Page 104 - Physical chemistry eng

CONCEPTUAL PROBLEMS 81

The intrinsic and transition excess heat capacities are determined by first extrapo-
D
N
lating the functions C (T) and C (T) into the transition zone between T and T .
P
P
2
1
This is indicated in Figure 4.8 by the solid line connecting the heat capacity baselines
above and below T . Once this baseline extrapolation is accomplished, dC int is the
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m
N
difference at a given temperature between the extrapolated baseline curve and C (T) .
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The value of dC trs is obtained from the difference between C (T) and the extrapo-
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P
lated baseline curve. The excess enthalpy of thermal denaturation is finally given by
T 2
trs
¢H den = dC dT . In other words the excess enthalpy of thermal denaturation is the
P
1
T 1
area under the peak in Figure 4.8 above the extrapolated baseline curve.
Vocabulary
bomb calorimeter endothermic intrinsic excess heat capacity
bond energy enthalpy of fusion standard enthalpy of formation
bond enthalpy enthalpy of reaction standard enthalpy of reaction
constant pressure calorimeter excess heat capacity standard reference state
denaturation exothermic standard state
differential scanning calorimetry Hess’s law transition excess heat capacity

Conceptual Problems

Q4.1 In calculating ¢H° R at 285.15 K, only the ¢H° f of 298 K. Why does this temperature rise not affect your
the compounds that take part in the reactions listed in Tables measurement of ¢H° R at 298 K?
4.1 and 4.2 (Appendix B, Data Tables) are needed. Is this Q4.10 Is the following statement correct? If not
statement also true if you want to calculate ¢H° R at 500. K? rewrite it so that it is correct. Because the reaction
Q4.2 What is the point of having an outer water bath in a H (g) + O (g) ¡ H O(l) is exothermic, the products are
2
2
2
bomb calorimeter (see Figure 4.3), especially if its tempera- at a higher temperature than the reactants.
ture is always equal to that of the inner water bath? Q4.11 The reactants in the reaction 2NO(g) + O (g) ¡
2
Q4.3 Is the following statement correct? If not rewrite it 2NO (g) are initially at 298 K. Why is the reaction enthalpy
2
so that it is correct. The standard state of water is H O(g). the same if (a) the reaction is constantly kept at 298 K or
2
(b) if the reaction temperature is not controlled and the heat
Q4.4 Does the enthalpy of formation of H 2 O(l) change if
flow to the surroundings is measured only after the tempera-
the absolute enthalpies of H 2 (g) and O 2 (g) are set equal to ture of the products is returned to 298 K?
100. kJ mol -1 rather than to zero? Answer the same
question for CO 2 (g). Will ¢H° R for the reaction H 2 O(l) + Q4.12 What is the advantage of a differential scanning
CO (g) ¡ H CO (l) change as a result of this change in calorimeter over a bomb calorimeter in determining the
2
2
3
the enthalpy of formation of the elements? enthalpy of fusion of a series of samples?
Q4.13 You wish to measure the heat of solution of NaCl
Q4.5 Why are elements included in the sum in Equation (4.14)
in water. Would the calorimetric technique of choice be at
when they are not included in calculating ¢H° R at 298 K?
constant pressure or constant volume? Why?
Q4.6 Why are heat capacities of reactants and products
Q4.14 Is the following statement correct? If not rewrite it
required for calculations of ¢H° R at elevated temperatures?
so that it is correct. Because the enthalpy of formation of
Q4.7 Is the following statement correct? If not rewrite it so elements is zero, ¢H° f (O(g)) = 0.
that it is correct. The superscript zero in ¢H° f means that the
reactions conditions are 298.15 K. Q4.15 If the ¢H° f for the chemical compounds involved in
a reaction is available at a given temperature, how can ¢H° R
Q4.8 Why is it valid to add the enthalpies of any sequence be calculated at another temperature?
of reactions to obtain the enthalpy of the reaction that is the
Q4.16 Is the following statement correct? If not rewrite it so
sum of the individual reactions?
that it is correct. If ¢H° R for a chemical reaction does not
Q4.9 In a calorimetric study, the temperature of the system change appreciably with temperature, the heat capacities for
rises to 325 K before returning to its initial temperature of reactants and products must be small.