Download Frequently asked questions with answers - THERMODYNAMICS and more Study notes Thermodynamics in PDF only on Docsity!
Frequently asked questions with answers - THERMODYNAMICS
(1) What is internal energy? Internal energy is the sum of all kinds of energies in a system. It is the total energy contained in a system. It is due to the translational, vibrational, and rotational motion of atoms in a molecule. It is also due to interactions between the molecules. It is represented by either E or U. (2) What is the relation between enthalpy and internal energy? Internal energy is the total energy contained in a system where enthalpy is the total heat content of a system. Enthalpy is equal to the sum of internal energy and product of pressure-volume work. H = U + PV The first law of thermodynamics states that a change in internal energy is equal to the heat added to a system minus work done by the system. U = Q – W Both are state functions and both are extensive properties. (3) What is the difference between energy and enthalpy? Energy is a state of matter whereas Enthalpy is an energy change between two states of a particular system. Energy is measured in Joules whereas enthalpy is measured in Joule/mol. Energy is a function of temperature whereas enthalpy is a function of temperature and pressure. (4) Should I use enthalpy or internal energy? When heat is added to a system at constant volume, we use the term internal energy because no work is done and it is a closed system.
When heat is added to a system at constant pressure then we use the term enthalpy because work is done and it is an open system. If q is the amount of heat absorbed by the system from surroundings, Heat (q) (at constant volume) = Internal energy (E) Heat (q) (at constant pressure) = Enthalpy (H) (5) What is the internal energy formula? The first law of thermodynamics states that any change in internal energy is equal to the heat added to a system minus work done by the system. dU = Q – dW Where dU = change in internal energy Q = heat added into a system dW = work done by the system (6) Is enthalpy just energy? Enthalpy is the sum of internal energy and product of pressure-volume work. Energy is a state of matter where enthalpy is an energy change between two states. So, enthalpy is not just energy. (7) What does it mean when enthalpy is zero? Zero enthalpy means reactants and products are identical. As enthalpy is a state function, if initial and final enthalpies are equal then the change in enthalpy is zero. It also means that there exists either no reaction or complete equilibrium. (8) Why is enthalpy used instead of internal energy? If a reaction occurs at constant volume, then added heat is equal to a change in internal energy. If a chemical reaction occurs at constant pressure, then enthalpy is used to measure the heat of the reaction instead of internal energy. (9) Can enthalpy be greater than internal energy? For an open system, the term enthalpy is used to calculate energy and it is always greater than internal energy. The enthalpy of an ideal gas is 40% greater than its internal energy.
Enthalpy of reaction is the difference between the enthalpy of products and the enthalpy of reactants. (16) Can enthalpy be negative? We cannot measure the absolute value of enthalpy. Only the change in enthalpy can be measured. So, the change in enthalpy may be positive or negative. For exothermic reactions, the change in enthalpy is negative whereas for endothermic reactions, the change in enthalpy is positive. (17) Can we define enthalpy for solids and liquids? We cannot define the enthalpy of solids and liquids. We can only define the enthalpy change for solids and liquids. (18) Is the internal energy of a solid and gas the same, provided the temperature and number of moles are the same? Internal energies of the solid and the gas are not same. When the temperature increases, kinetic energy increases and resulting is an increase in internal energy. However, the increase in internal energy in case of gases is more than solids because of the degree of freedom. (19) Why is internal energy considered to be an extensive property? Internal energy depends on the amount of substance, that’s why it is an extensive property. (20) What happens to the internal energy if work is done by the system? If heat is released from the system to surroundings and work is done by the system then internal energy decreases. (21) What is the significance of enthalpy? Enthalpy is used to calculate the heat of the reaction at constant pressure. It tells us whether the reaction is endothermic or exothermic. It is the measure of the amount of heat absorbed or released during a chemical reaction. (22) What is the difference between heat and energy? Heat is a transfer of energy from a body of high temperature to a body of lower temperature. Energy is the ability of a body to do work. (23) What is the difference between heat and enthalpy?
