Department of Chemistry and Physics

BELLARMINE COLLEGE

DEPARTMENT OF CHEMISTRY AND PHYSICS

CHEM115 - GENERAL, ORGANIC, AND BIOLOGICAL CHEMISTRY II

EXAM #1 ANSWER KEY 2/10/00

Do all the following questions. Please read each question carefully and apportion your time according to the relative worth of the question (shown in parenthesis after the question). Write legibly. Marks will be deducted for unclear, messy and illegible work. You must show all calculations (including units) where appropriate. Also, remember to show answers to the correct number of significant figures. Answer in the space provided after each question. The total marks for this exam is 50.

1.  You are at the beach on a hot summer's day. Why is it that the beach sand gets so hot that it will burn unprotected feet, but the water remains cool? (5)

The specific heat of sand is much less than the specific heat of water (water has one of the highest specific heats known). Therefore, it takes much less energy to heat up sand than it does water. Also, a factor is that there is an awful lot more water in the ocean than there is sand on the beach.

2.  You have just eaten a quarter-pounder cheeseburger, French fries, and a chocolate shake. The following table shows the nutrition content of these food items:

The caloric value for fats is 9 kcal/g, and 4 kcal/g for carbohydrates and proteins. If a 70.0-kg person expends 60 kcal/hr sleeping and 200 kcal/hr walking, how long would it take you (assuming for the sake of this question only that you weigh 70.0 kg) to "burn off" the kcal in this meal by a) sleeping and b) walking? (9)

Total protein = 45 g, total fat = 29 g, and total carbohydrate = 120 g.

Calories from protein = 45 g x 4 kcal/g = 180 kcal

Calories from fat = 49 g x 9 kcal/g = 441 kcal

Calories from carbohydrate = 120 g x 4 kcal/g = 480

Total calories = 1101 kcal

SLEEPING: 1101 kcal x hr/60 kcal = 18 hrs

WALKING: 1101 kcal x hr/200 kcal = 5.5 hrs

3. When 1.0 g of gasoline burns, 1.15 x 104 cal of energy are released. If the density of gasoline is 0.743 g/mL, how much energy (in kcal) could be obtained from 11.9 gallons of gasoline? (1.0 gallon = 3.784 L)

(5)

11.9 gal x (3.784 L/1.0 gal) x (1000 mL/L) x (0.743 g/mL) x (1.15 x 104 cal/g) x (kcal/1000cal) = 3.8 x 105 kcal

4.  Benzene is an organic liquid once commonly used as a solvent. However, it has since been proved to be a carcinogen and, therefore, its use has been severely curtailed. The melting point of benzene is 5.5oC its boiling point is 80.1oC.

a.  Sketch a heating curve for benzene from 0oC to 100oC. (3)

b.  How much energy (in kcal) would be needed to melt a 15.0-g block of frozen benzene at 5.5oC, heat the resultant liquid benzene to its boiling point, and then convert the liquid to benzene vapor at 80.1oC? The heat of vaporization of benzene is 94.1 cal/g, the heat of fusion of benzene is 30.45 cal/g and the specific heat of benzene is 0.416 cal/g.oC. (8)

MELTING: heat = 15.0 g x 30.45 cal/g = 456.75 cal

HEATING: heat = 15.0 g x 74.6oC x 0.416 cal/g.oC = 465.504 cal

BOILING: heat = 15.0 g x 94.1 cal/g = 1411.5 cal

TOTAL: 2333.754 cal = 2.33 kcal (to 3 SF)

5.  Gasoline reacts with oxygen in the air exothermically, yet there is no reaction when you are filling up your car with gasoline at the filling station where there is plenty of air for the gasoline to come into contact with. Why is there no reaction? Use an energy diagram to illustrate your answer. (5)

At room temperature, oxygen and gasoline molecules don't have enough energy when they collide to get over the activation barrier. It is only when a spark is supplied (as with a spark plug in a car engine) that enough energy is supplied to get the reaction going. Since the reaction is exothermic, some of the energy released when the first molecules react goes to helping the rest of the unreacted gasoline and oxygen molecules get over the activation barrier, i.e., a chain reaction ensues.

6.  A syringe has a volume of 10.0 cc (10.0 mL) at a pressure of 760.0 mm Hg. If the tip of the syringe were blocked so that air couldn't escape, what pressure would have to be exerted to decrease the volume to 2.00 cc? (5)

P1V1 = P2V2

P1 = 760.0 mm Hg, V1 = 10.0 cc, P2 = ?, and V2 = 2.00 cc

P2 = (P1V1)/V2 = (760.0 mm Hg x 10.0 cc)/2.00 cc = 3800 mm Hg (3.80 x 103 mm Hg to 3 SF)

7.  A balloon filled with helium has a volume of 1.50 L at sea level where the pressure is 760.2 mm Hg. Keeping the balloon in an insulated container so that its temperature remains constant, you climb to the top of Mt. Whitney in California where the atmospheric pressure is 439.6 mm Hg. What will be the volume of the balloon? (5)

P1V1 = P2V2

P1 = 760.2 mm Hg, V1 = 1.5 L, P2 = 439.6 mm Hg, and V2 = ?

V2 = P1V1/P2 = (760.2 mm Hg x 1.5 L)/439.6 mm Hg = 2.59 L (to 3 SF)

8.  A hot-air balloon has a volume of 875.0 L. When the balloon is heated to 56.0oC, its volume changes to 955.0 L. What was the temperature of the balloon before it was heated? (5)

T1/V1 = T2/V2

V1 = 875.0 L, T1 = ?, V2 = 955.0 L, T2 = 56 (+ 273) = 329 K

T1 = T2V1/V2 = (329 K X 875.0 L)/955.0 L = 301 K (-273) = 28.4oC (to 3 SF)