Transcript Chemical Reactions
Chemical Reactions
Chapter 4, part 2: Reactions in Aqueous Solution 1
The dissolving process
When a solid is put into a liquid, solute-solute attractions compete with solute-solvent & solvent-solvent attractions 2
Solubility
Let’s assume our solvent is water . . .
If solute-water attractions > solute-solute & water-water attractions, solute particles are pulled out one by one into the water: The solute is SOLUBLE in water 3
Solubility
But if solute-water attractions < solute-solute & water-water attractions, solute particles remain together: The solute is INSOLUBLE in water 4
Solution conductivity
Solution conductivity depends on type of solute particles 5
Solution conductivity
Ionic solutes (salts) made of cations & anions Ions DISSOCIATE (separate) during dissolving Molecular solutes made of molecules each solute particle that moves into solution is identical 6
Solution conductivity
Solutions of ionic solutes contain independent mobile ions Solution conducts electricity Solute is an ELECTROLYTE Solute also conducts when melted, but not when solid (ions can’t move) 7
Solution conductivity
Solutions of most molecular solutes contain independent neutral molecules Solution does not conduct electricity Solute is a NONELECTROLYTE Solute also does not conduct when melted or solid 8
Solution conductivity
Some molecular compounds (acids & bases) react with water to produce ions, as if they dissociated A few acids & bases do this very well, producing lots of ions = strong electrolytes, strong acids & bases Most acids & bases do this weakly, producing a few ions = weak electrolytes, weak acids & bases 9
Strong acids & bases
HCl STRONG ACIDS HNO 3 HBr H 2 SO 4 HI STRONG BASES LiOH NaOH Mg(OH) 2 Ca(OH) 2 HClO 4 KOH Sr(OH) 2 etc.
Ba(OH) 2 All other acids & bases are WEAK 10
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Predicting Electrolytes
All soluble salts and strong acids & bases are strong electrolytes Weak acids & bases are weak electrolytes All other molecular compounds are nonelectrolytes 12
Precipitation reactions
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Precipitation reactions
spectator ions precipitate 14
Predicting Precipitation Reactions
To predict whether a precipitate will form, you need to know which compounds are soluble (no ppt) and which are insoluble (ppt forms) Memorize the guidelines in Table 4.1 on page 154 in your text!
Add this guideline: CrO 4 2– acts like SO 4 2– 15
Solubility guidelines: soluble
Compounds of these ions are generally soluble and do NOT form precipitates : Alkali metals (group 1A, except Li 1+ ) Ammonium (NH 4 1+ ) Nitrates (NO 3 1– ) and acetates (C 2 H 3 O 2 1– ) Chlorides, bromides, iodides, except Pb 2+ , Ag 1+ , Hg 2 2+ Sulfates (SO 4 2– ) & chromates (CrO 4 2– ), except Sr 2+ , Ba 2+ , Pb 2+ , and Hg 2 2+ 16
Solubility guidelines: insoluble
Compounds of these ions are generally insoluble and DO form precipitates : Hydroxides (OH 1– ) and Sufides (S 2– ) Alkali metals (group 1A) and ammonium (NH 4 1+ ) are soluble Sulfides of group 2A metals are generally soluble Hydroxides of Ca 2+ , Sr 2+ , and Ba 2+ are soluble Carbonates (CO 3 2– ) and phosphates (PO 4 3– ) 17
Net ionic equations
AgNO 3 (aq) + NaCl (aq) AgNO 3 (aq) + NaCl (aq) AgCl + NaNO 3 AgNO 3 (aq) + NaCl (aq) AgCl (s) + NaNO 3 (aq) Ag The (aq) substances are dissociated: 1+ (aq) + NO 3 1– (aq) + Na 1+ (aq) + Cl 1– AgCl (s) + Na 1+ (aq) (aq) + NO 3 1– (aq) 18
Net ionic equations
Na 1+ and NO 3 1– are spectator ions Ag 1+ (aq) + NO 3 1– (aq) + Na 1+ (aq) + Cl 1– AgCl (s) + Na 1+ (aq) (aq) + NO 3 1– (aq) The net ionic equation omits spectators: Ag 1+ (aq) + Cl 1– (aq) AgCl (s) 19
Is that really a spectator?
