Why measure the total heat capacity of a calorimetry system

It is used to calculate the total heat absorbed by the warming system by the group of substances estimated and determined.

The structure of the oxygen bomb calorimeter The oxygen bomb calorimeter has automatic calorimeter, microcomputer automatic calorimeter, etc., and the calorimeter system is composed of oxygen bomb, inner cylinder, outer cylinder, temperature sensor, stirrer, ignition device, temperature measurement and control system and water. The main engine of automatic calorimeter is generally composed of a casing, an outer cylinder, an inner cylinder, a spare water tank (or a fixed container), an agitator, a temperature sensor, an ignition electrode, a water circulation system, a control circuit, etc. Some automatic calorimeters also have an outer cylinder water temperature system and an outer cylinder temperature control system, which can keep the water temperature of the outer cylinder and the temperature of the entire calorimeter system fluctuating in a small range, creating a relatively stable measurement environment for the entire calorimeter The working principle of the oxygen bomb calorimeter At present, most of the domestic usurped calorimeters are constant temperature type.

Its working principle is generally to load the coal sample and oxygenate to the specified pressure of the oxygen bomb into the inner cylinder system to start water circulation, stabilize the water temperature, and then inject water into the inner cylinder, after reaching the predetermined amount of water, start stirring, so that the water temperature of the inner cylinder is balanced to room temperature (the difference is not more than 1.).5) At this time, the temperature control head measures the water temperature and records it to the computer. When the water temperature of the inner cylinder is basically stable, the control system indicates that the ignition circuit is turned on, and after ignition, the sample burns rapidly under the support of oxygen, and the heat generated is transferred to the inner cylinder through the oxygen bomb, causing the water temperature of the inner cylinder to rise.

When all the heat in the oxygen bomb is released, the temperature begins to drop, and the computer detects the signal of the drop in the water temperature of the inner cylinder, and determines that the production, supply and marketing test is over, and the system stops stirring and releases the inner cylinder water. The computer processes the results of the collected temperature data.

As follows:

1. Water vapor.

If the air is mixed with water vapor, the specific heat capacity of the gas measured by the experimental results will be larger than the actual one.

2. If the period of the vibrating object is long, the original formula is not applicable. Because the period of the object vibration is too long, it means that the vibration of the problem is too slow, and the vibration of the object is too slow cannot be regarded as the adiabatic process, and the adiabatic equation is not applicable. Vibration that is too slow cannot be regarded as simple harmonic motion.

The applicable condition of this formula is that the object does simple harmonic motion in the adiabatic process.

3. Describe the physical quantity of the temperature change characteristics of the system when the gas exchanges heat with the outside world.

Calculate an ideal gas consisting of n molecules of the same kind.

If the specific heat capacity of the system fails, the partition function q of the molecule in Boltzmann's statistics is usually found first: and then the constant volume is obtained by deriving the temperature t while the volume v remains constant in the system.

Heat capacity attack: If the equation Naheng n is replaced by the Avogadro number, the fixed volume molar heat capacity is obtained, also known as the fixed volume molar specific heat capacity.

Introduce. The specific heat capacity of the definite volume is calculated by the energy equalization theorem. If a molecule is made up of n atoms, it has 3n degrees of freedom.

where the centroid is. There are 3 translational degrees of freedom, and translational kinetic energy has three quadratic terms, and the contribution to the molar specific heat capacity is .

Linear molecules have two rotational degrees of freedom, and rotational energy has two quadratic terms, and their contribution to the molar specific heat capacity is r. The non-linear molecule has three rotational degrees of freedom, and three principal rotational inertia axes can always be found, so that the rotational energy has three quadratic terms, so the contribution to the molar specific heat capacity is .

The remaining 3n-5 or 3n-6 are vibrational degrees of freedom, and a kind of simple coordinates can always be found, so that the vibrational energy vibration has 3n-5 or 3n-6 quadratic terms, so the contribution of the relative vibrational degrees of freedom of the atoms inside the molecule to the molar specific heat capacity is: linear molecules are (3n-5)r, and nonlinear molecules are (3n-6)r.

Because when one substance dissolves in another, the chemical bonds are broken and energy is released

The ratio of the heat exchanged between the system and the environment to the resulting temperature change without phase change and chemical change is called the heat capacity of the system. The amount of heat exchanged between the system and the environment should be related to the type of matter, the state, the amount of the substance, and the mode of exchange. Therefore, the heat capacity value of the system is affected by the above factors.

