CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 17. Thermochemistry

(∆H) in kJ for this reaction. Assume the densities of the solutions are 1.00 g/mL and that their specific heats are the
same as that of pure water.

Step 1: List the known quantities and plan the problem.

Known

• cp= 4.18 J/g•°C


• Vf inal= 25.0 mL + 25.0 mL = 50.0 mL


• ∆T = 32.0°C –25.0°C = 7.0°


• Density = 1.00 g/mL

Unknown

• ∆H =? kJ

The volume and density can be used to find the mass of the solution after mixing. Then, calculate the change in
enthalpy by using∆H = qsys=−qsurr=−(m×cp×∆T).

Step 2: Solve.

m= 50 .0 mL×

1 .00 g

1 mL

= 50 .0 g

∆H =−(m×cp×∆T) = -(50.0 g×4.18 J/g•°C×7.0°C) = -1463 J = -1.5 kJ

Step 3: Think about the result.

The enthalpy change is negative because the reaction releases heat to the surroundings, causing the temperature of
the water to rise.

Practice Problems

1. A rock is heated and dropped into a foam cup calorimeter containing 35.0 mL of water at 21.4°C. The
   temperature of the water rises to 24.8°C. How many joules of heat were released by the rock?


2. A 100.0 g sample of an unknown metal is heated to 95.00°C. This hot metal is then placed into a foam cup
   calorimeter containing 50.0 g of water at 20.00°C. The water and metal come to a final temperature of 31.67°C.
   Calculate the specific heat of the metal, and use the table above (Table17.1) to identify it.

Heats of Reaction

When methane gas is combusted, heat is released, making the reaction exothermic. Specifically, the combustion of
1 mol of methane releases 890.4 kilojoules of heat energy. This information can be shown as part of the balanced
equation.

CH 4 (g)+2O 2 (g)→CO 2 (g)+2H 2 O(l)+ 890 .4 kJ

The equation tells us that 1 mol of methane combines with 2 mol of oxygen to produce 1 mol of carbon dioxide and
2 mol of water. In the process, 890.4 kJ is released, so it is written as a product of the reaction. Athermochemical
equationis a chemical equation that includes the enthalpy change of the reaction. The process in the above
thermochemical equation can be shown visually below (Figure17.6 (A)).