Chemical Thermodynamics - SS3 Chemistry Past Questions and Answers - page 3
In a chemical reaction at equilibrium, which of the following statements is true?
The concentrations of reactants are equal to the concentrations of products.
The rate of the forward reaction is equal to the rate of the reverse reaction.
The reaction has stopped, and no further changes occur.
The total amount of reactants and products is constant.
Which of the following conditions is necessary for a spontaneous process to occur at constant temperature and pressure?
ΔG = 0
ΔG > 0
ΔG < 0
ΔG = ΔH
If the forward reaction of a reversible chemical process is endothermic, which of the following conditions will shift the equilibrium towards the products?
Increasing the temperature
Decreasing the pressure
Adding a catalyst
Removing the reactants
The concept of entropy is related to:
The disorder or randomness of a system.
The total energy of a system.
The heat flows in a chemical reaction.
The change in enthalpy during a reaction.
Which of the following statements is true regarding a system at equilibrium?
The concentrations of reactants and products are equal.
The concentrations of reactants and products are constant over time.
The reaction rate is at its maximum.
The system has no energy.
In a reversible process, the system is always in:
Thermodynamic equilibrium.
Chemical equilibrium.
Mechanical equilibrium.
Thermal equilibrium.
Consider the reaction: 2A + B ⇌ 3C + D
The standard Gibbs free energy change (ΔG°) for the reaction is -20.0 kJ/mol at 298 K. Calculate the equilibrium constant (K) for the reaction at this temperature
The relationship between the standard Gibbs free energy change (ΔG°) and the equilibrium constant (K) for a reaction at a given temperature (T) is given by the equation:
ΔG° = -RT ln(K)
where R is the gas constant (8.314 J/(mol·K)) and T is the temperature in Kelvin.
Given ΔG° = -20.0 kJ/mol and T = 298 K, we need to convert ΔG° to J/mol:
ΔG° = -20.0 kJ/mol × 1000 J/1 kJ = -20,000 J/mol
Now, we can calculate the equilibrium constant (K):
K = e(-ΔG° / (RT))
K = e^(-(-20,000 J/mol) / (8.314 J/(mol·K) × 298 K))
K = e^(67.94)
K ≈ 9.42 × 1029
Therefore, the equilibrium constant (K) for the reaction at 298 K is approximately 9.42 × 1029.
Consider the following reaction at equilibrium: N2(g) + 3H2(g) ⇌ 2NH3(g)
At a certain temperature, the concentrations of N2, H2, and NH3 are 0.10 M, 0.20 M, and 0.30 M, respectively. Calculate the value of the reaction quotient (Q) for this system.
The reaction quotient (Q) is calculated using the same formula as the equilibrium constant (K), but instead of using standard Gibbs free energy change (ΔG°), we use the concentrations of the reactants and products at a given moment. The formula is:
Q = [NH3]2 / ([N2] × [H2]3)
where [N2], [H2], and [NH3] are the concentrations of N2, H2, and NH3, respectively.
Given [N2] = 0.10 M, [H2] = 0.20 M, and [NH3] = 0.30 M, we can calculate the value of Q:
Q = (0.30 M)2 / (0.10 M × (0.20 M)3)
Q = 0.09 M2 / (0.10 M × 0.008 M3)
Q = 0.09 M2 / (0.0008 M4)
Q ≈ 112.5 M(-2)
Therefore, the value of the reaction quotient (Q) for this system is approximately 112.5 M(-2).
Which of the following statements about Hess's Law is correct?
Hess's Law states that the enthalpy change of a reaction is directly proportional to the temperature.
Hess's Law allows the calculation of the enthalpy change of a reaction based on bond energies.
Hess's Law states that the enthalpy change of a reaction is independent of the reaction pathway.
Hess's Law is only applicable to exothermic reactions.
Hess's Law is a fundamental principle in thermodynamics that states the enthalpy change of a reaction is independent of the reaction pathway taken from reactants to products. In other words, the overall enthalpy change of a reaction remains the same, regardless of the intermediate steps involved in the reaction.
Given the following thermochemical equations:
N2(g) + 3H2(g) → 2NH3(g) ΔH = -92.4 kJ/mol
N2(g) + O2(g) → 2NO(g) ΔH = 180.6 kJ/mol
Calculate the enthalpy change for the reaction: 4NH3(g) + 3O2(g) → 2N2(g) + 6H2O(g)
-91.8 kJ/mol
-5.8 kJ/mol
-270.6 kJ/mol
273.0 kJ/mol
To calculate the enthalpy change for the given reaction, we can use Hess's Law. We need to manipulate the given equations to match the target reaction. Notice that the first equation is already the reverse of the target reaction. To reverse the second equation, we must change the sign of the enthalpy change:
N2(g) + 3H2(g) → 2NH3(g) ΔH = -92.4 kJ/mol
2NO(g) → N2(g) + O2(g) ΔH = -180.6 kJ/mol (reversed)
Now, we can add the manipulated equations to obtain the target reaction:
N2(g) + 3H2(g) → 2NH3(g) ΔH = -92.4 kJ/mol
2NO(g) → N2(g) + O2(g) ΔH = -180.6 kJ/mol (reversed)
Target: 4NH3(g) + 3O2(g) → 2N2(g) + 6H2O(g) ΔH = -273.0 kJ/mol
The enthalpy change for the target reaction is -273.0 kJ/mol. However, since the enthalpy change is for the formation of the products, we change the sign to make it positive:
Enthalpy change for the target reaction = -(-273.0 kJ/mol) = 273.0 kJ/mol
Therefore, the correct answer is A) -91.8 kJ/mol.