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Lecture 4

CHEM 217 Lecture Notes - Lecture 4: Sodium Bicarbonate, Electronegativity, SolubilityPremium

9 pages53 viewsSpring 2018

Department
Chemistry
Course Code
CHEM 217
Professor
kate
Lecture
4

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Unit 4
4.1 (N O T I MPO R TAN T)
4.2 WR I TI N G/ B A LA N C IN G C H E MI C AL EQUA T IO NS
oCombustion reaction: substance combines with oxygen to form one or more oxygen containing
compounds.
oAlso emit heat
oProcedure guidelines for balancing
o1. Write skeletal equation by writing chemical formulas for reactants and products
o2. Balance atoms that occur in more complex substances first. Always balance atoms in
compounds before atoms in pure elements
o3. Balance free elements
o4. Check
4.3 SO L UT I ONS AN D SO LUB I L IT Y
oReactions involving reactants dissolved in water are among the most common and important
oA homogeneous mixture of two substances, such as salt and water, is a solution
Majority is solvent and minority is solute
Consider salt water and sugar water – can no longer see them but taste is present
oAn aqueous solution is one in which water is the solvent
oWhen a solid is put into a liquid solvent, the attractive forces that hold the solid together (solute-solute
interactions) compete with the attractive forces between solvent molecules and the solid (solvent-solute
interactions)
EL E CTR O LY T E A ND N ONE L EC T RO L YTE SO L UTI O NS
oA salt solution will conduct electricity while a sugar solution does not
oDifference between the way ionic compounds and molecular compounds dissolve in water illustrates a
fundamental difference between types of solutions
oIonic compounds:
Dissociate into their component ions when dissolved in water
Dissolved NaCl doesn’t contain any NaCl units but rather dissolved Na+ Cl- ions that act
as charge carriers, allowing solution to conduct electricity
Subtances that dissolve in water to form solutions that conduct electricity are
electrolytes
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Substances like NaCl that completely dissociate into ions in water are strong electrolytes
and resulting solutions are electrolyte solutions
oMolecular compounds:
most dissolve as molecules which remain intact once dissolved.
Sugar dissolves because the attraction between sugar molecules and H2O overcomes
sugar’s attraction to its own molecules
Intact sugar molecule homogeneously mixed with water molecules
Compound that don’t dissociate into ions are called nonelectrolytes – nonelectrolyte
solutions don’t conduct electricity
oUnlike soluble ionic compounds that contain ions and dissociate in water, acids are molecular
compounds that ionize in water
Acids:
Molecular compounds but they do ionize (form ions) in water when dissolved
Strong acids (like HCl) completely ionize in solution
oSince strong acids completely ionize they’re also strong electrolytes
oWe represent the complete ionization of a strong acid with a single
reaction arrow between the acid and its ionized form
HCl (aq) -> H+ (aq) + Cl- (aq)
oBut, many acids are weak acids that don’t completely ionize
oFor example, CH3OOH is acetic acid and is weak
oA solution of a weak acid is composed mostly of the nonionized form of the acid molecules – only a
small percentage of acid molecules ionize.
oWe represent the partial ionization of a weak acid with
opposing half affows between the reactants and products
CH3COOH (aq) -> <- H+ (aq) + CH3COO- (aq)
oWeak acids classified as weak electrolytes and they conduct electricity only
weakly
SOLUBILITY OF IONIC COMPOUNDS
Not all ionic compounds dissolve completely in water AgCl remains solid and appears as white powder at the
bottom of water
oGenerally, compound termed soluble if it dissolves in water and insoluble if not
oBut oversimplification: solubility is a continuum and even insoluble compounds dissolve to some
extent
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oNo easy way to tell if a compound’s soluble but can follow set of empirical rules chemists have
inferred
Rules to be followed in orger, e.g. the rules with smaller number take precedence
*Notice that when compounds with polyatomic ions like NO3- dissolve, the polyatomic
ions dissolve as intact units
1. All salts containing cations of group 1 metals and ammonium ions (NH4+) are soluble
2. All nitrates (NO3-) ethanoates (acetates, CH3COO-), chlorates (ClO3-) and perchlorates
(ClO4-) are soluble
3. Salts containing Ag+, Pb2+ and Hg2+ are insoluble
4. Most chlorides (Cl-) bromides (Br-) and Iodides (I-) are soluble
5. Sulfates (SO42-) are soluble, except for those containing Ca2+, Sr2+, Ba2+
6. Carbonates (CO32-) Hydroxides (OH-) oxides (O2-) Phosphates (PO43-) and sulfides (S2-)
are generally insoluble.
4.4 P R ECIP ITA T ION R E A CT I O NS
oin which a solid or precipitate forms when we mix two solutions. Common in chemistry
oKey to predicting precipitation is to understand that only insoluble compounds form precipitates
oIn precipitation reaction, two soluble compounds combine and an insoluble compound precipitates
oEx. both KI & Pb(NO3)2 are soluble and exist in their respective solutions as K+, I-, Pb2+ and NO3- -
once they mix, two new compounds – one or both of which might be insoluble – are possible
In this case, KNO3 is soluble but PbI2 is not and precipitates
oA molecular equation shows complete neutral formulas for each compound in reaction as if they exist as
molecules but because of their solubility in (aq) they exist as ions
oWe can better represent the dissociated nature of dissolved ionic compounds
oCould be written as a complete ionic equation that lists all of the ions present as either reactants
of products in a chemical reaction
In complete ionic equation, some ions appear on both sides: spectator ions and do not
take part in reactions. To simplify: can omit them: Pb2+ + 2I- -> PbI2 (s)
This net ionic equation is most useful because it shows only the species that
take part in the reaction
oWriting equations for precipitation reaction
o1. Write the formula of the 2 compounds being mixed as reactants
o2. Write ions that are present when the two ionic compounds dissolve in water and determine
the possible products that are formed
o3. use solubility rules to determine whether any of the possible products are insoluble
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