AP CHEMISTRY NOTES 2-1 ASSIGNING OXIDATION NUMBERS

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1 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 The sum of the oxidation number of all of the atoms in a compound is always equal to 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 The oxidation number of all elements in Group IIA is always The following elements always have the oxidation numbers indicated: Al = +3 Cd = +2 Zn = +2 Ag = 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 O In a binary ionic compound, the oxidation number of elements in Group VIIA is -1.

2 Examples: NaI FeCl 3 Br 2 NaNO 3 KClO 4 ZnCr 2 O 7 Na 2 O 2 S 8 Cr(MnO 4 ) 6 Pb(SO 4 ) 2

3 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 H 2 SO 4 sulfuric acid HNO 3 nitric acid HClO 4 perchloric acid H 2 CrO 4 chromic acid HMnO 4 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

4 EXAMPLES: Balance the following equations and write net ionic equations for each. CaCO 3 + HNO 3 > Ca(NO 3 ) 2 + CO 2 + H 2 O ICl + H 2 O > HCl + HIO H 2 C 2 O 4 + NaOH > Na 2 C 2 O 4 + H 2 O Fe + HCl > FeCl 2 + H 2 Na 2 CO 3 + HCl > NaCl + CO 2 + H 2 O Ba(C 2 H 3 O 2 ) 2 + K 2 SO 4 > KC 2 H 3 O 2 + BaSO 4

5 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 + H 2 O Mg + S > MgS As + H 2 > AsH 3 K 2 Cr 2 O 7 + H 2 O + S > KOH + Cr 2 O 3 + SO 3

6 WRITING REDOX HALF-REACTIONS AP NOTES 2-4 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 + AgNO 3 > Ag + Cu(NO 3 ) 2 HNO 3 + HI > NO + I 2 + H 2 O Zn + MnO 2 + NH 4 Cl > ZnCl 2 + Mn 2 O 3 + NH 3 + H 2 O K 2 Cr 2 O 7 + H 2 O + S > KOH + Cr 2 O 3 + SO 3

7 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 H 2 O 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(NO 3 ) 3 + Fe > Fe(NO 3 ) 2 + Al Co + Br 2 > CoBr 3

8 HNO 3 + HI > NO + I 2 + H 2 O (Acidic Solution) S + HNO 3 > SO 2 + NO + H 2 O (Acidic Solution)

9 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 H 2 O 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 - + O 2 > Ag(CN) H 2 O Cr + CrO 4 2- > Cr(OH) 3

10 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 nm. The accepted value was found to be 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 _ d 1 = x i - x x 1 = each measurement value _ x = average (mean) of values N = the number of measurements

11 Example: The following values were determined for the volume of a tin cylinder (in cm 3 ). Determine the standard deviation of the values. Value of Measurement x i Deviation _ d i = (x i x) Squared Deviation d i mean = sum =

12 Example: The mass of an object was measured six times and the following values were obtained: g, g, g, g, g, and g. Determine the standard deviatio