Chemistry Test Chemical Equilibrium 1 — Questions and Answers
Question 1: What is the correct definition of chemical equilibrium in a reversible reaction?
- The reaction has stopped completely
- The concentrations of reactants and products are equal
- The rates of the forward and reverse reactions are equal (Correct answer)
- All reactants have been converted to products
Correct answer: The rates of the forward and reverse reactions are equal
At chemical equilibrium, the forward and reverse reactions proceed at the same rate, so the net concentrations of reactants and products remain constant.
Question 2: Which expression correctly represents the equilibrium constant Kc for the reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)?
- Kc = [NH₃]² / ([N₂][H₂]³) (Correct answer)
- Kc = [N₂][H₂]³ / [NH₃]²
- Kc = [NH₃] / ([N₂][H₂])
- Kc = [N₂][H₂] / [NH₃]
Correct answer: Kc = [NH₃]² / ([N₂][H₂]³)
Kc is expressed as products over reactants, each raised to the power of their stoichiometric coefficients: [NH₃]² / ([N₂]¹[H₂]³).
Question 3: If Kc >> 1 for a reaction, what does this indicate about the equilibrium position?
- The equilibrium strongly favors the reactants
- The equilibrium strongly favors the products (Correct answer)
- Reactant and product concentrations are nearly equal
- The reaction does not proceed in either direction
Correct answer: The equilibrium strongly favors the products
A large Kc (>> 1) means the numerator (products) dominates the expression, so the equilibrium lies far to the right, favoring product formation.
Question 4: For the reaction H₂(g) + I₂(g) ⇌ 2HI(g), if [H₂] = 0.5 M, [I₂] = 0.5 M, and [HI] = 2.0 M at equilibrium, what is Kc?
- 4
- 8
- 16 (Correct answer)
- 32
Correct answer: 16
Kc = [HI]² / ([H₂][I₂]) = (2.0)² / (0.5 × 0.5) = 4.0 / 0.25 = 16.
Question 5: What is the relationship between Kp and Kc for a gas-phase reaction?
- Kp = Kc(RT)^Δn (Correct answer)
- Kp = Kc / (RT)
- Kp = Kc + RT·Δn
- Kp = Kc × Δn
Correct answer: Kp = Kc(RT)^Δn
Kp = Kc(RT)^Δn, where Δn is the change in moles of gas (moles of gaseous products minus moles of gaseous reactants), R is the gas constant, and T is temperature in Kelvin.
Question 6: Which of the following does NOT affect the numerical value of the equilibrium constant Kc?
- Increasing the temperature
- Adding a catalyst (Correct answer)
- Reversing the chemical equation
- Multiplying all coefficients by 2
Correct answer: Adding a catalyst
A catalyst lowers activation energy equally for both forward and reverse reactions, speeding up equilibration but not changing the value of Kc.
Question 7: For the equilibrium: CaCO₃(s) ⇌ CaO(s) + CO₂(g), which expression correctly represents the equilibrium constant Kc?
- Kc = [CaO][CO₂] / [CaCO₃]
- Kc = [CO₂] (Correct answer)
- Kc = [CO₂] / [CaCO₃]
- Kc = [CaCO₃] / ([CaO][CO₂])
Correct answer: Kc = [CO₂]
Pure solids are excluded from the equilibrium expression, so only [CO₂] remains, giving Kc = [CO₂].
Chemistry Test Chemical Equilibrium 1
What is the correct definition of chemical equilibrium in a reversible reaction?