The glass calorimeter is the instrument used in measuring the heat of reaction. If the heat of reaction is large, we can detect exothermic and endothermic reactions by feeling the temperature of the reaction vessel before and after the reaction. If the reaction is
- Exothermic, there is a rise in temperature so the reaction vessel feels warm;
- Endothermic, there is a fall in temperature so the reaction vessel feels cool.
To determine accurate values of ΔH, we use a calorimeter.
There are several types of calorimeters. The bomb calorimeter is very sensitive and is used in nutrition studies. The glass calorimeter is used to determine the ΔH of most chemical reactions. Reactants in stoichiometric amounts are placed in the calorimeter. As the reaction proceeds, the heat energy evolved or absorbed will either warm or cool the system. The temperatures of the system before and after the reaction are recorded.
Knowing the parameters below such as
- The temperature change,
- The mass of reactions, and
- The specific heat capacity of the reaction mixture, we can calculate the ΔH for the reaction.
Standard Conditions for Measuring ΔH
The heat of reaction depends not only upon the mass of the reactants, but also upon the temperature and pressure of the reacting system. Another important factor is the physical state of the substances involved in the reaction, since heat change accompanies the rearrangement of certain bonds between particles when a substance changes from one physical state to another.
For example, we can represent the formation of water as follows:
- H2(g) + ½ O2(g) H2O(l) ΔH = -286 KJ mol-1
- H2(g) + ½ O2 (g) H2O(g) ΔH = -242 KJ mol-1
In (a) liquid water is formed, while in (b) water vapour if formed. Liquid water absorbs heat from the surroundings to form water vapour. The latent heat of vaporization of water is 44 KJ mol-1.
H2O(l) ⟶ H2O(g) ΔH = +44KJ mol-1
Therefore, the formation of water vapor requires less energy than the formation of liquid water. This is seen in the values of ΔH in (a) and (b) which differ by 44KJ mol-1
Besides the three states of matter, the existence of the reactants and products of a reaction in the aqueous state also affects the heat of reaction. For example, the heat evolved during the neutralization of a given amount of sodium hydrogen solution by hydrochloric acid is greater when hydrogen chloride gas is passed into the solution than when hydrochloric acid solution is used. This is because hydrogen chloride gas dissolves in water with the evolution of heat. The heat evolved when a substance is dissolved in water is known as the heat of solution. This is of considerable importance since a vast majority of reactions occur in aqueous medium.
We now see why it is so important to include the state symbols: solid (s), liquid (l), gas (g) or aqueous (aq) after each substance when writing a chemical equation.
Since the value of ΔH is affected by several factors, it is necessary to define standard conditions for its measurement using a glass calorimeter. These conditions for determining ΔH are as follows:
- The temperature must be 298 K (25oC)
- If gases are involved, the pressure must be at 1.01 x 105Nm-2 (1 atm. At 760 mm Hg)
- If aqueous solutions are involved, their concentrations must be 1 moldm-3
The heat of reaction or enthalpy change obtained under such standard conditions is given the special symbol, ΔHӨ.
It should be noted that in practice, for many reactions, Δ cannot be measured at 298 K. Experimental ΔH values are converted to values at 298 K, or the temperature at which a ΔH value is given is specified within parenthesis.
The Glass calorimeter reads the momentum analyzed signal from muon straight-through events and energy information from electrons passing through the lead glass. Its resolution depends on the sampling ratio and on how tightly the blocks are grouped together.
Typical data from DC704 and glycerol show a clear signature of thermal history in the heating curves, where quenching faster yields a more pronounced peak than slow cooling. This effect can be corrected by a careful design of the glass material.
Thermodynamics of a Calorimeter
The basic idea behind a calorimeter is that when two objects of different temperatures are kept in contact heat will flow from the hotter object to the cooler one until they reach the same temperature. Calorimetry is the process of monitoring the transfer of energy, and is used to gather information about processes such as melting, glass transition, crystallization etc. In a glass calorimeter (which is a type of insulating sample container) the heat transferred can be measured using a thermocouple. The difference in the amount of heat exchanged between the calorimeter and its surroundings is equal to the sample’s specific heat. This can be compared to the enthalpy of fusion of ice and water (Hf) to find the total amount of energy stored in the sample.
A glass calorimeter is simple and cheap to construct. The basic principle is to stack two new Styrofoam cups inside each other with an insulating material in between them like a rectangular piece of Styrofoam or cardboard. Then you make a hole on the top of the insulated cup and insert a thermometer. The thermocouple measures the changes in the temperature of the sample during heating and cooling. This provides a wealth of valuable information about the glass-forming process as it is happening, including the location and characteristics of the glass transition.
When a supercooled liquid is quenched to the glass state at an appropriate cooling rate, a clear signature of the transition is observed as a shift in the temperature profile in the (T,dTdt)T,dTdt-traces shown in Fig. 1. The peak in the distribution is due to the fact that at these temperatures there are still a lot of small domains, whose structural relaxation time is dominated by molecular vibrations. This feature explains why the transition is so sensitive to cooling and crystallization conditions.
In contrast, the TC signal emitted by a molten glass is much less sensitive to these parameters. In the case of glycerol, a typical TC glass-transition signal is observed at Tg,TC which is higher than the experimental Tg and is dependent on the characteristic time scales associated with the cooling protocol (Fig. 5A).
It is also possible to detect the effect of a change in specific heat by plotting the sample’s heat capacity as a function of temperature (Fig. 4A-H). Our model shows excellent qualitative agreement with experimental data for the standard scanning protocols but also yields correct quantitative behavior for nonstandard cooling and heating rates.
