Reactions and reactions in aqueous solutions

1
Reactions and reactions in aqueous solutions

• General types of reactions
• Redox reactions
• Combustions
• Reactions in aqueous solution
• Net ionic equations

2
Coverage of reactions in class in your text

• Text coverage
• Disjointed several different places poor
  development
• Categorization of reactions ? artificial
• Not responsible
• Class Power Points
• Development based on knowing how to split
  matter re-combine pieces
• Build on acid-base and ion-exchange reactions (PP
  Chapter 5A)
• Apply dynamic reaction figure tool in alternate
  ways consistent with idea of pairing

3
Upgrading how to write reactions

• Reaction must be written with substances as they
  exist at standard conditions not in simplest form
• 4 Na O2 ? 2 Na2O NOT 2 Na O ? Na2O
• Species descriptors should give states as they
  exist at standard conditions OR what happens in a
  reaction
• 4 Na (s) O2 (g) ? 2 Na2O (s) (for reaction
  above)
• 2 Li3PO4 (aq) 3 Ca(NO3)2 (aq) ? 6 LiNO3 (aq)
  Ca3(PO4)2 (s)
• (from reaction in PP5A slide 9)
• when a solution of lithium phosphate is
  combined with a solution of calcium nitrate,
  calcium phosphate precipitates and lithium
  nitrate remains in solution
• Net ionic equations can express what has actually
  changed in terms of the status of species in a
  reaction
• Reaction above ? 2 PO43- (aq) 3 Ca2 (aq) ?
  Ca3(PO4)2 (s)
• when calcium ions and phosphate ions are in the
  same solution, calcium phosphate precipitates

4
Types of Reactions

• Redox reactions (oxidation reduction)
• reactions in which oxidation numbers on at least
  two species change
• for one species the oxidation number increases,
  for the other it decreases
• Examples
• Combining elements to make compounds (or
  reverse!)
• Combustions (reacting with O2) (or reverse!)
• Reactions in batteries (discharging) or reverse
  (charging!)
• Non-redox reactions
• no change in oxidation numbers
• Examples
• Acids reacting with hydroxides (and other
  acid/base reactions)
• Ion-exchange reactions
• Other exchange reactions

Need to add detail on status of species
5
How to spot redox reactions

• Elemental form ? Combined form (or vice versa)
• 2 Na (s) Cl2 (g) ? 2 NaCl (s)
• Zn (s) CuSO4 (aq) ? ZnSO4 (aq) Cu (s)
• MgI2 (aq) Br2 (aq) ? MgBr2 (aq) I2 (aq)
• From more H in substance ? to less H in
  substance (or vice versa)
• XH2 C2H4 ? C2H6 X (generalized reaction)
• From more O in substance ? to less O in
  substance (or vice versa)
• C6H12 (g) 9 O2 (g) ? 6 CO2 (g) 6 H2O (l)
• 6 KI (aq) 2 KMnO4 (aq) 4 H2O (l) ? 3 I2 (aq)
  2 MnO2(s) 8 KOH (aq)
• Zn (s) HgO (s) ? ZnO (s) Hg (l)
• Reaction for many camera, computer batteries

6
Components of oxidation-reduction reactions

• Oxidation half-reaction loss of electrons
• Reduction half-reaction gain of electrons

0
0
2
2-
Oxidation half-reaction (lose e-)
Reduction half-reaction (gain e-)
Electrons lost Electrons gained
7
Dynamic View of Redox Process
Oxidation half-reaction upper hemisphere of
electron transfer
2 Mg
2 Mg 2
4 e -
2 MgO
4 e -
O2
2 O 2-
Reduction half-reaction lower hemisphere of
electron transfer
8
Combustion reactions an important category of
redox reactions

• Combustion reaction with molecular oxygen (O2)
• Inorganic combustions
• combination reactions, oxidations
• Element O2 ? Compound (an oxide)
• 2 Cu (s) O2 (g) ? 2 CuO (s)
• S (s) O2 (g) ? SO2 (g)
• In both these cases other oxides could form (Cu2O
  SO3) in other cases there is only one oxide
  (Na2O)
• Oxide O2 ? Higher oxide (complete combustion)
• 2 Cr2O3 (s) 3 O2 (g) ? 4 CrO3 (s)
• 2 CO (g) O2 ? 2 CO2 (g)
• Organic/biochemical combustions
• Burning fossil fuels CxHy (X ¼ Y) O2 ? X
  CO2 ½ Y H2O
• Digestion respiration (many many individual
  steps)
• OVERALL REACTION for carbohydrate food
• C6H12O6 (glucose) 6 O2 ? 6 CO2 6 H2O

