Problem-Solving Example

Thermodynamics Gateway Page
In this module:
Problem 1
Problem 2
Problem 3
Goals: After working through these sample problems, you should be able to:
  • Apply the thermodynamics problem-solving procedure to unfamiliar types of problems
  • See how thermodynamics can be used in relevant, real-world situations

This module will give you some additional practice in using thermodynamics and applying the problem-solving procedure developed in this tutorial.

The problems are taken from the industrial process used to manufacture phosphoric acid (H3PO4). Phosphoric acid is used to make fertilizers, detergents, and water treatment compounds. In 1995, it was the 7th top chemical made in the United States at 26.2 billion pounds.

One way phosphoric acid is made is a four-step procedure called the dry process. Starting with a phosphorus-containing ore a 14 M solution of phosphoric acid is the end product.

Step 1: The ore (often a mixture of calcium phosphate and silica) is treated with coal to produce gaseous tetraphosphorus:

2 Ca3(PO4)2 (s) + 6 SiO2 (s) + 10 C (s) 10 CO (g) + P4 (g) + 6 CaSiO3 (s)

Step 2: The gaseous tetraphosphorus is cooled and condensed to give red phosphorus:

P4 (g) P4 (s, red)

Step 3: The phosphorus is burned in air to give tetraphosphorus decaoxide:

P4 (s, red) + 5 O2 (g) P4O10 (s)

Step 4: The tetraphosphorus decaoxide reacts with water to form phosphoric acid:

P4O10 (s) + 6 H2O () 4 H3PO4 (aq)

The following three problems ask about various aspects of this process. If you get stuck, each problem has three hints that should keep you on track.

Problem-Solving Example