properties of acids - texas a&m university · properties of acids ... properties of bases ......

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Properties of Acids

Solutions of acids have a sour taste

Don’t taste them in the lab !!!

They change the colors of many indicatorsAcids turn blue litmus to redAcids turn bromothymol blue from blue to yellow

They react with metals to generate hydrogen gas, H2

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Metal Activity Series

Active enough to displace hydrogen from an acid

Li, K, Ca, Na, Mg, Al, Li, K, Ca, Na, Mg, Al, MnMn, Zn, Fe, Co, Ni, , Zn, Fe, Co, Ni, PbPb,, H, Cu, Hg, Ag, Pt, AuCu, Hg, Ag, Pt, Au

Cannot displace hydrogen from an acid

More active

Less active

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Properties of Acids

They react with metal oxides forming the salt of the metal and water

CaO + 2HCl → CaCl2 + H2O

They react with metal hydroxides forming the salt of the metal and water

Ca(OH)2 + 2HCl → CaCl2 + 2H2O

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OxidesCompounds of oxygen and another elementThere are two ways to name oxides

Based on the oxidation number of the elementLi2O – lithium oxideBaO – barium oxideFeO – iron(II) oxideFe2O3 – iron(III) oxideBased on the number of atoms of each elementLi2O – dilithium oxideBaO – barium oxideFeO – iron oxideFe2O3 – diiron trioxide

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Example 1Name the following compounds:

BeO, Al2O3, Cu2O, OsO4, Cr2O3, CrO3

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Example 2Write formulas for the following compounds:

Potassium oxideBoron oxideDiindium trioxideCobalt(II) oxideDinitrogen pentoxideRhenium(VI) oxideXenon tetroxideCarbon monoxideCarbon dioxideManganese(VII) oxide

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Example 3Write total and net ionic equations for the reaction between cobalt (III) oxide and diluted hydroiodic acid

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Example 4Write total and net ionic equations for the reaction between dialuminumtrioxide and diluted hydrobromic acid

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HydroxidesConsist of metal cations and hydroxide (OH–) anions

Usually named based on the oxidation number

LiOH – lithium hydroxideBa(OH)2 – barium hydroxideFe(OH)2 – iron(II) hydroxide Fe(OH)3 – iron(III) hydroxide

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Example 5Name the following compounds:

Be(OH)2, Al(OH)3, CuOH, Cr(OH)3

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Example 6Write formulas for the following compounds:

Potassium hydroxideManganese(II) hydroxideCobalt(III) hydroxideStrontium hydroxide

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Example 7Write total and net ionic equations for the reaction between cobalt (III) hydroxide and hydrofluoric acid

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Example 8Write total and net ionic equations for the reaction between aluminum hydroxide and diluted sulfuric acid

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Properties of Acids - SummarySolutions of acids have a sour tasteThey change the colors of many indicatorsThey react with metals to generate hydrogen gas, H2

They react with metal oxides forming the salt of the metal and waterThey react with metal hydroxides forming the salt of the metal and waterAqueous solutions of acids conduct an electric current

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Acids as ElectrolytesStrong acids – strong electrolytesWeak acids – weak electrolytes

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Classification of AcidsAccording to the number of different elements they contain

Binary acids contain only 2 elementsHF, HCl, HBr, HI, H2S, H2Se, H2TeTernary acids contain 3 elementsHClO3, HIO4, H2SO4, H3PO4, HNO3

According to the number of protonsMonoproticHF, HCl, HBr, HI, HNO3, HClO4

PolyproticH2SiO3, H2SO4, H3PO4

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Ionization of AcidsMonoprotic acids ionize in one step:

HI(aq) + H2O(l) → H3O+(aq) + I–(aq)HNO3(aq) + H2O(l) → H3O+(aq) + NO3

–(aq)

Polyprotic acids ionize in two or three steps:H2SO4(aq) + H2O(l) → H3O+(aq) + HSO4

–(aq)HSO4

–(aq) + H2O(l) → H3O+(aq) + SO42–(aq)

The first step in the ionization of a polyproticacids always occur to a greater extent than the second or the third step

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Example 9Write sequence of reactions that takes place upon ionization of phosphoric acid

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Structure of AcidsStructures of binary acids usually are obvious

In most ternary acids the central atom is surrounded by oxygen atoms and then hydrogen atoms are connected to oxygen atoms

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Strengths of Binary AcidsThe strength of an acid reflects the ease of removing a proton from that acid

The H–X bond strength in hydrohalic acids:HF >> HCl > HBr > HI

Acid strength has the reverse trend:HF << HCl < HBr < HI

The same trend applies to the binary acids formed by the VIA elementsBond strength order:

H2O >> H2S > H2Se > H2TeAcid strength has the reverse trend:

H2O << H2S < H2Se < H2Te

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Strengths of Ternary AcidsIn most ternary acids the proton to be removed is bound to an oxygen atomTherefore, the strength of the acid depends on the strength of the O–H bond

HClO HClO2 HClO3 HClO4

Red – more negative (higher electron density)Blue – more positive (lower electron density)

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Strengths of Ternary AcidsTernary acid strength usually increases with:

an increasing oxidation state of the central atom for the acids of the same elementan increasing electronegativity of the central atom for the acids containing elements from the same group in the same oxidation state

ExamplesWhich acid in each pair is stronger:

HNO2 or HNO3?