9
Completing organic combustion reactions

• Carbon dioxide and water will ALWAYS be the
  products
• Make all the CO2 you can from the C in the
  organic compound
• DO NOT WORRY ABOUT THE O2 AT THIS TIME
• Make all the H2O you can from the H in the
  organic compound
• Count number of O atoms needed to make CO2 and
  H2O
• Add the number of O2 molecules that will make
  this number of O atoms
• Use ½ numbers if needed
• If the number of O2 is a half number DOUBLE
  EVERYTHING

C6H12 (l) O2 (g) ? ?
C6H14 (l) O2 (g) ? ?
2
C6H12
O2
9
C6H14
19
6
19/2
CO2
H2O
6
6
7
12
14
? O
18
19
C6H12 (l) O2 (g) ?
CO2 (g) H2O (l)
9
6
6
C6H14 (l) O2 (g) ?
CO2 (g) H2O (l)
2
19
12
14
10
Reactions in Aqueous Solution

• The Solvent Water
• Possible roles
• Reaction with added components
• Solvation of added components
• Media for transport of added components
• Dissolved components solutes
• In solution before or after reaction
• Expressed as actual species present in water
• Non dissolved components
• Solids, liquids or gases that enter as reactants
• Solids, liquids or gases that leave as products

11
Whats reacting and whats not?

• Whats not
• substance dissolving in water without changing
  the chemical characteristics of the species
  comprising the substance
• Ionic substances forming cations and anions in
  solution
• Covalent compounds dissolving in solutions
  without reacting with water
• Species that have the same status before and
  after other things have changed
• What is
• Any species that reacts with water
• Any species that changes location as a
  consequence of an interaction in water
  (precipitation)
• Any species that changes chemical properties by
  being placed in water
• Any species that reacts with another species in
  water

12
General reactions in water - 1

• Precipitations
• General form XY (aq)AB (aq) ? XB (aq)AY (s)
• Tip off XY and AB are both salts
• Metal followed by nonmetal ion or polyatomic ion
• Exception NH4X substances also salts
• Acid-base
• General form of one type of acid base reaction
• HY(aq) XOH(aq) ? H2O (l) XY (aq)
• any acid with strong hydroxide base
• Other acid base reactions later
• Tip off HY and XOH
• Upgrade to add species designations and get net
  ionic equations

13
General reactions in water - 2

• Oxidation-reduction (redox)
• Most difficult to spot
• General forms
• Xo AY ? AX Yo
• Bo AY ? BY Ao
• Ao Yo ? AY
• Tip off species in elemental state metals,
  dissolved nonmetals gases, liquids
• No reaction!
• No interaction with water or other species
• Cations anions that can coexist dissolved
• Reactions that actually go in opposite direction
• You can write all kinds of reactions but that
  does not mean that they occur as written

14
Obtaining balanced complete equations

• Determine type of reactant equation
• Know tip offs
• Pair matching parts
• Oppositely charged ions for precipitation
  reactions
• H and OH to make water from acid plus hydroxide
  cation and anion to make salt
• For redox reactions determine what is losing and
  what is gaining electrons
• Balance equation
• Balance charges of oppositely charged ions
• Balance number of H and OH
• Balance electrons lost vs electrons gained

15
Obtaining net ionic equations

• What stays together?
• Precipitated salts
• Water
• Covalent compounds that do not ionize (for now
  that is everything that is not an acid)
• Metals
• What is separated into pieces?
• Soluble ionic salts (solubility rules!) including
  hydroxides
• All acids for NOW!
• Some acids are strong electrolytes, others are
  weak ? LATER
• What is cancelled out?
• Only those species that are exactly the same in
  species designation (s, l, g, aq) in products and
  reactants

16
Completing Equations example with an exception!
NaCl (aq) AgNO3 (aq) ? ????????
NaCl (aq) AgNO3 (aq) ? NaNO3 (aq) AgCl (s)
17
Getting Net Ionic Equations
NaCl (aq) AgNO3 (aq) ? NaNO3 (aq) AgCl (s)

• Identify the stuff that is new
• NEW a different label than it or components had
  on the other side
• What did it come from (look at work box)

Ag (aq) Cl- (aq) ? AgCl (s)
18
Exercise in completing reactions getting net
ionic reactions (NIE)

• Step 1 determine what kind of reaction you are
  dealing with
• Step 2 do appropriate matching
• Step 3 balance components
• Step 4 cross out components that do not change
  to get NIE
• Can have NO NIE!
• Li3PO4 (aq) Fe(NO3)2 (aq) ?
• Mg (s) H2SO4 (aq) ?
• H2SO4 (aq) NaOH (aq) ?
• MgBr2 (aq) Cl2 (aq) ?
• BaCl2 (aq) Mg(NO3)2(aq) ?
• Ca (s) CuSO4 (aq) ?
• Cr (s) O2 (g) ?