Thermochemistry of Calorimeter
The calorimeter is a device for measuring heat transferred between two substances or systems. It is based on the principle that the total amount of energy given off or absorbed by a system is equal to its temperature change. It is used to determine the enthalpy of chemical reactions and other properties of solids.
A glass calorimeter is a small, insulated container designed to measure the total amount of heat given off or absorbed by a solid. The insulating material insulates the reaction from its surroundings, allowing accurate heat transfer measurements. Typically, the calorimeter will contain a trough for holding the liquid or solid of interest, a lid with an insulated hole to allow for stirring, and a thermometer. Commercial solution calorimeters are available for use in labs or industrial settings.
Using the calorimeter to measure the heat of reaction requires careful measurement and attention to detail. During the experiment, the temperature of the calorimeter and the reaction will change, but it is important to note how much each changes and to understand why the temperatures vary. The difference in the temperatures of the calorimeter and the reaction is what is measured as the heat of the reaction.
When the liquid or solid is heated, it undergoes structural relaxation as it moves to its melting point. The process is usually slow and gradual, but there are exceptions to this rule. At certain low temperatures, the structural relaxation is faster and the liquid begins to fall out of equilibrium. This is known as the glass transition, and is indicated on a thermodynamic plot by a discontinuous increase in the slope of the curve as the temperature approaches the melting point.
To determine the enthalpy of the glass transition, students use a calorimeter with the following experimental protocol. The insulated cup holds either water or a liquid chemical that can be dissolved in water (such as an acid) and is safe for the student to handle. The reaction is triggered by mixing the chemical and swirling the contents in the calorimeter. Students are instructed to record the initial temperature of the calorimeter and its trough as well as the sudden increase in the temperature and subsequent plateau. The temperature change DT is then calculated by extrapolating a straight line that passes through the maximum number of points on the plateau region of the graph (e.g., T 6degC on an HCl-NaOH plot).
Reactions of Glass Calorimeter
A Glass calorimeter is an instrument for measuring the enthalpy change of a reaction under conditions that maximize heat transfer between the reaction and surroundings. The reaction vessel is typically a Dewar flask which is immersed in a controlled temperature bath to provide a constant rate of heat leak correction for the system. The temperature of the reaction is measured using a sensor in the cap, which also provides a reference temperature for calibration purposes. A stroboscope and a tachometer monitor the stirring velocity and synchronize with the temperature sensors to provide accurate timing and data analysis.
Initially, the system is completely sealed and pressurized with excess oxygen to prevent the escape of gases during the reaction. The reaction is then started in a steel container, and the total volume of the vessel including the container, gas, and vapor is measured. In the old days, time-temperature readings were recorded at 100-s intervals on punched paper tape and printed on a teletype, and main periods of calibrations or reactions usually lasted 15-30 min. The corrected DT (temperature change of the calorimeter corrected for heat leak) was calculated at the end of each main period as a check to ensure that the system achieved the single exponential curve desired for rating periods.
In the laboratory, students construct a simple calorimeter using two polystyrene cups with an insulated cover, handheld stirrer, and thermometer. More sophisticated commercial solution calorimeters have well-insulated and fully enclosed reaction vessels, motorized stirring mechanisms, and a more accurate temperature sensor.
The sensitivity of the glass calorimeter to the cooling rate of the reaction determines how large a sample can be used. Larger samples require a longer time for the calorimeter to cool down and return to equilibrium with the environment. Smaller samples can be used with a shorter time, but the glass must be heated to a higher temperature in order to achieve equilibrium faster.
Students often observe that the enthalpy of fusion of a polymer, for example, increases with increasing reaction temperature, and they measure the enthalpy of fusion at various temperatures to find the melting point. This data is compared to the experimental enthalpy of fusion at the same temperature to calculate the molar mass of the polymer, a key factor in formulating design criteria for polymer production.
Applications of Glass Calorimeter
Calorimeters are essential to many physics experiments, and are used to measure both the energy of particles and their momentum. They are important for understanding the physics of colliding matter, determining the properties of matter at high energies and for studying the dynamics of nuclear reactions.
A calorimeter can be as simple as a glass tube or as complex as a large ring-shaped apparatus called a particle flow algorithm (PFA) calorimeter. The basic principle is that heat or mass flows into and out of the calorimeter at constant rates over a known time interval. The temperature of the calorimeter’s water increases proportionally to the amount of heat flowing into or out of the sample. The mass of the sample can also be measured by a scale attached to the calorimeter.
The specific application of a calorimeter depends on the experimental goals. A glass calorimeter is very useful in identifying the thermal transition point(s) of supercooled liquids, e.g., for locating their glass transition and melting points. This can be used to optimize the stability of a liquid against crystallization and determine its rheological properties, which are vital for the design of a polymer.
Another common type of calorimeter is the differential scanning calorimeter, a laboratory device that applies the conservation of energy to determine an unknown metal’s heat capacity by measuring the difference in temperature between a sample and its surroundings. This technique has a wide range of applications in chemistry, cell biology, pharmacology and nanoscience.
The simplest solution calorimeter consists of two thin-walled cups, each with an insulated cover and a handheld stirrer. The temperature of the liquid is monitored with a thermistor, and the calorimetry software measures the rate of increase in the sample temperature and the heat flow into or out of it.
A different type of calorimeter, called a bomb calorimeter, operates at constant pressure and is designed to measure the heat flow accompanying explosions or other vigorous reactions that would damage a conventional calorimeter. It consists of a robust steel container that contains the reactants, which is submerged in water. The reaction occurs inside the container, and the reaction’s heat is detected by a curtain of scintillation counters surrounding the calorimeter.