Chemistry 100
description
Transcript of Chemistry 100
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Chemistry 100
Aqueous Reactions
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Solutions
A solution is a homogenous mixture of two or more substances
One substance (generally the one present in the greatest amount) is called the solvent
The other substances - those that are dissolved - are called the solutes
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The Solution Process
Favourable interactions between the solute and the solvent drive the formation of a solution
Example: NaCl (an ionic solid) dissolving in water
Water is a polar fluid (i.e., possesses a permanent dipole)
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Electrolytes
Salt is an ionic compound. NaCl is dissolved in water - the ions
separate. The resulting solution conducts
electricity . A solute with this property is called an electrolyte
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Strong Electrolytes
Strong electrolytes - completely dissociated
Some molecular compounds dissolve in water to form ions. Dissolve HCl (g) in water. All the molecules dissociate. So it is also
a strong electrolyte.
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Weak and Nonelectrolytes
Weak electrolytes - only some of the molecules dissociate, i.e., acetic acid
Compounds that do not dissociate - nonelectrolytes Sugars Ureas Alcohols
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Acids
Acid - a substance that ionizes in water to form hydrogen ions H+.
HCl (aq) H+ (aq) + Cl(aq) What is H+? A hydrogen atom without
its electron - a bare proton.
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Monoprotic, diprotic, triprotic
One molecule of HCl gives one H+ ion:HCl H+ + Cl
We say that HCl is monoprotic - one proton
One molecule of sulphuric acid, H2SO4, has two hydrogens to give away. It is said to be diprotic.
Phosphoric acid, H3PO4 is triprotic.
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OO
O
S
O H
H
C
H
H
H C
O
OH only this H ionizes
both H's ionize
Some Chemical Structures
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Acetic Acid
Generally write as CH3COOH, not HC2H3O2. Weak acid - doesn’t dissociate
completely
CH3COOH (aq) ⇄ CH3COO- (aq) + H+ (aq)
The double arrow - the system is in chemical equilibrium!!!!
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Bases
Bases are substances that accept (react with) H+ ions. Hydroxide ions, OH, are basic. They react with H+ ions to form water:
H+ (aq) + OH (aq) H2O (l)
Ionic hydroxides like NaOH, KOH, Ca(OH)2 are basic. When dissolved in water they form hydroxide ions.
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Ammonia solution
When ammonia gas dissolves in water, some NH3 molecules react with water:NH3(aq) + H2O(l) ⇄ NH4
+ (aq) + OH–
(aq)
NOTE - only some NH3 molecules react with water. Ammonia is a
weak electrolyte.
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Strong and Weak Acids and Bases
Acids and bases that are strong electrolytes are called strong acids and strong bases.
Strong acids are more reactive than weak acids. Likewise for bases.
Note exception - HF, a weak acid, is very reactive
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Acids you should know
Chloric acid HClO3
Hydrobromic acid HBrHydrochloric acid HClHydroiodic acid HINitric acid HNO3
Perchloric acid HClO4
Sulphuric acid H2SO4
Acetic acid CH3COOH (weak)
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Bases you should know
Know the following bases:Strong basesa) Hydroxides of alkali metals: LiOH,
NaOH, KOHb) Hydroxides of the heavy alkaline earth
metals: Ca(OH)2, Sr(OH)2, Ba(OH)2
Weak base: ammonia solution NH3
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Metathesis reactions
A metathesis reaction is an aqueous solution in which cations and anions appear to exchange partners.
AX + BY AY + BXAgNO3 (aq)+ NaCl (aq) AgCl (s) +
NaNO3 (aq)
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Metathesis reactions (cont.)
Three driving forces Precipitate formation (insoluble
compound) AgNO3(aq)+ NaCl(aq) AgCl(s) +
NaNO3(aq)
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Metathesis Reactions (Cont’d)
Weak electrolyte or nonelectrolyte formation
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
Gas formation 2HCl(aq) + Na2S(aq) 2 NaCl(aq) +
H2S(g)
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Neutralization
Mix solutions of acids and bases - a neutralization reactions occurs.
acid + base salt + water Salt does not necessarily mean
sodium chloride!!!! Salt - an ionic compound whose cation
(positive ion) comes from a base and whose anion (negative ion) comes from an acid
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Precipitation Reactions
Some ionic compounds are insoluble in water.
If an insoluble compound is formed by mixing two electrolyte solutions, a precipitate results.
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Precipitation (Cont’d)
Solubility - maximum amount of substance that will dissolve in a specified amount of solvent.
Saturated solution of PbI2 contains 1 x 10-3 mol/L.
A compound with a solubility of less than 0.01 mol/L - insoluble.
More accurately - sparingly soluble.
