Chemistry 100. Aqueous Reactions. 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. The Solution Process.
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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
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)
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
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.
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.
CH3COOH (aq) ⇄ CH3COO- (aq) + H+ (aq)
The double arrow - the system is in chemical equilibrium!!!!
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.
NOTE - only some NH3 molecules
react with water. Ammonia is a
When ammonia gas dissolves in water, some NH3 molecules react with water:
NH3(aq) + H2O(l) ⇄ NH4+(aq) + OH–(aq)
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
Chloric acid HClO3
Hydrobromic acid HBr
Hydrochloric acid HCl
Hydroiodic acid HI
Nitric acid HNO3
Perchloric acid HClO4
Sulphuric acid H2SO4
Acetic acid CH3COOH (weak)
Know the following bases:
a) Hydroxides of alkali metals: LiOH, NaOH, KOH
b) Hydroxides of the heavy alkaline earth metals: Ca(OH)2, Sr(OH)2, Ba(OH)2
Weak base: ammonia solution NH3
A metathesis reaction is an aqueous solution in which cations and anions appear to exchange partners.
AX + BY AY + BX
AgNO3 (aq)+ NaCl (aq) AgCl (s) + NaNO3 (aq)
Three driving forces
Precipitate formation (insoluble compound)
AgNO3(aq)+ NaCl(aq) AgCl(s) + NaNO3(aq)
Weak electrolyte or nonelectrolyte formation
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
2HCl(aq) + Na2S(aq) 2 NaCl(aq) + H2S(g)
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
Some ionic compounds are insoluble in water.
If an insoluble compound is formed by mixing two electrolyte solutions, a precipitate results.
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.
NaCl, K2CO3, (NH4)2S are all soluble
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+.
Anion exception, salts of
NO3 nitrate none
CH3COO acetate none
Cl chloride Ag+, Hg22+,Pb2+
Br bromide Ag+, Hg22+,Pb2+
I iodide Ag+, Hg22+,Pb2+
SO42 sulphate Ca2+, Sr2+, Ba2+, Hg22+, Pb2+
Salts containing the following anions are soluble
Salts containing the following anions are insoluble
Anion exception, salts of
S2 sulphide alkaline metal cations,
NH4+, Ca2+, Sr2+, Ba2+,
CO32 carbonate alkaline metal cations, NH4+
PO43 phosphate alkaline metal cations, NH4+
OH hydroxide alkaline metal cations,
Ca2+, Sr2+, Ba2+,
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
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)
H2CO3(aq) CO2(g) + H2O(l)
Carbonates and bicarbonates may be thought of as the salts of carbonic acid H2CO3 – unstable!!
Consider the reaction
HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)
The above is known as the molecular equation
Note: the compounds are ionic (except water)!!
Let’s show ionic compounds as ions
H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq)
Na+(aq) + Cl–(aq) + H2O(l)
Some ions appear on both sides of the equation.
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
Note that the equation is balanced
for both mass and charge!!!
We are left with is the net ionic equation:
H+(aq) + OH–(aq) H2O(l)
Place zinc metal in a hydrochloric acid solution – hydrogen is evolved!!
Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2 (g)
Must work in moles to do chemical arithmetic.
Chemists - molarity as their unit of solution concentration
The volumes can be either millilitres (mL) or litres (L).
M1V1 = M2V2
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
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)
Many metals react with acids:
metal + acid salt + hydrogen gas
Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
Metals may also be oxidized by the salts of other metals. Recall your lab experiment
Fe(s) + CuSO4(aq) Cu(s) + FeSO4(aq)
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.
A metal can be oxidized by any ionbelow 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
Li Li+ + e
K K+ + e
Ba Ba2+ + 2e
Ca Ca2+ + 2e
Na Na+ + e
Mg Mg2+ + 2e
Zn Zn2+ + 2e
Fe Fe2+ + 2e
Pb Pb2+ + 2e
H H+ + e
Cu Cu2+ + 2e
Ag Ag+ + e
Au Au3+ + 3e
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
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.
Determine concentration of an unknown by reacting it with a second substance to form a ppt.
AgNO3(aq)+ NaCl(aq) AgCl(s) + NaNO3(aq)