3Mg(s) 3Mg2+(aq) + 6e− (oxidation)

2Fe3+(aq) + 6e− 2Fe(s) (reduction)

Add the half−reactions together.

2Fe3+(aq) + 3Mg(s) 2Fe(s) + 3Mg2+(aq)

Now, return the spectator ion, I−.

2FeI3(aq) + 3Mg(s) 2Fe(s) + 3MgI2(aq)Dr. Sapna Gupta/Electrolytes 30

Types of Redox Reactions

Displacement reactions

A common reaction: active metal replaces (displaces) a metal ion from a solution (use the activity series to predict if reaction will take place)

Mg(s) + CuCl2(aq) Cu(s) + MgCl2(aq)

Decomposition reactions

2KClO3(s) 2KCl(s) + 3O2(g)

Combination Reactions

2H2(g) + O2(g) 2H2O(l)

Combustion reactions

Common example, hydrocarbon fuel reacts with oxygen to produce carbon dioxide and water

CH4(g) + O2 (g) H2O(l) + CO2 (g)


Solved ExampleClassify the following reactions as precipitation; acid-base; or redox reaction and any other classification that can describe the reaction.

2H2 + O2 → 2H2O

Redox (combustion, combination)

Zn + H2SO4 → ZnSO4 + H2

Redox (single displacement)

H2O + NH3 → NH4+ + OH-

Acid-base (double displacement)

6FeSO4 + K2Cr2O7 + 7H2SO4 → Cr2(SO4)3 + 3Fe2(SO4)3 + K2SO4 + 7H2O

Redox

2NaCl + Pb(NO3)2 PbCl2 + 3NaNO3

Precipitation (double displacement)


Key Words and Concepts

• Ions in Aqueous Solution• Electrolytes

• Solubility Rules

• Molecular, Ionic and Net Ionic Equations

• Types of Chemical Reactions

• Precipitation Reactions

• Acid–Base Reactions

• Oxidation–Reduction Reactions• Oxidation

• Reduction

• Reducing agent

• Oxidizing agent

• Half reactions


chapter 4 electrolytes and aqueous...

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