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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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chemistry 100

Chemistry 100

Aqueous Reactions

solutions
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
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)
electrolytes
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
strong electrolytes
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.
weak and nonelectrolytes
Weak and Nonelectrolytes
  • Weak electrolytes - only some of the molecules dissociate, i.e., acetic acid
  • Compounds that do not dissociate - nonelectrolytes
    • Sugars
    • Ureas
    • Alcohols
acids
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.
monoprotic diprotic triprotic
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.
acetic acid
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!!!!

bases
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.
ammonia solution
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.

strong and weak acids and bases
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
acids you should know
Acids you should know

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)

bases you should know
Bases you should know
  • Know the following bases:

Strong 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

metathesis reactions
Metathesis reactions
  • 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)

metathesis reactions cont
Metathesis reactions (cont.)
  • Three driving forces
  • Precipitate formation (insoluble compound)

AgNO3(aq)+ NaCl(aq)  AgCl(s) + NaNO3(aq)

metathesis reactions cont d
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)

neutralization
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
precipitation reactions
Precipitation Reactions
  • Some ionic compounds are insoluble in water.
  • If an insoluble compound is formed by mixing two electrolyte solutions, a precipitate results.
precipitation cont d
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.
solubility fact 1
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

solubility fact 2
Solubility Fact 2

Salts containing the following anions are soluble

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+

solubility fact 3
Solubility Fact 3
  • 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+,

reaction forming gases
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
reactions forming h 2 s
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)

reactions involving co 2
Reactions involving CO2
  • Carbonates and bicarbonates may be thought of as the salts of carbonic acid H2CO3 – unstable!!

H2CO3(aq) CO2(g) + H2O(l)

ionic equations
Ionic Equations
  • 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)!!
ionic equations 2
Ionic Equations #2
  • 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.
out with the spectators
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
the net ionic equation
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!!!

another ionic reaction
Another ionic reaction
  • Place zinc metal in a hydrochloric acid solution – hydrogen is evolved!!

Zn (s) + 2HCl (aq)  ZnCl2 (aq) + H2 (g)

why use ionic reactions
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.
concentrations
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
molarity
Molarity
  • Must work in moles to do chemical arithmetic.
  • Chemists - molarity as their unit of solution concentration
dilution
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).

ionic concentration
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 CO32 2.0 M.
oxidation and reduction
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
redox reactions
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)
oxidation of metals by air
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?
oxidation of metals by acids
Oxidation of Metals - by acids
  • 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)

activity series
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.
activity series1
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 145) for more elements

Activity Series

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

some observations on the series
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
oxidation numbers
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.
assigning oxidation numbers
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
assigning ox numbers 2
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
assigning ox numbers 3
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
assigning ox numbers 4
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
balancing oxidation reduction redox equations 1
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.
balancing redox equations 2
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
balancing redox equations 3
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
balancing redox equations 4
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
balancing redox equations 5
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
titrations
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.
other definitions
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.
gravimetric analysis
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)