Chemistry 103---Quiz #5, Friday Oct. 8, 1999

Name ______________________ Ms. Average 8.0

Open book, individuals or teams of two. You may consult only your partner.

Due in class Friday, Oct. 8 or outside of Room 202 before class.

DH°f Na2O = -416 kJ

Using your textbook and the information above find DH° for each of the reactions below.

  1. 4Na (s) + O2 (g) ----> 2 Na2O (s)

    2. 2 x (-416 kJ) = __________-832 kJ________

    NOTE:This is for the reaction as written with 4 moles Na.
  2. 2 Na (s) + 2 H2O (l) ---> 2 NaOH (aq) + H2 (g)

    3. 2x(-470 kJ) - 2x(-285 kJ) = _________-370 kJ__________

     

  3. 2 Na (s) + CO2 (g) ----> Na2O (s) + CO (g)

-110.5 -416-(-393.5)= __________-132 kJ_________

Remember: DH°f of elements in their standand state { (Na (s) , O2(g) , H2 (g) }is 0.

Use your values to show why a water or carbon dioxide fire extinguisher might not be effective in putting out a sodium fire.

Adding water or carbon dioxide to a sodium fire results in an exothermic reaction which will produce even more heat and will make the fire worse. In addition, you are essentially adding more reactants to the reaction so the fire will burn until all the sodium is used up. Most fire extinguishers work by "smothering" the fire to deprive it of oxygen and non-reactive substances are sprayed at the base of the fire. It is not necessary to find a substance whose reaction is endothermic.

4. 5.70 grams of solid sodium are completely combusted in an atmosphere of pure oxygen in the reaction chamber of a calorimeter containing 200.0 mL of pure water at 25 °C. You may assume standard temperature and pressure, excess oxygen, and no heat loss. Find the temperature of the calorimeter water following the explosion.

Strategy: (See text pages 210-212 for more discussion and drawings)

  1. Find number of moles of sodium = 5.70 gx(1 mole/23.0 g) = .248 moles
  2. Use the stoichimetry of the first reaction (Na combustion) to find q rxn

    3. -832 kJ/ 4 moles Na so 208 kJ/ mole are released or 51.5 kJ for .248 moles.

  3. q sys = -51.5 kJ = -q surrounding = -q calorimeter water = + 51.5 kJ

    4. We expect the calorimeter water to heat up as a result of this exothermic reaction.

  4. Use the quantity of water and its specific heat along with q to find DT = Tfinal -Tinitial.

q = m water x Cp water x DT -----solving for DT gives us

DT = q / m water x Cp water = 51500 J / 200 g x 4.18 J / g °C = 62 °C

So the final temperature is 25°C + 62°C = 87°C (a hot, but reasonable value)

The most common mistake was a conceptual one in attempting to substitute values directly into the equation in 4). Please think before you jump to formulas!!!! Try to draw or conceptualize what is actually happening before writing your first formula. The writing assignments and quizzes in pairs are designed to encourage you to talk about the chemistry as you are formulating your answers so that you can avoid conceptual and mechanical errors. Many of you tried to use 5.7 g sodium for the mass in this equation, despite not having the specific heat of sodium. You then compounded the error by using water's specific heat. Several of you wrote a formula involving two-stage heating and again, it was not possible to find values to match this formula. Finally, there were numerous stoichiometric errors involving the 4 moles of Na in the original reaction. Please see me for further explanations.