H2SO4 or H2SO3?

H2SeO3 or H2SO3?

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Strengths of Ternary AcidsIt is important to remember that the comparison of acid strengths can be carried out only for the acids with similar structuresFor example, ternary acids of phosphorus represent an important exception

H3PO2 H3PO3 H3PO4

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The Preparation of AcidsThe binary acids are prepared by reacting the nonmetallic element with H2:

H2(g) + Cl2(g) → 2HCl(g)

Volatile acids, ones with low boiling points, are prepared by reacting salts with a nonvolatile acid like sulfuric or phosphoric:NaF(s) + H2SO4(conc.) → NaHSO4(s) + HF(g)NaCl(s) + H2SO4(conc.) → NaHSO4(s) + HCl(g)

We must use phosphoric acid to prepare HBr or HI:NaBr(s) + H3PO4(conc.) → NaH2PO4(s) + HBr(g)NaI(s) + H3PO4(conc.) → NaH2PO4(s) + HI(g)

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The Preparation of AcidsTernary acids are made by reacting nonmetal oxides (acid anhydrides) with water:

SO2(g) + H2O(l) → H2SO3(aq)N2O5(g) + H2O(l) → 2HNO3(aq)

Oxides of some metals in high oxidation state are acidic - they form acids when reacting with water:

Mn2O7(s) + H2O(l) → 2HMnO4(aq)CrO3(s) + H2O(l) → H2CrO4(aq)

Some nonmetal halides and oxyhalides react with water to give both a binary and a ternary acid:

PCl5(s) + 4H2O(l) → H3PO4(aq) + 5HCl(aq)POCl3(l) + 3H2O(l) → H3PO4(aq) + 3HCl(aq)

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Properties of Bases

Solutions of bases have a bitter tasteDon’t taste them in the lab !!!

Solutions of bases have slippery feeling

They change the colors of many indicators

Bases turn red litmus to blueBases turn bromothymol blue from yellow to blue

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Properties of BasesThey react with nonmetal oxides to form salt and water2NaOH + SO2 → Na2SO3 + H2O

They react with acids to form salt and waterCa(OH)2 + 2HCl → CaCl2 + 2H2O

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Bases as ElectrolytesAqueous basic solutions conduct electricityStrong bases – strong electrolytesWeak bases – weak electrolytes

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The Preparation of BasesStrong bases are prepared by reacting the metal or its oxide with water:

2Na(s) + 2H2O(l) → 2NaO H(aq) + H2(g)Ca(s) + 2H2O(l) → Ca(OH)2(s) + H2(g)

K2O(s) + H2O(l) → 2KOH(aq)

The other hydroxides can be obtained by metathesis reactions:

Cd(NO3)2(aq) + Ba(OH)2(aq) → Cd(OH)2(s) + Ba(NO3)2(aq)FeCl3(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3NaCl(aq)

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AmphoterismSome compounds can behave as both acids and basesThey are called amphotericamphoteric

Examples of amphoteric species are hydroxides of elements with intermediate electronegativity

Zn(OH)2 and Al(OH)3 behave as hydroxides in the presence of strong acids and as acids in the presence of strong bases

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Example 10Write total and net ionic equations for the reactions of zinc hydroxide with:

sulfuric acidsodium hydroxide

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There are four acid-base reaction combinations that are possible:

Strong acids – strong basesStrong acids – weak basesWeak acids – strong basesWeak acids – weak bases

General name – neutralization reactionsneutralization reactions

Most of these reactions result in the formation of salt and water

Acid-Base Reactions

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Type 1 – formation of a soluble salt:HNO3(aq) + NaOH(aq) → NaNO3(aq) + H2O(l)2HBr(aq) + Ca(OH)2(s) → CaBr2(aq) + 2H2O(l)

Type 2 – formation of an insoluble salt:H2SO4(aq) + Ba(OH)2(aq) → BaSO4(aq) + 2H2O(l)

Strong Acid + Strong Base

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Always form a soluble salt:HNO3(aq) + NH3(g) → NH4NO3(aq)H2SO4(aq) + NH3(g) → (NH4)2SO4(aq)

Reactions of acids with ammonia lead only to the formation of soluble salts

Water is not formed in these reactions

Strong Acid + Weak Base

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Always form a soluble salt:HNO2(aq) + NaOH(aq) → NaNO2(aq) + H2O (l)H2SO3(aq) + 2RbOH(aq) → Rb2SO3(aq) + 2H2O (l)

Weak Acid + Strong Base

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Always form a soluble salt:CH3COOH(l) + NH3(g) → NH4(CH3COO)(aq)HF(g) + NH3(g) → NH4F(aq)

But…

these reactions proceed in the opposite direction because the products are unstable in water

The salts formed by weak acid and weak base are not stable in aqueous solution!!!

Weak Acid + Weak Base

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Assignments & Reminders

Go through the lecture notes

Read Chapter 10 completely

Read Sections 4-5 & 4-6 of Chapter 4

Read Section 6-8 of Chapter 6