ap chemistry notes 2-1 assigning oxidation ......the oxidation number of any free element (including...
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AP CHEMISTRY 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 atoms in a compound is always equal to “0”.
3. The sum of the oxidation numbers of all of the atoms in a polyatomic ion is always equal to the charge
of that ion.
4. The oxidation number of all elements in Group IA (except hydrogen) 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 hydrides, 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
oxidation number of oxygen is -1. The peroxide ion is O22-.
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 ions (those that appear unchanged on both sides of the equation) are left out.
RULES:
1. Strong acids and bases ionize (break apart to form ions; ie. “split”)
a. Strong acids include HCl hydrochloric acid
HI hydroiodic acid
HBr hydrobromic acid
H2SO4 sulfuric acid
HNO3 nitric acid
HClO4 perchloric acid
H2CrO4 chromic acid
HMnO4 permanganic acid
(Note: Concentrated sulfuric acid does not ionize)
b. Strong bases include all hydroxides of Groups IA and IIA metals except magnesium
2. Soluble ionic compounds ionize (know your solubility rules!!)
3. Molecular compounds, solids, oxides, and pure liquids 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 electrons are transferred between
reactants (also known as a redox 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 Solution
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
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 water 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
Cr + CrO42- _______________> 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 molecule 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
xi
Deviation
_
di = (xi – x)
Squared Deviation
di2
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