C h a p t e rC h a p t e r C h a p t e rC h a p t e r 4 4 Reactions in Aqueous Solution Chemistry, 5 th Edition McMurry/Fay Chemistry, 5 th Edition McMurry/Fay

C h a p t e rC h a p t e r 44Reactions in Aqueous SolutionReactions in Aqueous Solution

Chemistry, 5th EditionMcMurry/Fay

Chemistry, 5th EditionMcMurry/Fay

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Some Interesting Chemical ReactionsSome Interesting Chemical Reactions

1. Production of Smog

N2 + O2 + heat → 2 NO

2 NO + O2 → 2 NO2 (brown gas)

2. The Greenhouse Effect

2 C8H18 + 25 O2 → 18 H2O + 16 CO2

CO2 transmits visible light but absorbs heat

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3. Reduction of Iron Ore

Fe2O3 + 3 CO → 2 Fe + 3 CO2

4. Depletion of Ozone

O3 + uv rays → O2 + O

CF2Cl2 + O3 → O2 + O

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5. Photosynthesis

6 CO2 + 6 H2O → C6H12O6 + 6 O2

6. Acid Rain

S + O2 + heat → SO2

SO2 + H2O → H2SO3 (acid)

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Types of Chemical ReactionsTypes of Chemical Reactions

• Precipitation Reactions: The chemical reaction that occurs when two solutions are mixed and a solid precipitate is formed.

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

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Precipitation ReactionsPrecipitation Reactions

• Net Ionic Equations: Break the reaction down into aqueous ions and precipitates. Eliminates “spectator ions,” thus simplifying the equation.

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

Ag+ + NO3- + Na+ + Cl- → AgCl (s) + Na+ + NO3

-

Net Ionic Equation: Ag+ + Cl- → AgCl (s)

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Precipitation ReactionsPrecipitation Reactions

Solubility Rules

Always form soluble compounds:

Group 1 cations, NH4+, NO3

-

Often form precipitates:

Ag+, Hg22+, Pb2

2+

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• Acid–Base Reactions: A reaction of an acid with a base, forming water plus a salt.

• The driving force of this reaction is the formation of the stable water molecule.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

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Acid-Base ReactionsAcid-Base Reactions

• Arrhenius Definition:

1. Acid releases H+ in water.

HCl → H+ + Cl-

2. Base releases OH- in water

NaOH Na+ + OH-

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• Brønsted Definition:

Acid donates a proton (H+), base accepts a proton.

HCl + H2O → H3O+ + Cl-

NH3 + H2O → NH4+ + OH-

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• Lewis Definition:

Acid accepts 2 electrons, base donates 2 electrons.

(later)

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pH - A Measure of Acidity pH - A Measure of Acidity

• The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration (in mol/L).

pH = –log[H+]

pH + pOH = 14

Acidic solutions: [H+] > 1.0 x 10–7 M, pH < 7.00Basic solutions: [H+] < 1.0 x 10–7 M, pH > 7.00Neutral solutions: [H+] = 1.0 x 10–7 M, pH = 7.00

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pH - A Measure of Acidity pH - A Measure of Acidity

• Calculate the pH of a HNO3 solution having a

hydrogen ion concentration of 0.76 M.

• The OH– ion concentration of a blood sample is

2.5 x 10–7 M. What is the pH of the blood?

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• Oxidation–Reduction (Redox) Reaction: A reaction in which one or more electrons are transferred between reaction partners.

Mg (s) + Cl2 (g) MgCl2(s)

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Redox Reactions Redox Reactions

Electrolyte: a solution that conducts electricity

- or –

An ionic compound dissolved in water

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Strong electrolyte: almost all molecules are

ionized.

HCl, HNO3, NaCl

Weak electrolyte: only a small proportion of the

ions are formed.

H2SO3, HC2H3O2

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Electrolytes in SolutionElectrolytes in Solution

• Why do ionic

compounds conduct

electricity when

molecular ones

generally do not?

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• Electrolytes: Dissolve

in water to produce

ionic solutions.