An ion is a spectator if and only if it is in exactly the same form in the products and reactants: Na CO 2 3 CO 2– 3 (aq) + BaCl 2 (aq) + Ba 2+ (aq) (aq) BaCO BaCO 3 3 (s) + 2 NaCl (aq) (s) Na 2 CO 3 Na 2 CO 3 (s) + 2 HCl (aq) (s) + 2 H 1+ (aq) 2 NaCl (aq) + H 2 Na 1+ (aq) + H 2 2 O + CO O + CO 2 2 (g) (g) Only Cl 1– is a spectator Na 1+ is not a spectator because it was (s), then (aq) 20
Examples
Indicate whether a ppt forms and if so, complete the reaction as a balanced net ionic equation: AlCl 3 (aq) + KOH (aq) K 2 SO 4 CaI Na 3 2 (aq) + Pb(NO 3 ) 2 PO 4 (aq) + AlCl 3 (aq) (aq) Al 2 (SO 4 (aq) + FeBr 3 ) 3 (aq) (aq) + BaCl 2 (aq) (NH 4 ) 2 CO 3 (aq) + Pb(NO 3 ) 2 (aq) 21
Acids
Acids produce H 3 O 1+ in aqueous solution: Strong acids are molecular compounds that react completely with water to produce H 3 O 1+ : HCl (g) + H 2 O H 3 O 1+ (aq) + Cl 1– (aq) For convenience, we often show acids as simply dissociating to produce H 1+ : HCl (g) H 1+ (aq) + Cl 1– (aq) There are only 6 strong acids (memorize them, pg 161 ) 22
Acids
Acids produce H 3 O 1+ in aqueous solution: Weak acids are molecular compounds that react incompletely with water: HC 2 H 3 O 2 HC 2 (aq) + H 2 O H 3 O 2 (aq) H 3 H 1+ O 1+ (aq) + C (aq) + C 2 H 3 2 H O 2 3 O 1– 2 1– (aq) (aq) All acids that are not strong are weak 23
Acids
H 1+ is a proton The reaction with water is proton transfer : HC 2 H 3 HCl (g) + H 2 O O 2 (aq) + H 2 O H H 3 3 O O 1+ 1+ (aq) + Cl (aq) + C 2 1– H 3 (aq) O 2 1– (aq) Acids with one H 1+ to transfer are monoprotic acids HCl HNO 3 HC 2 H 3 O 2 Acids with more than one H 1+ to transfer are polyprotic acids H 2 SO 4 H 3 PO 4 H 2 C 3 H 2 O 4 24
Acids
Polyprotic acids produce H 3 O 1+ H 2 SO 4 HSO 4 1– (aq) + H 2 O (aq) + H 2 O H 3 O 1+ H 3 O 1+ in steps: (aq) + HSO 4 1– (aq) + SO 4 2– (aq) (aq) For H 2 SO 4 the first step is strong and the second weak H 2 C 2 O 4 HC 2 O 4 1– (aq) + H 2 O (aq) + H 2 O H 3 O 1+ H 3 O 1+ (aq) + HC 2 O 4 1– (aq) + C 2 O 4 2– (aq) (aq) For all other polyprotic acids, the first step is weak and the second step is weaker 25
Bases
Bases produce OH 1– in aqueous solution: Strong bases are ionic hydroxide compounds that are completely dissociated in water: NaOH (s) Na 1+ (aq) + OH 1– (aq) The strong bases are the hydroxides of group 1A and 2A metals ( memorize them ) Weak bases are molecular compounds that react incompletely with water to produce OH 1– : NH 3 (aq) + H 2 O (l) NH 4 1+ (aq) + OH 1– (aq) Amines (–NH 2 ) are weak bases 26
Neutralization
Acids and bases neutralize each other The H 1+ from the acid is transferred to the base: HCl (aq) + NaOH (aq) HOH (l) + NaCl (aq) H 1+ (aq) + OH 1– (aq) HOH (l) The base is not always an OH 1– HCl (aq) + NH 3 (aq) NH compound: 4 1+ (aq) + Cl 1– H 1+ (aq) + NH 3 (aq) NH 4 1+ (aq) (aq) 27
Neutralization and net ionic equations
It is important to recognize strong acids and bases when writing net ionic equations H Cl H 1+ (aq) + Na OH (aq) (aq) + OH 1– (aq) H 2 O (l) + NaCl H 2 O (l) (aq) HC 2 H 3 O 2 HC 2 H 3 O 2 (aq) + Na OH (aq) (aq) + OH 1– (aq) H 2 O (l) + Na C 2 H 3 O 2 (aq) H 2 O (l) + C 2 H 3 O 2 1– (aq) The weak acid is not significantly ionized, so acetate is not a spectator (even if it is aq) 28
Examples
Write the molecular and net ionic equations for HNO 3 + NaOH HF + KOH HC 2 H 3 O 2 H H 2 2 SO C 2 4 O 4 + NH 3 + Ba(OH) 2 + NaOH HCHO 2 + Ca(OH) 2 29
Gas forming reactions
Some neutralizations produce a gas: CO 3 2– + 2 H 1+ H 2 CO 3 H 2 CO 3 H 2 CO 3 is unstable and decomposes immediately CO 2 (g) + H 2 O The overall reaction is CO 3 2– + 2 H 1+ CO 2 (g) + H 2 O 30
Gas forming reactions
Memorize these gas-formers (pg 166): SO 3 2– HSO 3 1– + 2 H 1+ + H 1+ SO 2 SO 2 (g) + H 2 O (g) + H 2 O burning sulfur smell CO 3 2– + 2 H 1+ HCO S 2– 3 1– + H 1+ + 2 H 1+ CO 2 (g) + H 2 O CO 2 (g) + H 2 O H 2 S (g) rotten egg smell ammonia NH 4 1+ + OH 1– NH 3 (g) + H 2 O 31
Redox
Redox (oxidation-reduction) reactions are those in which electrons are transferred The loss of electrons is oxidation The gain of electrons is reduction LEO says GER 32
Oxidation states
The oxidation state (O.S.) or oxidation number is a convenient but artificial way to describe the electron environment around an atom It is related to the number of electrons gained, lost, or apparently used in forming compounds Oxidation states are assigned using the rules on page 169 of your text ( memorize these
in order
) 33
Assigning oxidation states
1. The O.S. of each atom in an element is zero 2. The O.S. of a monoatomic ion is equal to its charge 3. The total of the O.S. of all atoms in any species (formula unit, molecule or ion) equals the charge on that species 4. In compounds, metals always have a positive O.S.
