The molecule is a microcosm object. Therefore direct measurement of its kinetic energy is impossible. Average kinetic energy is a statistical concept. This average value of kinetic energy of all molecules entering substance.
It is required to you
- - periodic table of chemical elements;
- - thermometer;
- - calculator.
Instruction
1. Find average kinetic energy, using value of average velocity of the molecules of substance. Calculate the mass of one molecule of substance. For this purpose determine its molar weight in kilograms on mol by means of the periodic table of chemical elements. For this purpose find relative atomic masses of all elements which are a part of a molecule of substance. They are specified in the corresponding cells of the table. Put them, and receive the relative molecular mass of a molecule. Divide this number into 1000 and receive the molar mass of substance in kilograms on mol.
2. Divide molar weight into Avogadro (NA=6,022∙10^23 1/mol) number and receive the mass of one molecule of substance m0 in kilograms. Count average kinetic energy of molecules, having increased the mass of one molecule m0 by a square of its speed of v, and divide result into 2 (Ek=m0∙v²/2).
3. Example. Calculate the average kinetic velocity of the molecules of nitrogen if their average speed is equal to 100 m/s. The molar mass of a diatomic molecule of nitrogen is equal to 0.028 kg/mol. Find the mass of one molecule 0.028 / (6.022∙10^23) ≈4.6∙10^ (-25) kg. Define average kinetic energy of the molecules Ek = 4.6∙10^ (-25) ∙100²/2=2.3∙10^ (-21) J.
4. Find average kinetic energy of molecules of gas through value of temperature. Measure this size by the thermometer. If the device measures in degrees Celsius, transfer value of temperature to Kelvins on an absolute scale. For this purpose to value of temperature in degrees Celsius add number 273. For example, if gas 23ºC temperature, then on an absolute scale its temperature is equal to T=23+273=296 K.
5. Define degree of freedom of molecule i. This size for a one-atomic molecule is equal to 3. For a diatomic particle – 5, triatomic and more – 6. Calculate average kinetic energy, having increased degree of freedom of a molecule by the absolute temperature of gas and Boltzmann (k=1.38∙10^ (-23) constant). Divide result into number 2 (Ek=i∙k∙T/2).
6. Example. Find average kinetic energy of molecules of diatomic gas at a temperature 85ºС. Determine gas temperature by an absolute scale of T=85+273=358K. Degree of freedom of diatomic molecule i=5. Make calculation Ek=5∙1.38∙10^ (-23) ∙358/2≈1.24∙10^ (-20) J.