Aerodynamics Questions and Answers – High Temperature Gases – Chemical Equilibrium

This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “High Temperature Gases – Chemical Equilibrium”.

1. At high temperature, there’s presence of only oxygen and nitrogen molecules in the air.
a) True
b) False

Explanation: While analyzing the chemical composition of air at normal temperature and pressure, the major composition is of oxygen and nitrogen molecules. Presence of water vapor, argon, helium etc. are very less thus they can be neglected. Although, at higher temperature (2500K – 9000K), oxygen and nitrogen undergo chemical reaction and there is presence of O, N, NO, NO+ etc.

2. Which of these methods is not used in describing the composition of air which is at a chemical equilibrium?
a) Partial pressure
b) Partial temperature
c) Mass fraction
d) Mole – mass ratio

Explanation: There are several ways to describe the composition of air which is at a chemical equilibrium. These are – partial fraction, concentration, mole fraction, mole – mass fraction and mass fraction. These help in determining the composition of the reacting gas.

3. What is Dalton’s law?
a) Total temperature is equal to sum of partial temperatures
b) Total entropy is equal to sum of partial entropies
c) Total pressure is equal to sum of partial pressures
d) Total number of moles is equal to sum of mole fractions

Explanation: Dalton’s law of partial pressure was formulated to find out the total pressure of the chemically reactive mixture. This is the sum of partial pressures of individual components in the mixture. Dalton’s law is only applicable or perfect gases which means that the intermolecular forces between individual molecules is negligible.

4. What is the equilibrium constant for the following reaction?

        XY ⇄ X + Y

a) $$\frac {p_X p_Y}{p_{XY}}$$ = Kp(T)
b) $$\frac {p_X + p_Y}{p_X p_Y}$$ = Kp(T)
c) $$\frac {p_X – p_Y}{p_X p_Y}$$ = Kp(T)
d) $$\frac {p_X + p_Y}{p_X – p_Y}$$ = Kp(T)

Explanation: Equilibrium constant for a reaction is a function of temperature and it given by the following formula:
Kp(T) = ∏ip$$_i^{v_i}$$
Where, vi is stoichiometric mole number for i species (this is negative for reactants and positive for products)
pi is partial pressure of each species
There for the above reaction the equilibrium constant is given by:
$$\frac {p_X p_Y}{p_{XY}}$$ = Kp(T)
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5. What is chemical equilibrium?
a) Change in temperature spontaneously
b) No change in chemical composition spontaneously
c) Change in pressure
d) Equilibrium between mole fractions

Explanation: In a mixture of gases (air), when there is no sudden change in the chemical composition spontaneously or over a short period of time, the system is known to be in chemical equilibrium. There are no changes in temperature, velocity, pressure in the system.

6. For the reaction below, what is the relation between the rate of forward and reverse reaction?

          N2 ⇆ 2N

a) Ratef = Rater
b) Ratef > Rater
c) Ratef < Rater
d) Ratef Rater = 1

Explanation: The reaction given is at equilibrium. For a reaction that is at chemical equilibrium, the rate of forward reaction is equal to the rate of reverse reaction.
This implies: $$\frac {k_f}{k_r} = \frac { [N]^2 }{ [ N_2 ] }$$

7. What happens to a reaction when reaction quotient is equal to the equilibrium constant?
a) Forward reaction
b) Equilibrium
c) Reverse reaction
d) No change

Explanation: The reaction quotient gives the same expression as the equilibrium constant for a reaction that is at chemical equilibrium. But, for a reaction when the reaction quotient (Q) is greater than the equilibrium constant (k), there’s more product than the reactant and reverse reaction takes place.