• Group 1A metals are always O.S. +1 and Group 2A metals are always O.S. +2 5. For nonmetals in compounds, • the O.S. of fluorine is –1.
• the O.S. of hydrogen is +1.
• the O.S. of oxygen is –2.
6. In binary compounds, the O.S. of a Group 7A element is –1, Group 6A element –2, and Group 5A element –3.
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Examples
What is the oxidation state of each element in What is the oxidation state of each element in S 8 S 2 O 3 2– Cr 2 O 7 2– Hg 2 Cl 2 Cl 2 O KO 2 KMnO 4 H 2 CO 35
Redox
In a redox reaction, atoms change (O.S.) The element oxidized loses electrons and its O.S. becomes more positive The element reduced gains electrons and its O.S. becomes more negative 36
Examples
Which of these reactions is a redox reaction? Identify the species oxidized and reduced.
HCl (aq) + NaOH (aq) H 2 O + NaCl (aq) 2 Pb(NO 3 ) 2 (s) 2 PbO (s) + 4 NO 2 NH 4 Cl (s) + NaOH (aq) NH 3 (g) + O (g) + H 2 2 (g) O + NaCl (aq) Identify the species oxidized and reduced: 5 VO 2+ (aq) + MnO 4 1– (aq) + H 2 O 5 VO 2 1+ (aq) + Mn 2+ (aq) + 2 H 1+ (aq) 37
Agents of Oxidation and Reduction
An agent makes something happen An oxidizing agent makes oxidation happen by being reduced A reducing agent makes reduction happen by being oxidized The “agent” is the entire species in which the oxidized or reduced atom appears 38
Examples
Identify the elements oxidized & reduced and the oxidizing & reducing agents in 2 NO 2 (g) + 7 H 2 (g) 2 NH 3 (g) + 4 H 2 O (g) 5 H 2 O 2 (aq) + 2 MnO 4 8 H 2 O + 2 Mn 2+ 1– (aq) + 6 H (aq) + 5 O 2 1+ (aq) (g) S 2 O 3 2– (aq) + 4 Cl 2 2 HSO 4 1– (aq) + 5 H (aq) + 8 H 1+ 2 O (aq) + 8 Cl 1– (aq) 6 Fe 2+ S 2 O 3 2– (aq) + 14 H 6 Fe 3+ 1+ (aq) + Cr 2 O 7 2– (aq) + 2 Cr 3+ (aq) (aq) + 7 H 2 O (aq) + 2 H 1+ (aq) S (s) + SO 2 (g) + H 2 O 39
Reactions
You now know how to write & balance 4 types of reactions combustion precipitation acid-base neutralization gas-forming and how to recognize redox reactions 40
Reaction quizzes
Reaction quizzes (RQ) will replace NQ I give you the names of the reactants You write the formulas of the reactant and the formulas of the products Cross out spectator ions Write the balanced net ionic equation You need not include state symbols such as (aq) or (g) 41
Example
Solutions of silver nitrate and potassium chloride are mixed AgNO 3 AgNO 3 (aq) + KCl (aq) (aq) + KCl (aq) AgCl (s) + KNO 3 (aq) cross out K 1+ Ag 1+ + Cl 1– and NO 3 1– AgCl as spectators check: it is already balanced 42
Example
Hydrochloric acid solution is added to solid sodium hydrogen carbonate HCl (aq) + NaHCO 3 HCl (aq) + NaHCO 3 (s) (s) H 2 O + CO 2 (g) + NaCl (aq) cross out Cl 1– as a spectator do NOT cross out Na 1+ H 1+ + NaHCO 3 H 2 because NaHCO O + CO 2 + Na 1+ 3 is solid check: it is already balanced 43
Example
Ethoxyethane burns in air C C 2 2 H H 5 5 OC OC 2 2 H H 5 5 + O + O 2 2 CO 2 + H 2 O there are no spectators in combustion balance C 2 H 5 OC 2 H 5 + 6 O 2 4 CO 2 + 5 H 2 O 44
Example
Solutions of nitrous acid and sodium hydroxide are mixed HNO 2 HNO 2 (aq) + NaOH (aq) (aq) + NaOH (aq) H 2 O + NaNO 2 (aq) cross out Na 1+ as a spectator do NOT cross out NO 2 1– because HNO 2 is a weak acid and is not significantly dissociated in solution!
HNO 2 + OH 1– H 2 O + NO 2 1– check: it is balanced 45