Reactions in Aqueous Solution Chemistry, 5th Edition McMurry/Fay
Some 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
Some Interesting Chemical Reactions 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
Some Interesting Chemical Reactions 5. Photosynthesis 6 CO2 + 6 H2O → C6H12O6 + 6 O2 6. Acid Rain S + O2 + heat → SO2 SO2 + H2O → H2SO3 (acid)
Types 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)
Precipitation 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)
Precipitation Reactions Solubility Rules Always form soluble compounds: Group 1 cations, NH4+, NO3- Often form precipitates: Ag+, Hg22+, Pb22+
Types of Chemical Reactions • 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)
Acid-Base Reactions • Arrhenius Definition: 1. Acid releases H+ in water. HCl → H+ + Cl- 2. Base releases OH- in water NaOH Na+ + OH-
Acid-Base Reactions • Brønsted Definition: Acid donates a proton (H+), base accepts a proton. HCl + H2O → H3O+ + Cl- NH3 + H2O → NH4+ + OH-
Acid-Base Reactions • LewisDefinition: Acid accepts 2 electrons, base donates 2 electrons. (later)
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
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?
Types of Chemical Reactions • Oxidation–Reduction (Redox) Reaction:A reaction in which one or more electrons are transferred between reaction partners. Mg (s) + Cl2 (g) MgCl2(s)
Redox Reactions Electrolyte: a solution that conducts electricity - or – An ionic compound dissolved in water
Redox Reactions Strong electrolyte: almost all molecules are ionized. HCl, HNO3, NaCl Weak electrolyte: only a small proportion of the ions are formed. H2SO3, HC2H3O2
Electrolytes in Solution • Why do ionic compounds conduct electricity when molecular ones generally do not?
Electrolytes in Solution • Electrolytes:Dissolve in water to produce ionic solutions. • Nonelectrolytes: Do not form ions when they dissolve in water.
Electrolytes in Solution • Dissociation: • The process by which a compound splits up to form ions in the solution.
Redox Reactions 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
Redox Reactions 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
Redox Reactions Reducing Agent: causes reduction (donates electrons) Oxidizing Agent: causes oxidation (accepts electrons) 2 Mg + O2→ 2 MgO Oxidizing Agent = O2 Reducing Agent = Mg
Redox Reactions 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…
Redox Reactions 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
Redox Reactions 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
Redox Reactions 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
Redox Reactions CALCULATING OXIDATION NUMBER 2. For atoms in the elemental form the oxidation number is zero. Mg (s): ON = 0 O2 (g): ON = 0
Redox Reactions CALCULATING OXIDATION NUMBER 3. Remember some common oxidation numbers: Group 1 → +1 Group 2 → +2 Halogens → -1 Oxygen → -2 Hydrogen → +1, -1
Redox Reactions CALCULATING OXIDATION NUMBER HSO3- ONS = ?? ONH + ONS + 3(ONO) = -1 ONH = +1, ONO = -2 So ONs = -1 – (1) – (3 x (-2)) = +4
Redox Reactions HALF REACTIONS Mg + Cl2 → MgCl2 Find ON of each atom: Mg0 + Cl20 → Mg2+Cl21- Break reaction into two separate (HALF) reactions showing electrons
Redox Reactions HALF REACTIONS Mg + Cl2 → MgCl2 Break reaction into two separate (HALF) reactions showing electrons Mg0 → Mg2+ + 2 e- Cl20 + 2 e- → 2 Cl-
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.
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.
Balancing Redox Reactions • Balance the following for acidic and basic solution: • ClO–(aq) + Cr(OH)4–(aq) CrO42–(aq) + Cl–(aq) • NO3–(aq) + Cu(s) NO(g) + Cu2+(aq) • MnO4–(aq) + IO3–(aq) MnO2(s) + IO4–(aq)