Thermochemistry

Thermochemistry refers to the transfer of energy through heat. We teach our students that energy is the ability to do work and/or produce heat and that the sum of these will always be constant.  With MicroLab equipment, they can explore these concepts first hand.  For example they can observe how a heated gas, contained in a cylinder, will expand and push the piston of a syringe out against a constant external pressure P. In this case, the work would be PDV and it can be related to the thermal energy input.

ThermoChem

Heat is always transferred from a hotter object to a colder object and represents a loss of kinetic energy from the hotter object, and an increase in kinetic energy of the colder object.  The transfer of energy from one system to another is always path dependent; it depends on how much energy is transferred in work and how much is transferred in heat. Through a series of experiments, students learn that the sum of these represents the change in the internal energy D of the objects, and this represents a ‘state’ function.

Temperature changes are currently measured on three different temperature scales, Fahrenheit, Celcius and Kelvin. These are well defined and commonly known.  For scientific work Celcius and Kelvin are usually used.  Temperature changes are commonly measured with four types of sensors; the common thermometer, (either mercury or alcohol types), thermistors, which are resistance based; thermocouple, which generate an electrical potential as a function of temperature, and infrared sensors which are based on the heat energy being radiated from the object.

MicroLab provides two types of temperature sensors for monitoring temperature changes.

  • Thermistors, or thermally-sensitive resistors, produce the largest response to temperature change of any thermal sensor, and operate from -10 to +100 C. These small and rugged temperature probes provide quick and accurate (± 0.2° C) thermal measurements with high resolution (± 0.01° C).
  • Thermocouples are usually thought of as extreme temperature sensors, used for example to map flame temperature. Type K thermocouples are quite linear over the range -200 to +1000° C and when used with a MicroLab 524, the resolution of a Type K thermocouple is 0.04° C.

Both sensors serve well in room-temperature biology or chemistry experiments requiring observation of small temperature changes.  All MicroLab software comes with a factory calibration file for the MicroLab thermistor (Model 103) that follows the Steinhart-Hart third-order log polynomial calibration equation. With its factory calibration it is accurate to ± 0.2° C in the range 0-70° C. Its calibrated range can easily be extended from – 10 to + 110° C by calibrating with MicroLab software.

Model103

The graph below shows the response of a MicroLab thermistor (Model 103) alongside a traditional thermistor moving from ice water to hot water at 73°C. Notice that the equilibration time (red) for the MicroLab thermistor was almost a factor of three faster than common educational sensors (blue), meaning students can better track the temperature response of their experiment.

Model103a

Chemical processes also involve changes in thermal energy. To evaluate these energy changes, we define for our students two new terms, system and surroundings. System is that portion of the universe that we are particularly interested in studying and in the experiment below, our system consists of a cup of water and a halogen light bulb. Surroundings represent all the rest of the universe. To quantitatively evaluate the changes in a system we must find some way to isolate the system from the surroundings so that we can measure the energy transfer that takes place between the system and the surroundings, or between two or more parts of the system. A measured amount of current is supplied to the halogen light bulb using the Electrochemistry Module (Model 272).  The electrical energy is transformed to heat and light.  The heat warms the water sample and is measured through temperature changes.  Students compare the electrical energy input with the thermal energy output and discuss the efficiency of this type of lighting.

ThermoChem2 Chemical energy can also be transformed into thermal energy when a bond breaks or forms in a chemical reaction.  When thermal energy is created as a result of a chemical reaction, the container heats up and transfers some of this energy to its surroundings.  That is to say the product molecules have lost energy and this change is called exothermic (exo = out of). When a reaction absorbs heat (and the container holding your reaction becomes cold), it gains energy and this change is called endothermic (endo = into). Exothermic processes are understood to represent energy stored within the bonds of the chemicals involved in the reaction being converted to thermal energy (random kinetic energy) via heat.