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Solubility Fact 1
All the common ionic compounds of the alkali metals are soluble in water. The same is true of the compounds containing the ammonium ion, NH4
+.
NaCl, K2CO3, (NH4)2S are all soluble
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Solubility Fact 2
Salts containing the following anions are soluble
Anion exception, salts ofNO3
nitrate noneCH3COO acetate noneCl chloride Ag+, Hg2
2+,Pb2+
Br bromide Ag+, Hg22+,Pb2+
I iodide Ag+, Hg22+,Pb2+
SO42 sulphate Ca2+, Sr2+, Ba2+,
Hg22+, Pb2+
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Solubility Fact 3
Salts containing the following anions are insolubleAnion exception, salts of
S2 sulphide alkaline metal cations, NH4
+, Ca2+, Sr2+, Ba2+, CO3
2 carbonate alkaline metal cations, NH4
+ PO4
3 phosphate alkaline metal cations, NH4
+
OH hydroxide alkaline metal cations, Ca2+, Sr2+, Ba2+,
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Reaction forming gases
A metathesis reaction can occur due to the formation of a gas which is not very soluble in water.
Examples involving hydrogen sulphide and carbon dioxide
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Reactions forming H2S
A metathesis reaction occurs when hydrochloric acid is added to a sodium sulphide solution.
2HCl(aq) + Na2S(aq) H2S(g) + 2NaCl(aq)
Net ionic reaction:2H+(aq) + S2(aq) H2S (g)
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Reactions involving CO2
Carbonates and bicarbonates may be thought of as the salts of carbonic acid H2CO3 – unstable!!
H2CO3(aq) CO2(g) + H2O(l)
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Ionic Equations
Consider the reactionHCl (aq) + NaOH (aq) NaCl (aq) + H2O
(l) The above is known as the molecular
equation Note: the compounds are ionic (except
water)!!
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Ionic Equations #2
Let’s show ionic compounds as ionsH+(aq) + Cl–(aq) + Na+(aq) + OH– (aq)
Na+(aq) + Cl–(aq) + H2O(l) Some ions appear on both sides of the
equation.
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Out with the spectators!
Remove ions that appear on both sides
H+ (aq) + Cl– (aq) + Na+ (aq) + OH– (aq)
Na+ (aq) + Cl– (aq) + H2O (l) The unchanged ions are called
spectators
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The Net Ionic Equation
We are left with is the net ionic equation:
H+(aq) + OH– (aq) H2O(l)Note that the equation is balanced
for both mass and charge!!!
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Another ionic reaction
Place zinc metal in a hydrochloric acid solution – hydrogen is evolved!!
Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2
(g)
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Why use ionic reactions?
They summarize many reactions. neutralization of any strong acid by a
strong base is given by H+(aq) + OH– (aq) H2O(l)
The chemical behaviour of a strong electrolyte behaviour of its constituent ions.
Ionic equations can be written only for strong electrolytes which are soluble.
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Concentrations
How do we express the concentration of a solution?
Percentage is one way. 2% milk 35% cream. (These are not true
solutions)!!! Some beer is 5% alcohol
Note: % measurements can be %w/w, %w/v, %v/v
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Molarity
Must work in moles to do chemical arithmetic.
Chemists - molarity as their unit of solution concentration
(L) solution of volumesolute of molesMolarity
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Dilution
Dilute a solution more solvent is added but the amount
(mass or moles) of solute is unchanged.M1V1 = M2V2
The volumes can be either millilitres (mL) or litres (L).
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Ionic Concentration
NaCl in water - totally ionized into Na+ and Cl ions.
A 2.0 M NaCl solution Na+ concentration will be 2.0 M Cl concentration also 2.0 M
A 2.0 M solution of K2CO3, K+ concentration will be 4.0 M The concentration of CO3
2 2.0 M.
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Oxidation and reduction
A piece of calcium metal exposed to the air will react with the oxygen in the air
2Ca(s) + O2(g) 2 CaO(s) Ca has been converted to an ion Ca2+
by losing two 2 electrons. Dissolve Ca in acid
Ca(s) + 2H+(aq) Ca2+(aq) + H2(g) Again the Ca has lost 2 electrons —
oxidation
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Redox reactions
In the last two reactions, the Ca atom lost two electrons. Where did they go?
When one substance is oxidized, another is reduced. An oxidation-reduction reaction occurs. Or a redox reaction occurs.
Oxidation: loss of electrons (more positive) Reduction: gain of electrons (less positive)
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Oxidation of Metals - by air
Many metals react with oxygen in the air. Na and K do so explosively!
Fe rusts - at a cost of $billions each year! Aluminum oxidizes
oxide layer forms a skin which prevent further oxidation. Al hides its reactivity.
