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Unformatted text preview: Silberberg, Chapter 4 The Major Classes of Chemical Reactions (Review) 4.1 The Role of Water as Solvent
Oppositely charged ions from an ionic compound separate from each other when
dissolved in water, become surrounded by water molecules, and spread randomly
throughout a solution. The movement of the ions enables the solution to conduct
• Electrolyte: a substance that conducts a current when dissolved in water.
• Solvated: the process by which an ion is surrounded by solvent molecules.
• Nonelectrolytes: covalent molecules that are polar in nature, dissolve in water, but do
Water is a polar molecule:
• The ionizing effect of water as a solvent is due to the distribution of it bonding electrons
and the overall shape of the water molecule.
1. Sharing of electrons between nonidentical atoms
in a covalent bond is unequal, with oxygen (O)
attracting the bonding electrons with greater
affinity than H.
2. This situation creates 2 poles of charge (+ for H,
- for O). H-O is a polar bond because of this.
3. Non-bonding electron pairs on O of water distort
molecule’s shape (O is group 6A, 2 bonding e-‘s, 4 nonbonding).
4. Water is a bent molecule to spatially accommodate the bonding H’s and 2
• Non-bonding electron pairs on O act to attract (+) charged ions in solution.
• Water separating the ionic compound greatly reduces the electrostatic force of attraction
between the ion pair. 4.2 Aqueous Ionic Reactions
Total ionic equation:
Net ionic equation:
2AgNO3 (aq) + Na2CrO4(aq) Æ Ag2CrO4 (s) + 2NaNO3 (aq)
+ - - 2Ag (aq) + 2NO 3 (aq) + 2Na+(aq) + CrO4 - (aq) Æ Ag2CrO4(s) + 2Na+(aq) + 2NO3 (aq)
2Ag+(aq) + CrO4 - (aq) Æ Ag 2 CrO4 (s)
- Spectator ions: Na+(aq), NO3 (aq)
† • Molecular equation: shows all the reactants and products as if they were intact,
• Total ionic equation: shows all soluble ionic substances dissociated into ions.
• Spectator ions: ions that appear on both side of the equation and not involved in the
actual chemical change.
• Net ionic equation: eliminates spectator ions and show the actual chemical change
4.3 Precipitation Reactions
Two soluble ionic compounds react to form an insoluble product or precipitate (e.g.
Ag2CrO4(s) from the previous example).
Predicting whether a precipitation reaction will occur:
1. Note the ions present in the reactants.
2. Consider the possible combinations.
3. Decide whether any of the combinations are insoluble.
See Table 4.1 pg. 139 for a list of precipitates. 4.4. Acid-Base Reactions
• Acid: a substance that produces H+ ions when dissolved in water.
HX æHæÆ H+(aq) + X- (aq)
æ • Base: a substance that produces OH- ions when dissolved in water.
MOH æHæÆ M+(aq) + OH- (aq)
• Strong acids and strong bases dissociate completely into ions in water.
• Weak acids and bases dissociate very little in water and remain intact.
• Acids and bases react together in a neutralization reaction, to form water and salt.
HX(aq) + MOH(aq) Æ MX (aq) + H2 O(l)
water • Titration: a basic (MOH) solution of known concentration (standard solution) is used to
determine the concentration of an acidic solution (HX) that is unknown.
• The equivalence point in the titration is reached when all the moles of H+ ions in the
solution have reacted with the measured amount of OH- ions added to the solution.
• The end point of the titration occurs when a tiny excess of base is added, generally
accepted to be the same as the equivalence point.
Oxidation-Reduction Reactions (redox)
• Redox reactions are the key chemical event for movement of electrons from one
reactant to another.
• Driving force in these reactions comes from the flow of electrons in a specific direction,
from least electronegative atom to the most electronegative atom.
X ætransfer oræ æelectronsÆY
æ æ shift of ææ
• X loses electrons • Y gains electrons • X is oxidized • Y is reduced † • X is the reducing agent
• X increases in oxidation
number • Y is the oxidizing agent
• Y decreases in oxidation number Rules for assigning oxidation number (O.N.):
1. For an atom in its elemental form (Na, O2, Cl2, etc.) O.N.=0
2. For a monatomic ion, O.N. = ion charge
3. The sum of O.N. values for the atoms in a compound equals zero. The sum of
O.N. values for the atoms in a polyatomic ion equals the ions charge.
Balancing Redox Equations:
1. Assign oxidation numbers to all elements in the reaction.
2. From the changes in oxidation numbers, identify the oxidized and reduced
3. Compute the number of electrons lost in the oxidation and gained in the reduction,
from the oxidation number changes.
4. Multiply one or both of these numbers by the appropriate factors to balance
electron gain with electron loss. Use these factors are balancing coefficients.
5. Complete the balancing by inspection, adding states of matter (solid, liquid, gas).
4.6 Elemental Substances in Redox Reactions
• Combination reactions: X + Y Æ Z
• Decomposition reactions: Z Æ X + Y
• Displacement reactions: X + YZ Æ XZ + Y
• Metathesis reactions (double displacement): WX + YZ Æ XZ +YW
4.7 Reversible Reactions: An Introduction to Chemical Equilibrium
See later chapters. ...
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