Imagine a reaction that results in a change in both volume and temperature:

8.26

(a) Has any work been done? If so, is its sign positive or negative?

(b) Has there been an enthalpy change? If so, what is the sign of ∆H? Is the reaction exothermic or endothermic?

Redraw the following diagram to represent the situation (a) when work has been gained by the system and (b) when work has been lost by the system:

8.27

Which arrow in the Hess’s law diagram corresponds to which step, and which arrow corresponds to the net reaction? Where are the reactants located on the diagram, and where are the products located?

Acetylene, C2H2, reacts with H2 in two steps to yield ethane, CH3CH3:8.28

8.29 Draw a Hess’s law diagram similar to the one in Problem 8.28 for the reaction of ethyl alcohol (CH3CH2OH) with oxygen to yield acetic acid (CH3CO2H).

(1) CH3CH2OH(l) 1/2 O2(g) CH3CHO(g) H2O(l) ∆Hº 174.2 kI

(2) CH3CHO(g) 1/2 O2(g) CH3CO2H(l) ∆Hº 318.4 kI

Net CH3CH2OH(l) O2(g) CH3CO2H(l) H2O(l) ∆Hº 492.6 kI

A reaction is carried out in a cylinder fitted with a movable piston, as shown here. The starting volume is V 5.00 L, and the apparatus is held at constant temperature and pressure. Assuming that and ∆E 34.8 kL redraw the piston to show its position after reaction. Does V increase, decrease, or remain the same?

8.30

The following drawing portrays a reaction of the type A B C, where the different colored spheres represent different molecular structures. Assume that the reaction has ∆Hº 55 kI. Is the reaction likely to be spontaneous at all temperatures, nonspontaneous at all temperatures, or spontaneous at some but nonspontaneous at others? Explain.

8.31

What are the signs of ∆H, ∆S, and ∆G for the following spontaneous change? Explain.

8.32

Consider the following spontaneous reaction of A3 molecules:8.33

(a) Write a balanced equation for the reaction.

(b) What are the signs of ∆H, ∆S, and ∆G for the reaction? Explain.