l6 chapt5-2 web

Chemistry 5

Chapter-5 Reactions in Aqueous Solutions Part-2 30 September 2002

Recognizing Acids & Bases ƒ Key Feature of Acids: • contain ionizable hydrogen Examples--

• not all molecules with hydrogen are acids Examples--

ƒ Key Features of Bases: • contain either ionizable OH group or element (e.g., N) that can accept a proton (H+) Examples--

• not all molecules with OH are bases Examples--

Additional A-B Reactions ƒ Acid-base reactions can occur between an insoluble base and soluble acid: • a number of metal hydroxides have limited solubility in water, yet they react readily with acids Al(OH)3 Zn(OH)2

ƒ Antacids • Alka-Seltzer, NaHCO3 • Milk of magnesia, Mg(OH)2 • Maalox, Mg(OH)2 + Al(OH)3 • Tums, CaCO3

Oxidation-Reduction Reactions ƒ Oxidation-reduction or redox reactions are common to all facets of chemistry and biology. For example:

ƒ In any redox reaction, there is always one species oxidized and one species reduced; cannot have one without other!

ƒ This concept of coupled oxidation and reduction of species in a redox reaction can be seen by assigning oxidation states to reactants and products. • Ca(s) + 2H+(aq) Æ Ca2+(aq) + H2(g) • MnO4-(aq) + C2O42-(aq) Æ Mn2+(aq) + CO2(g)

Redox Half-Reactions ƒ The concept of coupled oxidation and reduction of species in redox reactions can also be seen by separating the reactions involving oxidation and reduction from each other– • write two half-reactions, where one involves oxidation of a reactant and the other involves reduction of another reactant. • the sum of the half-reactions is the equal to the overall reaction.

ƒ Redox Half-Reactions • Zn(s) + Cu2+(aq) Æ Zn2+(aq) + Cu(s) Oxidation: Reduction: Overall: • Cu(s) + Ag+(aq) Æ Cu2+(aq) + Ag(s) Oxidation: Reduction: Overall:

Some Trends in Redox Reactions ƒ Reactions of metals in acidic solution React to yield hydrogen gas: • alkali metals (group-I) • alkaline earth metals (group-II) • Al, Zn, Fe, Sn, Pb No Reaction: • Cu, Ag, Au, Hg

ƒ Implications?

Balancing Redox Reactions ƒ Chemical reactions must conserve mass. ƒ In redox reactions, must also balance the

loss (oxidation) and gain (reduction) of electrons. ƒ Redox reactions can be balanced by the following general approach:

• write down two half-reactions: (1) reactant is oxidized and (2) other reactant is reduced. • use integer coefficients to balance numbers of all elements except “O” and “H”. • in acidic solution add H2O to balance oxygen in half-reactions • In acidic solution, add “H+ to balance hydrogen from water. • add electrons (e-) to balance charge. • use integer multipliers to make the number of electrons produced (oxidation) and used (reduction) the same. • add the two half reactions; cancel e- (and other species as appropriate).

Balancing Redox Reactions: Example ƒ Consider the following chemical reaction in acidic aqueous solution: Pb + PbO2 + SO42-

PbSO4

Identify the oxidation/reduction half-reactions, and balance the overall reaction.