Heat is a transfer of energy due to temperature differences. Enthalpy is the change in the amount of heat at constant pressure. (24) Why is internal energy directly proportional to temperature since internal energy increases during a phase change but the temperature does not? During a phase change, energy is absorbed or released. For example, change in phase from solid to liquid and from liquid to gas requires energy so internal energy increases. Temperature is directly proportional to the kinetic energy of particles. During a phase change, kinetic energy does not increase so does the temperature. (25) What is the reason for using the Cp value to find change in enthalpy for all non-flow processes? Why can’t we use Cv? In non-flow processes, constant pressure exists. So, we use Cp rather than Cv. (26) What is the specific enthalpy? Specific enthalpy is defined as enthalpy per unit mass of a substance. It is measured in Joule/kg. Specific enthalpy (h) = Enthalpy (H) / mass (m) (27) How will the internal energy change during the boiling process? In the boiling process, energy is required to convert liquid into the gas phase. As water molecules gain energy, they speed up and their internal energy changes. (28) In the adiabatic process what happens to internal energy when a gas expands adiabatically? In adiabatic expansion, no heat enters or leaves the system, and work is done by the gas. So, internal energy decreases and the temperature of the system falls. (29) In an isothermal process, the change in internal energy is zero. Why? In isothermal process, the temperature remains constant. As internal energy is directly proportional to temperature, so if the temperature change is zero, the change in internal energy is also zero. (30) Why is the ionization enthalpy of Tl greater than that of In? The nuclear charge in the case of TI is greater than that of In. This means that the shielding effect of TI is greater than In. So, removal of the outermost electron from TI is difficult. That’s why, it has more ionization enthalpy.
In an endothermic reaction, the energy of the products is greater than the energy of reactants. So it requires more activation energy. In an exothermic reaction, the energy of reactants is greater than the energy of the products. So it requires less activation energy. (38)Is the ice melting considered endothermic or exothermic, why? Melting of ice is an endothermic reaction as the ice absorbs energy from surrounding to change its state from solid to liquid. (39) What is the temperature inside the combustion chamber of an engine? The temperature inside the combustion chamber of an engine can reach 4500o^ F. (40) What is the difference between isentropic and polytropic processes in thermodynamics? Isentropic processes are adiabatic and reversible. They are ideal processes. No heat transfer and loss takes place in these processes. The processes other than isentropic processes are called polytropic processes. They are real processes and obey the relation PVn^ = C where P is pressure, V is volume, and the ‘n’ and ‘C’ are constants. (41) What is the relation between temperature-pressure for liquid? The relationship between temperature and pressure of the liquid is given by the Clausius- Clapeyron equation, ln (P 2 – P 1 ) = H/R (1/T 1 – 1/T 2 ) where H = change in enthalpy R = general gas constant P, T = pressure and temperature (42) If internal combustion engines work due to the expansion of gases in the cylinder why are they classified as heat engines?
Internal combustion engines do work on the principle of expansion of gases but this doesn’t mean that this phenomenon runs on its own. Sometimes, there are linkages between driving principles. The expansion phenomenon runs with the help of the thermal expansion principle of fluids. So, the main driving principle behind internal combustion engines is ‘heat’ not ‘expansion’. That’s why, they are called heat engines. (43) What is the gamma ratio of specific heats Cp, Cv of water? The ratio of specific heat capacity at constant pressure to the specific heat capacity at constant volume is called gamma. The value of gamma for water is 9.807. (44) What is the difference between thermodynamics and heat transfer? Thermodynamics deals with the relationship between energy and work. where heat describes the transport of energy. (45) Does global warming increase entropy on Earth or vice versa? Entropy is a function of temperature. When temperature increases, it may lead to an increase in entropy. An increase in entropy means more energy is spread out which increases global warming. (46) When do we use Cp and Cv in thermodynamic equations? Cp is used when there is a specific heat capacity at constant pressure. Similarly, CV is used when there is a specific heat capacity at a constant volume. Under Cv, the volume change is zero where under Cp the change in pressure is zero. (47) Are fire and electricity the same thing? Electricity is due to the flow of free electrons. Fire takes place when a substance reacts with oxygen. So, fire and electricity are different. (48) What happens to the entropy of a cube of ice as it is melted? Entropy is the measure of randomness in a system. The greater the randomness in a system, the greater will be its entropy. As ice melts, solid is converted into liquid. The molecules of a liquid are in a more disordered state so entropy increases. (49) What is the concept of adiabatic demagnetization?
