http://www.ck12.org Chapter 17. Thermochemistry
Cold packs are typically used to treat muscle strains and sore joints. The cold pack is activated and applied to the
affected area. As the ammonium nitrate dissolves, it absorbs heat from the body and helps limit swelling. For
ammonium nitrate,∆Hsoln= 25.7 kJ/mol.
Sample Problem 17.7: Heat of Solution
The molar heat of solution,∆Hsoln, of NaOH is−445.1 kJ/mol. In a certain experiment, 5.00 g of NaOH is
completely dissolved in 1.000 L of water at 20.0°C in a foam cup calorimeter. Assuming no heat loss, calculate
the final temperature of the water.
Step 1: List the known quantities and plan the problem.
Known
- mass of NaOH = 5.00 g
- molar mass of NaOH = 40.00 g/mol
- ∆Hsoln(NaOH) =−445.1 kJ/mol
- mass H 2 O = 1.000 kg = 1000. g (assumes density = 1.000 g/mL)
- Tinitial(H 2 O) = 20.0°C
- cp(H 2 O) = 4.18 J/g•°C
Unknown
- Tf inalof H 2 O =? °C
This is a multiple-step problem: 1) the mass of NaOH is converted to moles; 2) the resulting value is multiplied by
the molar heat of solution; 3) the heat released in the dissolving process is used with the specific heat equation and
the total mass of the solution to calculate∆T; 4) Tf inalis determined from∆T.
Step 2: Solve.
5 .00 g NaOH×1 mol NaOH
40 .00 g NaOH×
− 445 .1 kJ
1 mol NaOH×
1000 J
1 kJ=− 5. 56 × 104 J
∆T=
q
cp×m=
− 5. 56 × 104 J
4 .18 J/g·◦C×1005 g= 13. 2 ◦C
Tfinal= 20. 0 ◦C+ 13. 2 ◦C= 33. 2 ◦CStep 3: Think about your result.
The dissolving process releases a large amount of heat, which causes the temperature of the solution to rise. Care
must be taken when preparing concentrated solutions of sodium hydroxide because of the large amounts of heat
released.
Practice Problem- Calculate the final temperature of a solution prepared by dissolving 18.40 g of ammonium nitrate into 250. g
of water in a foam cup calorimeter. Both the ammonium nitrate and the water are initially at 22.0°C. Assume
that the heat capacity of the solution is the same as that of pure water.