• Nonelectrolytes: Do

not form ions when

they dissolve in water.

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• Dissociation:

• The process by

which a compound

splits up to form ions

in the solution.

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How are electrons transferred??

Observe charges on elements:

Mg0 (s) + Cl20 (g) MgCl2(s)

Mg0 (s) + Cl20 (g) Mg2+ + 2Cl-

Mg0 → Mg2+ + 2 e- 2 e- + Cl20 → 2 Cl-

So electrons are TRANSFERRED from Mg to Cl2

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How are electrons transferred??

2 e- + Cl20 → 2 Cl-

Cl2 is REDUCED, since the charge on each Cl is lowered

Mg0 → Mg2+ + 2 e-

Mg is OXIDIZED, electrons are removed

REDuction + OXidation = REDOX

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Reducing Agent: causes reduction (donates electrons)

Oxidizing Agent: causes oxidation (accepts electrons)

2 Mg + O2 → 2 MgO

Oxidizing Agent = O2

Reducing Agent = Mg

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Electrochemical Series: A table showing the relative

oxidizing strengths of different species.

Helps to predict the outcome of unknown redox

reactions

Zn + Cu2+ → ??

Check Table…

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Activity Series of ElementsActivity Series of Elements

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Zn + Cu2+ → ??

From Table: Zn is a stronger reducing agent than Cu.

Therefore the electrons are more likely to reside with Cu

in the product:

Zn + Cu2+ → Zn2+ + Cu

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OXIDATION NUMBER

Effective charge on any atom

NaCl: Na +1, Cl -1

MgCl2: Mg +2, Cl -1

H2O: H +1, O -2

MgO: Mg +2, O -2

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CALCULATING OXIDATION NUMBER

1. The sum of the O.N.s of all of the atoms in a molecule

or ion is equal to its total charge.

NaCl: ONNa + ONCl = 0 +1 + (-1) = 0

SO42-: ONS + 4 x ONO = -2 6 + 4x(-2) = -2

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2. For atoms in the elemental form the oxidation number

is zero.

Mg (s): ON = 0

O2 (g): ON = 0

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3. Remember some common oxidation numbers:

Group 1 → +1

Group 2 → +2

Halogens → -1

Oxygen → -2

Hydrogen → +1, -1

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HSO3- ONS = ??

ONH + ONS + 3(ONO) = -1

ONH = +1, ONO = -2

So ONs = -1 – (1) – (3 x (-2)) = +4

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HALF REACTIONS

Mg + Cl2 → MgCl2

Find ON of each atom:

Mg0 + Cl20 → Mg2+Cl2

1-

Break reaction into two separate (HALF) reactions

showing electrons

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HALF REACTIONS

Mg + Cl2 → MgCl2

Break reaction into two separate (HALF) reactions

showing electrons

Mg0 → Mg2+ + 2 e-

Cl20 + 2 e- → 2 Cl-

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Balancing Redox Reactions Balancing Redox Reactions

HALF REACTION METHOD

1. Break reaction into half reactions.

2. Balance each half reaction separately.

a) use H+ and H2O as needed in ACIDIC solution.

b) use OH- and H2O as needed in BASIC solution.

3. Balance charge in each half reaction by adding

electrons to one side of the equation.

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Balancing Redox Reactions Balancing Redox Reactions

HALF REACTION METHOD

4. Obtain equal number of electrons in each half reaction

by multiplying entire equation by a constant.

5. Add the 2 half reactions back together so that the

electrons cancel.

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Balancing Redox ReactionsBalancing Redox Reactions

• Balance the following for acidic and basic solution:

ClO–(aq) + Cr(OH)4–(aq) CrO4

2–(aq) + Cl–(aq)

NO3–(aq) + Cu(s) NO(g) + Cu2+(aq)

MnO4–(aq) + IO3

–(aq) MnO2(s) + IO4–(aq)

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C h a p t e rC h a p t e r C h a p t e rC h a p t e r 4 4 Reactions in Aqueous Solution Chemistry, 5 th Edition McMurry/Fay Chemistry, 5 th Edition McMurry/Fay