# Summary of week 2

This week, you have practiced the calculation of energy changes using energy balance equations. Many exercises for the calculation of energy changes upon temperature change are presented.

You learend how to calculate energy difference between two states of different temperature if we know the heat capacities. When the pressure is constant, integration of Cp with respect to temperature gives the energy changes upon temperature change within a single phase. This energy change under constant pressure is called sensible heat. Since we can only calculate the energy difference between two states, the concept of a reference state where the zero enthalpy is assigned might be particularly useful. Then the enthalpy of a material at a certain state is not an absolute term but the enthalpy difference from the reference state. For compounds, heat of formation is assigned for the convenience in tabulation.

The enthalpy changes in common chemical reactions can be expressed with the enthalpy of formations for the compounds involved by the Hess’s law.

In week 3, you will examine the second law of thermodynamics. While the first law of thermodynamics is intuitively comprehensible, the second law is tricky to understand. The second law is the entropy principle.

There are various statements escribing the second law. The second law is concerned with heat into work and the quality of energy, which is a amount of useful energy that can do work.

The change in quality of energy can be determined by the entropy function. You will examine the physical meaning of entropy.

In week 3, we focus on the traditional interpretation of entropy and the closely related topics such as heat engines, refrigerators and heat pumps.