is referred to as enthalpy, and the reaction enthalpy is the change in enthalpy arising from a chemical reaction. (57) Is enthalpy always greater than internal energy? The heat energy produced is often used in thermodynamics to improve the system’s energy or to do some useful work. The energy associated with an open system is called enthalpy, which is often greater than or equal to internal energy. (58) What is the significance of internal energy? Internal energy is important for understanding phase changes, chemical reactions, nuclear reactions, and many other microscopic phenomena, as the possible energies between molecules and atoms are important. Both objects exhibit macroscopic and microscopic energy in vacuum. (59) What factors affect internal energy? The internal energy can be altered by modifying the object’s temperature or volume without altering the number of particles inside the body. Temperature: As a system’s temperature increases, the molecules will move faster, thus have more kinetic energy and thus the internal energy will increase. (60) Is internal energy a state function? A state function defines a system’s equilibrium state, and thus defines the system itself as well. For example, internal energy, enthalpy, and entropy are state quantities since they quantitatively describe a thermodynamic system’s equilibrium state, regardless of how the system has arrived in that state. (61) How do you measure specific heat capacity? Specific heat efficiency is measured by the amount of heat energy required to raise one gram of one degree Celsius of a product. Water’s specific heat power is 4.2 joules per gram per Celsius degree or 1 calorie per gram per Celsius degree. (62) Which is the advantage of water’s heat capacity? Because water has a high heat capacity, increasing the temperature by one degree requires more energy. The sun sends out a more or less constant energy level which heats up sand faster and water faster. (63) What is the difference between heat capacity and specific heat capacity? Specific heat capacity is the heat needed to raise a substance’s temperature by 1 degree Celsius. Similarly, heat capacity is the ratio between the energy provided to a substance and the corresponding increase in its temperature.
(64) Why is the specific heat capacity of water higher than metal? This is because the metal spoon’s specific heat efficiency is much smaller than the soup liquid. Water has every liquid’s highest specific heat capacity. (65)hat is the SI unit of specific heat capacity? Specific heat efficiency (symbol: c) in SI units is the amount of heat required in joules to raise 1 gram of 1 Kelvin substance. It can be expressed as J / kg as well. · K. K. Specific heat capacity in calorie units per gram Celsius may be recorded. (66) What are the limitations of the first law of thermodynamics? It does not tell us about the direction of the flow of heat. It fails to explain why heat cannot be spontaneously converted into work. (67) What is the mathematical form of the First Law of Thermodynamics? dQ = dV+dW But dW = PdV Therefore, dQ = dV+PdV dQ is the heat absorbed by the system. dV is the internal energy of the system. dW is the work done by the system. P is the pressure. dV is the change in volume. (68) State first law of thermodynamics? The law states that the energy entering the system in the form of heat is equal to the sum of the increase in the system’s internal energy and the energy leaving the system in the form of work done by the system on its surroundings. (69) What are the applications of the first law of thermodynamics? Isobaric process Adiabatic process Isochoric process Isochoric process (70) First law of thermodynamics is the law of conservation of mass or energy? The first law of thermodynamics is the law of conservation of energy. (71) Define irreversible process.
