Chemical Thermometers

Chemical thermometers have been used to produce calibration charts at reaction pressures from 5 bar to 613 bar that correlate the temperature behind the reflected shock wave with the incident shock velocity. These charts are used to determine the temperature at which an experiment was performed.

• We need to know the reaction temperature to be able to interpret our experiments. Gases in the reaction zone of the high pressure shock tube do not behave ideally.
• Calculating the temperature based on the incident shock velocity and the ideal shock wave equations will lead to large errors.
• Chemical thermometers allow us to measure the temperature in the shock tube and account for all non-idealities.

A chemical thermometer is a molecule that decomposes in a known way and has the following characteristics:

• The high pressure rate coefficient is accurately known.
• The molecule decomposes unimolecularly to stable products i.e. no chain reactions occur.

What are Chemical Thermometers?

Calibration of post-shock temperature, T5, using 1,1,1-trifluoroethane at 5000 psi. T5 has been calculated from both the loss of parent and the formation of product.

Why do we need Chemical Thermometers?

• The chemical thermometer is introduced into the shock tube and the tube is fired many times to obtain different reaction conditions.
• The pressure and duration of the reaction are measured along with the velocity of the incident shock wave.
• The concentrations of the chemical thermometer in the shock tube before and after the reaction are measured.
• The concentrations of stable products are also determined.
• From the rate equation, the concentrations before and after the reaction and the reaction time the temperature that existed in the reaction zone is calculated.
• The temperature is correlated with the incident shock velocity to make a calibration chart.
• The reaction mixtures are very dilute and the concentrations of the reagents have only a negligible effect on the properties of the shock wave. Thus we can calibrate the temperature independently of our investigation into other molecules but obtain the reaction temperature from the calibration chart by measuring the incident shock velocity for each experiment.

How do we use Chemical Thermometers?

Both 1,1,1-trifluoroethane and cyclohexene have been used as chemical thermometers.

C₂H₃F₃

C₂H₂F₂ + HF

_{1150 - 1350 K}

_{Ea/R = 39.0}

c-C₆H₁₀

C₄H₈+ C₂H₂

_{1050 - 1150 K}

_{Ea/R = 33.4}

From loss of parent molecule

T= temperature

x= fraction of thermometer remaining

A= A factor

t= reaction time

T=(-Ea/R)ln(-ln(1-x))/At)