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AP NOTES 2-1 ASSIGNING OXIDATION NUMBERS

RULES FOR ASSIGNING OXIDATION NUMBERS:

1. The oxidation number of any free element (including diatomic elements) is always “0”.

2. The sum of the oxidation number of all of the in a compound is always equal to “0”.

3. The sum of the oxidation numbers of all of the atoms in a polyatomic is always equal to the charge of that ion.

4. The oxidation number of all elements in Group IA (except ) is always +1.

5. The oxidation number of all elements in Group IIA is always +2.

6. The following elements always have the oxidation numbers indicated: Al = +3 Cd = +2 Zn = +2 Ag = +1

7. The oxidation number of hydrogen atoms in a compound is usually +1. (In , however, the oxidation number of hydrogen is -1.)

8. The oxidation number of oxygen atoms in a compound is usually -2. (In peroxides, however, the 2- oxidation number of oxygen is -1. The peroxide ion is O2 .

9. In a binary ionic compound, the oxidation number of elements in Group VIIA is -1.

Examples:

NaI FeCl3

Br2 NaNO3

KClO4 ZnCr2O7

Na2O2 S8

Cr(MnO4)6 Pb(SO4)2

AP CHEMISTRY NOTES 2-2 WRITING NET IONIC EQUATIONS

A net ionic equation is one in which all species are shown in their ionized form (if appropriate – see below) and spectator (those that appear unchanged on both sides of the equation) are left out.

RULES:

1. Strong and bases ionize (break apart to form ions; ie. “split”)

a. Strong acids include HCl hydrochloric HI hydroiodic acid HBr hydrobromic acid

H2SO4

HNO3 nitric acid

HClO4 perchloric acid

H2CrO4 chromic acid

HMnO4 permanganic acid

(Note: Concentrated sulfuric acid does not ionize)

b. Strong bases include all of Groups IA and IIA metals except magnesium

2. Soluble ionic compounds ionize (know your rules!!)

3. Molecular compounds, solids, , and pure do not ionize

4. All “spectator ions” are removed from the equation

EXAMPLES: Balance the following equations and write net ionic equations for each.

______CaCO3 + HNO3 > Ca(NO3)2 + CO2 + H2O

______ICl + H2O > HCl + HIO

______H2C2O4 + NaOH > Na2C2O4 + H2O

______Fe + HCl > FeCl2 + H2

______Na2CO3 + HCl > NaCl + CO2 + H2O

______Ba(C2H3O2)2 + K2SO4 > KC2H3O2 + BaSO4

AP CHEMISTRY NOTES 2-3 OXIDATION-REDUCTION REACTIONS

OXIDATION-REDUCTION REACTIONS – the changes that occur when are transferred between reactants (also known as a reaction)

OIL RIG

Oxidation Is Loss of electrons ...... Reduction Is Gain of electrons

Oxidation and reduction ALWAYS occur simultaneously

Oxidation – the loss of electrons by a substance (the oxidation number rises, becoming more positive)

Reduction – the gain of electrons by a substance (the oxidation number lowers, becoming more negative)

Oxidizing Agent – the substance in a redox reaction which causes another substance to be oxidized (it itself is reduced)

Reducing Agent – the substance in a redox reaction which causes another substance to be reduced (it itself is oxidized)

EXAMPLES:

______HCl + NaOH > NaCl + H2O

Mg + S ______> MgS

______As + H2 > AsH3

______K2Cr2O7 + H2O + S > KOH + Cr2O3 + SO3 AP NOTES 2-4 WRITING REDOX HALF-REACTIONS

RULES:

1. Re-write the reactions as net ionic equations.

2. Find the substance oxidized and substance reduced.

3. Write the oxidation half-reaction and the reduction half-reaction and add electrons to balance.

EXAMPLES:

______Cu + AgNO3 > Ag + Cu(NO3)2

______HNO3 + HI > NO + I2 + H2O

______Zn + MnO2 + NH4Cl > ZnCl2 + Mn2O3 + NH3 + H2O

______K2Cr2O7 + H2O + S > KOH + Cr2O3 + SO3

AP NOTES 2-5 BALANCING REDOX REACTIONS – Acidic

RULES:

1. Write the net ionic equation for the reaction.

2. Write separate half-reactions for the oxidation and reduction processes (balance for substance changing charge).

3. Balance the half-reaction “by mass”:

a. Use H2O to balance oxygens where needed b. Add H+ to balance on the other side

4. Balance both reactions “by charge” (multiply each half-reaction by an appropriate number to equalize the number of electrons lost or gained).

5. Add the half-reactions and subtract terms that appear on both sides of the equation.

6. Check the final equation to be sure that atoms are conserved, charge is conserved, and all electrons have cancelled.

EXAMPLES:

______Al(NO3)3 + Fe > Fe(NO3)2 + Al

______Co + Br2 > CoBr3

______HNO3 + HI > NO + I2 + H2O (Acidic Solution)

______S + HNO3 > SO2 + NO + H2O (Acidic Solution)

AP CHEMISTRY NOTES 2-6 BALANCING REDOX REACTIONS – Basic Solution (UNIT 2)

RULES:

1. Write the net ionic equation for the reaction.

2. Write separate half-reactions for the oxidation and reduction processes (balance for substance changing charge).

3. Balance the half-reaction “by mass”:

a. Use H2O to balance oxygens where needed b. Add H+ to balance hydrogens on the other side c. Add the same number of OH- ions to both sides of the equation to equal the H+ ions added previously d. Convert the H+ / OH- ions to on one side of the equation and leave the lone OH- ions on the other side alone

4. Continue as for reactions in acidic solution.

EXAMPLES:

- ______- Ag + CN + O2 > Ag(CN)2 + H2O

2- ______Cr + CrO4 > Cr(OH)3

AP Chemistry NOTES 2-7

STATISTICAL ANALYSIS: PERCENT ERROR & STANDARD DEVIATION

PERCENT ERROR

Accuracy – how close a measurement or calculation is to the accepted value

One statistical method used to report / measure accuracy is percent error:

Example: In an experiment, the diameter of a was determined to be 0.267 nm. The accepted value was found to be 0.290 nm. What was the percent error?

STANDARD DEVIATION

Precision – how close together the values for multiple measurements of the same quantity are

One statistical method used to report / measure precision is standard deviation:

Where Σ = “the sum of” _ d1 = xi - x

x1 = each measurement value _ x = average (mean) of values

N = the number of measurements

Example: The following values were determined for the volume of a tin cylinder (in cm3). Determine the standard deviation of the values.

Value of Measurement Deviation Squared Deviation _ 2 xi di = (xi – x) di

10.11

10.13

10.10

10.12

10.15

10.11

10.12

mean = ______sum = ______

Example: The mass of an object was measured six times and the following values were obtained: 13.34 g, 13.08 g, 13.58 g, 13.42 g, 13.29 g, and 13.45 g. Determine the standard deviatio