In addition, the temperature range will also affect the heat capacity value, even if the temperature change range is the same, the system is in different states at the beginning and end, and the heat value exchanged between the system and the environment is different. Therefore, the heat capacity value calculated from the measured heat exchange value within a certain temperature variation range can only be an average value, which is called the average heat capacity. Namely.

c average = q δt

When the temperature changes, the average heat capacity can hardly reflect the true state of the system. For this reason, the concept of heat capacity is proposed, which is defined as:

c=δq/dt

The unit of heat capacity is j·k-1, which is the breadth property of the system. The heat capacity of 1mol of a substance is called molar heat capacity, which is expressed in cm and is expressed in J·mol-1·k-1, C=NCM; The heat capacity per unit mass of a substance is called the specific heat capacity.

The amount of heat required to increase the temperature of a substance by 1k (or 1) per unit mass is called the specific heat capacity of the substance. The specific heat capacity of the gas corresponds to different heating processes, and there are constant pressure specific heat capacity CP and constant volume specific heat capacity CV. Its ratio is the specific heat capacity ratio

r=cp/cv。

The determination of the specific heat capacity ratio of air plays an important role in the practical application of thermodynamic theory and engineering technology, for example, the efficiency of the heat engine and the propagation characteristics of sound waves in the gas are related to the specific heat capacity ratio of air.

Precautions for measuring the specific heat capacity of a substance.

When lifting a cylinder from the boiling water of a beaker, shake off the water droplets on the surface of the cylinder with a shake on the water surface, but not for too long, so as not to lower its temperature. Put the cylinder into the inner cylinder quickly, but don't splash the water outside.

During the experiment, the calorimeter should not be close to the alcohol lamp, so as not to affect the results of the experiment due to the baking of the alcohol lamp, and when the specific heat capacity is repeated, the inner cylinder of the calorimeter should be taken out to reduce the temperature in the calorimeter to room temperature before the experiment. Otherwise, the accuracy of the experimental results will be affected.

The temperature of the cold water used should preferably be slightly lower than room temperature. In this way, the temperature of the cylinder is close to room temperature after being mixed in water, which can reduce the experimental error caused by heat transfer, and care should be taken not to make the vacuole of the thermometer contact with the cylinder when measuring the mixing temperature.

1.Control changes in temperature and pressure. Reduce experimental errors.

2.No, in addition to the fact that deflation affects the pressure, the change in temperature may also affect the change in pressure3The deflation is not sufficient, and the resulting value is too large.

If the abandonment is insufficient, PO and P1 remain unchanged, P2 becomes larger, and the denominator becomes smaller, so R becomes larger.

4.This question is not very clear.

The specific heat capacity of a gas is determined as follows:

First, the content of the experiment.

1. The photoelectric receiving device is fixed on the vertical rod by using the macro square connecting block of the stove, and the fixed position is near the small hole of the hollow core glass tube.

2. Adjust the size of the air intake of the gas system, so that the steel ball vibrates up and down with the small hole as the center in the glass tube.

3. Use a photoelectric chronometer to measure the time required for 100 vibrations.

4. Measure the diameter d of the steel ball with a spiral micrometer, and weigh the mass m of the steel ball with an electronic balance.

Second, the key steps.

1) Adjustment of experimental instruments.

1. Connect the air pump and gas cylinder with rubber tubes, and insert the glass tube filled with copper balls into the spherical gas cylinder. The photoelectric receiving device is fixed on the vertical rod by a square connecting block, and the fixed position is near the small crack hole of the hollow core glass tube.

2. Adjust the bottom plate to talk about three horizontal adjustment end nails to make the bottom plate in a horizontal state.

3. Turn on the gas miscellaneous power supply, slowly adjust the adjustment knob on the air pump, after a few seconds, after injecting a certain pressure of gas into the gas storage cylinder, the steel ball in the glass tube leaves the spring and moves to the top of the tube.

2) Experimental measurements.

1. Setting: Turn on the power supply of the timing instrument and the connection between the photoelectric receiving device and the timing instrument. Turn on the chronograph and set the number of measurements to 100 times.

2. Process: Press the "Reset" button to start counting (the status display light flashes). When the status indicator stops flashing, the number displayed on the display is the time required to vibrate 100 times, and the measurement is repeated 5 times.

3. Measure the diameter d of the steel ball with a spiral micrometer and repeat the measurement 5 times.

4. Use an electronic balance to weigh the mass m of the steel ball.

3. Experimental principle.

The ratio of the specific constant pressure heat capacity cp to the specific volume heat capacity cv (also known as the adiabatic index) of the gas is called the specific heat capacity ratio, which is an important parameter in the thermodynamic process, especially in the adiabatic process. This experiment introduces the ingenious method of measuring the adiabatic index by using the principle of simple harmonic vibration of mechanics.