Second Law of Thermodynamics
Applied Science: BTEC Level 3 National Extended Diploma, topic 4.9.6
Second Law of Thermodynamics
The second law of thermodynamics dictates the direction of heat flow from objects at different temperatures.
Second law in terms of heat transfer
- The second law states that heat transfer occurs spontaneously from higher to lower temperature bodies.
- Heat transfer never occurs spontaneously in the reverse direction.
Second law examples
- A cup of coffee left to sit out never gets warmer.
- The heat energy from the coffee dissipates to the cup and its surroundings.
Entropy
Entropy is a measure of disorder. Entropy always increases during a reaction.
Entropy
- Entropy is a measure of the amount of disorder a system has.
- Entropy can also be thought of as a measure of how much energy is not available to do work.
- Entropy is given by the equation:
- Where Q is the work done on the process and T is the temperature.
Examples of entropy
- Ice has lower entropy than water as the H20 molecules in ice are arranged in a crystal structure.
- Water is more disordered than ice as the molecules are free to move around.
Second law in terms of entropy
- The total entropy of a closed system either increases or remains constant in any process; it never decreases.
- The second law of thermodynamics for entropy can be expressed as an equation:
Test yourself on Second Law of Thermodynamics
What happens to a bowl of ice cream that is left out on a table?
In which direction does heat energy flow spontaneously?
Which of the following is incorrect about the entropy of a closed system?
Which of the following equations for the entropy of a closed system is incorrect?
Which of the following substances has the largest entropy?