20013
1-2, 4-11 p. 547
1. How does the heat content of the products of a reaction system compare with the heat content
of the reactants when the reaction is
a)endothermic? –ANS: the heat content is higher.
b)exothermic?- ANS: the heat content is lower.
2. A)Distinguish between heats of reaction, formation, and combustion.
- ANS: ∆H is the quantity of heat released or absorbed during a chemical reaction. ∆Hf is the
heat released or absorbed when one mole of a compound is formed from its uncombined
elements at room temperature. ∆Hc is the heat released by the complete combustion of the
one mole of a substance.
B) On what basis are heats of formation and combustion defined?
ANS: The heat of combustion is defined in terms of one mole of reactant. Heat of formation is
defined in terms of one mole of product.
4. What factors affect the value of ∆H in a reaction system?
ANS: the change in the number of bonds breaking and forming and the strengths of these bonds
as the reactants form products.
5. Would entropy increase or decrease for phase changes in which the reactant is a gas or liquid
and the product is a solid? What sign would the entropy change have?
ANS: decrease: 6. How does an increase in temperature affect the entropy of a system?
ANS: it causes an increase in entropy
7. What combination of ∆H and ∆S values always produces a negative free-energy change?
ANS: a negative ∆H and a positive ∆S
8. Explain the relationship between temperature and the tendency for reactions to occur
spontaneously.
ANS: At low temperatures, T∆S is generally small in comparison with ∆H, so the sign of ∆H
determines the sign of ∆G. At high temperatures, the T∆S may be large enough to exceed the ∆H
value and thereby allow the sign an magnitude of ∆S to dictate the spontaneity of the reaction.
Assumptions regarding collisions and reactions between molecules
answers p. 548
#16 ans. 16 = 3.6e3J, and
#26 a. E forward = +80 Kj/mole
E reverse = - 80 kj/mole
Ea =100 kj/mole
Ea reverse = 20 kj/mole
#26 b. E forward = -40 Kj/mole
E reverse = +40 kj/mole
Ea =40 kj/mole
Ea reverse = 60 kj/mole
#27 on p. 549. I can show you the ans on ELMO