Question

Construct a cooling curve (looks the same as a heating curve) for a substance with a molar mass of 200 that has a gas phase specific heat capacity of 2.9 J/g-K, a boiling point of 525 K, a liquid phase specific heat capacity of 3.24 J/g-K, a freezing point of 352 K, a solid phase specific heat capacity of 3.3 J/g-K and molar enthalpies of fusion and condensation of -14.3 kJ/mol and -46.4 kJ/mol. Also what is the difference between a heating and cooling curve? Is a heating curve based on temperature and kJ while a cooling curve is based on temperature and time?

Answer 1

When the system is heated, energy is transferred into it. In response to the energy it receives, the system changes, for example by increasing its temperature. A plot of the temperature versus time is called the heating curve.

When a system contains only one phase (solid, liquid or gas), the temperature will increase when it receives energy. The rate of temperature increase will be dependent on the heat capacity of the phase in the system. When the heat capacity is large, the temperature increases slowly, because much energy is required to increase its temperature by one degree. Thus, the slope of temperature increase for the solid, liquid, and gases are different.

A cooling curve is a line graph that represents the change of phase of matter, typically from a gas to a solid or a liquid to a solid. The independent variable (X-axis) is time and the dependent variable (Y-axis) is temperature.^[1] Below is an example of a cooling curve used in castings

The initial point of the graph is the starting temperature of the matter, here noted as the "pouring temperature". When the phase change occurs there is a "thermal arrest", that is the temperature stays constant. This is because the matter has more internal energy as a liquid or gas than in the state that it is cooling to. The amount of energy required for a phase change is known as latent heat. The "cooling rate" is the slope of the cooling curve at any point.

In the part of the curve where the temperature decreases, the kinetic energy also decreases while the potential energy stays the same. However, at the phase transition, where the curve is flat, the kinetic energy stays the same while the potential energy decreases