Gold and platinum do not react with oxygen.
Silver tarnishes mainly because of H2S in the air.
What does copper do?
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Oxidation of Metals - by acids
Many metals react with acids:metal + acid salt + hydrogen gasMg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
Metals may also be oxidized by the salts of other metals. Recall your lab experimentFe(s) + CuSO4(aq) Cu(s) + FeSO4(aq)
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Activity Series
We has seen that some metals react with air, some also react with acids to give hydrogen.
We have seen that some metals can be oxidized by ions of other metals.
All this is summarized in the activity series.
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Activity Series
Li Li+ + eK K+ + eBa Ba2+ + 2eCa Ca2+ + 2eNa Na+ + eMg Mg2+ + 2eZn Zn2+ + 2eFe Fe2+ + 2ePb Pb2+ + 2eH H+ + eCu Cu2+ + 2eAg Ag+ + eAu Au3+ + 3e
A metal can be oxidized by any ion below it
Metals above H, react with acids to give H2
The further up the series, the more readily the metal is oxidized
See your textbook (p 136) for more elements
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Some observations on the series
Lead (Pb) is above H, so is Al. But these metals are not attacked by 6M HCl. They form very protective oxides.
Cu reacts with nitric acid (HNO3) because that acid is a strong oxidizing agent in addition to being an acid.
Gold (Au) and platinum (Pt) are valuable because they are (a) rare and (b) unreactive - they do not tarnish
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Oxidation Numbers
Oxidation number - a fictitious charge assigned to atoms either by themselves or when combined in compounds as an electron bookkeeping device.
There are a number of simple rules that chemists use to assign oxidation numbers.
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Assigning Oxidation Numbers
In any elemental form (atom or molecule), an atom is assigned a 0 oxidation number e.g. He, Cu, N in N2, S in S8
For a monatomic ion, the oxidation number equals the charge e.g., -1 for Cl in Cl-, +2 for Ca+2, -2 for S-2
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Assigning Ox. Numbers (#2)
Fluorine’s oxidation number is -1 in any compound. e.g. -1 for F in CF4, but 0 for F in F2
Oxygen’s oxidation number is -2 except when combined with fluorine or in peroxides. e.g. -2 for O in H2O and OH-, +2 for O in
OF2, -1 for O in H2O2
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Assigning Ox. Numbers (#3)
For elements in Groups IA, IIA & most of IIIA, oxidation numbers are positive and equal to the group number. e.g. +3 for Al in AlCl3, +1 for Na in NaCl, +2
for Mg in Mg SO4 Hydrogen has a +1 oxidation number.
Exceptions to this rule are the metallic hydrides, in which it is -1. e.g., +1 for H in H2O and CH3OH, -1 for H
in NaH
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Assigning Ox. Numbers (#4)
The sum of the oxidation numbers of the atoms in a neutral compound is zero; in a polyatomic ion, the sum equals the charge. e.g. see OH- and H2O above, +6 for S in
SO4-2
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Balancing Oxidation-Reduction (Redox) Equations (#1)
Assign oxidation numbers to all atoms in the equation. Note - polyatomic ion that is unchanged
in the reaction may be treated as a single unit with an oxidation number equal to its charge.
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Balancing Redox Equations (#2)
Isolate the ATOMS that have undergone a change of oxidation number A reduction in number indicates a
reduction An increase in number, an oxidation
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Balancing Redox Equations (#3)
Isolate the chemical species undergoing oxidation/reduction (note: separate into an oxidation and a reduction half-reaction).
Add the appropriate number of electrons to the half-reactions Oxidation – electrons on products side Reduction – electrons on reactants side
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Balancing Redox Equations (#4)
Remaining steps refer to the individual half reactions Balance for charges
Add H+ in acidic solution Add OH- in basic solution
Balance the H and the O atoms by adding water
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Balancing Redox Equations (#5)
Balance the number of electrons in the half-reactions Note: electrons lost = electrons
gained Add the half-reactions, eliminating the
electrons and obtaining the complete REDOX equation
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Titrations
Volumetric analysis technique based on volume measurements used to determine the quantity of a
substance in solution. Titration a solution of an accurately
known concentration is added gradually to a solution of an unknown concentration Reaction goes to completion.
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Other Definitions
Standard solution solution of accurately known concentration.
Equivalence point point at which unknown substance has completely reacted with standard solution. At the equivalence point reagents are
present in stoichiometric amounts.
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Gravimetric Analysis
Determine concentration of an unknown by reacting it with a second substance to form a ppt.AgNO3(aq)+ NaCl(aq) AgCl(s) +
NaNO3(aq)