Chemistry 103---Quiz #4, Friday Oct. 1, 1999

Name ______________________ 10 min., closed book, open calculators

  1. Reading question--write clearly, briefly, and grammatically. Formulas and drawings may clarify you answers.
  1. Explain the difference between system and surroundings in thermochemistry.

The system is the portion of the universe selected for thermodynamic study, usually containing a chemical reaction. The surroundings may exchange heat with the system. Thermometers and water baths are part of the surroundings.

B. Explain the difference between heat capacity and specific heat.

The specific heat is the number of joules required to raise 1 g of a substance 1 °C. (joules/g°C)

The heat capacity is the number of joules required to raise an object 1 °C (joule/°C)

  1. Balance (by any means necessary) the following equation and state clearly which elements are oxidized or reduced.

    2. 2 NaN3 (s) ---> 3 Na (l) + 3 N2 (g)

    Na goes form O. S. +1 to O. S. 0 so it is reduced.

    3 N's goes from O.S. -1 to O. S 0 so N is oxidized.

    All of these are neutral with a charge of 0. Don't add in charges!!

    _______N_______ is oxidized and _______Na_____ is reduced.

  2. A 60.0 g piece of Ag metal is heated to 90.0 °C and dropped into 120.0 g of 25 °C water (4.18J/g°C) in a well insulated container. The final temperature of the Ag-H2O mixture is 26.7 °C.

Find the specific heat of silver, Ag and state how the First Law of Thermodynamics applies to this problem.

q water = (26.7-25 °C)x(120g)x(4.18 J/g °C)= + 853 joules absorbed by water.

using conservation of energy q Ag = -q water = -853 joules lost = 60gx(26.7-90.0°C)x Cp silver

Solve for Cp silver--specific heats are always +!!!

_______+ 0.22 J/g °C_____ is the specific heat of silver.