Heating curves

Quantitative studies of solid-state organic reactions were performed by Glazman (267. 268). Equal amounts of acetic anhydride and 2-aminothiazole (grain diameter 0.15 mm) were mixed for 20 rain, and the mixture was heated in a glycerol bath at 0.5°C per minute. Heating curves showed that the reaction starts in the solid phase the use of an eutectic composition of organic reactants increases the yields. 

Fig. 3. Flame hardening (a) tempeiatuie—time heating curves of a 25 x 50 x 100 mm specimen at a feed of 75 mm /min and burner distance of 8 mm showing temperatures of A, surface B, 2 mm below surface and C, 10 mm below surface (b) hardness—depth curves for a 0.50% C steel 25 X 75 X 100 mm specimen at a feed of 50 mm /min, temperatures ia °C measured 10 mm below the surface, and burner distances ia mm, respectively, of A, 530 and 50 B, 540 and 12 C, 545 and 10 D, 550 and 8 and E, 565 and 6. Flame heating followed by water spray quenching. HV = Vickers hardness.

The period after which this can be repeated will depend upon the heating curve and the thermal time constant of the motor, i.e. the time the motor will take to reach thermal equilibrium after repeated starts (See Chapter 3). 

Phase transformations in the solid state (like those in iron), too, have latent heats. They may be small, but with sensitive equipment for measuring cooling curves or heating curves, they are easily detected. 

Fig. 17. The voltage-current relationship (a) and resistive heating curve (b) for a CFCMS monolith (sample 21-2B, 18% bum-off, 2.5-cm diameter x 7 5-cm length) [23]...

To identify the above effects, a differential temperature curve of a K2TaF7 -KF mixture (molar ratio 1 1) was plotted. The heating curve displays two effects, at 730 and 780°C (see Fig. 88, curve 4), whereas cooling provides only one effect, at 778°C (Fig. 88, curve 5). 

Eutectic arrest In a cooling (or heating) curve an approximately exothermal segment corresponding to the time interval during which the heat of transformation from the liquid phase of two or more conjugate solid phases is being evolved (or conversely). 

The enthalpies of fusion and vaporization affect the appearance of the heating curve of a substance. A heating curve is the graph showing the variation in the temperature of a sample as it is heated at a constant rate at constant pressure and therefore at a constant rate of increase in enthalpy (Box 6.1). 

The heating curve of a substance, like that in Fig. 6.26, shows how its temperature changes as heat is supplied at a constant rate, usually at constant pressure. How are these curves produced Simple laboratory heaters can be used to obtain a crude estimate of a heating curve. However, for accuracy, one of two related techniques is normally used. 

The output of a differential scanning calorimeter is a measure of the power (the rate of energy supply) supplied to the sample cell. The thermogram in the third illustration shows a peak that signals a phase change. The thermogram does not look much like a heating curve, but it contains all the necessary information and is easily transformed into the familiar shape. 

The temperature of a sample is constant at its melting and boiling points, even though heat is being supplied. The slope of a heating curve is steeper for a phase with a low heat capacity than for one with a high heat capacity. 

The following data were collected for a new compound used in cosmetics AHtlls = 10.0 kj-mol", AHvlp = 20.0 kj-mol heat capacities 30 J-mol 1 for the solid 60 J-mol 1 for the liquid 30 J-mol 1 for the gas. Which heating curve below best matches the data for this compound ... 

Fig. 8.8 Thermal effects on the formation of La3(B3N6) by metathesis reaction from equimolar amounts of LaCl3 and Li3(BN2). The strong exothermic effect in the heating curve (solid line) between 550 and 575 °C...

For the experiments in type C catalysts, the pellets were overfilled with cyclohexane and initially cooled to 230 K. They were then reheated in steps of 1 K and allowed to equilibrate for 10 min before each measurement. The signal was determined from 32 accumulations with an echo sequence of 20 ms echo time to ensure that the signal from the plastically crystalline phase of cyclohexane had decayed fully. The typical heating curves of cyclohexane in the fresh and coked catalyst are displayed in Figure 3.3.3(a) As the temperature is increased, larger and... 

Figure 23.45 The site power-to-heat curve for complete on-site power generation in steam turbines. (From Varbanov P, Perry S, Makwana Y, Zhu XX and Smith, 2004, Trans IChemE, 82A 784, reproduced by permission of the Institution of Chemical Engineers.)... 

A heating curve or a cooling curve may be used to emphasize the constant temperature of phase changes. In such a curve, temperature is plotted on the vertical axis against heat added on the horizontal axis (Fig. 18-1). 

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