Handout #4B (Fall 2011) Chem 6A - Prof. John E. Crowell

Chem 6A - Chapter 4 (Part B)

Acid Strength: Determining the amount of H+ Ions initially present in a Strong Acid after being neutralized

through reaction and titration by a Strong Base

A specific volume of 0.870 M HCl is reacted with solid Mg(OH)2. Addition of 18.0 g of Mg(OH)2 leaves some HCl unreacted. Treatment (i.e. titration) of the resulting solution with 208 ml of 0.500 M NaOH uses up the rest of the acid.

What was the volume of 0.870 M HCl to start with?

Oxidation-Reduction ReactionsOxidation: the LOSS (via electron transfer) of one or more electrons:

Reduction: the GAIN (via electron transfer) of one or more electrons:

3

( ) 1 ( )

( ) 3 ( )

Na s e Na s

Fe s e Fe s

2

32

( ) 2 2 ( )

( ) 6 2 ( )

Cl g e Cl s

N g e N s

OiL

RiG

Handout #4B (Fall 2011) Chem 6A - Prof. John E. Crowell

Chem 6A - Chapter 4 (Part B)

The redox process in compound formation

Oxidation Numbers

Handout #4B (Fall 2011) Chem 6A - Prof. John E. Crowell

Chem 6A - Chapter 4 (Part B)

Highest and lowest oxidation numbers of reactive main-group elements.

Oxidation Numbers

Terminology in redox reactions:

The element undergoing

oxidation is the reducing agent.

The element undergoing

reduction is the oxidizing agent.

Oxidation –Reduction Reactions

Handout #4B (Fall 2011) Chem 6A - Prof. John E. Crowell

Chem 6A - Chapter 4 (Part B)

Recognizing Oxidizing and Reducing AgentsIdentify the oxidizing agent and reducing agent in each of the following:

(a) 2Al(s) + 3H2SO4(aq) Al2(SO4)3(aq) + 3H2(g)

(b) PbO(s) + CO(g) Pb(s) + CO2(g)

(c) 2H2(g) + O2(g) 2H2O(g)

Strategy: Assign an O.N. for each atom and see which atom gained and which atom lost electrons in going from reactants to products.

An increase in O.N. means the species was oxidized (and is the reducing agent) and a decrease in O.N. means the species was reduced (is the oxidizing agent).

(a) 2 Al(s) + 3 H2SO4(aq) Al2(SO4)3(aq) + 3 H2(g)

0 0+6+1 -2 +3 +6 -2

The O.N. of Al increases; Al is oxidized; it is the reducing agent.

The O.N. of H decreases; H is reduced; H2SO4 is the oxidizing agent.

An active metal displacing hydrogen from water

Handout #4B (Fall 2011) Chem 6A - Prof. John E. Crowell

Chem 6A - Chapter 4 (Part B)

Displacing one metal by another

An aid in predicting the products of replacement reactions.

Activity series of the metals

1. Each element on the list replaces from a compound any of the elements below it.

2. The first five elements (Li - Na) are very active metals; react with cold water to produce the hydroxide & hydrogen gas.

3. The next four metals (Mg - Cr) are active metals; react with steam or hot water to form the oxide & H2 gas.

4. The oxides of all of these first metals resist reduction by H2.

5. The next six metals (Fe - Pb) replace hydrogen from HCl and sulfuric & nitric acids. Their oxides undergo reduction by heating with H2, C, and CO.

6. The metals Li - Cu can combine directly with O2 to form the oxide.

7. The last metals (Hg - Au) are often found free in nature, their oxides decompose with mild heating, and they form oxides only indirectly.