CHEM 2P32 Hints for Assignment 6

Oxo Acids and Anions; Ionic compounds

 
1. Hints. Remember that an oxo group is an oxygen attached to just one other atom. Hydrogen atoms in oxo acids are attached to oxygens, so the formula of an oxo acid like perxenic acid, H4XeO6, is more properly written as XeO2(OH)4. It is clear from the latter formula that perxenic acid has 2 oxo groups.
 
2. Hints. Refer to Sections 2.8-2.9 on Formulas and Names of Oxo Anions. See also Table 2.7 (Names and Formulas of Oxo Anions) and Table 2.8 (Relationship Between Names of Oxo Anions and the Oxidation Number of the Central Atom). Remember these general rules:

3. Hints. (a) and (b) are straightforward. Use the values of the electronegativity and the radius of the ion as found in the Tables in Wulfsberg.

For part (c), see Section 2.12 on speciation. Specific examples of forms of "cations" at various pH values are given in Figure 2.8 on p. 50; a more general table that applies to all "cations" is to be found on p. 53. See also Table 2.9, which shows the main forms of the elements in water at pH. 5.5-7 (this table only shows the form for the most common oxidation state of the element).

If an element exists as the hydroxide or the oxo ion, give the formula of that species.
 
 

4. Hints. (a) See Hints for Q. 2 above.

(b) One negative charge increases the basicity by a factor of 1010 approximately; one oxo group decreases the basicity by a factor of about 105, so two oxo groups cancel the effect of one negative charge. See Section 2.6.2 for more information.
Formula
After Cancelling 1 Negative Charge by 2 O's 
Basicity
MO4-, MO3-
MO2, MO
Nonbasic
MO42-, MO2-
M
Feebly basic
MO32-, MO64-
M-1/2, M-1
Moderately basic
negative charge remaining is more negative than -1
Very strongly basic

5. Hints. The general formula for calculating lattice energy is equation 3.11 on page 74 in Wulfsberg. The key is to determine the structure type corrrectly, and to use the correct values of the ionic radii. See Table 3.4 for the correlation of radius ratio, structure type (lattice type), and Madelung constant. Born exponents are given in Table3.3.
 
 

6, 7. Hints. The key here is Hess's Law, which states that when two or more reactions can be added to give an overall reaction, the enthalpy change for the overall reaction is the sum of the enthalpy changes for the individual reactions. So it's important to know what the individual reactions are. Then you can combine them so that they add up to the reaction for which you are trying to find the enthalpy change. See Section 3.5, Enthalpy and Precipitation, for an example.

Some reactions and the names of the enthalpy changes associated with them are given below. The compounds are assumed to be 1:1 for simplicity.

Reaction
Enthalpy Change for this Reaction
MX (s) ---> Mn+ (aq) + Xn- (aq) (dissolution)
Mn+ (aq) + Xn- (aq) ---> MX (s) (precipitation)
Mn+ (g) ---> Mn+ (aq) (hydration of cation)

Xn- (g) ---> Xn- (aq) (hydration of anion)
Mn+ (g) + Xn- (g) ---> MX (s)

The standard enthalpy of formation is defined as the energy change that results when one mole of a compound in its standard state is formed from the elements in their standard states (25oC, 1 atm pressure). In Question 7, the standard states of the product are specified as either aqueous or crystalline solid. As an example, consider the compound iron(II) formate, Fe(HCO2)2.
Fe (s) + H2 (g) + 2 C (s) + 2 O2 (g) ---> Fe(HCO2)2 (s) 
standard enthalpy of formation of solid Fe(HCO2)2:

Fe (s) + H2 (g) + 2 C (s) + 2 O2 (g) ---> Fe2+ (aq) + 2 HCO2- (aq) 
standard enthalpy of formation of an aqueous 1 m solution of Fe(HCO2)2:

So, to determine the enthalpy of precipitation of iron(II) formate, we combine these data as shown below. We have to reverse the second equation so its enthalpy change will be the negative of the enthalpy change for the reaction in the forward direction
1. Fe (s) + H2 (g) + 2 C (s) + 2 O2 (g) ---> Fe(HCO2)2 (s) 
2. Fe2+ (aq) + 2 HCO2- (aq) ---> Fe (s) + H2 (g) + 2 C (s) + 2 O2 (g)
Add 1 and 2: Fe2+ (aq) + 2 HCO2- (aq) ---> Fe(HCO2)2 (s)
Enthalpy change for this reaction is the enthalpy of precipitation, and is the sum of the enthalpy changes for reactions 1 and 2.

Notice that Question 7b is just like question 6, except that in Question 6 you are trying to find the lattice energy given all the other data, and in Question 7 you are trying to find the hydration energy given all the other data.
 
 

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