Exam 3 Practice

Thermochemistry, Heat

1. Suppose 1.30 kJ of energy is removed by a refrigeration unit from a 2.19 kg sample of brass (c_p(brass) = 0.380 J g^–1 °C) at a temperature of 24.6 °C. A scientist wants to calculate the new temperature of the sample.

   1. What are the surroundings in the above problem?
      1. the refrigeration unit
      2. the sample of brass
      3. the sample of brass and the refrigeration unit
      4. none of the above
   2. Which equation(s) applies/apply?
      1. q_cal = –q_rxn
      2. q = mc_pΔT
      3. q = C_calΔT
      4. none of the above
   3. Will the change in energy of the system, qsys, be positive or negative?
      1. positive
      2. negative
   4. Which of the given values must be converted to match the units of the specific heat?
      1. energy ( q )
      2. mass ( m )
      3. temperature ( T )
   5. Fill in the correct values for energy (in J), mass (in g), and temperature (in °C).
      1. Energy:
      2. Mass:
      3. Temperature:
   6. What is the new temperature (in °C) of the sample of brass?

   Solution

   Answer:

   1. 1
   2. 2
   3. 2
   4. 1, 2
   5.  (1) 1.30 × 10³ J; (2) 2.19 × 10³ g; (3) 24.6 °C
   6. 23.0 °C (see work below)

   \[\begin{align*} q &= m(\mathrm{brass}) ~ c_p(\mathrm{brass}) ~ \Delta T(\mathrm{brass}) \\[0.5ex] &= m(\mathrm{brass}) ~ c_p(\mathrm{brass}) ~ \bigr[ T(\mathrm{final}) - T(\mathrm{initial}) \bigr] \longrightarrow \\[0.5ex] T(\mathrm{final}) &= \biggl\{ q ~ \bigr[ m(\mathrm{brass}) ~ c_p(\mathrm{brass}) \bigr]^{-1} \biggl\} ~+~ T(\mathrm{initial})\\[0.5ex] &= \biggl\{ 1.30 \times 10^{3}~\mathrm{J}~ \biggr[ 2.19 \times 10^{3}~\mathrm{g} \left ( \dfrac{0.380~\mathrm{J}}{\mathrm{g~^{\circ}C}} \right ) \biggr]^{-1} \biggl\} ~+~ 24.6~^{\circ}\mathrm{C} \\[0.5ex] &= 23.\bar{0}37~^{\circ}\mathrm{C} = 23.0~^{\circ}\mathrm{C} \end{align*}\]

   Concept: thermochemistry basics; heat; specific heat capacity

   
   

2. If a system cools from 299 K to 294 K, what is the sign of q?

   1. positive
   2. negative
   3. impossible to know

   Solution

   Answer: B

   Concept: thermochemistry basics; heat

   
   

3. Solution

   Answer:

   Concept:

   
   

Questions are written by UGA Chemistry unless otherwise indicated and translated with minor tweaks into an online format by Eric Van Dornshuld.