Week 10 (Trimester 2)

These past few weeks in AP chemistry we have been introduced to the many aspects of thermochemistry and their applications to the real world. Inside and outside of class we have utilized various tools to understand these new concepts from every possible angle. Making connections on our own. These tools included videos, worksheets and class discussions, all of which offered new ways of thinking about ideas and the ways that different properties interact. As a result of learning this new material on thermochemistry, I now believe that it is one of the best connections from the microscopic world of atoms and molecules, and the macroscopic world of "us."

We first cleared all confusion on the ideas of heat and temperature. Heat is the transfer of kinetic energy between the particles of different systems, while temperature is a quantified measure of the speed of the particles in a particular system. We were then introduced to some key concepts behind thermodynamic calculations. The specific heat capacity (Cp) of a substance is the energy required to raise a particular mass of the substance a particular degree of temperature, typically J g-1 K-1. Enthalpy is the total energy within a system, and is generally considered useless on its own. Instead, scientists use the change in enthalpy (ΔH) of a system when calculating certain values. This value is special in that it includes the energy change from temperature as well as from a change in the state of the substance. Entropy is a measure of the number of separate microstates that a substance may be found in. In other words, it measures the number of different forms the atoms or molecules of a system may line up at various points in time. These values are each known as state functions, meaning that the final value is independent of the path taken for it to be achieved.

Enthalpy as a state function

Using the properties of thermodynamics that we had learned, we began to use certain equations in order to calculate the various hypothetical temperature changes within different experiments. The first equation we used multiplied the mass of the system by the specific heat by the change in temperature to find the energy change in the system, assuming the state did not change. Given any three variables of this equation, the last could be solved for with algebra. We then added the heat of fusion and vaporization, in energy per mass unit, to find the total enthalpy change of any system, with state changes included. We then moved on to Hess's law, stating that the enthalpy change required to form a compound is equal to the sum of the products minus the sum of the reactants (see link below). The next equation we used was Gibbs free energy, for calculating the ability of a reaction to do work. This reaction states that the ability of a reaction to do work is equal to the change in enthalpy minus the temperature multiplied by the change in entropy. I had previously never imagined that I would be able to accurately calculate these types of properties of reactions with so few variables and reference materials.

Combustion of magnesium in class

Practice with Hess' law: http://chemistry.about.com/od/workedchemistryproblems/a/Hesss-Law-Example-Problem.htm