Test: Thermodynamics And Thermochemistry - 1 - Chemistry MCQ

It is the definition of calorific value.

• The temperature of the system decreases in an adiabatic expansion.
• In isothermal process, temperature remains same.
• The temperature of the system increases in an adiabatic compression.

• An isothermal process is a change of a system, in which the temperature remains constant. For an ideal gas during an isothermal expansion the enthalpy, as well as internal energy, remains constant.
• During isothermal expansion of an ideal gas, ΔE = 0, ΔT=0
• From the definition of enthalpy,
  H=E+PV
  or ΔH=ΔE+Δ(PV)
  or ΔH=ΔE+Δ(nRT) {Since, PV=nRT for an ideal gas}
  or ΔH=ΔE + nRΔT
  or ΔH=0

In an isochoric process, ΔV=0.

Hence, work done PΔV = W = 0.

So, ΔE = q+0.

Hence, the increase in internal energy will be equal to heat absorbed by the system.

Internal energy, enthalpy, and entropy are state quantities because they describe quantitatively an equilibrium state of a thermodynamic system, irrespective of how the system arrived in that state.

An adiabatic process occurs without transfer of heat or mass of substances between a thermodynamic system and its surroundings. In an adiabatic process, energy is transferred to the surroundings only as work.

An isolated system neither shows exchange of heat nor matter with surroundings.

A process that does not involve the transfer of heat or matter into or out of a system, so that ΔQ = 0, is called an adiabatic process and such a system is said to be adiabatically isolated.

An intensive property is a property of matter that does not change as the amount of matter changes. It is a bulk property, which means it is a physical property that is not dependent on the size or mass of a sample. In contrast, an extensive property is one that does depend on sample size.

Bomb calorimeter is commonly used to find the heat of combustion of organic substances which consists of a sealed combustion chamber, called a bomb. If a process is run in a sealed container then no expansion or compression is allowed, so w = 0 and ∆U = q.

∆U < 0, w = 0

• The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic systems.
• The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed.
• The first law is often formulated
  ΔE = Q + W            

Isothermally (at constant temperature) and reversible work.

w = -2.303nrt log(P₁/P₂)

at constant temperature , ΔE = 0
ΔE = Q + W,

Q = −W = −965.84 cal

The heat required to raise the temperature of a body by 1 K is called thermal capacity. In other words, when q is the heat supplied to the body and the temperature rises by 1 K, then the thermal capacity of body is q.

H₂​O(l) ​⇋ H₂​O(g)​

As we know that, at equilibrium, ΔG=0

∴ΔH−TΔS=0

⇒ΔH=TΔS

According to the third law of thermodynamics (regarding the properties of systems in equilibrium at absolute zero temperature): 

The entropy of a perfect crystal at absolute zero is exactly equal to zero. At absolute zero (zero kelvin), the system must be in a state with the minimum possible energy, and the above statement of the third law holds true provided that the perfect crystal has only one minimum energy state.

  Entropy of gases is highest.

The iombustion of C₂H₄ can be derived as follows :

• Heat of neutralization of strong acid and weak base is less than 13.7 kcal/mol because some of energy used in dissociation of weak base.
• For strong acid and strong base heat of neutralization has constant values 57.1kJ/mol  and 13.7Kcal/mol

CH₄ required = (445.15 x 16) / 890.3 = 8gm

Heat of decomposition, ΔE = m.s.ΔT
= 1 x 1.23 x 6.12 = 7.5276 kJ
Molar heat of decomposition for NH₄NO₃ = 7.5276 x 80 = 602.2 kJ/mol